Continental shale gas enrichment is significantly influenced by pore structure, yet research on this topic remains limited compared to marine shale. This study investigates the complex pore structure of lacustrine Da'anzhai (DAZ) shale from the YB area, employing X-ray diffraction (XRD), scanning electron microscope (SEM), and low-temperature N2 adsorption. The fractal dimension (FD) of the pore structure, calculated using the Frenkel–Halsey–Hill (FHH) model, revealed two distinct dimensions: D1 (2.3438–2.7933) and D2 (2.5235–2.82). Both D1 and D2 exhibited positive correlations with total organic matter (TOC) content, thermal maturity, and other pore structure parameters, with micropores exhibiting the strongest influence. Results indicate that the DAZ shale comprises quartz (28.1%–42%), clay (41.5%–60%), and carbonate minerals (0.7%–11.2%), with TOC content ranging from 0.32% to 1.74%, indicating low to high thermal maturity. Total pore volume (PV) varies from 0.0105 to 0.0193 cm3/g, and specific surface area (SSA) from 3.14 to 11.33 m2/g. While micropores and mesopores dominate the pore size distribution, with micropores contributing most to SSA, mesopores and macropores significantly influence overall PV. Interestingly, regular pore morphology, primarily associated with well-sorted and rounded terrigenous quartz, negatively correlated with D1 and D2. Conversely, feldspar primarily contributed to macropore formation, exhibiting a negative correlation with both FD. Clay minerals, compacted over time, contribute to reduced pore size and increased micropore abundance, resulting in a positive correlation between intricate pore morphology and both D1 and D2. Compared to the marine Longmaxi (LMX) shale, the lacustrine DAZ shale exhibits lower organic matter (OM) and quartz but higher clay mineral content. Furthermore, pore types in the DAZ shale are dominated by intergranular pores within clay minerals, and the development of OM pores is significantly lower than in the LMX shale. These key differences result in higher pore structure complexity and FD in marine shale compared to lacustrine shale. This comparative analysis of DAZ shale provides valuable insights into the distinct pore structures of lacustrine shale and offers a framework for comparative analysis of pore structures in different sedimentary facies.
To investigate the geological characteristics and exploration potential of shale gas in the southern Sichuan Basin, we analyze the coupling relationship between the hydrocarbon generation and storage conditions of the Longmaxi Formation and discuss the preservation conditions from the lateral and vertical migration mechanisms of shale gas. According to the results, the organic-rich shale at the bottom was formed in a strongly euxinic and anoxic reducing environment, the oxygen content increased in the upper water body in which the Longmaxi Formation was deposited, and the water body became oxidized. The organic matter type in the shale is dominantly type I kerogen and a small amount of type II 1 kerogen. The organic matter content is more than 3.0% and is in the high-to postmature stage. The enrichment of siliceous organisms increases the organic matter and enhances the brittleness of shale, resulting in “superior hydrocarbons and a favorable reservoir”. Pyrolysis of organic matter promotes the formation of organic matter pores and dissolution pores, resulting in a coupled “source-reservoir” accumulation control system. The high vertical formation pressure guarantees the sealing of shale and restrains the lateral escape of shale gas. The high-angle intersection of the highly filled fractures and the current crustal stress can effectively enhance the fracture sealing and inhibit the vertical escape of shale gas, forming a three-dimensional effective closure. Hence, the area featuring a short tectonic uplift time, small amplitudes, large-scale underdeveloped fractures, and a high formation pressure coefficient is a favorable area for shale gas exploration, according to the analysis of three-dimensional preservation conditions.
The pore structure is an important factor affecting reservoir capacity and shale gas production. The shale reservoir of the Longmaxi Formation in the Changning area, Southern Sichuan Basin, is highly heterogeneous and has a complex pore structure. To quantitatively characterize the shale’s pore structure and influencing factors, based on whole rock X-ray diffraction, argon ion polishing electron microscopy observations, and low-temperature nitrogen adsorption-desorption experiments, the characteristics of the shale pore structure are studied by using the Frenkel-Halsey-Hill (FHH) model. The research reveals the following: 1) The pores of the Longmaxi Formation shale mainly include organic pores, intergranular pores, dissolution pores and microfractures. The pore size is mainly micro-mesoporous. Both ink bottle-type pores and semiclosed slit-type pores with good openness exist, but mainly ink bottle-type pores are observed. 2) The pore structure of the Longmaxi Formation shale has self-similarity, conforms to the fractal law, and shows double fractal characteristics. Taking the relative pressure of 0.45 (P/P 0 = 0.45) as the boundary, the surface fractal dimension D sf and the structural fractal dimension D st are defined. D sf is between 2.3215 and 2.6117, and the structural fractal dimension D st is between 2.8424 and 2.9016. The pore structure of micropores and mesopores is more complex. 3) The mineral components and organic matter have obvious control over the fractal dimension of shale, and samples from different wells show certain differences. The fractal dimension has a good positive correlation with the quartz content but an obvious negative correlation with clay minerals. The higher the total organic carbon content is, the higher the degree of thermal evolution, the more complex the pore structure of shale, and the larger the fractal dimension. The results have guiding significance for the characterization of pore structure of tight rocks.
Influences of the characteristics of organic matters and mineral compositions on the development of shale microscopic pores were discussed through performing low-temperature nitrogen adsorption, whole-rock "X" diffraction, and field emission scanning electron microscope on black organic-rich shale samples of Longmaxi Formation in the Dingshan area, southeastern Sichuan Basin, in combination with the characteristics of shale organic matters. The shale in the Dingshan area has complex mineral composition, which is mainly quartz and clay minerals. Both contents range from 23% to 72% and 36% to 70%, respectively. The vertical variation is obvious, and there are few feldspar and carbonate rocks; the pyrite content is more than 2%. The bottom shale has high brittleness and gradually decreases in the vertical direction. The brittleness index is between 0.481 and 0.627 and is concentrated above 0.5. It shows strong compressibility and is easy to form a complex network system in hydraulic fracturing. Quartz and feldspar mainly provide secondary dissolved pores, intercrystalline mineral pores, and nanoedge gaps in contact with organic matter. They have no obvious correlation with the specific surface area of shale but have a weak correlation with pore volume. They mainly control the development of macropores. Organic matter develops many hydrocarbon-generating pores, which strongly correlate with the specific surface area and a weak correlation with pore volume. It mainly controls the development of micromesopores. Clay minerals mainly provide a large number of interlaminar pores and interlayer fractures in the clay. The intergranular pores of clay and clay have a weak correlation with pore volume and specific surfaces. They contribute to the development of shale micropores, mesopores, and macropores. Pyrite mainly provides intercrystalline pores and mold pores. By restricting the interaction with organic matter, the development of shale pores is promoted within a certain content range. When the content exceeds this range, the development of micropores is inhibited. The conversion threshold in the Dingshan area is 5.0%.
The main geological factors controlling the accumulation and yield of marine‐facies shale gas reservoirs are the focus of the current shale gas exploration and development research. In this study, the Wufeng‐Longmaxi Formation in the Dingshan area of southeast Sichuan was investigated. Shale cores underwent laboratory testing, which included the evaluation of total organic carbon (TOC), vitrinite reflectance (Ro), whole‐rock X‐ray diffraction (XRD), pore permeability, and imaging through field emission scanning electron microscopy (FE‐SEM). Based on the results of natural gamma ray spectrum logging, conventional logging, imaging logging, and seismic coherence properties, the exploration and development potential of shale gas in the Dingshan area have been discussed comprehensively. The results showed that (1) layer No. 4 (WF2‐LM4) of the Wufeng‐Longmaxi Formation has a Th/U ratio <2 and a Th/K ratio of 3.5‐12. Graptolites and pyrite are relatively abundant in the shale core, indicating sub‐high‐energy and low‐energy marine‐facies anoxic reducing environments. (2) The organic matter is mainly I‐type kerogen with a small amount of II 1 ‐type kerogen. There is a good correlation among TOC, Ro, gas content, and brittle minerals; the fracturing property (brittleness) is 57.3%. Organic and inorganic pores are moderately developed. A higher pressure coefficient is correlated with the increase in porosity and the decrease in permeability. (3) The DY1 well of the shale gas reservoir was affected by natural defects and important late‐stage double destructive effects, and it is poorly preserved. The DY2 well is located far from the Qiyueshan Fault. Large faults are absent, and upward fractures in the Longmaxi Formation are poorly developed. The well is affected by low tectonic deformation intensity, and it is well preserved. (4) The Dingshan area is located at the junction of the two sedimentary centers of Jiaoshiba and Changning. The thickness of the high‐quality shale interval (WF2‐LM4) is relatively small, which may be an important reason for the unstable production of shale gas thus far. Based on the systematic analysis of the geological factors controlling high‐yield shale gas enrichment in the Dingshan area, and the comparative analysis with the surrounding typical exploration areas, the geological understanding of marine shale gas enrichment in southern China has been improved. Therefore, this study can provide a useful reference for shale gas exploration and further development.
Preservation conditions are the key factors that determine the effective accumulation of shale gas. The damage of faults formed by differential structures to the roof and floor and the shielding of lateral edges are the direct reasons for the difference in preservation conditions. Taking the organic-rich shale of the Wufeng–Longmaxi Formation in the south of the Sichuan Basin as an example, this paper reveals different types of shale gas-rich structures by using typical seismic profiles and puts forward the main controlling factors of different gas-rich structures and their influence on preservation. The results show that three kinds of gas-rich structures are developed in the Wufeng–Longmaxi Formation in southern Sichuan: positive type, negative type, and fault transformed slope type. The basin is dominated by a wide and gentle syncline, fault spreading fold, and low scope concealed anticlines. Wide and gentle anticline, arc anticline, and fault transformation slope are developed at the basin edge. Fault sealing is the main controlling factor for the preservation of shale gas in wide and gentle anticlines. The main controlling factors for the preservation of circular arc anticlines and hidden anticlines are anticline curvature and the distance between faults. The preservation of shale gas in a syncline is mainly controlled because it includes formation buried depth, foliation development degree, and formation dip angle. The preservation of fault transformed syncline is mainly affected by formation buried depth, dip angle, and fault sealing. Foliation and faults form a three-dimensional migration system, which jointly controls the intensity of gas escape. Positive structures such as wide and gentle anticline and circular arc anticline at the basin edge, and deep buried gentle syncline and low scope concealed anticline in the basin are favorable shale gas-rich structures.
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