Article

Classification of gravity-flow deposits and their significance for unconventional petroleum exploration, with a case study from the Triassic Yanchang Formation (southern Ordos Basin, China)

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... One type of gravity flow can transform into another type (Hampton, 1972;Haughton et al., 2003Haughton et al., , 2009Fan et al., 2018). For instance, debris flows may gradually become diluted and transform into turbidity currents through mixing with ambient water (Sumner et al., 2009;Zou et al., 2012;Henstra et al., 2016;Xu et al., 2016). ...
... The gravity flow deposits represent new exploration targets in the Weixi'nan Depression (Dong et al., 2014;Yang et al., 2019b). Little systematic research on gravity flow deposits using various types of data have been performed in this basin compared with other deep basins (Zavala et al., 2006;Zavala and Arcuri, 2016;Zou et al., 2012;Sang et al., 2013;Pu et al., 2014;Liu et al., 2014, 2016, Liu et al., 2017aTan et al., 2017;Fan et al., 2018;Xian et al., 2018a, c). In this paper, core, well logging and 3D seismic data are integrated to describe the features of the gravity flow deposits in the Weixi'nan Depression. ...
... SSDS are different from purely gravity-related deformation structures, such as load structures. SSDS are attributed to complex vertical adjustment and horizontal movement (Fan et al., 2018). SSDS are ascribed to slumping, collapse, liquefaction and fluidization. ...
Article
Gravity flows have been well investigated in deep-marine settings but less so in deep-lacustrine environments. To further understand the sedimentary characteristics and processes of gravity flows in a lacustrine rift basin, sublacustrine fans in the Weixi'nan Depression in the South China Sea were identified and studied by integrating core, well logging and 3D seismic data. Several aspects of gravity flow deposits were discussed in this study: (1) classification: seven lithofacies types were further grouped into four lithofacies assemblages/associations associated with emplacement by slumps, gravel-rich debris flows, high-density turbidity currents (which may transform into muddy debris flows) and low-density turbidity currents. (2) Proximal vs. distal position: the quantitative analysis of the lithofacies associations indicated that the gravity flow deposits are dominated by gravel-rich debris flow deposits and high-density turbidites in the proximal parts of the sublacustrine fans, whereas low-density turbidites dominate in the basin center. (3) Major triggers (humid climate and/or slope failures): the presence of active structures and a humid climate favored the formation of gravity flows, which originated from delta collapses and/or floods. (4) Lateral and vertical lithofacies distributions: the coarse-grained deposits represent a spectrum of different categories of gravity flows. In proximity to the slope break zone, slumps transformed into gravel-rich debris flows through liquefaction and deformation. As the gravity flows advanced toward the basin, the gravel-rich debris flows gradually became diluted with the surrounding water and transformed into high-density turbidity currents. When the flows reached the deep basin plain, the high-density turbidity currents transformed into low-density turbidity currents because the coarse grains could no longer remain in suspension. (5) An alternative mechanism: when the flows eroded the muddy substrate/basement, an alternative mechanism involving reverse-flow transformation occurred. In some cases, high-density turbidity currents transformed into muddy debris flows because the eroded light material became segregated toward the rear of the turbidity current and inhibited turbulence. This sedimentary model of gravity flows can be used for oil exploration in the study area and similar lacustrine rift basins.
... The use of high-resolution seafloor images and high-resolution threedimensional (3D) seismic datasets to investigate submarine sediment gravity-flow systems has led to substantial progress in understanding the morphology, stratigraphy, and architecture of such systems and in predicting the presence of hydrocarbon reservoirs within such systems (Alpak et al., 2011;Olariu et al., 2011;Sylvester et al., 2011;Wiles et al., 2013;Zhang et al., 2015;Chen et al., 2019). Petroleum exploration in continental basins has shown that deep-lacustrine gravity-flow systems have formed important hydrocarbon reservoirs in a number of basins and lake sequences worldwide (Feng et al., 1991;Feng et al., 2010a;Zou et al., 2012;Liu et al., 2015Liu et al., , 2016Meng et al., 2016;Pan et al., 2017Pan et al., , 2019Pan et al., , 2020Liu, L. et al., 2017;Liu, X et al., 2017;Yang et al., 2017a, b;Fongngern et al., 2018;Fan et al., 2018;Xian et al., 2018;Dodd et al., 2019;Yang et al., 2019;Liu et al., 2020). At present, research on gravity-flow deposits in lacustrine basins is mainly focused on genetic and depositional models, especially deposits related to slope failures; however, few studies have discussed deep-lake channel-levee-lobe systems associated with river floods, with the exceptions of Yang et al. (2017b), Fan et al. (2018) and Xian et al. (2018), who studied these systems in the Upper Triassic Ordos Basin, and Feng et al. (2010a), who described such systems in the first member of the Nenjiang Formation (K 2 n 1 ) within the Upper Cretaceous Songliao Basin, China. ...
... Petroleum exploration in continental basins has shown that deep-lacustrine gravity-flow systems have formed important hydrocarbon reservoirs in a number of basins and lake sequences worldwide (Feng et al., 1991;Feng et al., 2010a;Zou et al., 2012;Liu et al., 2015Liu et al., , 2016Meng et al., 2016;Pan et al., 2017Pan et al., , 2019Pan et al., , 2020Liu, L. et al., 2017;Liu, X et al., 2017;Yang et al., 2017a, b;Fongngern et al., 2018;Fan et al., 2018;Xian et al., 2018;Dodd et al., 2019;Yang et al., 2019;Liu et al., 2020). At present, research on gravity-flow deposits in lacustrine basins is mainly focused on genetic and depositional models, especially deposits related to slope failures; however, few studies have discussed deep-lake channel-levee-lobe systems associated with river floods, with the exceptions of Yang et al. (2017b), Fan et al. (2018) and Xian et al. (2018), who studied these systems in the Upper Triassic Ordos Basin, and Feng et al. (2010a), who described such systems in the first member of the Nenjiang Formation (K 2 n 1 ) within the Upper Cretaceous Songliao Basin, China. ...
... Hyperpycnal currents related to river floods occur more readily in lake basins than in marine basins (Mulder et al., 2003;Zavala and Arcuri, 2016), and carry clastic particles and organic matter together into the deep lake to form reservoirs and source rocks. Gravity-flow of sediment following slope collapse and related to river flooding can also carry sandy sediments into the deep lake, and may form reservoirs (Zavala and Arcuri, 2016;Yang et al., 2017b;Fan et al., 2018). ...
Article
The Songliao Basin is a large Cretaceous nonmarine petroliferous postrift downwarped basin in northeastern China. Deep lake channel-levee-lobe and slope fan deposits are identified within six depositional sequences that span from the Turonian Qingshankou Formation to the second member of the Campanian Nenjiang Formation. Research on the sediment gravity-flow systems in the Songliao Basin has broad application for understanding the sedimentary processes and pattern of sediment gravity-flow systems, and predicting the presence of good sandstone reservoirs within postrift downwarped lacustrine basins. Research results show that the channel-levee-lobe deposits are present within sequences in the first member of the Qinshankou Formation and the first member of the Nenjiang Formation. The straight channel levees and slope fans were on high-gradient slopes, whereas the sinuous channel levees were on low-gradient slopes. The straight-to-sinuous channel levees, fed by northeastern deltaic subaqueous distributary channels, extend for 15–70 km, predominantly within transgressive-to early highstand system tracts, which terminated in bifurcated or small lobes. The channel-levee-lobe and slope fan deposits sourced from western deltas were deposited basinward of the syndepositional flexural slope-break zones controlled by deep-seated faults. The falling trajectories of the paleoshorelines during the highstand and lowstand systems tracts and large sediment fluxes during the transgressive systems tracts resulted in the development of the slope fans and channel-levee-lobes. Lakebed paleorelief influenced the channel forms. Sand bodies within the channel-levee-lobes and slope fans, encased within organic-rich source rocks, represent new significant targets for hydrocarbon exploration.
... Recent research has emphasized on the importance of different origins of SSGF (Talling, 2014;Zavala and Arcuri, 2016), but the different SSGF types related to different origins have not been clearly distinguished (Liu et al., 2015). Moreover, how to recognize the origin of SSGF events based on the characteristics of the associated SSGF deposits is still poorly understood in lacustrine basins (Liu et al., 2015(Liu et al., , 2017bKremer et al., 2015;Fan et al., 2018). Because of the confusion associated with the types and origins of SSGF, the understanding of the factors that control the distributions pattern and depositional models of SSGF deposits only exists in general terms (Richards et al., 1998;Garcia et al., 2015;Liu et al., 2015;Yang et al., 2017). ...
... Therefore, these high-density turbidity currents are equivalent to bedload dominated hyperpycnal flows in the Jiyang Depression (Petter and Steel, 2006;Mulder and Chapron, 2011;Zavala and Arcuri, 2016). The low density turbidity currents can be subdivided into surge-like low density turbidity currents and quasi-steady low-density turbidity currents according to initiation processes and related deposits in the lacustrine basin (Fig. 15) (Mulder and Alexander, 2001;Gani, 2004;Mutti et al., 2009;Zavala and Arcuri, 2016;Fan et al., 2018). Deposits of surge-like low-density turbidity current are dominated by Ls, Gs, Cs and siltstones facies in LA 7 and LA 8, which are characterized by normal grading and Bouma-like sequences (Figs. 10, 12 and 14) (Talling et al., 2012;Zavala and Arcuri, 2016;Fan et al., 2018). ...
... The low density turbidity currents can be subdivided into surge-like low density turbidity currents and quasi-steady low-density turbidity currents according to initiation processes and related deposits in the lacustrine basin (Fig. 15) (Mulder and Alexander, 2001;Gani, 2004;Mutti et al., 2009;Zavala and Arcuri, 2016;Fan et al., 2018). Deposits of surge-like low-density turbidity current are dominated by Ls, Gs, Cs and siltstones facies in LA 7 and LA 8, which are characterized by normal grading and Bouma-like sequences (Figs. 10, 12 and 14) (Talling et al., 2012;Zavala and Arcuri, 2016;Fan et al., 2018). Deposits of quasi-steady low-density turbidity current are dominated by Gs, Cs and siltstones facies in LA 3 and LA 4, which are characterized by inverse to normal grading and stacking of climbing ripples with abundant thin laminar plant fragments (Figs. ...
Article
Subaqueous sediment gravity-flow (SSGF) deposits have recently been recognized as a major reservoir plays in lacustrine basins. Research on their genesis and depositional models are crucial to understand gravity flow evolution and the distribution of their deposits. This study analysed SSGF deposits in the third member of the Shahejie Formation in the Jiyang Depression, Eastern China, based on 3-D seismic data, well-log data, and core observations. The types and origins of SSGF, the distribution patterns and depositional models of SSGF deposits have been investigated. The dominant types of SSGF include: muddy debris flows, sandy debris flows, high density turbidity currents, surge-like low density turbidity currents, and quasi-steady low density turbidity currents, which are mainly caused by river floods and sediment failures. The overall distribution of SSGF deposits is controlled by the sediment supplies, basin structure, and sequence stratigraphic framework. Paleotopography, flood energy, triggering mechanism, and the slope angle of the delta front are also important. SSGF deposits associated with river floods can be further subdivided into elongated and fan-shaped deposits. Elongated sandstones are composed predominantly of bedload-dominated and suspension load-dominated hyperpycnal flow deposits. The fan-shaped sandstones can be subdivided into an inner fan, a middle fan, and an outer fan. Inner fan deposits are dominated by gully-filling debrites. Middle fan deposits are composed predominantly of channel-fill deposits, while outer fan deposits consist of thin bedded and laterally extensive lobe intervals. SSGF deposits triggered by sediment failures can be further subdivided according to their proximity to slumping: proximal, intermediate and distal parts. The proximal deposits are composed predominantly of slide and slump deposits. The intermediate deposits are dominated by sandy debrite tongues, while the distal deposits are composed predominantly of thin-bedded, surge-like, low-density turbidites. This study offers some insight into the types, origin of gravity-flow, as well as the distribution of their deposits in a lacustrine rift basin.
... However, the relationship between tectonic movements and geological events is not yet fully understood. How gravity flows influence OM accumulation has long been a focus of geological research (Chen et al., 2019;Fan et al., 2018;Ghadeer and Macquaker, 2012;Yang and Deng, 2013;Yang et al., 2017). It has long https://doi.org/10.1016/j.marpetgeo.2020.104472 ...
... The association between OM enrichment and gravity flows has been widely discussed (Chen et al., 2019;Fan et al., 2018;Ghadeer and Macquaker, 2012;Yang and Deng, 2013;Yang et al., 2017). Some researchers suggested that fine-grained deposits from gravity flows could bring organic fragments into deep water areas (Ghadeer and Macquaker, 2012;Stow and Tabrez, 1998). ...
Article
No detailed research has been carried out on the oil shales that are interbedded with gravity flow deposits and volcaniclastic rocks in the third Member of the Jiufotang Formation. In order to study the impact of basin evolution, gravity flows, and volcanism on oil shale formation, petrology, sedimentology, sequence stratigraphy, organic geochemistry, and elemental geochemistry were used to characterise the oil shales. Extensional tectonic subsidence is suggested as the controlling factor for basin evolution. The predominant maceral in the oil shales is sapropelinite, suggesting organic matter was derived from mainly aquatic organism (especially algae). Element indices (δU, (Cu + Mo)/Zn, Sr/Ba, Th (μg/g), Zr (μg/g), and the chemical index of alteration (CIA) show that the water conditions during deposition of the oil shales were anoxic and brackish, with little terrigenous detrital input and a cold and dry palaeoclimate. Gravity flows were accompanied by the input of terrestrial detritus and a degree of oxygenation of the bottom waters, reducing oil shale formation. Volcanic eruptions ejected intermediate-acidic ashes, not only leading to climate cooling, but also either enhancing algal blooms (by a fertilisation effect due to minor volcanic ash input) or inhibiting algal blooms (by a poisoning effect due to major volcanic ash input). There is synchronicity between the occurrence of rifting, gravity flows, volcanism, and earthquakes. The best periods for oil shale formation were during intermissions between tectonic movement, when there were few gravity flows, volcanic eruptions, or earthquakes. High bioproductivity and lake expansion along with an anoxic environment were the main controlling factors for high-quality oil shale formation. There is a series of oil shale-bearing rift basins formed in northeastern China that share a similar geological setting, so this research provides a useful example for future resource exploration of coeval oil shales in northeastern China.
... The Yanchang Formation serves as a good example for the study of low-permeability sandstone reservoirs. Several works have been published on its regional structure, stratigraphy, and depositional environment ( Ritts et al., 2009;Zou et al., 2012;Fan et al., 2018). However, studies on the controlling factors of reservoir quality are lacking. ...
... The Chang 10 1 reservoir is representative of China's low-permeability reservoirs( Yang et al., 2013;Fu et al., 2015). The current exploration has led to the discovery of a number of largescale oilfields with industrial values, suggesting that there are relatively high quality areas throughout the low porosity and low permeability reservoirs, that have great exploration potential in the future ( Zou et al., 2012;Fan et al., 2018). This paper shows that various factors jointly control the low porosity and low permeability of the reservoir, and reveal the influencing factors and distribution characteristics of the relatively high quality areas. ...
Article
There are many reservoirs with low porosity and low permeability in the world, and many large oilfields have been found in these reservoirs. Unlike conventional reservoirs, the genesis of low-porosity and low-permeability reservoirs and the controls of relatively high-porosity and high-permeability zones are still unclear. This study takes the Chang101 reservoir sandstones of the Shanbei area in the Ordos Basin, China, a typical low-porosity and low-permeability, as an example. Chang101 sandstones are fine-grained to medium-grained, moderately to well sorted arkose and lithic arkose deposited in a delta plain environment. It is a typical low porosity and low permeability reservoir with an average porosity of 10.21% and an average permeability of 2.88 mD. Pore types mainly include primary intergranular pores and secondary dissolved pores. The main controls on the reservoirs are the sedimentary environment and diagenesis. The sedimentary environment determines the initial properties of the reservoir sandstones, including the material composition and texture that determine the primary porosity and permeability. Diagenesis further transforms the reservoir quality to develop heterogeneity in the porosity and permeability. The reservoir sandstones have undergone complex diagenetic processes and are currently in stage A of mesodiagenesis. Compaction accounts for the major reduction of the primary porosity of the sandstones. Different types of cementation have both positive and negative influences on reservoir quality. Carbonate cementation, quartz overgrowths and some authigenic clay cementation(i.e., I/S and illite) are major pore-filling components, that reduce the reservoir quality. An appropriate amount of chlorite resists compaction and prevents quartz overgrowth, but excessive chlorite coatings fill the pores and block the pore throats. The laumontite cement is not only resistant to compaction but also easily dissolved to form a large number of effective secondary pores. The dissolution of feldspar and rock fragments plays a role in contributing to connecting the isolated intergranular pores and increasing the secondary pores. Relatively high porosity and high permeability zones are deposited in high-energy environment, mainly delta plain distributary channel sandstones with more detrital quartz, fewer rock fragments and coarser grains than other sandstones, were deposited in high-energy environment. There is weaker compaction and less or no carbonate cement, with the preservation of the primary pores by chlorite coatings and an increase in the pores by the dissolution of laumontite cement.
... The Ordos Basin is a rectangular syncline bounded by North-South striking structural features with a gentle slope in the east and a steep slope in the west (Fan et al., 2018;Yang et al., 2019;Yang, Jin, van Loon, Han, & Fan, 2017). Based on the present configuration of structural morphology and sequences of basin evolution history, the basin can be divided into six primary tectonic units: the Yimeng Uplift, the Weibei Uplift, the Jinxi Fold Belt, the Yishan Slope, the Tianhuan Depression, and the Western Thrust Belt (Liu, Li, & Liao, 2014) ( Figure 1). ...
... Comparative study and analysis of pore types in different sedimentary environments across the north and south of the study area show that intergranular pores in braided river deposits and F I G U R E 2 Stratigraphic and lithologic column of the Upper Palaeozoic in the study area (modified from Fan et al., 2018) meandering river deposits are 63.6% and 46.3%, respectively. The development of inter-crystalline pore in meandering river deposits is more than that in braided river deposits, 40% and 23.1%, respectively. ...
Article
The sedimentary characteristics and petrophysical properties of the reservoir of the second/lower member of the Lower Permian Shanxi Formation (P1s2) in Sulige Gas Field, Central China are studied by means of thin section petrography and scanning electron microscopy. This article highlights the relationship between reservoir quality and sedimentary environments, explains the differences in reservoir petrophysical properties in different sedimentary environments, discusses the influence of clastic components and authigenic minerals on the reservoir, and finally, provides the basis for the prediction of relatively high‐quality reservoir in P1s2 of the study area. The results show that the sandstones formed as a meandering river deposit in the south of the study area are fine‐grained, mainly composed of lithic fragments of quartz sandstone, metamorphic, and volcanic rock lithic fragment, with high content of matrix and poor sorting, average porosity of 5.09%, and average permeability of 0.20 × 10⁻³ μm². The braided river sedimentary deposits in the north are coarse‐grained, mainly composed of quartz sandstone sedimentary rock lithic fragment, with low content of matrix, good sorting, and more developed dissolution pores. The average porosity is 6.16%, and the average permeability is 0.53 × 10⁻³ μm². Among the authigenic minerals, siderite, calcite, and illite block, the pores between the grains reduce the porosity. Siliceous cement often indicates better reservoir petrophysical properties. The siliceous cement is more common in braided river deposits in the north area, in which the channel bar detailed facies are conducive to retain more primary pores or form secondary pores, thus forming better reservoirs.
... Lacustrine tight oil sandstones, especially those developed in deepwater gravity-flow deposits, are relatively unexplored and less well studied than marine tight oil reservoirs (Munawar et al., 2018;Wu et al., 2019;Yang et al., 2018). In recent years, lacustrine deep-water gravity-flow deposition has become a hot topic in the field of international academic research and petroleum exploration (Stevenson and Peakall, 2010;Bernhardt et al., 2012;Bell et al., 2018) because of the greater variability in composition and the more complex lithofacies patterns than in marine deposits (Sonnenberg, 2015;Kvale et al., 2017;Perry et al., 2017;Fan et al., 2018;Wang et al., 2018). Many of the tight reservoirs in China, such as those in the Ordos Basin, Songliao Basin, and Bohai Bay Basin, are composed of lacustrine deep-water gravityflow sediments Yang et al., 2018). ...
... Many of the tight reservoirs in China, such as those in the Ordos Basin, Songliao Basin, and Bohai Bay Basin, are composed of lacustrine deep-water gravityflow sediments Yang et al., 2018). Previous research achievements related to lacustrine gravity-flow reservoirs pertain mainly to sedimentary architecture analysis, lithofacies classification, and sedimentary transport processes (Liu et al., 2017;Fan et al., 2018;Xian et al., 2018); however, the relationships among diagenetic alterations, lithofacies, and reservoir quality of tight sandstones in lacustrine deep-water gravity-flow reservoirs are still poorly understood. The lithofacies of the lacustrine deep-water gravity-flow deposits have their unique characteristics. ...
Article
The tight oil sandstone reservoir of the Upper Triassic Yanchang Formation is one of the main targets for hydrocarbon exploration in the Ordos Basin, China. Sandstones of the Upper Triassic Yanchang Formation within lacustrine deep-water gravity-flow deposits are characterized by ultralow porosity, ultralow permeability, and complex diagenetic alterations and sedimentary lithofacies. However, the diagenetic alterations and their controls on the reservoir quality of the Chang 6 tight sandstones remain poorly understood. This study integrated a variety of techniques, such as casting thin section analysis, scanning electron microscopy (SEM), back-scattered electron imagery (BSE), X-ray diffraction (XRD), cathodoluminescence (CL) microscopy, and fluid inclusion, to investigate the lithofacies, diagenetic history, and diagenetic intensity, and also evaluate their controls on reservoir quality. The research results indicated that the Chang 6 sandstones exhibit variable intensity of diagenetic alterations among the various lithofacies, which leads to variable reservoir quality. Mechanical compaction was the most important factor reducing primary porosity. Sandstones of the Ss lithofacies (siltstone to very fine-grained sandstones with graded bedding or ripple laminations) showed the highest loss of primary porosity. Carbonate cements were commonly present in sandstones of the Sc (fine-grained and cross-bedded sandstones) and Sm (fine-grained and massive sandstones) lithofacies near the sandstone–mudstone bounding surface. Sandstones with more authigenic illite rims were able to resist compaction to some extent and were favorable to the preservation of porosity; however, abundant pore-filling illite aggregates commonly bridged pore throats, causing a significant reduction in reservoir quality. Feldspar dissolution porosity leads to a slight enhancement in reservoir quality. Intergranular pores and feldspar dissolution pores are most abundant in the sandstones away from the sandstone–mudstone bounding surface of the Sc lithofacies, resulting in the best reservoir quality, which is followed by sandstones of the Sm lithofacies. Ultimately, four evolution patterns of diagenetic alterations that formed best, moderate, or poor reservoir quality were summarized by linking diagenetic alterations to lithofacies. The research results are conducive to the evaluation of reservoir quality and to understanding the genetic mechanisms of tight sandstones, so they should facilitate hydrocarbon exploration and production in similar lacustrine deep-water gravity-flow reservoirs.
... Calcium carbonate minerals at Earth's surface conditions include the common polymorphs calcite, aragonite, and vaterite, and the less common hydrated phases of monohydrocalcite (CaCO3•H2O) and ikaite (CaCO3•6H2O) [15][16][17][18][19][20][21][22][23][24][25][26][27]. It has been reported that monohydrocalcite is metastable compared with calcite and aragonite at all temperatures and pressures [28]. ...
Article
The halophilic bacterium Halomonas smyrnensis from a modern salt lake used in experiments to induce biomineralization has resulted in the precipitation of monohydrocalcite and other carbonate minerals. In this study, a Halomonas smyrnensis WMS-3 (GenBank:MH425323) strain was identified based on 16S rDNA homology comparison, and then cultured in mediums with 3% NaCl concentration to induce monohydrocalcite at different Mg/Ca molar ratios of 0, 2, 5, 7, and 9. The growth curve of WMS-3 bacteria, pH values, NH4 + concentration, HCO3 − and CO3 2− concentration, carbonic anhydrase (CA) activity, and the changes in Ca 2+ and Mg 2+ ion concentration were determined to further explore the extracellular biomineralization mechanism. Moreover, the nucleation mechanism of monohydrocalcite on extracellular polymeric substances (EPS) was analyzed through studying ultrathin slices of the WMS-3 strain by High resolution transmission electron microscopy (HRTEM), Selected area election diffraction (SAED), Scanning transmission electron microscopy (STEM), and elemental mapping, besides this, amino acids in the EPS were also analyzed. The results show that pH increased to about 9.0 under the influence of ammonia and CA activity. The precipitation ratio (%, the ratio of the mass/volume concentration) of the Ca 2+ ion was 64.32%, 62.20%, 60.22%, 59.57%, and 54.42% at Mg/Ca molar ratios of 0, 2, 5, 7, and 9, respectively, on the 21st day of the experiments, and 6.69%, 7.10%, 7.74%, 8.09% for the Mg 2+ ion concentration at Mg/Ca molar ratios 2, 5, 7, and 9, respectively. The obtained minerals were calcite, Mg-rich calcite, aragonite, and hydromagnesite, in addition to the monohydrocalcite, as identified by X-ray diffraction (XRD) analyses. Monohydrocalcite had higher crystallinity when the Mg/Ca ratio increased from 7 to 9; thus, the stability of monohydrocalcite increased, also proven by the thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) analyses. The C=O and CO -C organic functional groups present in/on the minerals analyzed by Fourier transform infrared spectroscopy (FTIR), the various morphologies and the existence of P and S determined by scanning electron microscope-energy dispersive spectrometer (SEM-EDS), the relatively more negative stable carbon isotope values (−16.91‰ to −17.91‰) analyzed by a carbon isotope laser spectrometer, plus the typical surface chemistry by XPS, all support the biogenesis of these mineral precipitates. Moreover, Ca 2+ ions were able to enter the bacterial cell to induce intracellular biomineralization. This study is useful to understand the mechanism of biomineralization further and may provide theoretical reference concerning the formation of monohydrocalcite in nature.
... It is generally accepted that the coal-bearing rocks in the Shanxi Formation (lower Permian) and the dark shale rocks in the Yanchang Formation (upper Triassic) are two important source rocks for the petroleum resources of the Ordos Basin (Yu et al., 2017). The Yanchang Formation forms a 1,000 − 1,300 m thick succession of detrital riverdelta-lake sediments (Zhang et al., 2010;Yang et al., 2014;Fan et al., 2018;Shi et al., 2019) in a humid paleoclimate (Qiu et al., 2015a(Qiu et al., , 2015b. The Mesozoic hydrocarbon reservoirs in the Zhenjing area (southwestern Ordos Basin) were sourced mainly from black shales of the Yanchang Formation Qiu et al., 2013;Shi et al., 2019;Wang et al., 2017), which is also a resource for unconventional oil and gas (Yang et al., 2017). ...
Article
This study provides an integrated interpretation of the Mesozoic ‐ Cenozoic tectonothermal evolution of Permian strata in the Qishan area of the southwestern Weibei Uplift, Ordos Basin. Apatite fission‐track and apatite/zircon (U‐Th)/He thermochronometry, bitumen reflectance, thermal conductivity of rocks, paleotemperature recovery and basin modeling were used to recover the Meso‐Cenozoic tectonothermal history of Permian Strata. The Triassic AFT data has a pooled age of ∼180 ± 7 Ma (one age peak) with P(χ2) = 86%. The average corrected apatite (U‐Th)/He age of two Permian sandstones is ∼168±4 Ma and a zircon (U‐Th)/He age from Cambrian strata is ∼231±14 Ma. Bitumen reflectance and maximum paleotemperature of two Ordovician mudstones are 1.81%, 1.57% and ∼210°C, ∼196°C respectively. After undergoing a rapid subsidence and increasing temperature in the Triassic by a thermal abnormal, the Permian experienced four stages of cooling‐uplift history after the time when the maximum paleotemperature reached in the late Jurassic: (1) A cooling stage (∼163 Ma ‐ ∼140 Ma) with temperatures ranging from ∼132 °C to ∼53 °C and cooling rate of ∼3 °C/Ma, erosion thickness of ∼1900 m and an uplift rate of ∼82 m/Ma; (2) A cooling stage (∼140 Ma ‐ ∼52 Ma) with temperatures ranging from ∼53 °C to ∼47 °C and cooling rate less than ∼0.1 °C/Ma, erosion thickness of ∼300 m and an uplift rate of ∼3 m/Ma; (3) (∼52 Ma ‐ ∼8 Ma) with ∼47 °C to ∼43 °C and ∼0.1 °C/Ma, erosion thickness of ∼500 m and an uplift rate of ∼11 m/Ma; (3) (∼8 Ma – present) with ∼43 °C to ∼20 °C and ∼3 °C/Ma, erosion thickness of ∼650 m and an uplift rate of ∼81 m/Ma. Tectonothermal evolution history of the Qishan area in the Triassic was controlled by the interaction of the Qinling Orogeny and the Weibei Uplift, and the exposed Permian strata had the earliest uplift‐cooling time compared to other parts within the Weibei Uplift. The early Eocene at ∼52 Ma and late Miocene at ∼8 Ma, as two significant turning points after which both the rate of uplift and the rate of temperature changed rapidly, were two key time of uplift‐cooling history for the exposed Permian in the Qishan area.
... In fact, the mineral precipitation process mediated by organic components occupies an important position in both present and ancient environments [56][57][58][59][60][61][62][63][64][65][66][67][68][69][70]. The adsorption of Ca 2+ and Mg 2+ ions on the bacterial cell surface is the initial step of biomineralization. ...
Article
Removal of calcium and magnesium ions through biomineralization induced by bacteria has been proven to be an effective and environmentally friendly method to improve water quality, but the process and mechanism are far from fully understood. In this study, a newly isolated probiotic Bacillus licheniformis SRB2 (GenBank: KM884945.1) was used to induce the bio-precipitation of calcium and magnesium at various Mg/Ca molar ratios (0, 6, 8, 10, and 12) in medium with 30 g L −1 sodium chloride. Due to the increasing pH and HCO3 − and CO3 2− concentrations caused by NH3 and carbonic anhydrase, about 98% Ca 2+ and 50% Mg 2+ were precipitated in 12 days. The pathways of bio-precipitation include extracellular and intracellular processes. Biominerals with more negative δ 13 C values (−16%� to −18%�) were formed including calcite, vaterite, monohydrocalcite, and nesquehonite with preferred orientation. The nucleation on extracellular polymeric substances was controlled by the negatively charged amino acids and organic functional groups. The intracellular amorphous inclusions containing calcium and magnesium also contributed to the bio-precipitation. This study reveals the process and mechanism of microbial desalination for the removal of calcium and magnesium, and provides some references to explain the formation of the nesquehonite and other carbonate minerals in a natural and ancient earth surface environment.
... The study area is located in the southwestern Ordos Basin, covering two secondary tectonic units, the Tianhuan Depression and the Yishan Slope (Figure 1(a)). The Upper Triassic Yanchang Formation was deposited in a large inland lacustrine system (Fan et al., 2018;Yang et al., 2017b;Zou et al., 2010), and records a complete sedimentary cycle. The Yanchang Formation can be divided into 10 members based on lithologic characteristics (Yang, 2004). ...
Article
Full-text available
The Chang-8 and Chang-6 members of the Upper Triassic Yanchang Formation (lower part) are regarded as the main oil producing members of the Ordos Basin. Recently, new hydrocarbon discoveries have been made in the upper part of the Yanchang Formation (e.g., Chang-3) in the southwestern Ordos Basin, implying that this interval also has a good potential for hydrocarbon exploration. However, studies on the origin of the high-quality reservoir, hydrocarbon migration, and accumulation patterns remain insufficient. In this study, integrated petrological, mineralogical, and fluid inclusion tests are employed to evaluate reservoir characteristics, and reconstruct the history of hydrocarbon migration and accumulation during oil and gas reservoir formation. The results reveal that the Yanchang Formation is characterized by low porosity (8 − 14%), medium permeability (0.5 − 5 mD), and strong heterogeneity; the reservoir properties are controlled by secondary porosity. Two types of dissolution are recognized in the present study. Secondary pore formation in the lower part of the formation is related to organic acid activity, while dissolution in the upper part is mainly influenced by atmospheric fresh water associated with the unconformity surface. The Yanchang Formation underwent hydrocarbon charging in three phases: the early Early Cretaceous, late Early Cretaceous, and middle Late Cretaceous. A model for hydrocarbon migration and accumulation in the Yanchang reservoirs was established based on the basin evolution. We suggest that hydrocarbon accumulation occurred at the early stage, and that hydrocarbons migrated into the upper part of the Yanchang Formation by way of tectonic fractures and overpressure caused by continuous and episodic hydrocarbon expulsion during secondary migration, forming potential oil reservoirs during the later stage.
... Lithofacies analysis may help solve the problem, but the results are also evidence rather than proof. A marine setting for the Lingshandao Formation has been proposed by Lu et al. (2011Lu et al. ( , 2012 on the basis of its flysch architecture (alternations of dark-gray autochthonous shales and thin light-gray turbidites), but turbidites may also occur in lacustrine settings (e.g., Yang et al., 2014Yang et al., , 2017bFan et al., 2018). The occurrence of turbidites can consequently not be used as a criterion to distinguish between a marine and a continental setting. ...
Article
Interpretation of the continental or marine character of sediments on the basis of the paleosalinity is increasingly based on the concentration of some trace elements. The paleosalinity can be affected, however, also in deep-marine environments, by discharge peaks of major sediment-laden rivers. These induce hyperpycnal flows that run down the basin slope. The huge volume of fresh water gradually mixes with the ambient saline seawater, diminishing the paleosalinity. The temporary lowered paleosalinity may be preserved in the sedimentary record in the form of ‘diverging’ concentrations of trace elements and trace-element ratios. This is exemplified for the Early Cretaceous Lingshandao Formation on Lingshan Island (western Yellow Sea). It was found during field work and geochemical analysis that hyperpycnal currents may not only lower the salinity of a deep-marine environmental setting, but may also carry along continental plant fragments and other biological remains, which may obscure the marine character of the sediments. It is deduced that detailed facies analysis in the field is consequently required to reveal the true character of a marine environment.
... Biotic limestone well developed in this area, and many types of biotic limestones have Comparative study on thermal behaviors between micrites and thrombolites using… 1231 largely appeared. The characteristics of limestones are diverse with the obviously different sedimentary environments and formed causes [43][44][45][46][47][48][49]. Therefore, this area is an ideal place to study biotic limestones. ...
Article
Although micrites (abiotic limestone) and thrombolites (typical biotic limestone), according to the sedimentation and biological action, can be primarily distinguished from intuitively field observation and rock slices analysis. However, further analysis of thermal decomposition unique characteristics to distinguish them has rarely been reported. Here, the comparative studies of thermal decomposition behavior between micrites and thrombolites were carefully investigated using thermogravimetric analyzer in nitrogen atmosphere at multi-heating rates of 5, 10, 20 and 30 K min-1 from 323.15 to 1273.15 K. Moreover, the mineralogical compositions were both analyzed by powder X-ray diffraction followed by the results of calcite acting as the basic constituent for further verified. The kinetic model function and kinetic parameters were calculated by Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose and Popescu methods, respectively. The results showed that the kinetic parameter activation energy (E) of thrombolites was obviously higher than that of micrites, which suggests the former have a higher crystallinity. In addition, the results of difference significance analysis showed extremely significant differences (P \ 0.01) via statistical analysis using SPSS v19.0 for the E values between micrites and thrombolites calculated by different methods. These above results further confirmed that the thermal stability and crystallinity of thrombolites were obviously higher under the inducing effects by typical microorganisms. This provided very important useful information for understanding the mechanisms of abiotic and biotic limestones formed in nature; in particular, the thermal analysis of thrombolites maybe also provides an important guiding significance for both indoor and outdoor similar studies.
... Controlling factors for the development of hyperpycnal flows include tectonic setting, climate, and the density difference between the turbidity currents from river flooding and the ambient water in the lake (Bourget, Zaragosi, & Mulder, 2010;Pouderoux, Jean-Noël, Geoffroy, Alan, & Helen, 2012;Yang et al., 2017). The Luanping Basin is a small half-graben basin with a relatively high gradient compared to that of a depressed lacustrine basin such as the Ordos Basin (Fan, Yang, van Loon, Yin, & Han, 2018;Yang et al., 2017). Sedimentary structures (wave ripples and low-angle lamination) and interbedded greyish and greyish-green laminated mudstone indicate a shallow lacustrine environment. ...
Article
Full-text available
Hyperpycnites are extrabasinal turbidites related to hyperpycnal flows which form during river floods. Compared to intrabasinal turbidite systems in deep marine or lacustrine settings, hyperpycnites in shallow lakes settings are not understood. In this work, we studied the typical sedimentary characteristics of shallow lacustrine hyperpycnal flow deposits in outcrops of the Luanping Basin, Northeast China. Based on the sedimentary architecture and analysis of the facies associations in these outcrops, the hyperpycnites can be classified into the following three types: (a) bedload‐dominated erosional channel deposits, (b) bedload‐ and suspended‐load‐dominated channel‐to‐lobe transitional deposits, and (c) suspended‐load‐dominated lobe deposits. The erosional channels in proximal locations are filled by mud clasts, parallel‐laminated sandstones, massive sandstones, cross‐bedded sandstones, massive siltstones, and mudstones. The channel‐to‐lobe transitional deposits in a middle location are composed of parallel‐laminated sandstones, cross‐bedded sandstones, mud clasts near internal erosional surfaces, ripple cross‐bedded sandstones, massive siltstones, and mudstones. The lobe deposits in distal locations are composed of ripple cross‐bedded sandstones, massive siltstones, and mudstones. The sedimentary process of the shallow lacustrine hyperpycnal flows shows obvious response to periodic flood energy fluctuations. Gentle slope, small capacity of flood discharges and semiarid paleoclimate may be the possible reason for the observed sedimentary architecture of the shallow lacustrine hyperpycnal flow deposits. Therefore, this study offers new insights into the sedimentology of shallow lacustrine deposits in rift basins.
... The biggest difference between biomineralization and general mineralization lies in the participation of bio-macromolecular metabolism, cells, and organic matrix. The product of biomineralization, namely, biominerals, can be carbonate, phosphate, sulfate, sulfide, silicate, hydroxide or oxide, and involve different species of cations such as calcium, magnesium, iron, and/or manganese [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. In the process of bacterially induced mineralization, microorganisms can release metabolic products to react with metal ions in the environment, and the process usually results in the subsequent precipitation of mineral crystals [16][17][18][19][20][21]. ...
Article
Biomineralization induced by microorganisms has become a hot spot in the field of carbonate sedimentology; however, the mechanisms involved still need to be explored. In this study, the bacterium Bacillus subtilis J2 (GenBank MG575432) was used to induce the precipitation of calcium carbonate minerals at Mg/Ca molar ratios of 0, 3, 6, 9, and 12. Bacillus subtilis J2 bacteria released ammonia to increase pH, but the ammonia released only made the pH increase to 8.25. Carbonic anhydrase was also produced to catalyze the hydration of carbon dioxide, and this process released carbonate and bicarbonate ions that not only increased pH but also elevated carbonate supersaturation.The biominerals formed at a Mg/Ca molar ratio of 0 were spherulitic, elongated, dumbbell-shaped, and irregularly rhombohedral calcite; at a Mg/Ca molar ratio of 3, the biominerals were calcite and aragonite, the weight ratio of calcite decreased from 26.7% to 15.6%, and that of aragonite increased from 73.3% to 84.4% with increasing incubation time. At higher Mg/Ca molar ratios, the biominerals were aragonite, and the crystallinity and thermal stability of aragonite decreased with increasing Mg/Ca molar ratios. FTIR results showed that many organic functional groups were present on/within the biominerals, such as C–O–C, N–H, C=O, O–H, and C–H. HRTEM-SAED examination of the ultra-thin slices of B. subtilis J2 bacteria showed that nano-sized minerals with poor crystal structure had grown or been adsorbed on the EPS coating. The EPS of the B. subtilis J2 strain contained abundant glutamic acid and aspartic acid, which could be deprotonated in an alkaline condition to adsorb Ca2+ and Mg2+ions; this made EPS act as the nucleation sites. This study may provide some references for further understanding of the mechanism of biomineralization induced by microorganisms.
... Biotic limestone well developed in this area, and many types of biotic limestones have Comparative study on thermal behaviors between micrites and thrombolites using… largely appeared. The characteristics of limestones are diverse with the obviously different sedimentary environments and formed causes [43][44][45][46][47][48][49]. Therefore, this area is an ideal place to study biotic limestones. ...
Article
Although micrites (abiotic limestone) and thrombolites (typical biotic limestone), according to the sedimentation and biological action, can be primarily distinguished from intuitively field observation and rock slices analysis. However, further analysis of thermal decomposition unique characteristics to distinguish them has rarely been reported. Here, the comparative studies of thermal decomposition behavior between micrites and thrombolites were carefully investigated using thermogravimetric analyzer (TGA) in nitrogen atmosphere at multi-heating rates of 5, 10, 20 and 30 K min -1 from 323.15 to 1273.15 K. Moreover, the mineralogical compositions were both analyzed by powder X-ray diffraction (XRD) followed by the results of calcite acting as the basic constituent for further verified. The kinetic model function and kinetic parameters were calculated by Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and Popescu methods, respectively. The results showed that the kinetic parameter activation energy (E) of thrombolites was obviously higher than that of micrites, which suggests the former has a higher crystallinity. In addition,the results of difference significance analysis showed extremely significant differences (P < 0.01) via statistical analysis using SPSS v19.0 for the E values between micrites and thrombolites calculated by different methods. These above results further confirmed that the thermal stability and crystallinity of thrombolites were obvious higher under the inducing effects by typical microorganisms. This provided very important useful information for understanding the mechanisms of abiotic and biotic limestones formed in nature, especially the thermal analysis of thrombolites maybe also provide an important guiding significance for both indoor and outdoor similar studies.
... The Ordos Basin is the second largest sedimentary basin in China with abundant oil and gas resources (Qiu et al., 2015;Wang et al., 2017;Fan et al., 2018;Shi et al., 2019). The Ordovician Majiagou Formation consists of six lithologic parts from bottom to top, which is a complete marine transgression-regression cycle (Fig. 1). ...
Article
The geochemical characteristics, genesis and accumulation mechanism of the pre-salt gases of Ordos Basin have been rarely studied in China. Components and carbon isotope values of pre-salt gases are investigated and compared with those of the Upper Paleozoic and post-salt gases. Homogenization temperatures and carbon isotope values of gases of the pre-salt fluid inclusions were also measured. Based on the geochemical parameters, combined with the geological conditions, the genesis and accumulation mechanism of pre-salt gases are studied. The components, carbon isotope values and the first two homogenization temperature ranges (90–120 °C and 120–160 °C) of the pre-salt gases contain those of the Upper Paleozoic and post-salt gases. The pre-salt inclusions have an extra homogenization temperature range (160–200 °C) and the pre-salt rocks also have some hydrocarbon generation capacity, which means the pre-salt rocks may also generate some gases. The pre-salt gases are mixed gases generated from the Upper Paleozoic coal strata, the post-salt and pre-salt Lower Paleozoic Majiagou Formation, which is confirmed by the geochemical characteristics and the geological settings.
... From the Middle to the Late Permian, sedimentation took place in a fluvial-deltaic to lacustrine environment adjacent to the ancient sea (Yang et al., 2014a). During the Mesozoic times, deposition within the Ordos Basin was dominated by continental fluvial, deltaic and lacustrine environments (Yang et al., 2014b(Yang et al., , 2017aFan et al., 2017Fan et al., , 2018. Finally, during Cenozoic times, the Ordos Basin was uplifted as a plateau with a peripheral rift valley. ...
Article
Tight sandstone gas reservoirs constitute an important type of unconventional gas resources and may be of crucial significance as environment-friendly source of energy in the world. However, the dynamics of densification and evolution of the low-porosity and low-permeability sandstone reservoirs are intensively debated in literature. Recently, the question of sequence of reservoir densification and hydrocarbon accumulation has arisen. A typical tight sandstone reservoir (the first member of the Shanxi Formation and the eighth member of Shihezi Formation) in the Permian Sulige gas field in Central China provides an excellent case study to probe these questions. By using a variety of different methodologies such as scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence, fluid inclusion analysis and confocal Raman spectrometer analysis, we identified different authigenic minerals such as illite, kaolinite, dickite, chlorite and smectite illitization coexisting with siliceous and calcitic cements. These authigenic minerals precipitated at different diagenetic stages, related to the changing conditions during compaction and lithification, and therefore, changed the porosities of the host rocks during each of these stages. The types of pores in the Sulige gas field are dominated by illite and kaolinite intercrystalline pores, matrix solution pores, and primary intergranular pores. The results of this study suggest that compaction and cementation are the reasons for the most distinct loss in porosity. On the other hand, kaolinite and illite preserve their intercrystalline pores, which could be identified as the main reserve space of the tight gas reservoirs. Therefore, the secondary porosity of the sandstone reservoirs forms the most important contributor to the gas reservoir potential in the Sulige gas field. In addition, it could be inferred that the tight gas accumulation took place during the densification of the reservoirs. The implications of the presented results may also help to understand the formation of other tight reservoirs world-wide.
... Lithofacies analysis may help solve the problem, but the results are also evidence rather than proof. A marine setting for the Lingshandao Formation has been proposed by Lu et al. (2011Lu et al. ( , 2012 on the basis of its flysch architecture (alternations of dark-gray autochthonous shales and thin light-gray turbidites), but turbidites may also occur in lacustrine settings (e.g., Yang et al., 2014Yang et al., , 2017bFan et al., 2018). The occurrence of turbidites can consequently not be used as a criterion to distinguish between a marine and a continental setting. ...
Conference Paper
Chemical parameters are increasingly used to determine depositional conditions of ancient deposits, and to help interpret the depositional environment. The conclusions based on such chemical parameters can, however, be incorrect if no additional field checks are carried out. An example is the way in which the continental or marine character of sediments is established on the basis of the paleosalinity preserved in the sediments: the concentration of some trace elements-and even more the ratio between some trace elements-is a proxy for the paleosalinity of the depositional environment, and consequently commonly also for the proximity to the adjacent continent. This is because fresh water, discharged into the sea by rivers, in most cases gradually mixes with the ambient sea water so that the near-coast relatively low salinity of the sea water is raised with increasing distance from the coast. Discharge peaks of major sediment-laden rivers that induce hyperpycnal flows at their mouth, can, however, affect this simple picture fundamentally. The hyperpycnal flows run down the basin slope, occasionally over tens of kilometers or even more. The huge volume of fresh water that can thus be introduced into a marine environment tends to mix with the ambient saline seawater only slowly, diminishing the paleosalinity even far away from the coast. This may be a local and temporary effect, but it may significantly affect the bio-environment and consequently the fossil record. It may also hamper the correct interpretation of the full-marine character of the depositional environment, which may, moreover, become obscured by the introduction of riverine organisms that are brought along by the hyperpycnal flows. The temporary lowered paleosalinity may be preserved in the sedimentary record in the form of 'diverging' concentrations of trace elements and trace-element ratios, which parameters are commonly used to determine the salinity of the depositional environment. A full-marine character of the marine depositional environment may thus be obscured. This is exemplified for the Early Cretaceous Lingshandao Formation on Lingshan Island (western Yellow Sea) 1. It is deduced that detailed facies analysis in the field is consequently required to reveal the true character of a marine environment.
... The Ordos Basin has an Archaean and Proterozoic metamorphic crystalline basement covered by Mesoproterozoic, Neoproterozoic, Palaeozoic, Mesozoic and Cenozoic sediments with an average thickness of~6000 m (Ding et al. 2016;Dai et al. 2016b;Yang et al. 2017;Fan et al. 2018;Du et al. 2019). The Lower Palaeozoic strata are mainly marine carbonates, and the Upper Palaeozoic-Mesozoic strata are mainly terrestrial clastic rocks (Dai et al. 2016b). ...
Article
The carbon isotope value of ethane in the southern part of the Jingbian gas field is lower than that in the northern part, indicating a carbon isotopic reversal in the southern Jingbian gas field (δ ¹³ C methane > δ ¹³ C ethane ). Through comparing the geochemical characteristics of gases in the southern and northern parts of the gas field, the reasons for the carbon isotopic reversal in the southern Jingbian gas field were determined to be high thermal maturity and mixing action. When thermal maturity reaches a critical value, the carbon isotope value of ethane becomes relatively more depleted with thermal maturity. Although the carbon isotope value of methane increases with thermal maturity, the extent is relatively smaller. Finally, the rare phenomenon of δ ¹³ C methane > δ ¹³ C ethane occurs. High thermal maturity leads to the secondary thermal cracking of gases. Mixing of the cracked gases and primary gases also leads to carbon isotopic reversal. Both of the above mechanisms share a common premise, which is high thermal maturity.
... The Ordos Basin is the second largest sedimentary basin and also the petroliferous basin in China Wang et al. 2017a;Yang et al. 2017a;Fan et al. 2018Fan et al. , 2019Shi et al. 2019). It is a multi-cycle cratonic basin with gentle internal structure, stable sediment, and less fractures (Zhao et al. 2010;Yang and Liu 2014;Qiu et al. 2015;Li et al. 2017a, b;Guo et al. 2018;Han et al. 2019a). ...
Article
Full-text available
The Ordos Basin is one of the most petroliferous basins in China, and the cumulative gas yield of the Jingbian gas field was 91.36 × 109 m3, accounting for nearly 30% of total gas production in the Ordos Basin. The ethane carbon isotopic value of gases in the Ordovician carbonate reservoirs in the Jingbian gas field is abnormally depleted (δ13C1 > δ13C2 < δ13C3), which has directly led to the controversy over the source and genetic types of natural gas in the Jingbian gas field. To find out the genetic mechanism of anomalies of ethane carbon isotope, components and carbon isotopic values of natural gas in the Jingbian gas field were measured. On this basis, natural gas components and stable carbon isotopic values of the Wushenqi and Jingbian gas field were comprehensively measured and compared. The results show that higher thermal maturity leads to further cracking of the oil generated earlier and the residual kerogen. These secondary cracked gases mixed with the early generated gases lead to the anomalies of ethane carbon isotope. When thermal maturity reaches a critical value, it will have different effect on carbon isotopic values of methane and ethane. The carbon isotopic value of methane will continue to increase; however, that of the ethane will gradually decrease, which will eventually lead to the anomalies of ethane carbon isotope. Migration fractionation will also lead to the abnormally depleted of carbon isotopic values of ethane.
... The study of deep-water sediments in the Ordos Basin began in the 1970s, and divergent views exist on the large scale development of sandstones attributed to deep-water gravity flows in the basin. Chen et al. concluded that the deep-water sandstones of the Chang 6 and Chang 7 intervals are primarily turbidite deposits including slope displacement turbidite fans and slumping turbidite fans [37][38][39][40][41]. Zou et al. reported expansively developed debris flow sand bodies in the Chang 6 and Chang 7 intervals in the center of the basin [42][43][44][45][46]. Recent studies indicate that turbidity flow and debris flow deposits exist in the Ordos Basin [47][48][49][50][51][52]. In addition, Yang et al. found a gravity flow-induced sandstone different from those of debris flow and slumping turbidite sediments of the Chang 6 and Chang 7 intervals in the southern part of the basin, which are considered as hyperpycnal flow sediments [53,54]. ...
Article
Full-text available
The Chang 7 interval of the Upper Triassic Yanchang Formation in the Ordos Basin represents a typical deep lacustrine depositional sequence. On the basis of field outcrops, cores, well logs, light/heavy mineral provenance analysis, and petrological studies, we evaluated the characteristics of deep-water gravity flow deposition of the Chang 7 interval and constructed a depositional model. The sediments mainly came from the northeast of the study area, and multiple sublacustrine fans were deposited in the center of the basin. Different from the deep-marine fan, the sublacustrine fan in the study area develops under the background of gentle slope without any erosional canyon between the fan and delta front. Gravity flow deposits in the study area can categorised into three groups: sand debris flow deposits, turbidity current deposits, and deep-water mudstone deposits. The main channel and branch channel are mainly developed with thick massive sandy debris sandstone, while the channel lateral margin and branch channel lateral margin are mainly developed with middle massive sandy debris sandstones and turbidite sandstones, which from bottom to top, the thickness of sand layer becomes thinner and the grain size becomes smaller. Thin mudstone is developed between channels; the lobe fringe includes sheet-like turbidite sandstones and deep lake mudstones. The widely distribute, good quality source rocks (TOC=2%–6%) developed in deep lacustrine have attained the peak stage of oil generation (Ro=0.9%–1.2%). The superimposition of the sublacustrine fan sand bodies and the wide distribution of good quality source rocks favor the formation of large lithologic reservoirs characterized by source–reservoir integration, self-generation and self-storage, and near-source accumulation.
... In the semi-deep to deep lake facies, the samples affected by gravity flows have higher 19/(19 + 23)TT, 24TeT/(24TeT + 23TT), and Pr/Ph ratios, and a lower relative abundance of gammacerane, compared to the samples with no gravity flow disturbance, especially the oil shale samples (Fig. 11). Therefore, in a more dynamic sedimentary environment, such as in a shallow lake environment or where there are gravity flows, freshwater inflows and the gravity flows could bring in oxygen and terrigenous clasts (Biscara et al., 2011;Fan et al., 2018;Chen et al., 2019;Li et al., 2020). This directly increased the dissolved oxygen content of the upper part of the water column (Schmidtko et al., 2017), and disrupted the salinity stratification, thus enhancing vertical ventilation of the water column and increasing bottom-water oxygen content (Stramma et al., 2008;Schmidtko et al., 2017;Li et al., 2020). ...
Article
Airfall volcanic ash deposited in the form of tuff layers is widely distributed in oil-shale bearing basins in western Liaoning Province, northeast China. In order to investigate the impacts of volcanic ash on algal productivity and the formation of organic-rich oil shales, the organic petrology and organic geochemistry of lacustrine sediments of the Lower Cretaceous Jiufotang Formation were investigated. Molecular geochemical data (the pristane/phytane ratio, the gammacerane index and the relative amount of β-carotane) indicate that they were formed in suboxic/anoxic depositional environments, with salinity fluctuations from freshwater to saline. Oil shales in the semi-deep to deepwater lake facies were mainly deposited under still and anoxic bottom water conditions, concomitant with water column salinity stratification. The relative abundances of diterpenoids and regular steranes indicate a limited terrigenous conifer plant contribution and a predominantly algal contribution to the organic matter. Blooms of algae contributing to high primary productivity, and anoxic depositional environments contributing to organic matter preservation, were key to organic-rich oil shale formation. Fish bones, ostracod shells, and alginite are present below the thick tuff layer. Alginite richness gradually increased from very low just above the tuff layer, to a higher concentration further above the tuff layer, indicating that during major volcanic eruptions, large quantities of volcanic ash led to water column poisoning, causing the sudden mass mortality of some aquatic organisms that were sensitive to turbid, acidified and toxic water. The increased proportion of brackish/saline-water dinoflagellates, which is corroborated by the high concentration of telalginite, abundant dinoflagellate-specific biomarkers (C31 4,22,23,24-tetramethylcholestanes), high values of the C27/C29 ααα-20R and C28/C29 ααα-20R sterane ratios, 4-methylsterane index, dinosterane index, and tetramethylsterane index, suggest that brackish/saline-water dinoflagellates show a positive response to volcanic ash input. Some specific brackish/saline-water dinoflagellates are present in organic-rich tuff laminae, indicating that during periods of minor volcanic eruptions or in intermissions between major eruptions, a minor volcanic ash input was more likely to result in rapid and efficient volcanic fertilisation effects and high-quality oil shale formation. Although the lacustrine oil shale system of the Jiufotang Formation shares some similar petrological and geochemical characteristics with the classical saline lacustrine oil shale system of the Green River Formation in the United States of America, the significant influence of airfall volcanic ash on algal productivity makes the formation mechanism of oil shales within the Jiufotang Formation strata quite different from that of oil shales within the Green River Formation.
... Ongoing loading may eventually lead to pseudonodules (Van Loon, 2009), either or not induced by a seismic shock. Such pseudonodules have, indeed, been found in the deep-lacustrine mudstones of the Chang 6 Oil Member in the Ordos Basin (Fan et al., 2018). The Yanchang Formation contains pseudonodules also at many other levels. ...
Chapter
Sandy mass-transport deposits occur worldwide in deep-marine and deep-lacustrine basins. Reconstruction of the various subaqueous mass-transport processes from the sedimentary record remains a challenge. Investigation of the lacustrine Ordos Basin in central China indicates that mud-coated intraclasts and other structures in the thick massive sandstones of the Yanchang Formation are viable criteria to interpret the depositional processes. The mud-coated structures tend to consist of a lithic core and a mud coating. The core is commonly composed of mudstone or sandstone. The coatings tend to be composed of shale or other fine-grained sediment that forms a nearly concentric envelope. The depositional features suggest that the sediments with mud-coated structures were deposited by sediment gravity flows and were supported during transport by the strength of the plastic medium. Consequently, mud-coated structures are one of the best criteria to distinguish sandy debrites as the origin of the deep-water thick, massive sandstones in the Yanchang Formation, which may help to distinguish them elsewhere.
Article
The Ordos Basin, central China, is a large-scale Late Triassic intracratonic lacustrine basin, and in which the Upper Triassic Yanchang Formation comprises interbedded sandstones, mudstones and tuffs, representing deposits of a complete lacustrine life cycle. Based on observation of extensive well cores, various types of soft-sediment deformation structures have been identified in the lacustrine deposits of the southwest basin, including deformed lamination/bedding, load casts and flame structures, pseudonodules, microfaults, autoclastic brecciation, sand dykes, dyke-sill complexes, and sandy and muddy slumps. Liquefaction and fluidization of unconsolidated permeable sediments, together with brittle fracturing of semi-consolidated sediments, are the major deformation mechanisms that generated these structures. Both the scale and prevalence of the deformed sediments, as well as the geological setting of the lacustrine basin, suggest that seismic events (synsedimentary tectonic activity) were the main trigger to initiate deformation processes, although sedimentary overloading could also have been a minor localized trigger. Many volcanic tuffs preserved in both the primary undisturbed deposits and deformed sediments serve as support for synsedimentary tectono-volcanic activities that induced deformation in these lacustrine sediments. The deformation structures usually form vertical clusters of several different types, representing active syndepositional tectonic periods. Stratigraphic correlation shows three such deformation clusters to be widespread in the area detailed herein, and likely recorded three major tectonic periods during deposition. A thick extensive oil shale, with various deformation structures, also reveals another active tectonic period. These four intervals coincide well with the transition of the sedimentary architecture in the area, indicating four phases of most intense tectonic periods and their influence on lacustrine evolution.
Chapter
One of the most important topics still to be clarified regarding the sedimentology of the Yanchang Formation in the Ordos Basin is the occurrence of hyperpycnites. The present contribution shows convincingly that hyperpycnites are well developed in Oil Member 7 of the formation. They extend from distributary estuaries into the deep lake and have intercalations of dark shales and tuffs, and they coexist with debrites and turbidites as a result of the progradation of subaqueous fans. The study makes clear that the origination of hyperpycnal flows was controlled mainly by episodic tectonic movements and the humid climate. The hyperpycnites have important implications for the possible presence of unconventional petroleum reservoirs because the flows supplied not only large amounts of coarse grains and organic material to the deep-water fine-grained central lake sediments, but also resulted in a high total organic carbon content. The coarse-grained layers serve as reservoir rocks, whereas the fine-grained layers form seals.
Article
During the deposition of the Chang 7 Member of the Yanchang Formation, pronounced lake invasion occurred, resulting in the formation of the largest lake in the Ordos Basin. This change has caused ambiguity in the source rock characteristics among the different sedimentary environments represented, thereby restricting the exploration of oil–source correlations and hydrocarbon migration pathways corresponding to oil and gas in this formation. In this study, we comprehensively assessed the characteristics of source rocks from different sedimentary environments and selected effective biomarker parameters to allow for a detailed exploration of oil formation and hydrocarbon migration. The source rock biomarkers differed notably among the different sedimentary environment, with source rocks from semi-deep to deep lakes having high ratios of pristane to phytane and ∑C21-/∑C22+ n-alkanes, relatively high C27 regular sterane contents, and low (C19+C20)/C21 and C30-rearranged hopane contents. Source rocks from these facies developed in reductive environments and had high V/Cr, Ni/Co, V/(V+Ni), and U/Th values, while those from the delta facies developed in a semi-oxidative to oxidative environment and thus showed the opposite characteristics. The Chang 6 tight oil in the Dingbian-Wuqi area primarily migrated vertically near its source; the western block originated from semi-deep to deep lake source rocks, while the eastern block formed from delta front source rocks. These findings should guide future research in this reservoir, especially with respect to high-resolution oil–source correlations across different sedimentary environments.
Article
Organic-rich lacustrine mudstones and shales in Chang 7 member of the Upper Triassic Yanchang Formation are potential shale oil reservoirs. Petrographic and geochemical data were used to study their lithologic and geochemical characteristics, mineral composition and sedimentary environments. Three different types of deep lacustrine deposits are identified, including unstratified massive mudstone (muddy debrite), normally graded mudstone (muddy turbidite) and laminated shale. Mudstones and shales contain relatively abundant brittle minerals (quartz, feldspar, pyrite and carbonates), which are favorable for the hydraulic fracturing. Terrigenous input and diagenesis conversion process are the main sources of felsic minerals. Laminated shales yield the highest TOC values (average of 7.47%) as well as S1 and S2 values. Most of the mudstones and shales contain Type II organic matter, while contain little Type I organic matter. The Ro and Tmax values combined with OEP versus CPI and C29ββ/(ββ+αα)sterane versus αααC29sterane20S/(20S+20R) discrimination diagrams and Ts/(Ts+Tm) ratios, indicate that the mudstones and shales are mature. Ph/C18 versus Pr/C17 and Pr/Ph versus gammacerane/C30-hopane discrimination diagrams together with U/Th, V/(V+Ni) and Ce/La ratios show that mudstones and shales reflect a reducing depositional environment. N-alkanes of ΣnC21-/ΣnC22+ values and C27-C29 regular sterane distributions suggest that plankton and terrestrial plants are the parent materials of organic matter. In addition, small amounts of extended homohopanes (>C31) and gammacerane combined with Sr/Ba values support a fresh water depositional environment. These conditions have resulted in high quality and widespread distribution of organic mudstones and shale, which are favorable for shale oil exploration.
Chapter
The Ordos Basin, situated in the western part of the North China Block, has a long history that started in the Mesoproterozoic and extends to the Quaternary. It started as a marine basin, but was transformed into an intracratonic (largely lacustrine) basin when plates had collided during the Triassic. During the Carboniferous and Permian, but also during the Jurassic, huge coal deposits were formed. These are exploited in hundreds of coal mines but also resulted in several economically exploitable gas fields. The Triassic and the Jurassic contain large oil fields, particularly in the deposits of lacustrine deltas, but increasing interest has grown in tight oil and gas reservoirs that consist of fine-grained, deep-lacustrine deposits (mainly formed by turbidites and hyperpycnites extending from the front of the deltas to the deep center of the lake). The paleogeographical development of the Mesozoic lake is detailed.
Chapter
Research into transport and deposition mechanisms of fine-grained mass-flow deposits is hampered by the absence of an easily applicable classification scheme. We propose here such a scheme on the basis of the mud content in combination with the presumed transport mechanism. The scheme comprises 12 types of gravity-flow deposits. The practical applicability of this classification is shown for the Triassic lacustrine Yanchang Formation in the Southern Ordos Basin (China). This formation contains abundant slump deposits, debrites, turbidites, and hyperpycnites. The analysis of the fine-grained gravity-flow deposits indicates that (1) mud can be deposited under active hydrodynamic conditions; (2) lacustrine mudstones and shales consist in a large part of gravity-flow deposits; (3) fine-grained gravity flows contribute much to the transport and deposition of organic matter, and thus are important mechanisms involved in the generation of hydrocarbons; (4) the mechanisms of transport and deposition, and consequently the resulting sorting, largely determine the reservoir capability of fine-grained gravity-flow deposits.
Chapter
The deep-lacustrine sandstones of the Triassic Yanchang Formation deposited by sediment gravity flows have an ultralow porosity and permeability. The sedimentary facies are complex and the diagenetic alterations and their controls on the reservoir quality are still poorly understood. They have been studied by thin-section analysis, scanning electron microscopy, back-scattered electron imagery, X-ray diffraction, cathodoluminescence microscopy, and analysis of fluid inclusions. Differences in the intensity of the diagenetic alterations lead to differences in reservoir quality. In particular, compaction reduced the primary porosity. Fine-grained turbidites lost most of their primary porosity. Sandstones with authigenic illite rims resisted compaction to some extent and preserved porosity, but pore-filling illite aggregates reduced the reservoir quality significantly; feldspar dissolution increased it slightly. Intergranular pores and feldspar dissolution pores result in the best reservoir quality. Four diagenetic evolution patterns resulted in good, moderate, or poor reservoir quality. These results help in evaluating reservoir quality and understanding the genesis of tight sandstones.
Chapter
The successive stages of compaction- and lithification-induced porosity reduction (densification) and hydrocarbon accumulation have been investigated in a tight sandstone reservoir in the Permian Sulige Gas Field in central China. Thin-section analysis, scanning electron microscopy, X-ray diffraction, cathodoluminescence, fluid-inclusion analysis, and confocal Raman spectrometer analysis were used to identify the authigenic minerals involved in the reservoir development: illite, kaolinite, dickite, chlorite, and smectite illitization, in combination with quartz and calcite cement. The types of pores are dominated by illite and kaolinite intercrystalline pores, pores originating from matrix dissolution, and primary intergranular pores. Particularly compaction and cementation resulted in porosity loss, but kaolinite and illite preserved intercrystalline pore spaces. Consequently, the secondary porosity contributes most to the reservoir quality. It is also inferred that the tight gas accumulation took place during the densification of the reservoirs. These findings may help understand the formation of other tight reservoirs.
Chapter
The Middle-Late Triassic Yanchang Formation, present in the large intracratonic Ordos Basin, is built of alternating sandstones, mudstones, and tuffs, representing a complete lacustrine cycle. Cores from wells in the south-western part of the basin show various types of soft-sediment deformation structures, particularly contorted lamination/bedding, load casts, and associated structures (flame structures, pseudonodules), penecontemporaneous faults, injection structures, and sandy and muddy slumps. They originated by liquefaction and fluidization of unconsolidated sediments, as well as by brittle fracturing of semiconsolidated sediments. Seismic events resulting from synsedimentary tectonic activity were the main triggers but sedimentary overloading may locally have been a minor trigger. The volcanic tuffs support tectono-volcanic activity. The deformation structures are clustered in three stratigraphic levels, indicating phases of intensified seismic activity. A thick extensive oil shale, with various deformation structures, reveals another active tectonic phase. These four intervals coincide well with the transition of the sedimentary architecture in the area.
Article
In order to unravel the Mesozoic sedimentation in response to multistage deformation of the western Ordos fold-thrust belt, combined zircon U-Pb and fission track analyses of single zircon grain are conducted for seven sandstone samples in the Helanshan area. Zircon FT ages are calculated using absolute uranium concentration determined by LA-ICP-MS during the procedure of zircon U-Pb dating. The newly-obtained zircon FT data show age features that are comparable with the previous zircon FT ages obtained by conventional FT methods (by which uranium contents are determined through an external detector to map induced fission tracks). The newly-obtained zircon FT ages have four subordinate FT age peaks (i.e., 225 Ma, 165 Ma, 125 Ma and 80 Ma) which are well matched with the regional geodynamic events occurred in the Late Triassic, Late Jurassic, Late Cretaceous and Early Cenozoic, respectively; and the zircon U-Pb ages have two main peaks at the Paleoproterozoic and the Late Paleozoic. The zircon U-Pb ages, integrating with the FT ages and available paleocurrents, suggest that the Paleoproterozoic basement rocks and Late Paleozoic plutonic rocks of the Alxa Block are the source rocks for the Triassic sandstones, and the Jurassic sandstones are associated with a multi-cycling sedimentation. The Jurassic sandstones contain recycled zircons that firstly deposited in the Triassic strata and subsequently exhumed during the Late Triassic-Early Jurassic.
Article
The tight sandstone of gravity-flow deposits in Miocene Mbr 2 of the Huangliu Fm, Ledong area is one of the focuses future exploration in the Yinggehai Basin. Reservoirs have complex sedimentary and diagenesis processes, experienced multi-stage fluid influence. However, the influence of different diagenesis on reservoir quality, and distribution patterns of different diagenetic facies in Mbr 2 of the Huangliu Fm are still short in research. In this paper, a variety of methods, such as thin sections analysis, electron microscopy (SEM), electron probe micro-analyzer (EPMA), cathode luminescence (CL), x-ray diffraction measurement (XRD), in situ carbon and oxygen isotopes composition of cements (SIMS), fluid inclusion homogenization temperature, and high-pressure mercury injection (HPMI), are used to analyze reservoir quality, diagenetic evolution stages, diagenesis, fluid filling processes, etc. Studies have shown that there are obvious differences in diagenetic evolution process, including carbonate cementation, organic acid and hydrothermal dissolution, etc. Which ultimately leads to large differences in physical properties. Although sandstone texture has an impact on reservoir quality, strong compaction and early carbonate cementation were the main reasons for reservoir densification. Early calcite was precipitated in alkaline seawater, and later ferrocalcite was formed by the recrystallization of calcite and polyframboidal pyrites and euhedral pyrites formed under organic acid charged. After hydrothermal fluid charged, dolomite, iron dolomite, cement pyrites and dispersed granular pyrites precipitated. Besides, illite filled the throat. The dissolution of organic acids and late CO2-rich hydrothermal fluids improved the quality of reservoirs. According to the difference of diagenetic process and the quality of the reservoirs, Mbr 2 of the Huangliu Fm is divided into 4 types of diagenetic facies. High-quality reservoirs are mostly developed in H2-Ⅰ, H2-Ⅱ, and H2-Ⅴ sand formation near faults. Early carbonate cements were less developed, which could support primary intergranular pores. Feldspar, rock fragments, and early carbonate cements were strongly dissolved. It includes two diagenetic facies, low cementation - strong dissolution reservoirs and slightly cementation - weaken dissolution reservoirs. On the contrary, tight reservoirs mostly develop in H2-Ⅲ, H2-IV, and H2–V sand formation far away from faults, including late compacted tight reservoirs and early carbonate cementation tight reservoirs. The research results are helpful to analyze the diagenetic evolution of tight sandstone reservoirs, and conducive to the study of similar gravity-flow reservoir quality with the influence of hydrothermal fluids.
Article
Mixed siliciclastic-carbonate sediments have been widely studied, especially those of marine origins. However, it is rarely investigated in continental settings. Boreholes from the Qinhuangdao-A (QHD-A) block in central Bohai Bay Basin sampled lacustrine mixed sediments during the Eocene Es1-2s period (32-38Ma), providing an opportunity to facilitate this aspect of research. Here, an integrated study of seismic, core data, and microscopic analysis revealed the formation and evolution of the mixed sediment system. Core observations and microfossil identifications show that mixed sediments characterizing volcaniclastic sand and gravel mixed with bioclastic debris are composed of shallow-depth biota assemblages. The relatively warm climate, low tectonic activity and low siliciclastic influx may result in prevailing occurrences of nearshore biota. Preexisting siliciclastics featuring a well-rounding shape experienced secondary transportation to mix with the nearshore biota, forming the major mixing type: “compositional mixing”. Seismic data, well-logs, and lithofacies logs divided mixed sediments into three evolutional units (U1-U3). U1 and U2 were dominated by “compositional mixing”. Within the U1, it occurred locally due to high sediment input during the low-level period. Peak took place in the U2 because lake-level rise led to increasing accommodation and decreasing siliciclastic influx. With further deepening of water depth in the U3, shallow-depended biota was not well developed and carbonates increased. Lake-level change results in a distinct depositional sequence of mixed sediments that bioclastic particles mixed with siliciclastics in shallow period and centimeter-scale carbonates interbedded with siliciclastic beds in the highstand. These findings bring new insights into mixed deposits, especially in continental settings.
Article
Fluid mobility is one of the most important factors in evaluating the potential for recovering tight oil. However, quantitative effects of multiple factors that influence movable fluid saturation in different pore-throat combinations and their relationships with lithofacies in tight sandstones, especially for lacustrine deep-water gravity-flow deposits, remain controversial due to the strong heterogeneity and complex pore structure of tight reservoirs. Core samples obtained from the Upper Triassic Yanchang Formation in southern Ordos Basin were evaluated by using a variety of techniques, including nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), impregnated thin sections, and helium porosity and nitrogen permeability measurements. Based on NMR T2 distributions under water-saturated and centrifugal conditions, four types of pore-throat combinations are identified by using the relaxation time thresholds Ts1 and Ts2 from NMR: large intergranular pore-dominated pore-throat combinations (LIP), small intergranular pore-dominated pore-throat combinations (SIP), intragranular pore-dominated pore-throat combinations (IAP), and micropore-dominated pore-throat combinations (MP). Among them, SIP is the dominant pore space favorable for fluid flow. Movable fluid saturation in SIP pore space varies among different lithofacies. The best fluid mobility commonly occurs in fine-grained, cross-bedded sandstones (Sc), whereas the worst fluid mobility usually appears in siltstones to very fine-grained sandstones (Ss). The quantitative effects of a total of 41 factors that influence movable fluid saturation of different pore-throat combinations were investigated through the analysis of the Pearson correlation matrix. The results demonstrate that porosity and permeability mainly affect the movable fluid saturation in the pore space of SIP and MP. In addition, maximum mercury intrusion saturation from PCP (Smax) and tortuosity (λ) are the critical pore structure parameters affecting the movable fluid saturation in the pore space of SIP and MP, whereas the average pore throat radius ratio (η) is the critical factor affecting the movable fluid saturation in LIP pore space. Overall, movable fluid saturation in different pore-throat combinations is characterized mainly by different microscopic pore structure parameters. A general pore network model for different lithofacies with different fluid mobility is established to facilitate assessment of the heterogeneity of tight sandstones and to further guide hydrocarbon exploration and development in similar lacustrine deep-water depositional settings.
Article
To gain a better understanding of nanoscale pore structure characteristics in Chang7 lacustrine shale, Ordos Basin, China, 33 shale samples from 15 wells are examined and analyzed by total organic carbon (TOC), X-ray diffraction (XRD), mercury porosimetry, helium pycnometry, low-pressure N2 adsorption experiments and fractal method. The results show that Chang7 lacustrine shale pores diameter is mainly from 1.5 to 5 nm, and mesopores give the largest contribution to the total pore specific surface area (SBET) and volume (VBJH), and micropores have the second contribution to SBET but little contribution to VBJH, macropores have very little contribution to SBET but make a good contribution to VBJH. According to the occurrence and origin of shale pores, micro-pores can be divided into mineral intragranular pore, minera intercrystalline pore and organic pore. Two fractal dimensions (D1 and D2) are obtained from N2 adsorption isotherms analysis using FHH method. Relationships between fractal dimensions and shale compositions, pore structure parameters are investigated, which show that both D1 and D2 have good positive relationships with SBET and VBJH, but obvious negative correlations with the average pore diameter. That is, the smaller the pore diameter, the more complex the pore structure is, resulting in the larger pore surface and volume. D1 and D2 are both positively correlated with total clay, illite, mixed-layer of illite-smectite, whereas no obvious relationships with chlorite, indicating that the layer and flocculent structure increase the complexity of nanoscale pore structure. TOC content has positive correlation with D1 but no obvious relationship with D2, indicating that the pores in organic matter are mostly micropores. Fractal dimensions are negatively correlated with total brittle mineral, quartz and feldspar content, that is because the surface of brittle minerals is smoother and more homogenous than that of clay minerals.
Article
Pebbles up to 10 cm in diameter, as the main clastic component of gas reservoirs in the Permian Sulige Gas Field, deposited more than 150 km away from their northward provenance (Yinshan Mountains) in the northern Ordos Basin. The transportation and depositional mechanisms of these pebbles have been unclear for decades. Based on core examinations, facies analysis, and mathematical modeling (stress field calculation of pebbles), it is thought that (1) these coarse-grained deposits were transported and deposited in braided rivers; (2) the size of coarse-grained sediments and pebbles in the Sulige Gas Field is in accordance with the size of flash flood; (3) flash flood is regarded as a crucial important transporting mechanism of pebbles; and (4) the “flash flood genesis” braided river-braided delta sedimentary model provides a reasonable understanding for prediction of coarse-grained reservoirs for gas exploration not only in the Permian Sulige Gas Field but also in similar coarse-grained sediments elsewhere.
Article
Lithotypes, organic matter, and paleoenvironment characteristics are essential attributes in organic-rich lacustrine mudstone. Understanding the covariant relationship among them is significant not only for revealing the main controlling factors of organic matter accumulation but also for optimization of the favourable interval of unconventional petroleum resources. Herein, we present a systematic petrological and geochemical analysis of 24 samples collected from the Chang73 submember in the Triassic Yanchang Formation. Base on the texture, bedding, and composition of mudstone, the collected samples can be further divided into four lithotypes (Lithotype1 to Lithotype 4). Toc and rock pyrolysis data indicate that those four lithotypes mainly contain a high content of type I and II1 kerogen, good to excellent hydrocarbon generation potential, and are at the low mature to mature stage. Paleoenvironmental parameters, represented by paleosalinity (Sr/Ba, B/Ga), sediment provenance (Al2O3/TiO2 and Th/Sc–Zr/Sc), paleoclimate (C-value, Sr/Cu), paleoproductivity (P/Al, P/Ti, Ni/Al, Ni/Ti, Cu/Al, and Cu/Ti), bottom-water redox conditions (V/Cr, V/(V + Ni), and UEF-MoEF), and sedimentation rate ((La/Yb)N), indicate that the four lithotypes were all deposited in freshwater environments and sourced from felsic volcanic rocks but different in the paleoclimate, paleoproductivity, bottom-water redox conditions, and sedimentation rate. According to the ratios of C-value, B/Ga, Al2O3/TiO2, P/Ti V/Cr and (La/Yb)N with TOC, The degree of organic matter accumulation is positively correlated with the humidity of the paleoclimate, paleoproductivity, and reducibility of bottom water but negatively related to the sedimentation rate. Comparing the differences between the organic matter characteristics and paleoenvironment of the four lithotypes, we conclude that Lithotype1 is the most favourable target lithotype for developing unconventional oil resources.
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Growing evidence suggests that land generated sediment gravity flows are the most important source of clastic sediments into marine and lacustrine sedimentary basins. These sediments are mostly transferred from source areas during exceptional river discharges (river floods). During floods rivers discharge a sediment-water mixture having a bulk density that often exceeds that of the water in the receiving water body. Consequently, when these flows enter a marine or lacustrine basin they plunge and move basinward as a land-derived underflow or hyperpycnal flow. Depending on the grain-size of suspended materials, hyperpycnal flows can be muddy or sandy. Sandy hyperpycnal flows also can carry bedload resulting in sandy to gravel composite beds with sharp to gradual internal facies changes laterally associated with lofting rhythmites. Lofting occurs because flow density reversal due to the buoyant effect of freshwater when a waning turbulent flow loses part of the sandy load. On the contrary, muddy hyperpycnal flows are loaded by a turbulent suspension of silt and clay. Since the concentration of silt and clay don’t decrease with flow velocity, muddy hyperpycnal flows will be not affected by lofting and the flow will remain attached to the sea bottom until its final deposition. The last characteristics commonly result in cm-thick graded shales disposed over an erosive base with dispersed plant debris and displaced marine microfossils. Deposits related to hyperpycnal flows are hyperpycnites. Although hyperpycnites display typical and diagnostic characteristics that allow a clear recognition, these deposits are often misinterpreted in the literature as Sandy debrites, shoreface, estuarine of fluvial deposits. The correct identification and interpretation of hyperpycnites provides a new frontier for the understanding and prediction of conventional and unconventional reservoirs.
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Fine-grained sediments from the Late Triassic Yanchang Fm. in the Ordos Basin (central China) were studied by core analysis and geophysical logging. Part of the mudstones in this formation are stratified, part of them are unstratified; the various mudstones can be subdivided into eight types on the basis of their structures and textures. They represent a variety of environments, ranging from delta fronts and subaqueous fans to deep-water environments. Part of the sediments were reworked and became eventually deposited from subaqueous gravity flows, such as mud flows, turbidity currents and hyperpycnal flows that easily developed on the clay-rich deltaic slopes. The sediments deposited by such gravity flows show abundant soft-sediment deformation structures. Understanding of such structures and recognition of fine-grained sediments as gravity-flow deposits is significant for the exploration of potential hydrocarbon occurrences. Because fine-grained deposits become increasingly important for hydrocarbon exploration, and because the sediments in the lacustrine Yanchang Formation were deposited by exactly the same processes that play a role in the accumulation of deltaic and prodeltaic fine-grained sediments, the sedimentological analysis provided here is not only important for the understanding of deep lacustrine sediments like the Yanchang Formation, but also for a better insight into the accumulation of fine-grained prodeltaic deep-marine sediments and their potential as hydrocarbon source rocks and reservoir rocks.
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The Triassic Yanchang Formation contains the main oil-bearing strata in the Ordos Basin, central China. But the sedimentology of the Upper Triassic is still under debate, and flood-generated, hyperpycnal-flow deposits and their implications for unconventional petroleum development have long been overlooked. Our study indicates that hyperpycnites are well developed in the seventh oil member of the Yanchang Formation. They are characterized by couplets of upward-coarsening intervals and upward-fining intervals , separated by microscale erosion surfaces. The origination of hyperpycnal flows was controlled mainly by episodic tectonic movements and the humid climate. The deposits extend from distributary estuaries into the deep lake, have intercalations of dark shales and tuffs, and coexist with debrites and turbidites as a result of the progradation of subaqueous fans. The hyperpycnites have implications for unconventional petroleum reservoirs, because the flows supplied not only large amounts of coarse grains and organic material to the deep-water, fine-grained central lake sediments but also affected the ecosystems, resulting in a higher total organic carbon content in the sediments.
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The fine-grained autochthonous sedimentation in the deep part of a Late Triassic lake was frequently interrupted by gravity-induced mass flows. Some of these mass flows were so rich in water that they must have represented slurries. This can be deduced from the soft-sediment deformation structures that abound in cores from these lacustrine deposits which constitute the Yanchang Fm., which is present in the Ordos Basin (central China). The flows and the resulting SSDS were probably triggered by earthquakes, volcanic eruptions, shear stress of gravity flows, and/or the sudden release of overburden-induced excess pore-fluid pressure. The tectonically active setting, the depositional slope and the high sedimentation rate facilitated the development of soft-sediment deformations, which consist mainly of load casts and associated structures such as pseudonodules and flame structures. Sediments with such deformations were occasionally eroded by slurries and became embedded in their deposits.
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Rapid erosion in mountain forests results in high rates of biospheric particulate organic carbon (POC) export by rivers, which can contribute to atmospheric carbon dioxide drawdown. However, coarse POC (CPOC) carried by particles >~1 mm is rarely quantified. In a forested pre-Alpine catchment, we measured CPOC transport rates and found that they increase more rapidly with water discharge than fine POC (<1 mm) and dissolved organic carbon (DOC). As a result, decadal estimates of CPOC yield of 12.3 ± 1.9 t C km–2 yr–1 are higher than for fine POC and DOC, even when excluding 4 extreme flood events. When including these floods, CPOC dominates organic carbon discharge (~80%). Most CPOC (69%) was water logged and denser than water, suggesting that CPOC has the potential to contribute to long-term sedimentary burial. Global fluxes remain poorly constrained, but if the transport behavior of CPOC shown here is common to other mountain streams and rivers, then neglecting CPOC discharge could lead to a large underestimation of the global transfer of biospheric POC from land to ocean.
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The fine-grained sedimentary rock has an important position in sedimentary rocks, but it has been often overlooked because of its seemingly simple appearance. Meanwhile, due to limitation of ultra-microcosmic experimental conditions, deposition and diagenesis of fine-grained sediments remain a relatively weake research field in sedimentology, even in geology. With exploration and development of shale oil and gas, the study of fine-grained sedimentary rocks becomes increasingly urgent. However, there are several issues deserving of attention in the current study, such as to utilize sedimentology, reservoir geology and stratigraphy comprehensively, to combine high-resolution observation instruments with geophysical recognition techniques, experimental simulation and other methods, to search for theoretical breakthrough and method innovations, the standardize the related concepts and terminology of fine-grained sedimentary rocks, to establish systematic and scientific classification schemes of fine-grained sedimentary rocks, to reconstruct dynamic processes of fine-grained material deposition and diagenesis, and to set up stratigraphic specification of deepwater fine-grained materials. In addition, a great emphasis should be placed on interdisciplinary researches and association between scientific significance and industrial value so as to make fine-grained sedimentary rocks receive extensive attention, gain breakthrough in progress, and serve shale oil and gas exploration and development better.
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Using a seismic database from the Qiongdongnan Basin in the South China Sea, this study demonstrates that shelf-edge trajectories and stratal stacking patterns are reliable, but understated, predictors of deep-water sedimentation styles and volumes of deep-water sand deposits, assisting greatly in locating sand-rich environments and in developing a more predictive and dynamic stratigraphy. Three main types of shelf-edge trajectories and their associated stratal stacking patterns were recognized: (1) flat to slightly falling trajectories with negative trajectory angles (Tse) (-2° to 0°) and negative shelf-edge aggradation to progradation ratios (dy/dx) (-0.04 to 0) and associated progradational and downstepping stacking patterns with low clinoform relief (Rc) (150-550 m [492-1804 ft]) and negative differential sedimentation on the shelf and basin (As/Ab) (-0.6 to 0); (2) slightly rising trajectories with moderate Tse (0°-2°) and medium dy/dx (0-0.04), and associated progradational and aggradational stacking patterns with intermediate Rc (250-400 m [820-1312 ft]) and intermediate As/Ab (0-0.6); and (3) steeply rising trajectories with high Tse (2°-6°) and high dy/dx (0.04-0.10) and associated dominantly aggradational stacking patterns with high Rc (350-650 m [1148-2132 ft]) and high As/Ab (1-2). Each trajectory regime represents a specific stratal stacking patterns, providing new tools to define a model-independent methodology for sequence stratigraphy. Flat to slightly falling shelf-edge trajectories and prograda-tional and downstepping stacking patterns are empirically related to large-scale, sand-rich gravity flows and associated bigger and thicker sand-rich submarine fan systems. Slightly rising shelf-edge trajectories and progradational and aggradational stacking patterns are associated with mixed sand/mud gravity flows and moderate-scale slope-sand deposits. Steeply rising shelf-edge trajectories and dominantly aggradational stacking patterns are fronted by large-scale mass-wasting processes and associated areally extensive mass-transport systems. Therefore, given a constant sediment supply, then rse, dy/dx, Rc, and As/Ab are all proportional to intensity of mass-wasting processes and to amounts of mass-transport deposits, and are inversely proportional to the intensity of sand-rich gravity flows and to amounts of deep-water sandstone. These relationships can be employed to relate quantitative characteristics of shelf-edge trajectories and stratal stacking patterns to deep-water sedimentation styles. Copyright ©. The American Association of Petroleum Geologists. All rights reserved.
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The Chang 7 Member of Triassic Yanchang Formation in Ordos Basin is taken as an example to study the distribution rule and major controlling factors of the fine-grained sedimentary system by core description, thin section observation, X-ray diffraction, geochemical testing and TOC well logging quantitative calculation. The main sedimentary pattern of organic-rich shale in Chang 7 Member is transgression–water stratification. The sedimentary facies, water depth, anoxic condition and lacustrine flow are the major controlling factors for the formation and distribution of organic-rich shale. During the deposition of the Chang 73 Member, lake water invaded rapidly, lake depth and scope sharply increased. Because of the temperature difference, the circulation between upper surface water and lower water in deep lake was restrained, then large-area anoxic environment was formed in deep lake, which was favorable for the development of organic-rich shale. In silent deep lake where is far away from the delta front, organic-rich shale well developed, with high organic carbon content and mainly type I kerogen. In deep lake where sandy debris flows developed, organic-rich shale was inter-bedded with sandstone, which contains high organic carbon content, mainly with type I-II1 kerogen. In semi-deep lake close to the delta front, wavy-massive silty mudstone developed, mainly with type II kerogen.
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Based on the analysis of numerous drill cores and drilling data, lacustrine gravity flow depositional systems were analyzed comprehensively in the Chang6 and Chang7 oil members (Triassic Yanchang Formation) in the southern part of the Ordos Basin. The gravity flow depositional systems in these members are made up of slides, slumps, sandy debris flows, liquefied flows, turbidity current etc, forming well developed units in the study area. Successive beds characterized by, from bottom to top, massive bedding (MB), graded bedding (GB) and horizontal bedding (HB) form well developed sequences; parallel bedding (GB) and ripple bedding (RB) are rare. It turns out that the depositional sequences are quite different from turbidites with a Bouma sequence: (1) MB represents a sandy debris flow, (2) GB deposits in a turbidity current, (3) PB and RB are deposits that were reworked by bottom currents (traction flow), and (4) HB represents a deep-water environment rather than gravity flows. Deposits in the proximal part of the subaqueous lacustrine fan consist mainly of slides, slumps and massive sandy debris flows. Deposits at the middle part of the fan are characterized by an MB-GB-HB sequence of massive sandy debris flows, graded turbidites and horizontally bedded lacustrine mudstones. Deposits at the end of the subaqueous lacustrine fan were mainly graded turbidites and horizontally bedded lacustrine mudstones (GB-HB sequence). Sandy gravity flow deposits mainly developed on the delta front and in the basin plain, extending for dozens of kilometers. They directly cover the source rock in the Chang7 oil members, which has the advantage of near-source oil accumulation. The sandstones at the bottom of each sedimentary cycle are worth further exploration because of their good reservoir properties and high oil content.
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Based on analysis of successive and whole cores over six hundred meters from the second Member of Kongdian Formation (Kong 2 Member) of fine-grained facies zone in central lake basin of Cangdong sag, ten thousands of systematic and joint analysis data, and the matching logging data, the sedimentary characteristics in the fine-grained facies zone is examined by means of traditional petroleum geology, sedimentology, and new theory and method of tight oil and gas generation. The Kong 2 Member is the sedimentation during the maximum lake flooding in the Kongdian Formation. Fine-grained sedimentation of half-deep lake to deep lake are widely distributed, the fine-grained feldspar-quartz contained sedimentary rock, fine-grained mixed sedimentary rock, and dolomite are developed, and the clay rock is poorly developed. The fine-grained rocks have these features as follows: more rock compositions, less advantageous minerals; more debris minerals, less clay minerals; more analcites, less pyrites; more brittle minerals, less quartz; more rock types, less oil shale; more tight reservoirs with less non-cracked reservoirs; more high-quality source rocks, less non-source rocks; more high-frequency cycle sediments, less single sediments. These understandings updated the former view that only mud shale source rock was dominantly developed in the closed continental lake. With the help of geologic research and matched engineering technologies, several exploration wells have produced industrial oil flows, showing a good prospect of tight oil exploration in fine-grained facies zones of continental lacustrine basins. © 2016 Research Institute of Petroleum Exploration & Development, PetroChina.
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Fine grained clastic sediments are very common in the interior deposits of ancient epicontinental seas. Not only do they make up the gross lithology in these basins, but they can also be traced for more than 1000 km offshore from basin margins. Given that epicontinental seas were overall shallow and in many parts most likely less than 100 m deep, basin floor slopes can safely be expected to be in the 0.01 to 0.001° range for much of depositional history. Known processes that bring muds to the basin margin and beyond are hypopycnal river plumes, hyperpycnal fluvial discharge events, storm-setup relaxation flows, and gravity-driven fluidized muds. With the exception of river plumes, all of these processes require the presence of sufficient slope for sustained movement. Due to that constraint, these processes combined might in the majority of situations have been able to move muddy sediments on the order of 100 km offshore. Whereas this is sufficient to distribute mud across marginal shelf seas, it becomes problematic in the case of much larger epicontinental seas. For example, those of Upper Devonian or Upper Cretaceous times extended in places for thousands of kilometers, and thus a process is needed that can move muddy sediments the rest of the way. Flume studies of the bedload transport of mud, combined with observations from the rock record, suggest that wind or tide induced bottom current circulation was most likely essential for moving muddy sediments from the periphery of epicontiental seas to their interiors. Remobilization of seafloor muds during frequently recurring lowering of sea level is likely to have aided in this process.
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This paper aims to make clear the shale gas exploration potential of the Yanchang Formation in the Ordos Basin. Based on outcrop investigation, core sample analysis, and old well data reexamination and in combination with organic geochemistry and rock analysis, gas content measurement, and isothermal adsorption curves, etc. , a systematic evaluation was conducted of formation distribution, petrological characteristics, geochemical characteristics, reservoir characteristics and other geological features like gas bearing in this study area. The following results were achieved. (1) The major target pay zones in this area are the members of Chang 4+5, Chang 7, Chang 9 in the Mesozoic Yanchang Formation, among which Chang 7 has the best quality in shales. (2) There is a wide distribution area of the lacustrine shales with great thickness, the sapropelic as the kerogen type, high TOC content, big distributional difference, strong adsorption ability, and the gas content of lacustrine shales is equivalent to that of marine shales. (3) By use of the volumetric method, the gas-in-place in the member of Chang 9 is calculated to be 0.4145 trillion m3, that of Chang 7 is 1.1506 trillion m3, that of Chang 4+5 is 0.2551 trillion m3; the resource reserves in the Chang 7 accounts for 63% of the total in the Yanchang Formation so that it can be selected as the first target zone in the shale gas exploration and development in this basin. In conclusion, it is considered that the shale gas resources are rich in the Yanchang Formation, Ordos Basin, which will be a new area for natural gas exploration and development in the future.
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Shallow-water deltas and central-basin sandbodies in large lacustrine depression basins have become the most important target for the stratigraphic reservoirs exploration in China. Based on the analysis of typical lacustrine shallow-water deltas in modern lakes and the study of lacustrine shallow-water deltas and central basin sandbodies in the Meso-Cenozoic large lacustrine depression basins in China, several recognizes have been obtained as following: (1) Based on the analysis of the geological settings for the shallow-water deltas formation, nine types of lacustrine deltas have been classified according to the feeder systems, lake depth and delta-front gradient; (2) Depositional model for shoal-water profile river deltas has been established which presents the terminal distributary channels and terminal splays; (3) Controls for the large-area shaltow-water deltaic sandbodies have been analyzed and the distribution model of shallow-water deltas and central-basin sands in open lacustrine depression basin has been established, and it is proposed that open lake is the most important basis for the development of large-scale shallow-water deltas in the basin center and outflow channels control the alignment of the central-basin sandbodies; (4) Fluvial, delta, lake current and bottom density current are the main traction currents that formed sandbodies in the basin center, and flooding turbidite fans and slumping turbidite fans are the main sedimentary gravity deposits; (5) The third-order sequence boundaries in the large shallow-water deltas have great influence on the distribution of lithological or diagenetic traps, and superimposed sandbodies on the sequence boundary are abundant of lithological reservoirs in the delta plains which show the great potential for oil and gas exploration. These understandings are helpful to the development of lacustrine sedimentology and the exploration of stratigraphic reservoirs in large lacustrine depression basins.
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Ordos basin has developed during middle - late Triassic and early Cretaceous and has been reformed since late Cretaceous. It mainly possesses the characters of intra-craton and is a residual basin with multi-stage and different reformation. Ordos basin also belongs to multi-superimposed basin which overlapped on the early and late Paleozoic basins. Therefore, Multi-energy mineral deposits are abundant and petroleum, coal and uranium are beared in the same basin. According to the the main geological structure features and geological events in Ordos baisn and peripheral areas, together with the comprehensional research of the fission track ages in various district of the basin, the paper concludes that there exists four evident stages of structural fluctuation at least during basin development and also four stages of evolution and sedimentation in the basin. middle - late Triassic and early - middle Jurassic Fuxian and Yan'an stage is the two most prosperous stages during which the lake is broad and basin deposits extensively with an area over two times of that in present, the main oil and coal bearing strata develop. The two stages are divided by the regional early Jurassic uplift fluctuation, which causes interruption in deposition and development of the roughness of relief on the upper part of the Yanchang formation by reason of forceful and uneven cutting erosion. The uplift fluctuation is weak and the depositional interruption and erosion continuous time is short in the late Yan'an stage. Then basin subsides and enters the third evolution stage of middle Jurassic Zhiluo - Anding stage in which sedimentary range is still broad and the lake area is minish in the basin. There happens strongly structural movement and develops the thrust-nappe belt in western margin of the basin in late Jurassic during which the different depth of conglomerate deposit in the foredeep part in the east side of the western margin, erosion reform occurs in the middle and east of the basin. The structural movement causes the form of structure framework of regional east uplift and west subside in the west area of Yellow river. Broad sediments unconformablely overlap on the west margin thrust belt and sideral uplift of the basin north and south at the early Cretaceous stage. Ordos basin uplifts wholely to disappear and enters the late reformation period. In the first three stages, the depocenters lie near or to the east of Yan'an city, while deposit center in the west of the basin near provenance. The two centers differs in space in different stage. Until early Cretacious, the location of sedimentary depocenters and accumulation center is nearly the same which is on the middle-south part of the west of the basin. There happen six major geological events in Ordos basin in late reformation period since late Cretaceous. (1) The main part of the basin uplifts continuously, episodically and differently, which causes the strong and uneven erosion, for example, the max erosion depth in Mesozoic can be two thousand meters near Yellow river on the east. (2) This uplift forms two or three stages of regional erosion-planation surfaces with ages of E13-E21, E32 and N12'. (3) The side of Ordos block subsides and the peripheral faulted basins form successively to deposit with heavy depth. (4) The regional east uplift and west subside movement continuing two bill years has been reversed which means the east subsides to widely deposit red clay and Liupianhshan, western margin of Ordos block and west part of basin uplift successively. The reverse event embodies the important influence of the regional structural movement of the west of China. (5) Aeolian red clay and loess begin to sediment broadly in 8Ma BPand 2. 5Ma BP, respectively. Also the red mud semi-plateau, loess plateau and loess plateau surface develop successively. (6) Drainage system of Yellow river grows and fluid outside to erode the side rock. According to the above major geological events and the evolution of dynamics environment, five evolution stages is divided since late Cretaceous in Ordos basin which are late Cretaceous to Paleocene, Eocene to Oligocene, Early - Middle Miocene, late Miocene to Pliocene and Quaternary. The happening of these geological events and structural fluctuation are closely related with the compound superimpose, change with time and strongweak of major structural movement of peripheral structural domain especially that of the east and west of China. These activity and reformation change the Mesozoic prototype basin topography greatly. The main times and stages of the occurrence and formation and reset of miltiple energy resources have evident recall and decoupling relations with those of the Mesozoic and Cenozoic evolution and reformation in Ordos basin. The whole different uplift and regional erosion in and after the late of basin evolution own the important influence to the formation and distribution of the milti-mineral deposits and the coaction among them in Ordos basin.
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Based on cores and logging data, and gravity flow theory, the sedimentary characteristics and facies model of gravity flow deposits in the Longdong area in southwestern Ordos Basin were analyzed. Five types of gravity flow deposits are recognized: slides, slumps, sandy debris flows, muddy debris flows and turbidity currents. Affected by multiple provenances, paleogeomorphology and triggering mechanisms jointly, the gravity flows resulted in sublacustrine fans and slump olistoliths in the basin center. Containing channels, the sublacustrine fans can be divided into three subfacies, inner fan, middle fan and outer fan and subdivided into six microfacies, main channel, main channel lateral margin, distributary channel, distributary channel lateral margin, inter-channel and sheeted turbidite sand. It is inferred that they are caused by seasonal floods. The slump olistoliths, with no channels, consist four parts: slide rock, slump rock, debris flow lobe and sheeted turbidite sand, and are inferred to be the product of collapse of break belts triggered by events like earthquakes. Combining facies models with types of gravity flow deposits can reveal the sedimentary characteristics and genetic models of gravity flow deposits in deep water more directly and provide theoretical basis for deep water oil and gas exploration.
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The Chang 7 Member of Triassic Yanchang Formation in Ordos Basin is taken as an example to study the distribution rule and major controlling factors of the fine-grained sedimentary system by core description, thin section observation, X-ray diffraction, geochemical testing and TOC well logging quantitative calculation. The main sedimentary pattern of organic-rich shale in Chang 7 Member is transgression-water stratification. The sedimentary facies, water depth, anoxic condition and lacustrine flow are the major controlling factors for the formation and distribution of organic-rich shale. During the deposition of the Chang 73 Member, lake water invaded rapidly, lake depth and scope sharply increased. Because of the temperature difference, the circulation between upper surface water and lower water in deep lake was restrained, then large-area anoxic environment was formed in deep lake, which was favorable for the development of organic-rich shale. In silent deep lake where is far away from the delta front, organic-rich shale well developed, with high organic carbon content and mainly type I kerogen. In deep lake where sandy debris flows developed, organic-rich shale was inter-bedded with sandstone, which contains high organic carbon content, mainly with type I-II1 kerogen. In semi-deep lake close to the delta front, wavy-massive silty mudstone developed, mainly with type II kerogen.
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Despite a globally growing seismic and outcrop analogue dataset, the detailed (centimetre to decametre) internal stratal make up of deep-marine basin-floor ‘channelized-lobe’ strata remain poorly known. An ancient analogue for modern, mixed-sediment, passive margin, deep-marine basin-floor fans is the well-preserved Neoproterozoic Upper and Middle Kaza groups in the southern Canadian Cordillera. This succession is a few kilometres thick and comprises six sedimentary facies representing deposition from different kinds of sediment-gravity flows. Representative lateral and vertical assemblages of one or more of these facies comprise six stratal elements, including: isolated scours, avulsion splays, feeder channels, distributary channels, terminal splays, and distal and off-axis fine-grained turbidite units. The internal characteristics of the various stratal elements does not differ from more distal to more proximal settings, but the relative abundance of the various stratal elements do. The difference in relative abundance of stratal elements in the kilometre-scale stratigraphy of the Kaza Group results in a systematic upward change in architecture. The systematic arrangement of the stratal elements within the interpreted larger bodies, or lobes, and then lobes within the basin floor fan, suggests a hierarchical organization. In this article a hierarchy is proposed that is based on avulsion but, also importantly, the location of avulsion. The proposed avulsion-based hierarchical scheme will be a useful tool to bridge the scalar gap between outcrop and seismic studies by providing a single stratigraphic framework and terminology for basin-floor stratal elements.
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At the northern margin of the North China Block (NCB), the Xilamulun Fault (XMF) is a key belt to decipher the tectonic evolution of Central-Eastern Asia, as it records the Paleozoic final closure of the Paleo-Asian Ocean, and localizes a Late Triassic intracontinental deformation. In this study, structural analysis, 40Ar–39Ar dating, and paleomagnetic studies were performed to investigate the kinematics of the XMF and to further discuss its Triassic geodynamic significance in the Central-Eastern Asia framework after the Paleozoic Central Asian Orogenic evolution. The structural analyses reveal two phases of ductile deformation. The first one (D1), which displays N-verging and E–W trending folds, is related to the Early Paleozoic collisional event between the NCB and the Songliao–Hunshandake Block (SHB). The second phase (D2) displays a high-angle foliation and a pervasive sub-horizontal E–W stretching lineation with kinematic criteria indicative of dextral strike–slip shearing. The 40Ar–39Ar dating on mylonitic granite places the main shearing event around 227–209 Ma. This D2 shearing is coeval with that of the dextral strike–slip Bayan Obo–Chifeng Fault (BCF) and the Chicheng–Fengning–Longhua Fault to the south, which together constitute a dextral shearing fault system on the northern margin of the NCB during the Late Triassic. The paleomagnetic study performed on the Middle Permian Guangxingyuan pluton, located between the XMF and BCF, documents a local clockwise rotation of this pluton with respect to the NCB and SHB. Our multidisciplinary study suggests an NNW–SSE shortening and strike–slip shearing dominated tectonic setting on the northern margin of the NCB during the Late Triassic. Combining the contemporaneous dextral strike–slip movements of the XMF and BCF in northern China and the sinistral strike–slip movement of East Gobi Fault (EGF) in southeastern Mongolia with the large-scale tectonic framework, a Late Triassic NNW–SSE shortening-eastward extrusion tectonic model for Central-Eastern Asia is firstly proposed. The NNW–SSE shortening results in the eastward extrusion of the continental wedge bounded by the BCF and EGF, which is accommodated by the different kinematic patterns of the southern (XMF and BCF) and northwestern (EGF) bounding faults. This shortening-extrusion tectonic framework is tentatively interpreted as the result of the far field forces associated with three Late Triassic lithosphere-scale convergences in East Asia: i) northward intracontinental subduction between the NCB and South China Block, ii) collision of the Qiangtang Block with the Qaidam Block, and iii) southward subduction of the Mongol–Okhotsk Ocean beneath the Mongolia Block.
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Mudstone is the most abundant sedimentary rock and variously acts as sources, seals, and shale gas reservoirs in petroleum systems. Many important physicochemical properties of mudstones are strongly influenced by the mineralogy and size of deposited grains, and by diagenetic changes (precompaction and postcompaction); these are commonly predictable. The diverse composition of mudstones reflects input and hydrodynamic segregation of detrital materials to basins, primary production witliin basins, and diagenetic processes [both precipitation and dissolution) in the sediment. High-magnification observations both in modern and ancient sediments demonstrate that mudstones are texturally and mineralogically heterogeneous; this variability is not always readily apparent. Although some mud is indeed deposited by suspension settling out of low-energy buoyant plumes, textural analyses reveal that it is commonly dispersed by a combination of waves, gravity-driven processes, and unidirectional currents driven variously by storms and tides. Such dispersal mechanisms mean that muddy successions are typically organized into packages that can be interpreted using sequence stratigraphy. Early bioturbation homogenizes mud, whereas early chemical diagenesis can result in highly cemented zones developing, especially at stratal surfaces. The nature of deeper burial diagenesis, which involves compaction, mineral dissolution, recrystallization, mineral reorientation and lithification, and petroleum generation, is preconditioned by depositional and early diagenetic characteristics of the mud. Copyright © 2011. The American Association of Petroleum Geologists. All rights reserved.
Article
Channel-levee systems are responsible for constructing deep sea fans, among the largest sedimentary deposits on Earth. Levee height plays a key role in defining the volume and texture of the material that is deposited in the bounding levees, and thus the morphology of submarine fans. Models of channel formation and evolution generally assume that the levees aggrade in response to the cumulative overspill of turbidity flows, and that their height is controlled by these flows. In contrast, we show that levee growth in the Ursa Basin (Gulf of Mexico) is limited by the mechanical strength of the levee, not the fl ow behavior. While many studies document sidewall failures in levee systems, our poro-mechanical model is the first to demonstrate that collapse of levees is a large-scale, deep-seated process driven by the interaction of levee formation and high fluid pressure. Rapid deposition of a regional sand unit induced large fluid overpressure in the underlying mud, which preconditioned the system for levee failure, which then fed a large volume of sediment back into the channel-levee system. Long-lived levee failures continually reintroduced previously deposited levee material back into the channel system. This implies that a large volume of sediment is continuously recycled, which has not been previously understood. Turbidite flow models generally assume that flows progressively lose their fine-grained component due to levee overspill as they traverse the channel. In contrast, we show a mechanism by which fi ne-grained material can re-enter the system in large quantities, and this has significant and broad importance for models of channel and fan evolution. We also show that that levee failure introduces significant unconformities, in contrast with the common assumption that levees offer complete and high-resolution records of climate, tectonics, and sea level.