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Lower Paleozoic dark shale is developed in the western Middle Yangtze Block, which lays a material foundation for the enrichment and accumulation of marine shale gas. In order to ascertain the control action of geological structures on the differential preservation of shale gas and reveal the key factors in shale gas preservation, this paper firstl...
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Context 1
... to thrust faultesubtle thrust þ fault-propagation fold. Thrusting occurred in the major Qiyueshan fault, being a high-angle strong thrust, which resulted in strong uplifting and denudation of strata in the east, while subtle thrusting took place in the frontal branch faults, controlling the faultpropagation folds, such as the Jiannan anticline (Fig. 3a). In the Jiaoshiba section, it appears as thrust fault -subtle thrust þ faultedetachment folds, that is, the front branch of the Qiyueshan fault is detached along the strata in the Precambrian, resulting in the development of box-shaped anticline in the caprock (Fig. 3b). The fault structure in the Nanchuan section to the south of ...
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... faults, controlling the faultpropagation folds, such as the Jiannan anticline (Fig. 3a). In the Jiaoshiba section, it appears as thrust fault -subtle thrust þ faultedetachment folds, that is, the front branch of the Qiyueshan fault is detached along the strata in the Precambrian, resulting in the development of box-shaped anticline in the caprock (Fig. 3b). The fault structure in the Nanchuan section to the south of Jiaoshiba is a thrust faultesubtle thrust þ fault-propagation fold, and the front branch faults control the Pingqiao fault-propagation anticline (Fig. ...
Context 3
... Qiyueshan fault is detached along the strata in the Precambrian, resulting in the development of box-shaped anticline in the caprock (Fig. 3b). The fault structure in the Nanchuan section to the south of Jiaoshiba is a thrust faultesubtle thrust þ fault-propagation fold, and the front branch faults control the Pingqiao fault-propagation anticline (Fig. ...
Citations
... The study area is also located at the edge of Sichuan Basin. These styles include broad and gentle anticlines, tight anticlines, eroded synclines, monoclines, faulted anticlines, broad and gentle eroded synclines, fold zones, fault propagation tips in box folds, and limbs of gentle synclines related to thrust (Hu, 2019;Chen et al., 2020;He et al., 2020a;Guo et al., 2021;Xiang et al., 2021). ...
... Considering that the four synclines in the study area have similar TOC and R o , causing so many differences, this must be related to preservation conditions. Multiple factors are favorable for shale gas accumulation, including a small number of faults, suitable burial depths, and slight denudation (Shu et al., 2018;He et al., 2022;Pang et al., 2019;Chen et al., 2020;Yang et al., 2021a;Feng et al., 2021). In contrast, factors including productive tension joints in the anticline core, intensive tectonic uplift and shallow burial, and more abundant detachment layers are unfavorable for shale gas preservation (Chen, 2017;Hu et al., 2017;Yang et al., 2021b). ...
... Through numerous orogenies, including the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan, this region developed with two sets of basin-controlling faults trending NNE and NW. The basin is actually a residual basin formed by the modification and superposition of several prototype basins [32][33][34][35]. The Western Hubei region is adjacent to the Sichuan Basin in the northwest (with the Qiyueshan fault as its border), the Huangling anticline-Yichang slope belt to the northeast (with the Tianyangping-Jianli fault as its border), and the Jiangnan-Xuefeng overthrust belt to the southeast (with the Cili-Baojing fault as their boundary) [34,36]. ...
... The basin is actually a residual basin formed by the modification and superposition of several prototype basins [32][33][34][35]. The Western Hubei region is adjacent to the Sichuan Basin in the northwest (with the Qiyueshan fault as its border), the Huangling anticline-Yichang slope belt to the northeast (with the Tianyangping-Jianli fault as its border), and the Jiangnan-Xuefeng overthrust belt to the southeast (with the Cili-Baojing fault as their boundary) [34,36]. It contains four secondary tectonic units: the Lichuan complex syncline, Central complex anticline, Huaguoping complex syncline, and Yidu-Hefeng complex anticline (Figure 1c). ...
To evaluate the reservoir characteristics of siliceous shale in the Dalong Formation within the late Permian intra-platform rift trough in Western Hubei (China), we studied a drill core from well ED-2 in Western Hubei. To analyze the physical characteristics, pore structure, methane adsorption performance, and their influences on the siliceous shale reservoir, we performed X-ray diffraction, total organic carbon (TOC) content, vitrinite reflectance (Ro, indicating thermal evolution), total porosity and permeability, field emission scanning electron microscopy, CO2 and N2 physical adsorption, and methane isothermal adsorption analyses, among others. Our results show that the Dalong Formation in Western Hubei is an organic-rich (2.6–14.3 wt.%), highly thermally evolved (Ro = 2.59–2.76%), siliceous shale containing mainly type-I and type-II1 organic matter. The Dalong siliceous shale has low porosity and permeability and belongs to a larger reservoir with low horizontal permeability (0.002–335.209 mD) and porosity (1.2–7.8%). Pores in the shale are mainly organic, inorganic, and microfractures; the organic pores are very developed. The pore volume and specific surface area of the shale are mainly due to micropores and mesopores and are positively correlated with TOC and clay mineral contents and weakly negatively correlated with quartz and carbonate contents. The micropores and mesopores are well developed, improving the methane adsorption capacity, which, in turn, is strongly positively correlated with TOC content. Comprehensive analysis shows that the high organic matter content of the Dalong siliceous shale has the greatest influence on its pore structure; the many organic pores generated after hydrocarbon generation have controlled the development of micropores and mesopores, which is conducive to the adsorption and storage of shale gas. The development of brittle minerals resistant to compaction, such as siliceous minerals, helps preserve organic pores. This study is informative for basin-scale petroleum system investigations, which are essential for understanding oil and gas exploration possibilities and regional petroleum systems.
... Generally, faults are not conducive to reservoir continuity and preservation conditions, www.nature.com/scientificreports/ especially normal faults 55,56 . The wide and gentle monocline is the best gas enrichment area, and the well-sealed anticline core is also conducive to gas accumulation 57,58 . ...
Superimposed accumulation mechanism and model of vertical source rock–reservoir system of coal-measure gas is crucial to evaluate the exploration potential, and also the basis of co-exploration and co-production of coal measure gas. This work investigates the formation mechanism of various types of reservoirs (coalbed methane, shale gas, tight sandstone) in the Taiyuan Formation (Yushe-Wuxiang Block, eastern Qinshui Basin). Source rocks (coal seams and coal-measure mudstones) in the study area are characterized by type III kerogen, organic rich and over-mature, and reach a gas generation peak during the Early to Late Cretaceous. Coalbed methane mainly adsorbs on the surface of micropores, shale gas mainly occurs in micropores, macropores and micro-factures in adsorbed and free states, and tight sandstone gas mainly occurs in macropores in a free state. The combinations of successions are identified, coalbed methane, shale gas, and tight sandstone gas horizons are divided into a mudstone-sandstone reservoir (combination I), a coal-mudstone-sandstone reservoir (combination II), and a coal-mudstone reservoir (combination III). This division occurs from top to bottom in the succession and is identified on the basis of lithology, total organic carbon content (TOC) of mudstones, gas logging, superimposition relationships, and the source rock-reservoir-caprock assemblage. The strata thickness, continuity, and gas logging results of combination III comprise the most favorable conditions for fairly good development potential, followed by combination I. The development potential of combination II is poor due to the small strata thickness and poor continuity. The identification of superimposed reservoirs can provide an engineering reference for the exploration of coal-measure gas.
... The shale gas in southern China has distinct characteristics from the shale gas in North America. After the reservoir formation, the shale underwent significant deformation in response to multi-stage tectonism, which was also influenced by the combined effects of Huangling paleo-uplift, Guizhong paleo-uplift, Jiangnan-Xuefeng paleo-uplift, and Dabashan orogenic belt during the Mesozoic and Cenozoic periods (Yuming et al., 2013;Kongquan et al., 2020). The subsequent development of intricate faults, folds, strong uplifts, and denudational processes in the western part of the Middle Yangtze block resulted in the rupture of the shale strata and the destruction of the preservation conditions of the original shale gas reservoir (Xusheng, 2014;Zhihong, 2015;Zou et al., 2015;Guo, 2016;Zhai et al., 2017;Xiaoxi et al., 2018). ...
Jingmen block is an essential location for the development and exploration of shale gas. The regional structure, the tectonic characteristics, and their influence on shale gas preservation in the Jingmen block were summarized based on observations from drilled cores, seismic data interpretation, FMI imaging logging results, and other methods of subsequent analysis. The study results show that: 1) the overall structure of the study area is simple. The shale corresponding to the Wufeng and Longmaxi formations of the Upper Ordovician and Lower Silurian periods were subjected to compression deformation in Indosinian and early Yanshanianand, following extension and strike-slip transformation in the late Yanshanian period. Structures, including back-thrust, echelon, parallel, inversion structures, and rifts, could also be observed 2) Fractures with different tectonic stages and scales have different effects on the preservation degree of shale gas in the study area, resulting in differences in the nature, scale and zone of influence of faults in different regions and 3) The crack associated with fault structure include interlayer low-angle detachment fracture, conjugate shear fracture, reticular fracture, high-angle shear fracture, high-angle strike-slip shear fracture, and high-angle fracture zone. The location and type of fracture development in different stages vary, which affect the shale gas preservation and migration 4) Based on the influence of structures on shale gas preservation, the study area is divided into four categories viz. two favorable areas and two more favorable areas.
... The Wufeng Formation-Longmaxi Formation in western Hubei is characterized by a large thickness of organicrich shale, with a high organic carbon content and high organic maturity, which serves as an important replacement area for shale gas exploration in southern China after the Sichuan Basin [12][13][14]. Earlier studies of this area have mainly focused on the tectonic background and shale gas preservation conditions [15,16], biostratigraphic classification and comparison [17,18], lithofacies paleogeography [19][20][21], and organic geochemistry [22], whereas relatively little research has been conducted on the characterization of the rock types. Moreover, the classification criteria for rock types has not been unified [14,23], and no systematic study has been conducted on the formation environments and development patterns of the different types of shales. ...
... The black siliceous shales are rich in siliceous minerals (quartz + feldspar), with a mean content of 60.6%, and are dominated by quartz (mean content of 54.6%; Figures 3 and 4). The quartz grains are mostly microcrystalline and powder crystalline grains of biological origin in a circular-subcircular shape [14,16], accompanied by minor amounts of subangular terrigenous quartz grains [9]. The carbonatite content is low (mean of 6.3%), occurring as calcite and dolomite cement. ...
By performing scanning electron microscopy, microscopic observations, whole-rock X-ray diffraction analysis, organic geochemistry analysis, and elemental analysis on drill core specimens and thin sections, in this study, we classified the shale types of the Wufeng Formation-Member 1 of the Longmaxi Formation in western Hubei, southern China, and explored the development characteristics and formation environments of the different shale types. The results show that (1) the shales of the Wufeng Formation-Member 1 of the Longmaxi Formation are composed of three types of shale: siliceous shale, mixed clay-siliceous shale, and clay shale. The siliceous shale is a type of shale unique to deep-water environments; clay shale is the main type of shale formed in shallow-water environments; and mixed clay-siliceous shale falls between the two. (2) The changes in shale type are characterized by multiple depositional cycles in the vertical direction with strong heterogeneity and an obvious tripartite character, and the siliceous shales gradually thicken as they laterally extend northwestward, with their last depositional cycle gradually ending at a later time. (3) The Late Ordovician-Early Silurian paleoenvironment can be divided into six evolutionary stages (A, B, C, D, E, and F) from early to late. In particular, the sea level was relatively lower in stages A and F when the bottom water was mainly oxygen rich with higher terrigenous inputs and a lower paleoproductivity, which led to the formation of clay shales poor in organic matter but rich in terrigenous quartz clasts. The sea level was higher in stages B, C, and D when the bottom water was anoxic with lower terrigenous inputs and a higher paleoproductivity, which led to the formation of siliceous shales rich in organic matter and biogenic silica. The total organic carbon (TOC) contents of siliceous shales decrease in the order of stage C > stage D > stage B, which is mainly attributed to the different degrees of water restriction in the three stages and the consequently different paleoproductivities. Stage E corresponds to the mixed clay-siliceous shales, the depositional environment of which is between those of the siliceous shales and the clay shales, thereby resulting in the mineral composition and TOC content of the mixed clay-siliceous shales being between those of the other two shale types.
... The NE-striking QYSF extends to the Wushan fault-fold belt in the north and the Daloushan fault-fold belt in the south, forming the present eastern margin of the Sichuan Basin (Fig. 1b). Generally, from north to south, the QYSF can be divided into three segments, namely the northern, central, and southern segments, which exhibit variations in the structural framework and deformation style (Wei et al., 2019;Chen et al., 2020). The southern segment is buried under the hinterland, also known as the Xishui-Gulin concealed fault. ...
Structural deformation is one of the main factors controlling shale gas preservation in Silurian Longmaxi Formation in the Sichuan Basin. The exploration practices demonstrate that the shale gas preservation condition in the areas to the west of the Qiyueshan Fault (QYSF) (along the eastern margin of the Sichuan Basin) is better than that to the east. In this study, we synthesize seismic interpretation, apatite fission track (AFT), and triaxial rock mechanics experiment to determine the structural segmentation, deformation time, and shale fracturability of the QYSF belt. Seismic interpretation shows that the QYSF can be divided into three segments, among which, the central segment is transitional deformation zone, the burial depth and deformation structure difference of Silurian strata on the two sides of the fault are not variable obviously, and shale gas preservation conditions are good. AFT simulation shows that the initial deformation time of chevron syncline-thrust structure to the east of the QYSF was 160–120 Ma, while that of the chevron anticline-thrust structure deformation between the Qiyueshan and the Huayingshan was 100–80 Ma, and it gradually changes from the central to the north and south. Rock mechanics test reveals that under the same stress field, the difficulty of fracture is influenced the dip angle of the shale bedding plane. Shale is most likely to produce fracture when the dip angle of bedding plane 34°–37°, but horizontal formation is difficult to rupture. This proves that gentle anticline and syncline are more conducive to shale gas preservation. Therefore, shale gas is mainly enriched in the core of gentle anticline and synclinal. Combined with the analysis of typical wells, we believed that the two sides of the QYSF are different in deformation time, uplift amplitude, and later multi-directional stress reworking. These cause differences in the direction, scale, and density of fractures in the strata, thus affecting shale gas preservation on two sides of the QYSF.
In order to reveal the restriction in shale gas enrichment of the Wufeng-Longmaxi Formation in the northern Guizhou province, the influence model of detachment layer was established through field geological investigation, core observation, logging, sample analysis, and geological background data. The response relationship between the detachment layer and the shale gas enrichment model in different structural formats was analyzed. The results show that the thickness of the Wufeng-Longmaxi Formation’s detachment layer is influenced by the conditions near the fault zones and mineralogical characteristics. The lithofacies of the detachment layer shows mainly a combination of clay-rich shale facies. This indicates that lithofacies type is one of the main factors influencing the variation in slip layer thickness. The detachment layer exhibits distinct well logging response characteristics and is influenced by nitrogen enrichment. The development of detachment fractures allows atmospheric nitrogen to infiltrate shale gas. It leads to poor gas saturation in the shale gas. In addition, the overall tectonic deformation in the northern Guizhou province was found to gradually intensify from Northwest to Southeast, and there were two tectonic models: a slot-shift tape transition belt and a spacer type deformation belt. The influence of decollements on shale gas preservation was barely found in the northern Guizhou province. It is mainly controlled by buried depth of the target layer, conditions of the cover layer, structural type, and deformation intensity.
The in-situ stress and formation pressure are important parameters in shale gas development. They directly affect the well wall stability, the direction of horizontal well drilling, and the fracturing effect during the shale gas development process. There are abundant shale gas resources in the southeastern Chongqing-Sichuan area, but the structure in the area is complex, and it is difficult to predict the in-situ stress and formation pressure. Therefore, in this paper, a finite element simulation model was established based on a large number of seismic, logging, and experimental rock mechanics data and the prediction accuracy of the stress field simulation was effectively improved. The construction of the stress field was based on the combined spring model, as well as the data related to the measured in-situ stress and the formation pressure obtained during drilling. The coupling relationship between the in-situ stress, the formation strain, and the formation pressure were derived to carry out the prediction of the distributions of the formation pressure and the formation pressure coefficient. The prediction results showed that the present-day maximum principal stress direction in the study area was about NE65°–110°, and the present-day maximum principal stress was 56.12–93.79 MPa. The present-day minimum principal stress direction was about NE335°–20°, and the present-day minimum principal stress was 48.06–71.67 MPa. The formation pressure was 2.8–88.25 MPa, and the formation pressure coefficient was 0.74–1.55. The formation pressure distribution was greatly affected by fault, tectonic location, in-situ stress and rock petrophysical properties, and the overpressure areas of the formation were distributed in the synclines and the deeply buried areas. This study shows that the finite element based formation pressure prediction method is effective.
The Tarim Basin in northwest China is a craton with complete Precambrian crystalline basement and well-developed Neoproterozoic sedimentary cover. Many Neoproterozoic volcanic rock outcrops were found around Tarim basin and their forming implied the Rodinia supercontinent assemblage and break-up. There are also some hypotheses for the nature of Tarim basement and central aeromagnetic anomaly. To test these hypotheses, lithogeochemical analyses of major elements, rare earth elements and trace elements and SHRIMP U-Pb zircon dating were performed for three samples of Precambrian basement rocks (QM1-1, migmatitic granite, from Well Qiman 1 in northern Tarim Basin, and TD2-1, granodiorite, from Well Tadong 2 and YD2-1, granodiorite, from Well Yingdong 2 in eastern Tarim Basin). The results show that the rock-forming ages are 1850.4 ± 9.1 Ma, 733.6 ± 5.7 Ma and 744.2 ± 4.5 Ma for QM1-1, TD2-1 and YD2-1, respectively. QM1-1 is measured to contain SiO2 and K2O of 70.12 and 5.90, respectively, higher than TD2-1 (SiO2, 63.91; K2O, 3.01) and YD2-1 (SiO2, 63.72; K2O, 3.16), and also contain Al2O3, CaO and NaO of 12.57, 2.61 and 1.86, respectively, lower than TD2-1 (16.24, 3.65 and 3.60) and YD2-1. All three samples exhibit a right-leaning light rare earth element distribution pattern, and represent relatively strong differentiation of LREE and low fractionation of heavy rare earth element. Generally, they have similar lithogeochemical characteristics of trace elements, and display strong enrichment of large ion lithophile elements (Rb, Ba, K and Pb) and relative depletion of high field strength elements (Nb, Ta, Sr, P and Ti) on the primitive mantle-normalized trace element spider diagrams. Their similarity in rare earth element and trace element distribution patterns indicates that the granodiorites (TD2-1, YD2-1) might be originated from recrystallization of the ancient crustal melt represented by QM1-1 after mixing with a higher proportion of mantle-derived melt. Consequently, the granodiorites inherited the lithogeochemical characteristics of the ancient continental crust. The rock-forming age and lithology indicate that TD2-1 and YD2-1 are products of the Neoproterozoic Rodinia supercontinent break-up and also prove the central aeromagnetic anomaly where they locate as a rift valley in the period. According to the rock-forming age of QM1-1 and the 1.8 Ga medium to shallow metamorphic rocks found commonly in the outcrops around the basin, as well as the duration of the Columbia supercontinent convergence, it is concluded that the Tarim oldland was merged during the Columbia supercontinent convergence and has undergone a unified tectonic evolution since then.
