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... Both sediment gravity flow deposits related to floods and slope failures have been discovered in sedimentary basins in China, including the Ordos Basin, the Bohai Bay Basin, and the Songliao Basin (Zou et al., 2009;Xian et al., 2012;Yang et al., 2015aYang et al., , 2015bLiu et al., 2017;Yang et al., 2017;Pan et al., 2017;Yuan et al. 2018). Active faults are important triggers for the sediment gravity flows in continental rift basins in eastern China and they take prominent roles in determining depositional sites and sand-body geometry (Xian et al., 2012;Yuan et al., 2018). ...
... Unfortunately, the MS1 are structureless, making it extremely difficult to decipher their origins or sedimentary processes. It is widely accepted that slide-slumps or debris flows tend to transform into turbidity currents downslope (Hampton, 1972;Shanmugam et al., 1994;Piper et al., 1999;Zou et al., 2009;Xian et al., 2012;Yuan et al., 2016;Liu et al., 2017), and thus turbidites are increasingly common basinwards (northwestwards) (Fig. 6). Similarly, the MS1 become more developed basinwards; moreover, they are generally interbedded with turbidites (Fig. 6a, e). ...
... Lithofacies interpretation indicates that slide-slumps and debrites disappear gradually as turbidites are increasingly developed basinwards (Fig. 6). These characteristics probably imply the transformation of slide-slumps or debris flows into turbidity currents downslope (Hampton, 1972;Shanmugam et al., 1994;Piper et al., 1999;Stow and Johansson, 2000;Zou et al., 2009;Xian et al., 2012;Yuan et al., 2016;Liu et al., 2017). The ISGF deposits are mainly distributed in the centre of the study area (Fig. 14a). ...
Deep-water sandstones related to sediment gravity flows are becoming an increasingly important exploration domain in continental rift basins in China. However, ambiguities remain regarding the initiation, evolution, and deposition of sediment gravity flows on faulted slopes, where step faults are commonly developed. To address the uncertainties related to these processes, grain size analysis, lithofacies analysis, logging interpretation, and seismic interpretation were employed to study Paleogene sediment gravity flow deposits from well cores on a faulted slope in the Zhanhua Sag of the Bohai Bay Basin. Twelve lithofacies related to sediment gravity flows were recognized and attributed to intrabasinal sediment gravity flows (ISGFs) induced by slope failures and extrabasinal sediment gravity flows (ESGFs) generated by floods. Active faults were important triggers for ISGFs on the faulted slope in the Zhanhua Sag. The fault activity rate had a prominent role in determining the volume of re-transported sediments, which further influenced the evolution of the resulting flows. Travelling across multiple faults downslope, ISGFs generally underwent considerable velocity losses, with the result that almost all sediments were deposited in shallow water. Moreover, unconsolidated ISGF deposits could be transported again to generate sediment gravity flows, obscuring the depositional characteristics of ISGFs. ESGFs originated from rivers during floods and involved the transformation of cohesive into turbulent hyperpycnal flows. In humid and warm climates, active sag-border faults favoured the generation of long-lived ESGFs. These flows usually travelled along intra-sag faults and therefore considerable volume of sediments could be transferred into deep water. The resulting thick-bedded hyperpycnites contributed substantially to the reservoirs of Bonan Oilfield. Meanwhile, short-lived ESGFs usually generated in the regions where sag-border faults were extinct. They generally travelled down the faulted slope and deposited sediments in shallow water. These features suggest that the initiation, evolution, and deposition of sediment gravity flows on the faulted slope are mainly controlled by faults and climate. Hyperpycnites can serve as excellent reservoirs for hydrocarbon accumulation and they are probably common in other sags of the Bohai Bay Basin.
... The deep-water gravity flow sediments are new fields for oil and gas exploration and have attracted great attention around the world over the past decades (Li et al., 2010b(Li et al., ,c, 2011(Li et al., , 2013Lowe, 1979Lowe, , 1982Shanmugam and Moiola, 1995;Shanmugam, 1997Shanmugam, , 2000Zou et al., 2009). They are widely developed in the centre of the large lacustrine basins in China. ...
... Later, they were considered to be deep lacustrine turbidite fans and seismite deposits (Chen et al., , 2012Ding et al., 2011;Li et al., 2010a;Xia et al., 2007;Yang et al., 2005;Zhao et al., 2008). Recently, they are reconsidered to be the deposition of the sandy debris flow (Chen et al., 2012;Li et al., 2010bLi et al., ,c, 2011Li et al., , 2012Zou et al., 2009). The researchers who believe that the study area is braided river delta front lack sufficient data of the study area and concluded that the deep lake is in the northern part of the study area. ...
... In recent years, the characteristics of sandy debris flow sediments have also been found in the Southern Ordos Basin (Chen et al., 2012;Li et al., 2010bLi et al., ,c, 2011Li et al., , 2012Zou et al., 2009). These include (1) shear structures in the bottom of the sandstones; (2) floating mud clasts in the top or bottom of the sandstones with inversely graded bedding; (3) the mud clasts are elongated; (4) sharp top and bottom contacts. ...
In recent years, the Ch6–Ch7 deep-water gravity flow deposits of the Yanchang Formation in the Southern Ordos Basin have been characterised as sandy debris flow sediments, a modification from previous sedimentary facies characterisations of braided river delta fronts, deep lacustrine turbidite fans and seismite deposits. This leads to the necessity of a detailed interpretation of the origin and reservoir properties of the deep-water deposits of the Jinghe Oil Field in the Ordos Basin. A large number of core images were analysed, identifying 15 lithofacies and 3 main sedimentary facies, including the sandy debris flow microfacies, turbidite microfacies and seismite-slump microfacies. Sedimentary facies determination was proved by particle size analysis and vertical and horizontal microfacies distributions. The sedimentary process can be described by earthquake and gravity deformation inducing a slide of the large deposit of delta front sediments on the slope break down the slope. Simultaneously, ambient lake water penetrated the sediments,
forming seismite-slump microfacies with load structures, liquefied structures, and slump deformation structures. With continuous sediment liquefaction, sandy debris flow microfacies,
which were massive bedding sandstones, were formed during transportation. Leading the sediments,
turbidite flows resulted from flow transformation which were possibly remolded by weak bottom currents in intermittent periods. Studies of reservoir properties and oil shows indicate that sandy debris flow sandstones have the best reservoir properties and oil shows, followed by turbidite sandstones, with seismite-slump sandstones being the poorest. The sandy debris flow and part of the turbidite sandstones have good oil production potential.
Keywords: Gravity flow sediments, sandy debris flow, turbidite, seismite-slump sandstone, Yanchang Formation, Ordos Basin
... As indicated by the microscopic characteristics, the detrital mineral content during the deposition of the Chang 7 1 is high, and correspondingly, Ca is strongly enriched; for Ti, this is indicative of the stability of terrigenous detrital supply, the content of TiO 2 is similar to the PAAS mean content (EF = 0.8), which indicates that terrigenous detrital supply is stable during deposition of the Chang 7 Mb. [35]; for Fe, this is often related to pyrite and commonly used to represent the redox condition, the content of TFe 2 O 3 is relatively high and indicates that the overall lake basin is in a reductive state at that time [36]; in addition, P 2 O 5 is enriched in the Chang 7 3 and Chang 7 2 and yet deficient in the Chang 7 1 , with an average EF of 1.17, which demonstrates the high content of organic matter and primary productivity of the Chang 7 shale [37]. ...
... An effective index of non-aluminosilicate detrital materials can be developed by dividing the Ti content by the Al content. High TiO2/Al2O3 values reflect high terrigenous input and high deposition rates [35]. The TiO2/Al2O3 values of the M, CM, CM-1, and S-2 lithofacies are 0.043, 0.039, 0.038, and 0.043, respectively, which implies that in the lithofacies with higher detrital content, the TiO2/Al2O3 values are relatively higher, indicating higher terrestrial input ( Figure 10B). ...
In order to clarify the characteristics of fine-grained sedimentary lithofacies and the depositional models in lacustrine environments of the Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, we focus on the lacustrine lithofacies classification and controlling factors. Based on the typical field sections of the southern Ordos Basin, combined with the methods of organic geochemical analysis, polarizing microscopic observation, XRD and elemental geochemistry, we summarize the main controlling factors of the lithofacies under different sedimentary environments and establish the sedimentary model under different sedimentary backgrounds. Results show that the Chang 7 Member of the Ordos Basin includes six major lithofacies types, i.e., quasi-laminated clayey shale facies, blocky silty mudstone facies, quasi-laminated silty shale facies, laminated silty shale facies, blocky mixed mudstone facies, and laminated argillaceous siltstone facies. Al2O3, MgO, CaO, Na2O, K2O, and SiO2 are relatively depleted, while TiO2, TFe2O3, and P2O5 are relatively enriched in the Chang 72+3. Meanwhile, the trace elements are relatively enriched in Cd, Cu, Mo, U, and V. The fine-grained sedimentary facies were influenced by various sedimentary environments, such as paleoclimate, redox conditions, productivity and terrigenous input. According to lithofacies types and genetic mechanisms of the Chang 7 Member in the study area, two types of lithofacies assemblages are identified. The rapid rise in lake level during the deposition of the Chang 72+3 resulted in an anoxic water column, high productivity, and low terrigenous input under a humid climate and weak-to-moderate weathering conditions. Therefore, clayey shale lithofacies assemblage is developed in the lower Chang 7 Member.
... As early as the 1970s, largescale lithologic oil and gas reservoirs with deep-water turbidites as the main reservoir were discovered in Jichang and Xifeng (Fu et al., 2008;Sun et al., 2020). In recent years, a large area of deep-water, oil-bearing sandstone has been discovered in the Chang 6 3 oil-bearing group in the Huaqing and Baibao areas (Zou et al., 2009). The understanding of the gravity flow development types of the Yanchang Formation in the Ordos Basin has passed through four development stages. ...
... They quantitatively characterized the structural characteristics and distribution of dominant sand bodies. Qu et al. (2021) (Fu et al., 2010;Fu et al., 2015;Liu et al., 2015;Wang et al., 2019;Wang, Wu, et al., 2020a;Wang, Yue, et al., 2020b;Yang et al., 2014;Yang et al., 2017;Zhang et al., 2021;Zou et al., 2009) division schemes of gravity flows in the study area, this paper analyzes the petrological characteristics, rock composition, pore-throat characteristics and microheterogeneity of FROSF, FROMT and FROSS. To clarify the characteristics of reservoir petrology, porethroat structures and other aspects of gravity flow genesis in the depression lake basin are examined. ...
The tight sandstone reservoirs have become the focus of today's oil and gas exploration. To clarify the microscopic characteristics of the tight sandstone reservoirs of the Chang 63 deep water gravity flow origin in the Huaqing area, the Chang 63 gravity flow origin reservoirs in the study area are divided into three categories. They are the formation reservoir of sandy clastic flow (FROSF), formation reservoir of muddy debris flow deposits and turbidites (FROMT) and formation reservoir of slides and slumps (FROSS). In this study, core and logging data, thin sections, field emission scanning electron microscopy (FE‐SEM), and mercury intrusion capillary pressure (MICP) were combined to investigate the rock composition and pore‐throat characteristics of tight sandstone reservoirs with different types of gravity flow origins. The reservoir micro‐heterogeneity is also investigated in this paper. The results show that the reservoirs with different gravity flow origins are similar in lithology and are mainly composed of feldspar lithic sandstone and lithic feldspar sandstone. The pore types are mainly intergranular pores and feldspar‐dissolved pores. There are significant differences in the porosity, permeability, microscopic pore‐throat characteristics and microscopic heterogeneity of the reservoirs. From FROSF to FROMT and FROSS, the porosity, permeability, mercury inlet saturation, discharge pressure, median pressure and mercury removal efficiency gradually decrease. The tightness of the reservoir gradually increases. The storage capacity is gradually reduced. The disparity between the throat radius and pore radius gradually increases. The pore‐throat structure becomes more complex, and the microscopic heterogeneity of the reservoir gradually increases.
... The rheological characteristics of sandy debris flow belong to Bingham plastic flow, which is supported by the matrix strength, dispersive pressure and buoyancy. The Bingham plastic flow represents the continuous sequence from viscous to nonviscous debris flow, which is between the argillaceous debris flow and the particle flow (Shanmugam 2002;Zou et al. 2009;Li et al. 2010). The sandy debris flow is marked by overall frozen sediment with block sandstone, and usually developed at the slope and the area below slope-break belts of the basin (Shanmugam 2002;Zou et al. 2009). ...
... The Bingham plastic flow represents the continuous sequence from viscous to nonviscous debris flow, which is between the argillaceous debris flow and the particle flow (Shanmugam 2002;Zou et al. 2009;Li et al. 2010). The sandy debris flow is marked by overall frozen sediment with block sandstone, and usually developed at the slope and the area below slope-break belts of the basin (Shanmugam 2002;Zou et al. 2009). The identification marks of sandy debris flow in strata have been summarized by previous researches (e.g., Shanmugam and Moiola 1995) as follows: (1) the blocky sandstone abruptly contacts with mudstone, which represents the overall plastic flow; (2) presence of ripped-up fragile boulder clay, which reflects that the whole boulder clay in the debris flow does not experience obvious particle collision and rounding during the process of slumping; (3) presence of floating gravel in medium-fine sandstone; and (4) reverse graded sequence with gravels concentrated in the upper part. ...
Ancient deep-lacustrine fans serve as prolific hydrocarbon reservoirs and provide records of climatic and tectonic activity. The deep-lacustrine fans of the Lower Jurassic Sangonghe Formation in Turpan Basin have become the most important exploration targets as stratigraphic traps. An integrated analysis of field profile investigation, core examination, well logging and seismic reflection profile interpretation, and grain size distribution patterns has enabled the identification of a series of deep-lacustrine fans. The deep-lacustrine fans of the Lower Jurassic Sangonghe Formation, deposited in the Turpan Basin consist of sandy debris flow, bottom current reworking and turbidite lithofacies. The deep-lacustrine fans are depicted by the following geological and geophysical features: (1) abundant soft-sediment deformation structures, and dish structures, normal graded beddings and inverse graded bedding structures developed in the strata, (2) the grain size distribution on the C–M diagram is parallel to the C = M baseline and cumulative probability curves display a bi-segment pattern with the suspension components to be the dominant parts, (3) gamma-ray logging curve are inverse-cycle features and seismic reflection profiles are characterized of poor continuity, high amplitudes, scattered and lenticular shapes. The deep-lacustrine fans are favorable target reservoirs, and have become the most important exploration targets in the Turpan Basin.
... In spite of inevitable defects and controversy, the new understanding of gravity flow sediments put forward by sedimentologists represents a relatively scientific and reasonable interpretation of the deep-water gravity flow sedimentary phenomenon at the present stage. The above new theory was applied in study of gravity flow sedimentary model in Ordos Basin (Zou et al., 2009;Li et al., 2011Li et al., , 2019, Bohai Bay Basin (Xia et al., 2017), and Songliao Basin (Du, 2015) of China by petroleum exploration departments and has played a crucial role in the distribution prediction of deep-water oil-bearing sandstone. ...
... This paper integrates the latest progress in the study of the gravity flow sediments domestically and internationally (Shanmugam and Moiola, 1995;Shanmugam, 2006;Zou et al., 2009;Li et al., 2011) and summarizes four types of deep-water gravity-driven processes: slide, slump, debris flow and turbidity current (Table 1). There are conspicuous differences and respective identification criteria in fluid rheology, flow state, nature of movement, nature of moving material, sediments granularity, depositional mode and location of deposition between different types of gravity flows (Table 1). ...
Sandstone thickness and reservoir quality in the third member of Paleogene Shahejie formation (E2s3) of different wells in BZ25 oil field are significantly variable. The uncertain cognition of high quality reservoir distribution restricts the development progress. By inspiration and application of latest theory progress of gravity flow, we carried out studies on lithological characteristic, lithofacies combination, paleo-geomorphology, sand period and evolutionary model of E2s3 gravity flow in BZ25 area. We identified three facies types of gravity flows in E2s3 of BZ25 area including slump, sandy debris flow and turbidity current. The above three types of gravity flows formed five typical lithofacies combinations in BZ25 area. Based on research of paleo-geomorphology and lateral facies distribution, we conclude that the process of gravity flow deposition from slope to deep-water environment in BZ25 area can be recovered as “slump - sandy debris flow - turbidity current”, and the sandy debris flow body distributes as special tongue-like shape with two branches, while the turbidity current body is characterized by fan-shaped. The sand-rich sandy debris flow in BZ25 area can be divided into two periods. The east branch of second period of the sandy debris flow is less exploited, and its central body has great development potential thickness.
... Sand body of deep-water gravity-flow refers to gravity-driven sands deposited in deep-water areas below the storm wave base plane, which is currently a key area for unconventional petroleum exploration (Haughton et al., 2009;Zou et al., 2009;Zou et al., 2012;Cao et al., 2017). The gravity-flow deposit is affected by various factors, including the lake basin bottom shape, provenance supply, tectonic activity, and climate, which means that gravity-flow itself is a key indicator of paleotectonic movement, paleo-source input, and paleoclimatic evolution (Amy et al., 2005;Ma et al., 2017;Yang et al., 2020). ...
... The Chang 7 and Chang 6 members of the Yanchang Formation in the Ordos Basin in central China are typical sections of gravity-flow deposits and the key targets for deepwater tight oil and gas reservoirs exploration. Chang 7 oil group in the Jiyuan area is a lithologic reservoir controlled by a large-scale turbidity current discovered earlier (Fu et al., 2008;Zou et al., 2009;Zhang et al., 2016). Then, large areas of deepwater oil-bearing sandstones in Chang 6 oil group in Bai Bao and Huaqing areas were discovered (Zhao et al., 2008;Li et al., 2009;Liu et al., 2015), further expanding the exploration field of the deep-water tight reservoir. ...
Gravity-flow can carry a large number of sediments and organic matters from shallow water to deep lakes with its strong transporting energy, directly or indirectly facilitating the formation of deep-water tight reservoirs and shale reservoirs. Therefore, studying the genetic types, dynamic mechanisms, and depositional models of gravity-flow deposits is essential in the exploration of unconventional petroleum in large lacustrine basins. This research studied the genetic types, dynamic mechanisms, and sedimentary models of the gravity-flow deposits of the Chang 6 oil group in the Heshui Area, Ordos Basin, China, aiming to reveal its petroleum geological significance. Core observation, microscopic thin section identification, particle size analysis, and determination of rare earth elements were carried out. As a result, three types of gravity-flow deposits are detected, namely, slide-slump, sandy debris flow, and turbidity current. A certain slope gradient in bed form is the necessary geomorphic condition for gravity flow formation, and determines its development level, distribution range, and flow transformation efficiency. Sufficient provenance lays the material foundation and determines its depositional composition and development type. Other factors include earthquakes, volcanoes, and floods, which serve as triggering forces. In addition, fragmentation, liquefaction, and fluid mixing are the main dynamic mechanisms driving flow transformation. Based on the flow type of gravity flow, particle size characteristics, gravity-flow transformation relations, development mechanism, and spatial distribution pattern, we distinguished two depositional gravity-flow models, i.e., slump turbidite body and sublacustrine fan. Re-portrait the spatial distribution of deep-water gravity flow in the study area. From the perspective of sedimentology, explain the genesis of sand bodies in the northeast and southwest. The sandy debris flow in the middle fan braided channel microfacies of the sublacustrine fan sways the development of thick massive sand bodies in the study area. Hybrid event beds formed by the fluid transformation in a slump turbidite are the potential dessert area for deep-water tight oil and gas.
... Furthermore, they are mainly observed at the translational domain of the MTDs, which are generally considered to represent unstable sediments with greater impedance difference than chaotic and transparent facies internally. Through some researchers have also proven that sandy MTDs are an important part of deep-water deposition (Zou et al., 2009;Liu, 2014). However, due to the lack of well data we are unable to confirm the actual lithology within the MTDs. ...
Focusing on the Zengmu-Beikang Basin in the southern South China Sea, we summarized the regional sequence stratigraphic framework and identified mass-transport complex (MTC) that had developed since the Late Miocene (10.5 Ma), by using high-resolution multibeam bathymetric and 2D seismic datasets. Based on the analysis of high amplitude and continuous seismic reflections within the mass-transport deposits (MTDs), we identified 14 basal shear surfaces (BSS) and divided the MTC into 14 units (MTD1–MTD14 from the youngest to the oldest in numerical order). Two types of seismic facies and five seismic features within the MTDs were recognized, including chaotic and transparent facies (debris flow), chaotic and semi-transparent facies (debris flow), thrust-fold systems, blocks, escarpments, pressure ridges, and normal faults. Analysis of the depositional area, frequency, and vertical extent of the escarpments of the fourteen MTD units revealed their variations in dimensions. Specifically, MTD14–MTD4 exhibited a gradual increase in the size of the deposition unit, with larger-sized MTD7–MTD4; while MTD3–MTD1 showed a gradual decrease in the size. The evolution of the MTDs in the study area is primarily controlled by several factors, including erosion of mass-movement processes (e.g., slide, slump, and mass flows), sea-level fluctuation and sediment supply, tectonic activity, development of mud diapirs, and topographic influence. It is also plausible that the large-sized MTD7–MTD4 might be formed in the Pliocene due to steep slope gradient and instability caused by the development of mud diapirs, whereas in the Quaternary, the influence of mud diapirs in the shallow strata diminished, and smaller-sized MTD3–MTD1 were formed.
... Ga is a special biomarker in saltwater environments, which mainly occurs in highsalinity water environments and indicates stratified water [46][47][48]. The gammacerane index (GI) of all samples is <0.2, indicating that the water body is fresh water and lacks stratification [49,50]. This is roughly the same as the water environment indicated by the ETR index, and there is a significant positive correlation between the ETR index and the GI. ...
The Baise Basin is a Paleogene pull-apart basin with numerous strike-slip faults which are not favorable for hydrocarbon preservation. The Nadu Formation, research object of this paper, is generally rich in oil and contains a large number of high-angle joint fissures. Analyzing the origin of residual oil in high-angle joint fissures can reveal the hydrocarbon migration and accumulation characteristics of the pull-apart-type basins. Molecular geochemical composition characteristics of crude oil and oil source of the Nadu Formation were discussed based on the saturated hydrocarbon biomarker compound and stable carbon isotope distribution of n-alkanes. The studied samples were selected from four members (E2n1, E2n2, E2n3up, and E2n3low) of the Nadu Formation. The results suggested that the average oil content of E2n1 fissures is 0.32 mg/cm², and the oil distribution is not uniform. The distribution of oil on the fissures of E2n2 and E2n3 is uniform and complete, and the oil content reaches 0.53 mg/cm². The oil in the joint fissures of the Nadu Formation is heavy, as the light hydrocarbon is seriously lost during migration. Thus, the oil in the joint fissures is residue after crude oil loses light components during migration. By comparing the molecular biomarker characteristics and stable carbon isotopic compositions, crude oil of the Nadu Formation can be classified into three categories: E2n1, E2n2 + E2n3up, and E2n3low. The E2n1 oils have the lowest maturity and are sourced from the E2n1 source rocks. Moreover, the maturity of E2n2 and E2n3 samples are relatively high. Biomarker and carbon isotope characteristics of the E2n2 and E2n3up oils are similar, indicating that they are derived from the E2n2 + E2n3up source rocks. The E2n3low oils are the mixture of the crude oil generated from the E2n3up source rocks and the E2n3low source rocks. Results presented show that the residual oil of high-angle joint fissures in the Nadu Formation is contributed by adjacent source rocks. The crude oil discharged from the Nadu Formation can only migrate upward along high-angle joints in a short distance, and the migration distance is usually less than 5 m. In conclusion, although the Nadu Formation has developed a large number of high-angle joint fissures, crude oil in the Nadu Formation has not vertically migrated for long distance along the joint fissures. The well-preserved fractures as important shale oil storage spaces indicate that the Nadu Formation has good shale oil exploration potential. The results may provide insights into the origins of hydrocarbons in the Nadu Formation from the Baise Basin and enhanced knowledge for optimizing future exploration and production.
... The Chang 7 member is a good target to study the deposits of deep lake gravity flows. Under the guidance of sequence stratigraphy, the Chang 7 stratigraphic framework was established in the study area [27][28][29]. The Chang 7 member was divided into three base level cycles, which were further finely divided. ...
Gravity flow deposits are important hydrocarbon reservoirs in deep lacustrine deposits. Previous studies have paid much attention to the hydrocarbon reservoirs in those intrabasinal classic turbidite deposits. However, relatively little is known about the distribution of oil reservoirs in those extrabasinal hyperpycnal flow deposits. With the help of cores and wireline logging data, the present study undertakes a description and interpretation of subsurface shale oil reservoirs in the deep lake deposits in Chang 7 member, Yanchang Formation, Ordos Basin. Parallel bedded fine sandstone (Sh), massive bedded fine sandstone (Sm), massive bedded fine sandstone with mud clasts (Smg), deformed bedded siltstone (Fd), wave-lenticular bedded siltstone (Fh) and black shale (M) were found and interpreted in those deep lake deposits. The deposits were interpreted as hyperpycnal flow deposits which developed in channel, levee and deep lacustrine facies. The development of the Chang 7 sand body increased gradually, and the sand body of Chang 71 was found to be the main position of sandy hyperpycnites. The fine description of the sand body indicated a channelized sedimentary pattern. The thick sandy hyperpycnites mainly developed in the middle of those channels, and the eastern part of the study area was found to be the main deposition position of the hyperpycnal flow deposits. From the perspective of plane overlap and single well analysis, a thick sand body is the favorable position for the development of an oil reservoir, which has a significant control effect on the reservoir scale and oil production. This research can aid in understanding the facies distribution of hyperpycnal flows and has implications for hydrocarbon reservoir exploration.
... Therefore, studying deep-water gravity flow deposits in lacustrine environments is critical [1][2][3]. The theory of deep-water gravity flow deposits has advanced rapidly, and since the 1970s, gravity flow sedimentation theory has been extensively applied in numerous continental lacustrine basins in China, including the Songliao Basin [4], the Ordos Basin [5,6], the Bohai Bay Basin [7,8], and the Tarim Basin [9]. In recent years, the field of deep-water sedimentology has witnessed the emergence of new concepts and models [10,11], such as sandy clastic flows and muddy clastic flows, which are frequently used to explain the transport and deposition processes of deep-water blocky sandstones in continental lacustrine basins. ...
This article studies the sedimentary characteristics and models of the delta and gravity flow system of the third member of the Shahejie Formation in the Niuzhuang Sag area. Through seismic, logging, and core observation methods, a thorough investigation is conducted to examine the lithologic characteristics, grain size characteristics, sedimentary structure characteristics, and sedimentary facies distribution characteristics of this region. The results show that the third middle member of the Shahejie Formation in the Niuzhuang Sag can be classified into four sedimentary types: sliding, collapse, clastic flow, and turbidity flow. This article aimed to establish the distribution characteristics and depositional models of the deltaic and gravity flow depositional systems within the study area. The findings reveal that slip deposition primarily occurs near the delta front, while collapse and clastic flow depositions are concentrated near the far slope. Moreover, turbidity flow deposition is found near the far slope. This study significantly contributes to our understanding of the sedimentary characteristics and models associated with deltas and gravity flow systems in faulted lacustrine basins. Furthermore, it enriches existing theories related to gravity flow and provides a valuable reference for the investigation of deep-water sedimentation in continental faulted lacustrine basins.
... Sandy debris flow deposits are dominated by massive fine-grained to medium-grained sandstone (Fig. 14a) intercalated with pebbly coarse-grained sandstone and siltstone, which reflects laminar flow transport unloaded by massive consolidation (Shanmugam, 2000(Shanmugam, , 2002(Shanmugam, , 2013Zou et al., 2009;Talling et al., 2012Talling et al., , 2013. Mud gravel, mud clasts, mudstone liquefaction veins and flame structures are observable, but an obvious Bouma sequence is not observed (Figs. ...
Deep‐water gravity depositional processes and evolution in arc systems have become topics of intense research focus in recent years. This study discusses the co‐evolution of volcanism and deep‐water gravity flow deposits at the southern margin of the Junggar Basin, based on petrology, geochronology and geochemical analyses. The results show that a massive collapse of unstable sediments from the slope was triggered by volcanism, resulting in the formation of slumping gravity flows. The occurrence of volcanic beds in the slump deposits confirm that synchronous volcanism likely affected sediment instability, triggering gravity flows. The Th/Yb, Ta/Yb and Th/Ta elemental ratios, U‐Pb ages of detrital zircons and paleocurrent directions indicate that the North Tianshan (NTS) island arc represents the provenance of the Qianxia Formation. Moreover, statistical data on the pyroclastic components in the gravity flow deposits reveal an intensity index of volcanism, indicating that volcanism is strongly related to gravity flow deposits, especially in terms of the type and distribution of the deposits. A model for volcanically‐triggered deep‐water gravity flow deposits is established, in order to provide a more in‐depth understanding of the co‐evolution of volcanism and gravity flow deposits within the depositional setting of the late Paleozoic NTS oceanic subduction margin in the Junggar Basin.
... In recent years, with the increasing demand for oil and natural gas, the difficulty of exploration and development has also increased. The global oil and gas exploration and development has shifted from conventional oil and gas to shale oil, tight oil and other unconventional oil and gas (Fu et al., 2005;Yang et al., 2013;Zou et al., 2009). The pore structure of reservoir rocks has a significant impact on the physical properties of the reservoir, the migration and storage capacity of oil and gas, and the development of the oil field. ...
... In the process of tight oil exploration and development, the Open Access Library Journal study of reservoir has become a crucial step, and the search for high-quality "sweet spot" reservoirs in relatively tight reservoirs has become the main goal of oil and gas exploration [1]- [6]. The Ordos Basin is one of the typical representatives of tight sandstone reservoirs in China and is rich in dense oil resources, which are mainly located in the Chang 6 and Chang 8 oil formations of the Mesozoic Triassic Yanchang Group in the longitudinal direction and concentrated in the lake basin sediment center of the Triassic Yanchang Group in the planar direction, with great exploration potential [7] [8]. In previous studies of tight oil in Ordos ...
... Based on the matrix type of structureless clast-supported conglomerate lithofacies, it can be divided into mud-supported conglomerate (A1) and sand-supported conglomerate (A2, Table 1). The former was mostly developed in the mud-rich gravity flow sedimentary environment of the fan delta plain subfacies with fewer in the upper fan delta front [64]. The latter mainly formed under the alteration of sand-rich gravity flow and debris flow. ...
Fan deltas of the Lower Cretaceous area in Saihantala sag, Erlian Basin have been identified as major petroleum exploration opportunities. The sedimentary evolution is, however, still debatable, which hinders insights into its controlling factors. This research employed new core observations, thin section observations, and grain size analyses of 28 wells in the Saidong sub-sag, together with numerous borehole and seismic data points, to explore lithofacies types, subfacies, and microfacies characteristics, thus leading to a further investigation of the sedimentary facies evolution of the sag and its controlling factors. The findings showed there are 3 categories, 12 sub-categories, and 20 fine lithofacies types in the Saidong sub-sag. Additionally, various sand-conglomerate lithofacies were characterized by lower composition and texture maturity. With dentate-shaped, box-bell-shaped, and other morphological well-logging responses, fan deltas were mostly developed in the A’ershan Formation and the Tengge’er Formation, which could be subdivided into three subfacies and eight microfacies. Given the sedimentary features and lithofacies characteristics of each microfacies, it can be determined that three main stages occurred in formations from the A’ershan to the Tengge’er: the water transgression, the water oscillation, and the water regression. Moreover, fan delta deposits were regulated primarily by semi-arid hygrothermal and semi-arid paleoclimate and paleotectonic factors.
... 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]. ...
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.
... The mudstone section is mainly gray-black, black layered, and massive mudstone (Figure 1c, Figure 1d, Figure 1e). During the reaction deposition period, the water body is deeper, which is the characteristic of sandy debris flow in typical gravity flow [1] . ...
Nowadays, Longitudinal gravity flow deposits, as a kind of important deep-water reservoirs, can form a certain scale of lithologic oil and gas reservoirs, which is of fundamental significance in the exploration of oil in Saihantala Depression. The evolution and sedimentary characteristics of the longitudinal gravity flow in the east subsag of Saihantala Depression are analyzed based on the core date, well logging, seismic data, regional geological data, and the method of seismic sedimentology. The sedimentary model of the longitudinal gravity flow and the exploration potential of the lithologic reservoir of longitudinal gravity flow are also discussed. The reservoir is mainly composed of medium-fine sandstone with developed secondary pores. The geological conditions for the formation of axial gravity flow are sufficient provenance, the development of slope breaks and the negative axial topography. The updip direction of the longitudinal gravity in the area has a short extension of the sand body, and the downdip direction has thick massive fine sandstones, which opens up pathways for lithologic reservoir exploration.
... First is the establishment of new models of lake basin multi-type sedimentary systems and lithofacies paleogeography 581 recovery. Combining with modern sedimentation investigation, flume experiment, fabric feature dissection and genetic model research of different sedimentary facies, the theory has perfected the lake basin sedimentary system pattern and proposed new sedimentation models such as sandy debris flow [59] , large shallow-water delta [6061] , beach-bar sedimentary system [62] , density flow [63] and coarse-grained fan [64] , revealing the distribution rules of different types of lake basin sedimentary systems and providing an important basis for objective evaluation in the oil and gas exploration. ...
China's continental oil and gas geological theory occupies an important academic position in the world's academic circle of petroleum geology. China's oil and gas resources are dominated by continental resources. Chinese geologists have successfully explored and developed complex continental oil and gas, and developed a continental oil and gas geological theory system. This paper summarizes the development history and theoretical achievements of continental oil and gas geological theory since the 1940s and proposes that the development of this theory should be divided into three stages (i.e., proposal, formation and development). The China's continental oil and gas geological theory has formed a basically perfect theoretical system consisting of five parts, i.e., continental basin structure theory, continental basin sediments and reservoirs theory, continental oil generation theory, continental oil and gas accumulation theory, and continental sandstone oil and gas field development geology. As an advanced geological theory, it has a universal significance globally. This paper focuses on the major discoveries of oil and gas exploration and development and the production growth in the basins of the Central and Western China in the past 30 years as well as the major advances in the continental oil and gas geological theory, including the continental basin tectonics of Central and Western China under the compression background, special reservoir geology such as various types of lake basin sedimentary systems and deep conglomerate, new fields of continental hydrocarbon generation such as coal-generated hydrocarbons, continental oil and gas enrichment regularity such as foreland thrust belts and lithologic-stratigraphic reservoirs, continental unconventional oil and gas geology and continental low-permeability oil and gas development geology. These major advances have greatly developed and enriched the continental oil and gas geological theory and become an important part of it. © 2018 Research Institute of Petroleum Exploration & Development, PetroChina
... The formation of deep-water sandstones has attracted considerable attention in both the oil industry and the field of sedimentology (Dalla and Gamberi 2010;Bourget et al. 2010;Talling et al. 2012). In total, 945 major oil and gas fields were discovered around the world from 1859 to 2007, of which 341 (34%) were in passive-continentalmargin deep-water environments (Zou et al. 2009). In addition, large oil and gas reservoirs related to deep-water sandstones have been discovered in numerous continental basins in China (Fu et al. 2013;Yang et al. 2012;Pan et al. 2013). ...
Deep-water deposition is a current issue in sedimentological research. Sandy-debris-flow sandstones and turbidity-current sandstones are the main types of sandstone that are the focus of considerable disputes in this research. Previous studies mainly focused on description of the macroscopic sedimentary structure and theoretical derivation of the formation mechanisms. The microscopic petrological characteristics, reservoir properties, and formation mechanisms of deep-water sandy-debris-flow and turbidity-current sandstones have been studied in the Yanchang Formation of the Ordos Basin, China, by means of field outcrop surveys, thin-section identification, geochemical element analysis, and porosity and permeability measurements under overburden pressure. The content of detrital grains in the sandy-debris-flow sandstones is high, whereas the contents of mica sheets and matrix are low. The fine-grained matrix is distributed unevenly within the pores. A considerable number of residual intergranular pores are preserved in the middle of single sand bodies, resulting in relatively better reservoir properties. The total number of detrital grains in the turbidite sandstone is low, while it contains abundant mica sheets and matrix. The mica sheets and fine-grained matrix are distributed evenly within the pores, resulting in serious damage to pores and poor reservoir properties. The sandy-debris-flow sandstones in the center of the lake basin form a high-quality reservoir; thus, this area is suitable for oil and gas exploration.
... The sediment at fan delta fronts is unstable due to its rapid accumulation. It often slides and forms gravity flows under exogenous trigger mechanisms (Zou et al., 2009). Fine-grained terrigenous particles are deposited from suspension and form organic-rich calcilutite with phytoplankton in semi-deep lakes and deep lakes (Fig. 6). ...
Based on observations made on cores and cuttings from several wells in the lowermost part of the third member of the Shahejie Formation, several rock types, specifically clast-supported rudstone, matrix-supported rudstone, mixed-source rudstone, calcisiltite/calcarenite, massive calcilutite and laminated calcilutite, have been identified in the Shulu sag. According to the sedimentary structures and distribution characteristics of these rocks, the carbonate breccias fall into two categories, based on their origins: one formed by fan-delta channel sedimentation, whereas the other formed by earthquake-induced slump fan deposition. Clast-supported rudstone and matrixsupported rudstone are the main lithologies deposited by braided rivers in the fan delta plain and front, of which the pore space is mainly dissolution pores within gravels and tectonic fissures. Clastsupported rudstone, matrix-supported rudstone and mixed-source rudstone are the main lithologies of the earthquake-induced slump fans. These carbonate breccias developed along with soft-sediment deformation structures, which are interpreted as seismites and are widely distributed in the sag, in which intercrystalline pores, intergranular pores and fissures created from diagenetic shrinkage are developed. The two kinds of rudstones have different reservoir characteristics and oil/gas testing results. The rudstones generated in the fan delta have higher porosity and permeability, as well as better oil/gas testing results. Thus, they are key targets for petroleum exploration.
... The sandy debris flow deposits are the most widely-distributed type of gravity flow sand bodies, and due to high density of fluid, it is often distributed in slope zones of basin with irregular tongue shape, the sand bodies are continuous. Classic turbidites constitute a relatively small proportion in the gravity flow deposits, due to low fluid density, it can extend to plain areas of the basin, fans with water channels on the plain are the main difference from sandy debris flow deposits, sand bodies occur as isolated lenticular or thinbedded sheet-like shapes (Zou et al., 2009). Slump deposits are deformable formations developed in the deep-water environment due to sliding and/or slumping, it is mixed with sand and mud, resulting in poor sorting. ...
Tight oil refers to a petroleum play that occurs in a free or adsorbed state in source rocks or tight reservoir rocks (e.g., sandstone and carbonate rock) interbedded with or close to source rocks. Tight oil has generally not experienced large-scale, long-distance migration. According to such a definition and its characteristics, 10 key indices are proposed for tight oil resource evaluation in China. Tight oil reservoirs are divided into three groups in terms of porosity and permeability. Tight oil can be classified into three types according to the contact relationship between the tight oil reservoirs and source rocks, i.e., tight lacustrine carbonate oil, tight deep-lake gravity flow sandstones oil, and tight deep-lake deltaic sandstones oil. In China, tight oil resources are widely distributed and significant exploration discoveries have been achieved in the sixth member and seventh member of the Triassic Yanchang Formation in the Ordos Basin, the Permian Lucaogou Formation in the Junggar Basin, the Middle-Lower Jurassic strata of the Sichuan Basin, and the Cretaceous Qingshankou and Quantou Formations in the Songliao Basin. The total geological resources of tight oil in China assessed by using the “analog” method are estimated to be (10.67−11.15) ×10⁹ tones. Taking into account of the future prospects of petroleum development, tight oil may become a realistic alternative to the conventional oil resources in China.
... Evolution of basin controls assemblages of depositional facies and development characteristics of sand bodies. Delta-front and gravity flow sand bodies are mainly developed in the 7th member in the basin and the 6th member in the center of the basin (Zou et al., 2009;Li et al., 2010;Li et al., 2011), and thick laminated sand bodies are formed in the center of the lake basin (Fu et al., 2010). The area of lake basin reached the largest during deposition of the 3th sub-member of the 7th member (Yang et al., 2010), then its gradually shrank thereafter. ...
Efficient large-scale development of ultra-low-permeability reservoirs (0.3–1 mD) has been achieved in the Changqing Oilfield, Ordos Basin of China. According to unique features of petroleum exploration and development in this basin, tight oil herein refers to petroleum that occurs in oil-bearing shales and interbedded tight sandstone reservoirs adjacent to source rocks with ambient air permeability 1×109 t of oil resources in shale in the 7th member of the Yanchang Formation and approximately 0.9×109 t and 1.1×109 t of tight sandstone oil resource in the 6th and 7th members of the Yanchang Formation, respectively. These tight oil resources are the realistic resources addition for the oilfield, which can ensure an annual production of 50×106 t of oil and gas equivalent and maintain long-term stable oil production in the Changqing Oilfield, Ordos Basin, China. Key words: tight oil, tight sandstone reservoir, shale reservoir, resource potential, Yanchang Formation, Ordos Basin
... However, at the same time, abundant discoveries and breakthroughs have been made in nonmarine lacustrine basins in China. These include the Bohai Bay, Erdos, the Erlian Basins, and so on (Feng & Xu, 2006;Li, Zhang, Song, et al., 2004;Lin, Zheng, Ren, et al., 2003;Zou, Zhao, Yang, et al., 2009). ...
Large sublacustrine fan deposits have been identified within the lacustrine successions of the second member of the Palaeogene Dongying Formation in the Liaozhong Depression. In this study, by using comprehensive and detailed analyses of the borehole lithology, limited cores, wireline logs, and seismic facies, 4 types of sublacustrine fan deposits were identified and characterized. Then, an integrated depositional model was established based on the above investigations. The result indicates that the geophysical characteristics of the different sublacustrine fan deposits differed from each other, in terms of their internal configuration and external geometry of seismic reflections, and stacking patterns of wireline logs, and stratigraphic position in the vertical successions. Subsequently, the model was established according to the comprehensive analysis of the geophysical characteristics and stratigraphic position. The discussion of developing conditions and hydrocarbon discoveries within different types of sublacustrine fan deposits provided robust insights regarding how to locate and evaluate reservoirs in such deposits. Furthermore, the results of this study may potentially assist in achieving a new understanding of how to identify sublacustrine fan deposits in similar lacustrine basins and may also assist in making further decisions regarding the terms of hydrocarbon migration and accumulation.
The geological structure of interbedded shale oil reservoirs is complex, later characterized by high reservoir heterogeneity and diverse reservoir spaces. These distinctive features are primarily attributed to their unique source–storage configuration. This paper comprehensively investigates the pore structure characteristics and controlling factors, which are beneficial for realizing efficient and sustainable resource utilization. The pore structure characteristics and main control factors of interbedded shale oil in the Heshuinan (HSN) area of the Ordos Basin are studied by analyzing thin sections and scanning them under an electron microscope, and using XRD analysis, a high-pressure mercury injection, a constant-rate mercury injection, and a nitrogen adsorption method. The influence of sedimentation and diagenesis on the pore structure is analyzed. Research shows that the interbedded shale oil reservoirs of the Triassic Chang 7 in the HSN area have an average porosity of 8.47% and an average permeability of 0.74 × 10⁻³ μm². The reservoirs are classified as typical ultra-low porosity, ultra-low permeability reservoirs. The various pore types in the study area are mainly residual intergranular pores and feldspar dissolution pores. The pores are mostly in the shape of parallel slits and ink-bottle-shaped. The pore-throat radii range from 0.02 μm to 200 μm. Sedimentation and diagenesis jointly control the pore structure in the study area. Sedimentation determines the material foundation of the study area. Diagenesis affects later pore development. Early compaction greatly reduces the intergranular pores, but the chlorite envelope reduces the influence of compaction to some extent. The compacted residual intergranular pores are further reduced by clay minerals, carbonate minerals, and siliceous minerals. Late dissolution promotes pore enlargement, which is the key to the formation of high-quality reservoirs. Furthermore, on this basis, this paper outlines the genetic mechanism of the Chang 7 high-quality reservoir in the HSN area to provide guidance for the exploration and development of interbedded shale oil and gas.
The Shaximiao Formation in the Southwestern Sichuan Basin is rich in natural gas resources, but the genetic types of channel sandbodies remain unclear. This study investigated the sedimentary characteristics and sandbody genetic types of the Jurassic Shaximiao Formation using well core, logging, and reservoir property data. The results revealed that: (1) The Shaximiao Formation is predominated by shallow-water delta front facies, with delta plain facies present only in the Sha-1 Member. Lithologically, it exhibits a “mudstone-overlying-sandstone” pattern with channel sedimentary sequences portraying normal grading and various microfacies. (2) The sandstones of the Shaximiao Formation developed seven types of lithofacies. Based on the characteristics of lithofacies associations, six genetic types of sandbodies, i.e., deeply incised abrupt-transition, deeply incised gradual-transition, growth gradual-transition, progradational superimposed, progradational mouth bar, and suspension accretion sandbodies are recognized. The deeply incised abrupt-transition and deeply incised gradual-transition sandbodies have the best reservoir physical properties, while the suspension accretion sandbody has the worst. (3) These genetic types of sandbodies are controlled by the relative position of the channels and their relation to mouth bars, with reservoirs in deeply incised abrupt-transition and deeply incised gradual-transition sandbodies developed in the delta plain. (4) High-quality reservoirs are found in deeply incised abrupt-transition and gradual-transition sandbodies, where sedimentary processes strongly influence reservoir physical properties. This study provides new insights into the relationship between tight sandstone reservoirs and channel sandbody genetic types, offering guidance for exploration and development in similar regions.
Many types of sedimentary systems occur in the middle of the third member of the Shahejie Formation (E2S32) of the Paleogene in the Dongying Sag east of the Bohai Bay Basin. Due to the topography and material supply, traction and gravity flow depositions are intertwined in this area, and the sand body types are complex and diverse, making it challenging to improve the accuracy of their description and prediction and restricting oil reservoir exploration and development. Therefore, this paper documents our systematic study of the sedimentary characteristics of the southern slope of Dongying Depression, the formation mechanism of different sand body types, and the prediction of sand body distribution. First, according to the coring well’s single-well facies and vertical rock sequence, nine single lithofacies types and five lithofacies association types were identified. Combined with the well logging facies marks of all wells, the depositional models of delta and gravity flow depositional systems were established in the study area. Then, the gravity flow was divided into slip, collapse, debris flow, and turbidity flow according to its development mechanism. Finally, the distribution law of the gravity flow sedimentary facies type was predicted. Gravity flow sliding deposits are primarily distributed near the delta front, slump and clastic flow deposits are distributed near the far slope, and turbidity current deposits are distributed at the far slope. With the gradual shrinkage of the water body in the north-west direction and the continuous advancement of the river delta, the gravity flow sand body gradually disappears in the late E2S
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and transits to delta plain deposition.
Microfacies and depositional architecture of the lacustrine deepwater gravity flows implicate exploration and development of hydrocarbon reservoirs in lacutrine environments. The seventh oil layer group of the third member of the Yanchang formation of the upper Triassic (Chang 7) is crucial for shale oil exploration from the Heshui Area of the Ordos Basin, China. Based on logging data interpretation, core description, and reservoir laboratory tests, this paper analyzed the sedimentary microfacies and reservoir characteristics of the Chang 7. According to the thickness of sand bodies, lithology characteristics, and logging curves response, we recognized two sedimentary facies including sublacustrine fan and lacustrine facies, three sub-facies including middle fan, semi-deep, and deep lakes, and four microfacies including distributary channel, proximal margin of distributary channel, and distal margin of distributary channel, and lacustrine mud, spatial and temporal associations of which represent depositional processes and environments in space and time at the Heshui Area of the Ordos Basin. Using the multivariate comprehensive parameter method, the Chang 7 reservoirs were divided into four categories including type I, type II, type III, and type IV, and the potential of their oil productivity decreases in turn. As main producing objectives, type I and type II reservoirs are chiefly distributed in the middle, northwest, and north of the study area with the strip and block shapes. The research results will provide an effective basis for shale oil exploration and development of the Chang 7 in the Heshui Area of the Ordos Basin, China.
This study describes and characterises the spatial distribution and evolution of Carboniferous deep-water sediment gravity flows and their associated deposits in the West Junggar region of north-western China. It uses acombination of outcrop sections, cores and seismic profiles to define nine lithofacies associated with a differentsediment gravity flow types, including turbidity currents, debris flows, hyperpycnal flows and supercritical flows.The subduction and closure of the remnant West Junggar Ocean basin demonstrated variable palaeogeographicalcharacteristics and configurations during different periods of the Carboniferous. Fossil assemblages and zircondating data indicate that the target strata have completely recorded the evolution of the Westermn Junggarremnant ocean in the Carboniferous. The basin evolution is divided into the stable ocean basin stage, the slowsubduction and flling stage and the fast subduction and filling stage. Active faulting and varying slope gradient;affected the occurrence and types of sediment gravity flows during each stage. Tectonic and seismic activitiesassociated with subduction caused continental slope instabilities and basinward transport of sediment by gravityflows with active faults on the continental slope triggering sediment gravity flows. At the beginning of subduction, the ability of gravity flows to transport coarse debris was limited owing to the shallow gradient of thecontinental slope. Sediment transport via gravity flows was dominated by sandy debris flows, muddy debrisflows, turbidity currents and less frequent flood-controll3ed hyperpycnal flows. As the basin entered the fastsubduction and filling stage, the slope gradient steepened and the frequency of active thrust faulting increasedwhich triggered high-density turbidity currents and supercritical flows. Additionally, the warm and humicpaleoclimate led to the development of hyperpycnal flows during the fast-filling stage. This study demonstratesthe importance of understanding the character and distribution of sediment gravity flows and associated deposits, especially, the implications for the hydrocarbon industry.
To reveal the development characteristics and distribution of gravity flow sedimentary system under micro-paleogeomorphologic units of the Chang 7 Member of Triassic Yanchang Formation in the southwestern Ordos Basin, on the basis of the restoration of the paleogeomorphological form of the Chang 7 depositional period by the impression method, each micro-paleogeomorphologic unit was depicted in-depth, and the characteristics and development models of gravity flow deposits in the study area were studied in combination with outcrop, core, mud logging and log data. The results show that: (1) The paleogeomorphology in the Chang 7 depositional period was an asymmetrical depression, wide and gentle in the northeast and steep and narrow in the southwest. Three sub-paleogeomorphologic units were developed in the basin, including gentle paleo-slope, paleo-slope and paleo-depression, and they can be further subdivided into eight micro-paleogeomorphologic units: bulge, groove, slope break belt, plain of lake bottom, deep depression of lake bottom, paleo-channel, paleo-ridge of lake bottom, and paleo-uplift of lake bottom. (2) There are 9 types of lithofacies and 4 types of lithofacies assemblages of Chang 7 Member. According to lithofacies composition and lithofacies vertical combination, the gravity flow deposit is further divided into 5 types of microfacies: restricted channel, unrestricted channel, natural levee, inter-channel, lobe. (3) Paleogeomorphology plays an important role in controlling sediment source direction, type and spatial distribution of sedimentary microfacies, genetic types and distribution of sand bodies in Chang 7 Member.
Layered intrusive masses and sublacustrine fans have similar reflection features on seismic data. Thus, distinguishing them has been a big challenge for lithologic hydrocarbon reservoir exploration in faulted basins. In this study, we took the Hailar Basin as an example to achieve quantitative identification and prediction between such two geologic bodies. First, we established geologic models and performed seismic forward modeling to find the relationship between the models and the highlighted seismic body (HSB). Then, a quantitative chart was plotted and applied in the study. The research results showed that HSB values greater than 630 represented the response from layered intrusive masses, those less than 380 represented the response from lacustrine mudstone, and the values between the two matched the response from sublacustrine fans. We concluded that the above forward-modeling-based HSB had advantages in distinguishing and predicting layered intrusive masses and sublacustrine fans. The prediction result was proven by subsequent well drilling. The method has popularization and application values in similar geological areas.
The middle and eastern part of Chang 63 sub member in Huaqing area of Ordos Basin is controlled by the Northeast provenance, with large thickness and good contiguity of sand bodies. The proved reserves have been submitted to 633 million tons, and most of them have been developed. The unclear genesis and distribution of sand bodies in the western area restrict the exploration and development process of Chang 6 in this area. In this study, clastic components, heavy minerals and imaging logging data were used to determine the provenance of the area; paleosedimentary thickness recovery method was used to restore the bottom shape of Chang 6 sedimentary Lake Basin; and gravity flow sedimentary characteristics were further determined by core, grain size analysis and rock mineralogy identification. The results show that: in the west of Huaqing area, Chang 63 is mainly affected by three provenance systems: Northeast, northwest and southwest. In the northwest provenance control area, the bottom shape is relatively wide and gentle; in the northeast and southwest provenance control area, the bottom shape fluctuates greatly, which divides the area into several small accommodation spaces; in addition, the Northeast provenance control area is mainly the slope area of the lake basin The results show that the source of debris supply is sufficient, and the sand body is large in scale. The plant debris and stem fossils are common in the core observation of mudstone, and the imbricate structure of sandstone is common, which is dominated by flood type gravity flow, and the sand body is banded distribution. The Southwest and northwest provenance has weak debris supply capacity, and mainly develops slump gravity flow deposition, with abundant slump deformation structure and mudstone tearing debris. The sand body scale is significantly controlled by the bottom shape of the lake basin Large scale sandbodies are developed in the lower part of the bottom, and they are distributed in lobate shape. The study on the provenance and sedimentary characteristics of Chang 63 in Western Huaqing area provides an important basis for the distribution law of dominant sand bodies in this area and the next exploration and development.KeywordsOrdos BasinHuaqing areaChang 63 reservoirProvenance analysisGravity flow depositionSand body distribution
The first member of Shahejie Formation of Eocene–Early Oligocene (Es1, Nanpu Sag) consists of fluvio‐deltaic to deep lacustrine shale‐sandstone accumulations, and the deep lacustrine deposits include sandy debris flow (SDF), turbiditic and mudrock facies. In the process of hydrocarbon exploration, it was found that that SDF sandstones in the deep lacustrine deposits can constitute excellent hydrocarbon reservoirs. Therefore, the identification of these lithofacies, especially SDF sandstones, is crucial for hydrocarbon exploration. We document here parameters that discriminate lacustrine SDF sandstones from turbiditic and mudrock deposits. Data from cores, wireline logs, and seismic sections are integrated and interpreted. Core‐based identification indicators of the SDF sandstones include massive structures with upper and lower sharp contacts with interbedded mudrocks, lateral pinch‐outs, locally imbricated to floating pebble‐size grains, and elongated (teared) mudrock clasts within the middle and upper parts of normally‐ or inversely‐grading sandstone and poor to high matrix content. These properties are attributed to deposition of non‐Newtonian flows with sediment‐support mechanism of dispersive pressure, matrix strength and buoyancy, sediment‐transportation mechanism, and sediment‐settling properties (hindered settling and freezing of the SDF). Log‐facies identification indicators of the SDF sandstones are characterized by higher RLLD/RLLS values (40–80 Ω m), higher SP values (35–70 mV), lower Gamma ray (GR) values (70–120 API), and strongly serrated curve motifs. Seismic facies identification indicators of the SDF sandstones have mound‐like, lenticular, or wedge‐shaped external geometries, and interior worm‐like and fusiform chaotic reflections with medium to strong amplitude, medium to high frequency, and medium to low continuity. The results and interpretation of the integrated core‐log‐seismic data as identification indicators are useful tools for identifying SDF sandstones and effectively distinguishing them from other deep lacustrine deposits.
Shale oil is an unconventional oil resource that needs to be developed and utilized urgently. However, the Chang 7 shale in the Ordos Basin, as the most typical continental source rock in China, is limited by the study of organic matter (OM) enrichment factors in continental lacustrine facies, and there are still controversies about the controlling factors, which limit the progress of oil and gas exploration. This paper aims to reconstruct the paleoenvironment of Chang 7 shale in the southern margin of Ordos Basin and reveal the controlling factors of organic rich shale by organic and elemental analysis, X-ray diffraction (XRD) analysis, thin section observation, and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) analysis. The results show that during the deposition period of Chang 7 shale, the climate was warm and humid, the lake water has strong reducing, low salinity and rapid depth changes. Total organic carbon (TOC) is positively correlated with salinity and hydrothermal action and inversely proportional to terrigenous input. The high productivity, low consumption and low dilution result in high enrichment of shale OM in the southern margin of Ordos Basin.
The Paleogene Shahejie Formation (Shahejie Fm) of Bohai Bay Basin has ignited unprecedented interest among scholars owing to another breakthrough achieved in shale exploration besides Kongdian Fm. However, this breakthrough, particularly high output submember-the third Member of the Paleogene Shahejie Formation (Mbr 3) lacks elaborate study on paleoenvironment and its covariations with organic matter (OM) concentration for shales, which impedes the further acquaintances of OM distribution and shale oil-gas possibility. The present study adopts advanced approaches to gain insights into the deposition process of shales in the Mbr 3, Qikou Sag, Bohai Basin. Petrological characteristics, organic geochemistry, and an abundant element database offer a solid foundation for performing master controlling factor analysis and establishing deposition modes for OM accumulation and preservation. Our results indicate that the entire Mbr 3 has potential. Shales in the Mbr 3 of the Shahejie Fm were deposited under a temperate environment with weak weathering and sediment influx surrounded by suboxic freshwater. However, the organic-enriched shale in the upper Mbr 3 is a more desirable exploration layer. Disparate paleoproductivity support and clastic detritus sediment afforded a diminished consequence of OM accumulation between the upper Mbr 3 (L1-L2 in this study) and middle Mbr 3 (L3 in this study). Our investigation suggests that the OM concentration in the shales was dominated primarily via paleoproductivity and secondarily via redox reactions. Clastic sediment influx sets the primarily passive effect, while effect of salinity was minimal. Two sedimentary modes were established to interpret the variability between L1-L2 and L3 during the two depositional stages. The lake expansion period of L3 resulted in shales being deposited mainly on a gentle slope with a shallow water column containing insufficient OM susceptible to redox condition compared to the optimal state in the semideep lake in L1-L2 stage. The results provide an integrated reconstruction of paleosedimentary environment for the target layer and offer a solid basis for further exploration through coupling relationship between sedimentary environment and OM enrichment in the study area.
Fluid mobility has been important topic for unconventional reservoir evaluation. The tight sandstones in Chang 7 Member of the Ordos Basin has been selected to investigate the fluid mobility based on the application of core flooding-NMR combined method and core centrifugation-NMR combined method, and the porous structure is studied using optical microscope, field emission scanning electron microscope (FE-SEM), CT and mercury injection. Our results include: (i) Feldsparrock fragments dissolution pores, calcite dissolution pores, clay mineral dissolution pores, intergranular dissolution expansion pores, inter-granular pores, intra-kaolinite pores, and intra-illite/smectite mixed layer pores are developed in Chang 7 tight sandstones; 3D CT pore structure shows that the pore connectivity is positively related to physical properties, and the overall storage space is connected by the throat with diameter between 0.2 and 0.3 µm. The percentage of storage space connected by throats with diameter less than 100 nm can reach more than 35%. (ii) Movable fluid saturation of Chang 7 tight sandstones is between 10% and 70%, and movable oil saturation is between 10% and 50%. Movable fluid saturation may cause misunderstanding when used to evaluate fluid mobility, so it is recommended to use movable fluid porosity in the evaluation of fluid mobility. The porosity ranging from 5% to 8% is the inflection point of the fluidity and pore structure. For samples with porosity less than 8%, the movable fluid porosity is generally less than 5%. Moreover, the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.1 to 1 µm. For samples with porosity greater than 8%, the porosity of the movable fluid is more than 5%, and the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.2 to 2 µm. (iii) The movable fluid saturation measured by core flooding-NMR combined method is generally higher than that measured by core centrifugation-NMR combined method. The former can evaluate the mobility of the oil-water two-phase fluid in samples, while the latter can better reflect the pore structure and directly evaluate the movable fluid in the pore system controlled by different throat diameters. All these results will provide valuable reference for fluid mobility evaluation in tight reservoirs.
The Chang 6 member sandstone is one of the main oil reservoirs of the Ordos basin, China, and consists of a lacustrine gravity flow deposited in Huangling area. Carbonate cement is volumetrically the most important diagenetic parameter controlling reservoir quality in the sandstones.
Three generations of carbonate cements were recognized. The massive microcrystalline calcite, poikilotopic calcite and microcrystalline dolomite from the supersaturated alkaline pore water precipitate with basement cements at an early diagenesis stage. The ferroan calcite is mainly pore-filling and replacement cementation in the middle diagenesis stage, and ferroan dolomite is pore-filling cementation in the late diagenesis stage.
The four types of reservoir facies and the distribution patterns were identified in the Chang 6 member sandstones. (1) Massive carbonate cemented facies with high contents of carbonate cements are distributed along the edges of the sandstone bodies, which are in the channels of the turbidite and sandy debris flow deposits. (2) Partial carbonate cemented facies occur along the margins of the massive carbonate cemented facies. (3) The porous facies with the lowest percentages of carbonate cements in sandstone are distributed in the middle of the sand-bodies of the sandy debris flow deposits. (4) High muddy debris facies that have high percentages of carbonate-cemented and high muddy debris sandstones occur essentially in the proximal overbank and lobe of the turbidite deposits.
The reservoir quality of sandstones depended upon time of precipitation of carbonate cements on and diagenetic processes. Massive carbonate cements and the high muddy debris of sandstones undergo lots of carbonate cements and strong compaction in the early diagenesis stage, respectively. There was weak water rock reaction of dissolution in the later diagenesis process. Partial carbonate cemented facies and the porous facies of sandstones have sufficient water rock reaction and more dissolution pores.
The lithofacies characteristics of fine sedimentary rocks affect the hydrocarbon generation capacity and reservoir performance of tight oil and are closely related to the sedimentary environment. On the basis of field outcrops and core observations, combined with logging data, thin section identification, grain size analysis, organic carbon analysis and physical property tests, the fine sedimentary facies of the Chang 7 Member in the Ordos Basin are studied. Nine lithofacies types are identified in the study area, including planar and trough cross-bedding sandstone facies (Spt), massive bedding sandstone facies (Sm), slump deformation fine sandstone facies (Ssd), sand-mud interbedded facies (Sb), rippled siltstones facies (Fr), horizontal bedding mudstone facies (Mh), massive bedding mudstone facies (Mm), horizontal bedding shale facies (S) and tuff facies (T). Lithofacies types Spt, Ssd, Fr are developed in the gentle slope belt in the northeastern study area, which is a delta front-semi-deep lake sedimentary environment and mainly contains underwater distributary channel and estuary dam sedimentary microfacies types. Lithofacies types such as Sm, Sb, Mh and T are developed in the southwestern steep slope zone of the study area, which is a channel-type gravity flow sedimentary environment, and sedimentary microfacies types such as near-source channels, far-source channels and leading-edge dolomites are developed. In the vertical direction, during the Chang 73 sedimentation period, the lake surface area and water depth reached their maxima, and S and Mh lithofacies developed, which were conducive to the formation of high-quality hydrocarbon source rocks for tight oil. During the Chang 72-Chang 71 sedimentation period, the lake basin shrank, and the reducibility of the water body weakened. The delta front sand body composed of Spt lithofacies in the northeast and the sandy clastic flow sand body composed of Sm in the southwest constitute the main tight oil reservoirs. High-quality hydrocarbon-generating lithofacies and favourable reservoir lithofacies are spatially superposed and developed, forming two source-reservoir combinations in the Chang 7 tight oil source-reservoir formation and near source-reservoir formation.
In recent years, owing to the advances in technological development, research and development of unconventional energy resources have been activated. In particular, development of shale gas and tight oil in the United States will have a great influence on the energy supply in the world in the future. Interest in tight oil is increasing in China because China has fascinating unconventional energy resources, and among them, tight oil has the potential to become a promising energy resource. In China, it is expected that reservoir characteristics and availability of tight oil are clarified with the progress of the geological survey, and that the development of tight oil is advanced. In this paper, we examine the availability and potential capacity of tight oil resources in China. On the basis of the characteristics of tight oil reservoir in the United States, the situation and potential of Chinaʼs tight oil resources are examined through the comparison of the geological availability environment of tight oil reservoirs in China and the United States.
Our current knowledge of deep-water depositional systems is mainly built of extensive achievements obtained from marine basins. Although lacustrine basins are important oil&gas productive areas of the world, gravity-induced deposits in these basins are poorly understood in the literature when compared with those documented in marine basins. Based on new insights from 3D seismic data, this paper attempts to investigate the effect of the interaction between gravity-induced flows and lake-floor topography on the resulting external geometry of gravity-induced deposits in the Cretaceous Qingshankou and Nenjiang formations of the Songliao Basin (SLB).
Eight different shapes of gravity-induced deposits were identified in the SLB, and which were in turn grouped into four distinct categories according to external shape, internal architecture and spatial assembly: (1) Channel-fan complexes; (2) Fan-form complexes; (3) Strip-form complexes; and (4) Pond-form complexes. Furthermore, fan-form complexes can be subdivided into four subcategories (isolated fan with compressional ridges, isolated fan without compressional ridges, mother-son fans and stacked fans). Strip-form complexes can be subdivided into two subcategories, including strip-form controlled by gully system and strip-form controlled by growth faults.
Sediment transport mechanism (density flows or mass transport flows) and lake-floor topography together influence the pathways, runout distances, discharge places and internal characteristics of gravity induced deposits, which could ultimately influence external geometries of gravity-induced deposits in the SLB. Consequently, sublacustrine gravity-induced deposits does not exhibit classic fan-like geometry. Frequent and powerful hyperpycnal flows associated with flood events are fairly common, and develop channel-distal fan complexes. MTDs exhibit various external geometries, which are significantly affected by growth faults, slope gullies, gradient, and basin-floor irregularities. The new depositional model built from the SLB is vital to understanding the distribution of gravity-flow deposits in a sublacustrine slope.
Based on new theory of deep water sandstone, the sedimentary process of the gravity flow sandstone of the Shahejie Formation in the Huanghua depression was divided into four sandstone zones of genetic types in this work: sliding, slumping, sandy debris flow and turbidity flow by comparing sedimentary structures of typical cores, variation of grain sequence, difference of the reservoir physical properties, lithologic vertical combination sequences, and diagrams of well logging facies. The geometry and contact relationship of sandstones of different genesis were studied, and the evolution model of gravity flow sedimentation process was figured out. Combined with dynamic production data, the pattern of oil and gas differential enrichment was analyzed.
The seventh oil layer of the Upper Triassic Yanchang Formation (Chang-7) tight oil sandstone reservoirs is a major exploration target. A significant amount of hydrocarbons has been discovered in these reservoirs in the southwestern Ordos basin in China. The Chang-7 tight sandstones are characterised as tight with low porosity, low permeability, and strong heterogeneity. This study investigates the sedimentary facies, diagenesis, and their impact on the reservoir quality of the Chang-7 tight oil sandstones. The sandstones were deposited in a deltaic-lacustrine depositional system. Three major depositional facies are identified consisting of delta front fed by braided rivers and meandering rivers, and slump turbidite fans. The depositional environment exerts a key control on reservoir quality. The distinct low-energy sedimentary environment produced fine to very fine-grained sandstones with high matrix and mica contents, characterised by low initial porosity and permeability. Diagenesis mainly comprised mechanical compaction and cementation by quartz, carbonate minerals and various clay minerals. The reservoir properties of the Chang-7 sandstones are generally poor, with porosity of 1.4–20.7% (average porosity 8.6%) and permeability of 0.001–116.7 mD (average 0.2 mD), which are attributed to significant compaction and cementation. Mechanical compaction was more important than cementation for reducing porosity, whereas secondary dissolution porosity was significant for the Chang-7 tight oil sandstones due to closer proximity to the underlying Chang-73 source rocks.
The Shulu Sag is located in the southwestern corner of the Jizhong Depression, Bohai Bay Basin of East China. The lower part of the Shahejie Formation developed massive conglomerate characterized by low porosity and low permeability with carbonate fragment as its main constituent. According to the sedimentary structure and distribution characteristics, etc., the carbonate breccia may fall into two genetics: one formed by fan-delta channel sedimentation, whereas the other is formed by earthquake-induced slump fan deposition. The braided river is the main sedimentary body of the fan delta and the typical characteristics are imbricate structures with normal graded bed sequence. The latter appears along with typical seismites widely distributed in the sag, which include soft sediment deformation structures (sedimentary dikes, hydraulic shattering, etc.), and brittle deformation (synsedimentary faults).
Based on the integrated analysis of seismic, drilling and core data, a large channel-fan system of hyperpycnal flow origin was found in the Qijia-Gulong area of the Nen 1 Member of the Cretaceous Nenjiang Formation in the Songliao Basin. The hyperpycnal flow in this area, which originated from the edge of the basin and then passed the northern delta, formed a complete channel-fan system in the deepwater area. The channel-fan system comprises straight channels and meandering channels extending from north to south over a straight distance of more than 80 km with a width of 100900 m, and distal fan lobes at the channel tip cover a maximum area of 20 km 2. This system, which is dominated by fine-grained deposits, contains massive sandstone, sedimentary structures of flow-water origin, and internal erosion surfaces; it also contains abundant continental organic clasts and exhibits evidence of bed-load and suspended-load transportation mechanisms. The hyperpycnite sequence contains pairs of coarsening-upward lower sequences and fining-upward upper sequences, reflecting the dynamic features of cycles in which floods first strengthen and then weaken. A new sedimentary model has been built for hyperpycnites in the Songliao Basin.
Based on seismic, logging, formation testing, core and lab test data, this study analyzed the sequence division, facies features of deep water deposits and modes, development of large-scale gravity flow, reservoir physical properties and their main controlling factors, and proposed a classification standard and prediction method of favorable exploration areas in deep water area of the Bin1 oil layers of the lower sub-member of the first member of Paleogene Shahejie Formation in Banqiao-Qibei slope zone of Qikou sag, Bohai Bay Basin. The Bin1 oil layers can be divided into three fifth-order sequences, each less than 100 m thick; a set of gravity flow deposits were formed under deep water background in the slope zone, which contains sedimentary micro-facies such as main channel, distributary channel, channel margin, inter-channel mudstone, and turbidite sand sheet in areas without channels, and, in space, has inherited and constructive development features of multistages. It is a sedimentary sequence of fan delta – distal subaqueous fan – deep lake, and every distal subaqueous fan formed by gravity flow can be divided into inner-, middle- and outer fans. The cross-facies transported sands which are sourced from higher-sand-content major sands of delta front can form high quality reservoirs with an average porosity of 15.1% and geometric average permeability of 5.1×10⁻³ μm². The main channel and distributary channel of distal subaqueous fan are the most favorable exploration zones.
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