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... The South China Sea (SCS) is surrounded by an extensional margin to the north, a subduction margin to the east, a compressional margin to the south and a transtensional or transform margin to the west (Xie et al., 1995;Yuan et al., 2009). The SCS basins developed and evolved as a result of the kinematics and interactions among the Eurasia Plate, the IndiaeAustralia Plate and the Pacific Plate in the Cenozoic (Honza and Fujioka, 2004;Yuan, 2007;Li et al., 2012a). ...
... with sand storage forming a self-generation and self-storage accumulation combination (Fig. 8). Because this combination is deeply buried in the area, the secondary pores are characterized by an enhanced development after strong digenesis, which generally have a porosity of 5%e10% and permeability of 3e5 mD (Li et al., 1995;Xing et al., 2011), the permeability of argillaceous rocks is generally 10 À1~1 0 À3 mD. ...
... The Yan'an Formation in the Jurassic system is the main coal-bearing stratum in the study area, there are five coal seams in this formation (coal seams II-VI). The Yan'an Formation can be divided three depositional system tracts [14]: a lowstand systems tract (LST), a lacustrine-expanding systems tract (EST), and a highstand systems tract (HST). The LST and the HST are dominated by river deposition, while the EST is dominated by lacustrine delta deposition [14]. ...
... The Yan'an Formation can be divided three depositional system tracts [14]: a lowstand systems tract (LST), a lacustrine-expanding systems tract (EST), and a highstand systems tract (HST). The LST and the HST are dominated by river deposition, while the EST is dominated by lacustrine delta deposition [14]. Yan'an Formation is made up of sandstone, mudstone, and coal . ...
The rare earth element (REE) behavior and the related Ce anomalies of two profiles in bauxite of the Xinmo Syncline from Wuchuan-Zheng'an-Daozhen (WZD) area (Northern Guizhou, China) have been studied. The bauxite has diaspore as its main ore mineral, with lesser amounts of boehmite. Clay minerals, including kaolinite, chlorite, illite and smectite, are a minor mineral constituent. Furthermore, a significant Ce-bearing mineral parisite (Ce2Ca(CO3)(3)F-2) is found near the bottom of the profiles. The bauxites have flat HREE (Er-Lu) shape relative to NASC, with variable weak depletion to weak enrichment patterns of LREE (La-Nd) and MREE (Sm-Ho) across the profiles. Strong positive Ce-anomalies (normalized to NASC) are also noticeable in the uppermost part of the profiles. Mass balance calculations suggest that an obvious fractionation exists among different REE and LREE are more mobile than HREE during the leaching process. In addition, the samples collected along a vertical profile show that a downward increase for the REE concentration and a remarkable enrichment of REE are found at the basement of the bauxite deposits. A similar downward increase trend is also observed in the values of La-N/Yb-N and Gd-N/Yb-N. The above characteristics show that the REE losses caused by leaching decreased gradually from the top to the bottom of the profile. The higher mobility of LREE in the profile might be due to differences in the stabilities of the original REE-bearing minerals during leaching process. The REE enrichment at the basement of bauxite deposits is attributed to the increase of pH around the parent rock as well as the presence of mineral ligands during bauxitization. The value of [Ce/Ce*](NASC) decreased downwards, and increased again near the bottom of the profile. The positive Ce-anomaly in the uppermost part of the profile has been attributed to the redox change of Ce3+ to Ce4+ and the consequent precipitation of cerianite (CeO2). A decrease in the oxidation state downwards causes the decrease of the value of [Ce/Ce*](NASC). The formation of parisite near the bottom of the profile leads up to the increase in the value of [Ce/Ce*](NASC) again. The precipitation of parisite may occur via reactions between the fluoride complexes (CeF2+ or CeCO3F0) and Ca2+ and HCO3-.
... The Yan'an Formation in the Jurassic system is the main coal-bearing stratum in the study area, there are five coal seams in this formation (coal seams II-VI). The Yan'an Formation can be divided three depositional system tracts [14]: a lowstand systems tract (LST), a lacustrine-expanding systems tract (EST), and a highstand systems tract (HST). The LST and the HST are dominated by river deposition, while the EST is dominated by lacustrine delta deposition [14]. ...
... The Yan'an Formation can be divided three depositional system tracts [14]: a lowstand systems tract (LST), a lacustrine-expanding systems tract (EST), and a highstand systems tract (HST). The LST and the HST are dominated by river deposition, while the EST is dominated by lacustrine delta deposition [14]. Yan'an Formation is made up of sandstone, mudstone, and coal . ...
... The theoretical models of sequence stratigraphy proposed by early scholars were mainly based on marine sedimentary basins (Adabi et al., 2010;Galloway, 1989;Van Wagoner et al., 1988;Wang et al., 2022), and many scholars have conducted exploratory researches on the application of sequence stratigraphy principles to terrestrial petroliferous basins (Borsa et al., 2023;Gu, 1995;Guo et al., 1998;Jia, 2009;Li et al., 1995;Ren et al., 2017;Zhu et al., 2011). However, due to the inherent problems in the theory and technology of sequence stratigraphy, such as the research objectives of sequence stratigraphy, the distribution location of lowstand system tract (LST for short), the definition of sequence stratigraphic grading, the issue of lithological structure in the systems tracts, the issue of sea level rise and fall curves, etc, the standardized industrial application of sequence stratigraphy has been seriously restricted (Catuneanu et al., 2009;Catuneanu and Zecchin, 2020;Wang, 2007;Wu, 2017). ...
Based on the research results of stratigraphic sedimentary facies, plane maps of sedimentary facies for four periods of the Neogene terrestrial foreland basin in Southwest Tarim were compiled. Research results conclude that the Neogene Miocene developed alluvial fan facies, fan delta facies, and lacustrine facies successively from the piedmont belt to the Maigaiti Slope. During the sedimentation of the Miocene Anju’an Formation, the lake range expanded to its maximum. According to the new division scheme for five-order sequences in prototype sedimentary basin, the stratigraphic sequence of a terrestrial foreland basin was divided and compared, and a sequence development and evolution model was established. Research results show that: (1) the Neogene–Quaternary is a typical type I sedimentary sequence with huge thickness; (2) the Neogene is a period of tectonic compression and flexural subsidence sedimentation in the foreland basin; with the maximum lake flooding surface at the top of the Miocene Anju’an Formation as the boundary, the lower part is a second-order super-sequence of the lacustrine transgression system tracts (TST), the upper part is a second-order super-sequence of the highstand system tracts (HST), and the Quaternary is a second-order super-sequence of the lowstand system tracts (LST) in forced lacustrine regression caused by the Late Himalayan tectonic movement; (3) the classic sequence stratigraphy made an error in establishing the theoretical model of marine type I sequences, as it placed the LST in the wrong position. The corrected type I sequences are in the order of TST, HST, and LST. (4) Finally, two examples were applied to demonstrate the development position of the LST, indicating that whether it is a marine or terrestrial prototype sedimentary basin, the LST was developed during the late stage of tectonic compression and uplift in a basin, and is the sedimentary sequence at the top of the prototype sedimentary basin.
... Therefore, the thickness and lithology of those Lower Jurassic sediments show significant differences (Zhao et al., 1999). The Jurassic succession in the Ordos Basin comprises the upward suite of the Fuxian, Yan'an, Zhiluo, and Anding formations (Li et al., 1995). Paleontology, sedimentology, and geochemistry studies indicate that the Ordos Basin was located in a warm temperate and humid climate zone during the early through middle Early Jurassic (Hettangian-Pliensbachian), and the environment changed into a tropical-subtropical semiarid and semi-humid climate zone during the Toarcian (Deng et al., 2012(Deng et al., , 2017. ...
... (a) Evolutive-spectrum analysis and spectral analysis on GR curves; (b) Evolutive-spectrum analysis and spectral analysis on tuning time successions The Yan'an Formation is divided into 10 oil members (Figure 3), based on the coal seams deposited in the strata. These members primarily reflect an upward transition from the braided river facies into a meandering river to river-lake delta facies ( Figure reserve is over 700 billion tons (Li, Lin, Xie, Yang, & Jiao, 1995). ...
The coal‐bearing Yan'an Formation in the Ordos Basin of the Middle Jurassic was deposited in a river delta–lacustrine sedimentary environment with relatively stable accumulation rates. Spectral analysis and wavelet transform analysis of the natural gamma‐ray (GR) curves from several boreholes through the Yan'an Formation yielded five main sedimentary cycles with wavelengths of 34–57, 10–18, 8–14, 2.9–4.1, and 2–3 m. These cycles were controlled by orbital cycles of eccentricity (405, 128, and 97 kyr), obliquity (35 kyr), and precession (22 kyr), and the long‐eccentricity period of 405 kyr was the most significant cycle. Filtering the sedimentary records for the long‐wavelength cycle and short‐wavelength cycle implies that there are 11 long‐eccentricity cycles and 35 short‐eccentricity cycles preserved in the Yan'an Formation. Assigning the 405 kyr long‐eccentricity cycle to the filtered 11 primary long‐wavelength cycles implies that the Yan'an Formation spanned 4.4 Myr. The orbital–climate cycles were critical factors in the sediment facies changes and episodes of coal formation within the basin. The long‐eccentricity cycles of the Milankovitch cycle are the predominant control factors of the coal seams deposited in the Middle Jurassic Yan'an Formation.
... The mega-sequence ends with the purplish-red and variegated sediments (sandstone, mudstone, and marl) of the Anding Formation, which were deposited in a lacustrine setting under semi-arid climate conditions. A variety of plant, sporopollen, and freshwater bivalve fossils suggest Early to Middle Jurassic ages for the Fuxian, Yan'an, and Zhiluo Formations (Li et al., 1995a;Wang, 1996;Zhang et al., 1998). ...
Exploring the relationship between coal deposits as an important terrestrial carbon sink and orbital forcing of climate is critical for understanding the global carbon cycle and climate change. The Jurassic greenhouse period, characterized by extensive coal reserves widely distributed in the mid-latitude terrestrial basins, marks a significant coal-forming interval in Earth’s history. However, understanding of the processes that controlled the formation and distribution of coal at this time is inadequate. The Yan’an Formation of the Ordos Basin in north central China is among the largest and most extensively studied Jurassic coal reservoirs of the world. Here we establish a high-resolution age framework for the Yan’an Formation derived from integrated, high-precision U-Pb zircon geochronology using chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on interstratified ash beds and cyclostratigraphy based on centimeter-scale magnetic susceptibility. Accordingly, the main coal-forming interval of the Yan’an Formation spanned ca. 174.0 Ma to <171.7 Ma, which coincided with the onset of the Middle Jurassic. The spectral analyses of the Yan’an Formation coal seams demonstrate a strong correlation to minima in the 405 k.y. orbital eccentricity cycles, suggesting a strong climate control on lake level fluctuations and clastic sediment input. Finally, we explore the cyclicity of a large set of published marine carbon isotope data from western Tethys and its phase relationship to cyclic coal deposition in the Ordos Basin. Our resutls underscore the role of terrestrial organic carbon burial in the global carbon cycle during the Middle Jurassic.
... ly Proterozoic metamorphic crystalline basement, which is located below the Upper Proterozoic, Palaeozoic, Mesozoic, and Cenozoic sedimentary cover. Apart from the absence of Silurian-Devonian sedimentation, the cover from Proterozoic to Tertiary is complete. The entire sedimentary sequence is more than 6,000 m thick (H. D. Chen, Li, & Zhang, 2011;S. Li, Lin, Xie, Yang, & Jiao, 1995 Only carbonate rock without any terrigenous clastic rock existed at the top (C. L. Zhang et al., 2017;J. Zhang, Li, Liu, & Feng, 2011). On the Upper Cambrian, the Huaiyuan movement began as the southern Ordos Basin uplift occurred, causing the ancient geographical pattern in the south to be higher than that in the north (Su et al., 2011 ...
There are widely developed carbonate formations in the lower Palaeozoic, Ordos Basin, China. However, previous studies have paid more attention to the Ordovician formations and largely ignored the Cambrian formations, which also have a larger sediment thickness. Recently, with the continuous discovery of dolomite gas fields in the Ordovician Majiagou formation, more investigations have begun to focus on the deeper dolomite in the Cambrian Sanshanzi Formation (Є3s). This study looks into the origin of the dolomite through a combination of petrology, major–trace elements, and stable isotope and rare earth element (REE) analysis of samples from five outcrops. The results show that the main dolomite types in the study area are dolarenite (Type 1), micrite‐fine crystalline dolomite (Type 2), and medium crystalline dolomite (Type 3). The δ¹³C values and δ¹⁸O values of each dolomite formation are close to those of seawater‐derived dolomite and have slightly higher Na²⁺ concentrations, suggesting that dolomite formation is closely related to the denser seawater. The REE compositions are characterized by enrichment for light REEs and depletion for heavy REEs, exhibiting an apparent negative Eu anomaly and a slight negative Ce anomaly, indicating that the dolomites of the Є3s may form in a weak‐reduction to the weak‐oxidation environment. The total REE (ΣREE) concentrations of Type 3 dolomites are lower than other types, suggesting that those dolomites may mean that the REEs are lost during the diagenesis. Type 1 dolomite has lower MgO and CaO values and a higher Mn²⁺ concentration than the other two types of dolomites, indicating that Type 1 dolomite suffered the action of atmospheric fresh water. Type 2 dolomite has lower Mg and Ca values than Type 3 dolomite, suggesting that Type 3 dolomite has a higher dolomitization degree. Further study into the combined palaeogeographical background confirms the dolomitization model of the study area: Type 1 and Type 2 dolomites may have been formed by penecontemporaneous or seepage–reflux dolomitization during early‐stage diagenesis. Subsequently, stratigraphic uplift lead to the Type 1 dolomite suffering the action of atmospheric fresh water, but there was no dorag dolomitization. Type 2 dolomite became the main dolomite type by seepage–reflux dolomitization in this period. Later, Type 2 dolomite was converted into Type 3 dolomite during progressive burial.
... Generally, there are five main coal seams (coal seams II-VI) with the thickest exploitable seam being 7-8 m (Fig.1, Yang et al., 2006). The Yan'an Formation can be subdivided into three depositional system tracts (Li et al., 1995): a low stand systems tract (LST), a lacustrine-expanding systems tract (EST), and a high stand systems tract (HST). The Zhiluo Formation is a secondary coal-bearing stratum, with one main coal seam (coal seam I) in the lower Zhiluo Formation (Fig. 2). ...
... In the sedimentary period of the second member of Nantun Formation, lake basin underwent a transformation period of fault-folded depression, during which the tectonic subsidence slowed down and the basin margin fault activities became weaker so that the lake basin became wide and gentle, which embodies the characteristics of sedimentary paleoenvironment: wide basin and shallow water [12][13][14][15][16]. The fan delta scale became smaller, and the distribution was very much limited. ...
Tarbell Depression is a potentially lucrative exploration in outer blocks of Daqing Oil Field. However, the petroleum geological background is complex and there is little identified information before researches. In order to find out the oil and gas distribution of the studied area, it is necessary to study the regional depositional environment, sedimentary phase, and other characteristics and to establish an accurate model of the deposition. In our research, the core observation, particle size analysis, and electrofacies are used to analyze the hydrodynamic characteristics of the studied area, to build sedimentary facies model, and to predict sandbody distribution. Besides, seismic attribute analysis is also used to forecast inter-well sandbody distribution. Based on the recognition mentioned above, the sedimentary facies are predicted in the strata units of Tongbomiao, Nantun, and Damoguaihe formations. The results show that Tarbell Depression mainly develops alluvial fan, fan delta, delta, nearshore submarine fan, lake floor fan, and lacustrine depositional systems from Tongbomiao Formation to Damoguaihe Formation, in which fan delta system and lacustrine depositional system are the most developed types. Sedimentary facies research is the process of oil and gas exploration in the most important foundation, and it is of great significance for the future exploration and resources evaluation.
... (2) The Jurassic, which has a parallel unconformity contact with underlying Late Triassic strata; the tectonic setting is more stable than that of the Late Triassic. The sediment is mainly fluvial-delta depositional facies (Li et al., 1995). (3) The Upper Jurassic-Fenfanghe Formation, which is coarse clastic alluvial fan facies, reflecting basin-periphery strong tectonic activity and molasse formation caused by uplift. ...
... (2) The Jurassic, which has a parallel unconformity contact with underlying Late Triassic strata; the tectonic setting is more stable than that of the Late Triassic. The sediment is mainly fluvial-delta depositional facies (Li et al., 1995). (3) The Upper Jurassic-Fenfanghe Formation, which is coarse clastic alluvial fan facies, reflecting basin-periphery strong tectonic activity and molasse formation caused by uplift. ...
Major joint sets trending E-W (J1), ENE-WSW (J2), NE-SW (J3), N-S (J4), NNW-SSE (J5), NNE-SSW (J6), NW-SE (J7), and WNW-ESE (J8) respectively are recognized in Mesozoic strata within the southeast of Ordos Basin. Among them, the J1, J2 and J3 joint sets are systematic joints, while the other five joint sets (J4, J5, J6, J7, J8,) are nonsystematic joints. There are three groups of orthogonal joint systems (i.e. J1 and J4 sets, J2 and J5 sets, and J6 and J8 sets) and two groups of conjugate shear fractures (ENE-WSW and NNE-SSW, ENE-WSW and ESE-WNW) in the study area. Joint spacing analysis indicates that: (1) Layer thickness has an effect on the joint spacing, but the correlation of joint spacing and layer thickness is low; (2) Joint density of systematic joints is greater than nonsystematic joints, and the joint density of a thin layer is also greater than that of a thick layer; (3) The joints of Mesozoic strata in the basin are the result of tectonic events affected by multiple stress fields. All these joints in the Mesozoic strata are formed in the two main tectonic events since Late Mesozoic times. One is the westward subduction of the Pacific Plate beneath the Eurasia Plate, which formed the approximately E-W-trending compressive stress field in the China continent. The trends of the J1 joint set (E-W) and the bisector of conjugate shear fractures composed of ENE-WSW and ESE-WNW fractures are all parallel to the trend of maximum compressive stress (E-W). The other stress field is related to the collision of the Indian and Eurasian Plates, which formed the NE-SW-trending compressive stress field in the China continent. The trends of the J3 joint set and bisector of conjugate shear fractures composed of ENE-WSW and NNE-SSW fractures are all parallel to the trend of maximum compressive stress (NE-SW). Finally, we conclude that the J1 and J4 sets are formed in the E-W-trending compressive stress field, and the J2, J3, J5, J6, J7 and J8 sets are formed in the NE-SW-trending stress field.
... Ordos Basin is an oil, gas, coal, uranium and other energy mineral symbiotic rich sedimentary basin [1][2][3][4] , attracting broad interest of coal and petroleum geologists. A wealth of research result has been obtained recently. ...
Coal accumulation of Lower Member of Shanxi Formation in Northwest part of Ordos basin has been researched according to the principle of base level cycle based on outcrops, core description, and well log. Sedimentary face analysis, base level correlation, condense data controlling mapping were adopted. Coal forming has a close relation to base level cycle, face distribution, and coal-forming latter stage of short-term base level ascending cycle. Coal measure distribution has a complementary relationship with distributary channel. In peat swamp far from distributary channel, coal layer has greater thickness, with lower ash content, and lack of waste rock interlayer. Near the distributary channel, coal layer is thinner and sparsely-distributed, with high ash content, and transition to clastic rock. From shan2³ to shan2¹, thick coal layer has a trend of retreat towards north, showing that the lower member of Shanxi Formation constitutes mid-term base level ascending cycle.
... Therefore, these studies provide some geologic evidence and time-space constraint conditions on the syn-collision of flexural foreland and craton basin to research the basin-filling history, temporal and spatial evolution succession, as well as to understand further the plate collision process. The study on the orogen-basin couple is also the frontier of international geology; although there have been some previous studies and discussions on the process of foreland basement flexural subsidence by orogeny and their controlling factors [13,[36][37][38][39][40][41][42] , whose progress has made mainly in the nonmarine basin sequence and their architecture model [43,44] , the study on sequence architecture and their response to tectonics is still weak. ...
Two kinds of margin respectively occur in the Ordos Basin during the Middle-Late Triassic (Yanchang Age), one is foreland
margin developed under the background of flexural subsidence by thrusting intensively in the southwest margin, and the other
is intracratonic basin margin by stable subsidence in northern and central parts of the basin. The Middle-Late Triassic Yanchang
Formation can be divided into four regional third-order sequences, which are separated by gentle angular unconformity or regional
erosion surface, made up of lowstand system tract (LST), expanding system tract (EST) and highstand system tract (HST) from
lower to upper within a sequence. But there are distinct differences of the sequence framework between the southwest margin
and northern and central parts of the basin. The southwest margin develops heavy conglomerate layer and unconformity as a
result of orogeny by thrusting, and the intracratonic basin margin by stable subsidence in the northern and central parts
grows aggradational sandstone, conglomerate in fluvio-delta system and parallel unconformity. The depositional framework of
southwest margin reflects the tectonic evolution from flexural subsidence by thrusting to rebounded uplift. The formation
of sequence boundary is related to the resilient uplift and erosion. The sequence stratigraphic framework and depositional
system tract configuration in the foreland basin are controlled by structural activity of the fold and thrust belt, and the
sequence succession reflects episodic thrusting of the Middle-Late Triassic toward the foreland basin. The sequence evolution
in northern and central parts reflects the depositional succession of fluvio-delta system under intracratonic background,
composed of coarse-grained sediment in braided channel deposit at the lower, meandering channel deposit in the middle and
fine-grained sediment in the flood plain at the upper, dominated by lake level fluctuation. During the deposit of the LST
in the intracraton basin, accommodation space is limited, and results in abundant fluvial sediment migration laterally, erosion
and transport, forming laterally sandstone composite and aggradational deposit on the alluvial plain, which constitutes specific
erosion unconformity boundary.
The Middle Jurassic Yan'an Formation in the Ordos Basin (North China) is an important coal-bearing succession. Twenty coal samples and four samples of silty mudstones were collected from Coal Seam 2 of the Yan'an Formation in order to determine major, trace and rare earth elements; moreover, the geochemistry of the rare earth elements, and in particular yttrium, was investigated. The major elements investigation reveals that Si, Al, Na, K, and Ti are primarily found in clay minerals. Calcium is primarily found in calcite and iron is largely controlled by sulfur. The average REE contents in the coals and silty mudstones are 15.17 μg/g and 210.57 μg/g, respectively. The Upper Continental Crust-normalized distribution patterns of the REE (rare earth element) in the coals and silty mudstones show no obvious fractionation; however, they show negative Eu anomalies and negligible Ce anomalies. Correlations between the REE and the major elements indicate that the REE in the coals and silty mudstones are both derived from eroded rocks in the hinterland and mainly controlled by the presence of detrital minerals. We investigated some diagrams of major element, trace element, and REE from the coal seam to comprehend its provenance and tectonic setting. The findings indicate that they are mainly derived from felsic and intermediate rocks in a continental island arc setting. The REE analysis shows that various provenance areas supplied clastics when coal developed in the Yan'an Formation. The findings imply that provenance analysis should be taken into account when studying the sedimentary environments of coal-bearing successions.
Outcrop and drill hole data show that the Jurassic coal measures in the northeastern Ordos Basin are composed mainly of the Yan'an Formation and the lowstand system tract of the Zhiluo Formation, and there is a regional unconformity between them. The Dongsheng uranium deposit is associated with the Jurassic coal measures. Research data indicate that the Jurassic coal measures in the study area have a certain hydrocarbon-generating capacity, although the metamorphic grade is low (Ro=0.40%–0.58%). In the Dongsheng region alone, the accumulative amount of generated coalbed methane (CBM) is about 2028.29 × 10⁸ −2218.72 × 10⁸ m³; the residual amount is about 50.92 × 10⁸ m³, and the lost amount is about 1977 × 10⁸ m³. Analysis of the burial history of the host rocks and the evolutionary history of the Dongsheng uranium deposit suggests that the Jurassic coal measures generated hydrocarbon mainly from Middle Jurassic to Early Crataceous, which is the main mineralization phase of the Dongsheng uranium deposit. By the Late Cretaceous, a mass of CBM dissipated due to the strong tectonic uplift, and the Dongsheng uranium deposit stepped into the preservation phase. Therefore, the low-mature hydrocarbon-containing fluid in the Jurassic coal measures not only served as a reducing agent for the formation of sandstone-type uranium deposits, but also rendered the second reduction of paleo-interlayer oxidation zone and become the primary reducing agent for ore conservation. Regional strata correlation reveals that the sandstone-type uranium reservoir at the bottom of the Zhiluo Formation is in contact with the underlying industrial coal seams in the Yan'an Formation through incision or in the form of an unconformity surface. In the Dongsheng region with poorly developed fault systems, the unconformity surface and scour surface served as the main migration pathways for low-mature hydrocarbon-containing fluid migrating to the uranium reservoir.
The Chinese landmass, as a composite region, consists of multiple small continental blocks, such as Sino-Korea, Yangtze, Tarim, etc., and orogenic belts. Because of its distinctive tectono-sedimentary evolution, China's oil/gas-bearing regions differ remarkably from that elsewhere in the world. For instance, in comparison to the Middle East oil/gas-bearing regions which are characterized by Mesozoic-Cenozoic marine oil/gas-bearing beds, early oil and gas discoveries in China are distributed mainly in Mesozoic-Cenozoic continental sedimentary basins. Generation of oil from terrestrial organic matter, or terrestrial oil generation, and the formation of large oil/gas fields in continental sedimentary basins were previously the major characteristics of petroleum geology of China. However, in the past 20 years, a series of major oil and gas discoveries from marine strata have been made. Marine oil/gas fields in China are mainly distributed in the Tarim, Sichuan, and Ordos basins, which are tectonically stable and covered with Mesozoic-Cenozoic deposits. In these basins, hydrocarbon-bearing strata are of old age and the oil/gas fields are commonly deeply-buried. Cumulatively, 995 oil/gas fields have been found so far, making China the fourth largest oil-producing country and the sixth largest gas-producing country in the world. In terms of petroleum and natural gas geology, theories of hydrocarbon generation from continental strata, such as terrestrial oil generation and coal-generated hydrocarbons, etc., have been established. Significant progress has been made in research on the sequence stratigraphy of continental strata, formation mechanisms of ultra-deep clastic reservoirs, and hydrocarbon accumulation in the continental subtle reservoir. Regarding research on the marine petroleum geology of China, with respect to the major characteristics, such as deeply-buried reservoirs, old strata, and multiple phases of transformation, important advances have been made, in areas such as the multiple-elements of hydrocarbon supply, formation of reservoirs jointly controlled by deposition, tectonic activities, and diagenetic fluid-rock reactions, and oil/gas reservoirs formed through superimposed multi-stage hydrocarbon accumulation. As more and more unconventional hydrocarbon resources are discovered, unconventional oil and gas reservoirs are under study by Chinese petroleum geologists, who endeavor to come up with new discoveries on their formation mechanisms.
Guided by the theory and method of high-resolution sequence stratigraphy, the datum plane cycle sequence of all levels in Yanan Formation of Triassic in Huanxian county was researched and the interface characteristics, structural types, superimposition mode, spatial distribution, and the control of A/S over the reservoir are discussed. The results indicate that 1) the Yanan Formation of this area is consisted of three long-term datum plane cycle sequences, eight middle-term datum plane cycle sequences, and thirty-six short-term cycle sequences; 2) there exists three types of middle-short term datum plane cycle sequence structure types, which are upward deepening non-symmetric-type (mode A), upward shoaling non-symmetric-type (mode B), and symmetric-type with upward deepening first and then shoaling (mode C); 3) along the extending direction of this delta, the cycle sequence structure type changes from mode A to mode C, and then mode B. The advantageous place of reservoir is usually the superposition part and the lower part of cycle sequences.
Lower Cretaceous sediments in North Yellow Sea Basin show lacustrine facies. Based on drilling and high-precision 3D seismic data, the sequence characteristics, such as lowstand system tract, transgressive system tract, highstand system tract, initial transgressive surface and maximum flooding surface, can be clearly identified from the bottom of Lower Cretaceous strata. The lowstand system tract includes incised valley, lowstand progradational wedge and lowstand fans. The highstand system tract is not only confined by sequence top surface and maximum flooding surface, but also characterized by the development of progradational parasequence set. The transgressive system tract is located between lowstand system tract and highstand system tract, primarily characterized by retrogradational parasequence set. The following major control factors can be concluded through geological analysis: (1) According to the test ages of igneous rocks and seismic data interpretation, it is considered that magmatic activities from the late period of Late Jurassic to the early period of Early Cretaceous resulted in the strata uplifting and thinning, leading to the formation of structural slope-break belt. (2) Based on the lithologic data of four wells above the slope-break belt, it is found that the sediments in transgressive and highstand system tracts contain red mudstones. Seismic profiles indicate that the sequence top surface is overlapping bottom surface, and distribution range of the sequence is significantly smaller than that of overlying and underlying sequences. Meanwhile, the lowstand fan and progradational wedge can be clearly identified beneath the structural slope-break belt. These evidences prove that the lake level dropped down to the area beneath the structural slope-break belt, and the sedimentary center migrated in this period. (3) Persistent large provenance supply in the early period of Early Cretaceous is revealed through the slope fan identified by seismic data, the gravel grain support and the existence of muddy fillings and boulder clay confirmed by core data, as well as the higher sand content (60%-70%) verified by logging lithology. Therefore, three control factors for the formation of Lower Cretaceous sequence in North Yellow Sea Basin can be summarized as the generation of structural slope-break belt, lake-level drop and large provenance supply. © 2015, Editorial Office of ACTA PETROLEI SINICA. All right reserved.
Based on the fine description, measurement and analysis of selected outcrop observation points, whose cross sections were nearly perpendicular to the source direction, the distributary channel reservoir architecture of lake basin was studied in the classic shallow-water delta sediments of the Yanchang Formation of Yanhe and Shiwanghe section in the eastern margin of Ordos Basin. The results show that according to the distribution and origin, distributary channels in shallow-water delta are divided into three types, including trunk distributary channel which includes deeply incised distributary channel, inlet distributary channel and terminal distributary channel. Except deeply incised distributary channel, the relationship between width and thickness of other distributary channels is positively double-log with high correlation coefficient, up to 0.97.That is, the ratios of width and thickness of trunk distributary channel, inlet distributary channel and terminal distributary channel are similar and larger than those of the deeply incised distributary channel. There are two filling patterns of distributary channel, vertical and lateral accretion. And the predominant vertical accretion can be further divided into horizontal, concentric and bar-channel filling. There are three types of stacking patterns of distributary channel, lateral splicing-vertical cutted, lateral splicing-vertical superposed and isolated pattern. © 2015, Central South University of Technology. All right reserved.
Tarnan-Southbell depression is a potentially lucrative exploration outer block of Daqing Oil Field. As oil geological background is complex with less information, in order to identify the research area of the oil and gas distribution, it is necessary to study area depositional environment, sedimentary study phase and its characteristics, and to establish an accurate model of the deposition. According to the core observation, particle size analysis and log-phase, the hydrodynamic characteristics of the study area was analyzed, sedimentary facies model was built, and the sand-body distribution was predicted. By using the seismic attribute analysis, the inter-well sand-body distribution was forcasted. Based on the above mentioned recognition, the sedimentary facies was predicted in the 1st and 2nd member starta units of Nantun Formation. The results show that the Tarnan-Southbell depression mainly develops alluvial fan, fan delta, nearshore submarine fan, lake floor fan and lacustrine depositional system in which fan-delta system and lacustrine depositional system is the most developed type.
After high-resolution sequence stratigraphy was built, the authors studied the fine sedimentary facies and the oil source, trap conditions and formation of non-structural traps in the Liu-1 Member. The Liu-1 Member has abundant oil source, sufficient migration and charge of oil and gas, and excellent reservoir and cap association. The reservoirs include four types: fault-lithologic reservoir, unconformity reservoir, sandstone updip thinning out reservoir, and sand lens reservoir. The major reservoir layers are fan delta or delta sandy conglomerates. The characteristic of fault-lithologic traps is that fault strikes are normal to or intersect the direction of fan delta sources. The upriver sand bodies are eroded in unconformity traps. The downriver sandbodies uplift in sandstone updip thinning out traps. Sand lens lithologic traps are mainly formed by turbid sand distributed in deep lakes in the front of deltas. According to these characteristics, favorable targets are predicted.
In order to divide exactly seismic sequences of Qigequan formation and understand structural features inside these sequences in Sanhu area of Qaidam Basin, seismic sub-layer correlation is used to process and interpret seismic data. Combining with the Wheeler domain transformation map of seismic sub-layers, core data, well logging data and analysis test data, Qigequan formation is subdivided into five sequences from the bottom to the top, and the identification marks of sequence boundaries and flooding surfaces are determined. Each Qigequan formation sequence which has no obvious breaks is comprised of lacustrine transgressive system tract and lacustrine regressive system tract. And the lacustrine transgressive system tracts are mainly characterized by retroprogradation sequence while the lacustrine regressive system tracts mainly characterized by oprogradation sequence. Then the sequence mode of the research area is proposed. Combining with the updip pinchout of seismic sub-layers, sandbody correlation and structural analysis, our study points that the lithologic traps of work area mainly distribute in eastern Taijinar, eastern Seniehu, western Tuofengshan uplift area and northern Tuofengshan, southern Seniehu, western Salt Lake slope area.
Based on core, thin section, SEM, well logging data and performance analysis and guided by reservoir configuration theory, configuration analysis of thick mouth bar sandstone in the second block in Shengtuo oilfield was carried out at three levels including composite mouth bar, single mouth bar and internal accretion within single mouth bar. On this basis, reservoir quality distribution patterns of different configuration units were analyzed. Result indicates that reservoir parameters of different configuration units are significantly different. Class I and II reservoirs mainly occur in the major parts of mouth bars. Class III reservoir is generally distributed on flanks of mouth bar and under interlayers in the interior of mouth bars, while Class IV reservoirs mainly occur in sheet sands. In addition, dynamic characteristics of reservoir parameters in different configuration unit during waterflooding were discussed by using performance data. The reservoir porosity increased during waterflooding as a whole, but changed slightly. The largest increase appeared in the center of mouth bar, followed by bar flank, and nearly no changes occurred in sand sheet. In contrast, reservoir permeability showed different trends in different configuration units. Some units increased while others decreased. With constant flushing of injected water, effective permeability increased in high or ultra-high permeability reservoirs such as mouth bar center, decreased in middle or low permeability reservoirs like bar flanks and underwater distributary channels and remained unchanged in low-permeability sheet sand. The amplitude of porosity increase was much smaller than that of permeability. Median grain size of different configuration units all increased to some extent at both low and high water-cut stages, while clay content decreased. ©, 2015, Editorial Office of Oil and Gas Geology. All right reserved.
To explore the sequence stratigraphic architecture of Paleogene Liushagang Formation (Els) in the Fushan sag of Beibuwan basin, by using drilling, logging, seismic data, the isochronous sequence stratigraphic framework in systems tracts level has been established. Aiming at different tectonic units in Els of Fushan sag, with comprehensive consideration of tectonic activities, basin marginal background, provenance supply, etc., sequence strata architecture of Els can be extracted into four types, namely, fault controlling steep type, multi-stage fault terrace type, flexure break type, and gentle slope break type. The features of sequence architecture, different types of traps developed in different sequence architecture. It is of great significance to the hydrocarbon exploration in Els, Fushan sag.
Based on sequence stratigraphy theory, three types of identification signs of sequence interface were identified and middle-lower Jurassic was divided into five third-order sequences and ten fourth-order sequences, then the sequence stratigraphic framework was established in Turpan sag. The results show that the main source of Jurassic in southern region is plentiful, and the stratum is thick in north and thin in south and there are two developmental models of shallow lake basin with gentle slope background and deposit slope break background in Turpan sag. There develops five sets of major favorable reservoir sandbody of middle-lower Jurassic in Turpan sag, and the middle thick-thick sandstone layer of braided river-meandering river delta in lower part of Sanjianfang set deposition is the most favorable reservoir sandbody in the study area.
Extensive mass transport deposits (MTDs), which form a significant component of the stratigraphic record in ancient and modern deep water systems, mostly distributed in the continental margin of ocean basins worldwide. To date, no large-scale MTDs have been found in Chinese terrestrial basins. Based on integrated analysis of the seismic, drilling, core and logging data, this article report MTDs have been found in Songliao terrestrial basin (SLTB), Northeast China. During the depositional period of the Upper Cretaceous Qingshankou Formation, slope break of lacustrine basins was an ideal discharge place for gravity depositions. Regional sedimentary study of MTDs reveals that Cretaceous-age MTDs in SLTB were deposited in slope-break environments by slide, slump and debris flow, and MTDs were mixed with little turbidite simultaneously. Besides the massive aspect, key features of MTDs are the common presence of deformational structures or soft-sedimentary deformation, floating shale clasts, boudins, microfaults, distortion beddings, rotated sand debris. Detailed mapping of two three-dimensional (3-D) seismic surveys acquired in Yingtai (YT) and Haituozi (HTZ) areas reveals that MTDs are characterized by chaotic, mounded, and transparent seismic reflection. Growth fault, slope gully and geomorphology of the slope break greatly influenced the sedimentary process and external geometry of MTDs, and as a consequence, MTDs in SLTB exhibit classic fan-like geometry and some special (non-fan-like) external geometry. Furthermore, fan-like MTDs can be divided into three subcategories, including isolated fan, mother-son fan and stacked fan. Non-fan-like MTDs can be divided into two subcategories, strip-like MTDs and faulted-pit MTDs. A new sedimentary model has been built for MTDs in SLTB. MTDs have become a new exploration and development target in the SLTB.
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