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The assessment of gas storage capacity is crucial to furthering shale gas exploration and development in the eastern Sichuan Basin, China. Eleven organic-rich shale samples were selected to carry out the high pressure methane sorption, low-pressure N2/CO2 gas adsorption, and bulk and skeletal density measurements to investigate the methane storage capacity (MSC). Based on the relative content of clay, carbonates, quartz + feldspar, we grouped the 11 samples into three lithofacies: silica-rich argillaceous shale (CM-1), argillaceous/siliceous mixed shale (M-2), and clay-rich siliceous shale (S-3). The total porosity of the shale samples varies from 3.4% to 5.6%, and gas saturation ranges from 47% to 89%. The measured total gas amount ranges from 1.84 mg/g to 4.22 mg/g with the ratio of free gas to total gas amount ranging from 52.7% to 70.8%. Free gas with high content in the eastern Sichuan Basin may be the key factor controlling amount of shale gas production. The TOC content critically controls the MSC of shales, because micropore, mesopore volumes and the specific surface areas associated with organic matter provide the storage sites for the free and adsorbed gas. The methane sorption capacities of samples from different lithofacies are also affected by clay minerals and moisture content. Clay minerals can provide additional surface areas for methane sorption, and water can cause a 7.1%–42.8% loss of methane sorption capacity. The total porosity, gas-bearing porosity, water saturation, free gas and adsorbed gas number of samples from different lithofacies show subtle differences if the shale samples had similar TOC contents. Our results suggest that, in the eastern Sichuan Basin, clay-rich shale lithofacies is also prospecting targets for shale gas production. [ABSTRACT FROM AUTHOR]

Nanoscale organic matter‐hosted pores (OM pores) are the dominant pore type in the marine, organic‐rich shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation's first member in the Jiaoshiba Block, which provide important reservoir space for shale gas accumulation. In this study, field emission scanning electron microscopy (FE‐SEM), statistical analysis using ImageJ software, and gas adsorption tests were conducted to investigate the characteristics of OM pores, including pore shape, pore size distribution, pore quantity, and organic porosity. FE‐SEM images show that the OM pore sizes range between 2 and 900 nm. The predominant shape of the OM pores in the Wufeng shale is an irregular polygon, while elliptical and subrounded OM pores were observed in the Longmaxi Formation's first member shale. Average cross‐section area ratios of the OM pores to corresponding single‐particle organic matter (ØOMP/OM) for shale samples obtained via statistical analysis of FE‐SEM images range between 10% and 30%. We propose a new combination approach for the estimation of organic porosity, which gives organic porosities for the Wufeng shale and Longmaxi Formation's first member shale of 0.56%‐4.47% and 4.06%‐4.21%, respectively. To calculate the organic porosity for the OM pores with diameters of 10‐900 nm, the formula derived from the mass‐volume‐density‐organic matter relationships and ØOMP/OM of shale was used, while the organic porosity for pores with diameters of 0.3‐10 nm was estimated using carbon dioxide and nitrogen adsorption data. In addition, the reasons for the differences in the OM pore structures in the Wufeng shale and Longmaxi Formation's first member shale were examined for the study area and found to be the result of variations in total organic carbon (TOC) content and tectonic compression of the Wufeng shale. [ABSTRACT FROM AUTHOR]

The processes of shale gas transport and enrichment are closely related to the micro-fractures structure. Here, porosity, permeability test, gas adsorption, scanning electron microscopy (SEM), fluid spontaneous imbibition, Nano-CT, Micro-CT, and core observations were used to determine the types and characteristics of micro-fractures in the Longmaxi shales, Jiaoshiba area. The effects of micro-fractures on shale reservoir properties and shale gas accumulation were also analyzed. Diverse sizes of intra-particle (intraP), particle-edge, and transparticle micro-fractures were widely developed in shales. The difference of micro-fracture characteristics was mainly controlled by the mineral composition. The well-developed micro-fractures in the Longmaxi shales can act as a "bridge" to link nano-pores and macro-fractures. In such cases, a large-scale "escape network" composed of "nano-pores, micro-fractures, and macro-fractures" was formed. The escape of hydrocarbons along this network results in lower pore pressure. Most pores were dramatically compressed due to the compaction of overlying strata. As a result, the pore space and seepage capacity of organic-rich shales were disappointed, which is not benefit to the enrichment and production of shale gas. However, the limited development of micro-fractures will reduce the probability of forming "escape network". Thus, some of the pores could be remained and act as storage space along with micro-fractures for shale gas even within the tectonically deformed area. • Shale micro-fractures were systematically identified and quantitatively evaluated. • The micro-fractures have great influence on shale reservoir properties. • The micro-fractures can act as a "bridge" to link nano-pores and macro-fractures, thus forming a large-scale "escape network". [ABSTRACT FROM AUTHOR]

Widely developed in shale strata, laminae are the most typical shale sedimentary structure. To investigate their influence on shale gas reservoir quality, we conducted complementary studies by integrating microscopic observations, geochemical analyses, field-emission scanning electron microscopy (FE-SEM) imaging, and mercury intrusion capillary pressure (MICP) techniques. The microscopic observation of thin sections enabled us to detect the laminae characteristics, including the density and maximum thickness of individual lamina, and three types and stages of lamina development were identified. The total organic carbon (TOC) content, mineral composition, natural fracture, pore type, pore size distribution, porosity, permeability, and tortuosity were investigated in these different laminae types using FE-SEM, MICP and geochemical analysis. Combining the laminae characteristics and pore structure, we concluded that the organic pores and total pore area are mainly affected by the TOC content, whereas the development of the laminae controls the inorganic macropore scale and affects the pore size distribution. In addition, we found that the excessive development of shale laminae is not conducive to organic matter accumulation because of the change in redox condition, and because it has an adverse impact on the formation of complex artificial fractures during hydraulic fracturing. • Three laminae types were identified based on quantitative analysis. • Different laminae types show different pore structure characteristics. • Shale laminae influence the formation of natural fractures and artificial fractures. [ABSTRACT FROM AUTHOR]

The distribution and development of faults and fractures in organic-rich shales are important factors for the accumulation or loss of shale gas. In this work, merged three-dimensional seismic data, automated fault extraction, imaging logging, rock core examination, micro-CT scans, and field emission scanning electron microscopy were used to precisely characterize the distribution of faults and fractures of the Wufeng-Longmaxi shales from the Zilichang Block, Jiaoshiba area. The effects of faults and fractures on the accumulation and loss of shale gas were analyzed. The results show that the microfractures of shale samples from the JYA well of the Zilichang Block mainly consisted of intergranular, intragranular, and contraction fractures, and the fracture porosity of the samples is approximately 1.24%. These fractures help to increase the storage space of the shale reservoirs and are conducive for shale gas accumulation. In micro-CT scans, the microfractures were linked in a network-like structure and generally had excellent connectivity. Therefore, the microfractures could enhance the seepage capacity of the shale reservoirs and help the shale gas to migrate to hydraulically induced fractures. As a result, the productivity of the shale reservoirs could be dramatically improved. Macroscopic fractures were found to play a key role in the storage of shale gas. Many natural fractures, mainly are faults and open fractures, have developed in the caprock strata that directly overly the main production layers shales. The shale gas direct seal conditions are poor, which is not conducive to shale gas accumulation. Therefore, the Wufeng-Longmaxi shales in the JYA well is a relatively open shale system, with a formation pressure coefficient and gas saturation value of 0.96 and 37%, respectively. The single-well hydraulic fracturing productivity of the JYA well is low, with an absolute open flow value less than 0.5 × 104 m3/d. • Multiscale faults and fractures were identified. • Faults and macrofractures cause the loss of shale gas. • Microfractures enhance the seepage capacity of shale gas. [ABSTRACT FROM AUTHOR]

Both the characteristics of lithofacies and tectonic movements are important factors in controlling the development, destruction, and preservation of pores in shale reservoirs. However, the main factors that control the structure of shale pores are complex and highly disputed, restricting the understanding of the mechanisms that lead to the accumulation of shale gas. In this study, mineral composition and geochemical analyses, high resolution field emission scanning electron microscopy (FE-SEM), and low-pressure gas adsorption (N 2 and CO 2) were conducted to evaluate the pore systems of different types shale in the Jiaoshiba area, Sichuan Basin. A total of three shale groups, including silica-rich high production shales, clay-rich low production shales, and silica-rich low production shales were identified on the basis of lithofacies and gas-bearing characteristics. The highest values of TOC, pore volume, and specific surface area are found in the silica-rich high production shales, with an average of 4.182%, 33.04 × 10−3 cm3/g, and 38.71 m2/g, respectively. The total pore volume and specific surface area in the clay-rich low production shales are 19.85% and 28.62% lower than that of silica-rich high production shales. This suggests that the types of lithofacies have a more prominent influence on the specific surface area of the shales, while the pore volume plays a smaller role. However, the pressure in silica-rich low production shales is often released due to strong tectonic deformation. The OM pores in the silica-rich low production shales therefore tend to be both compressed and closed, with low surface porosity and of round shape. The total pore volume and specific surface area of silica-rich low production shales are 34.63% and 22.0% lower than that of the silica-rich high production shales. This indicates that tectonic movement has a significant influence on the shale pore volume, while specific surface area has a smaller influence. • Effects of lithofacies and preservation conditions on shale pores structure are analyzed. • Lithofacies mainly controls specific surface area and adsorbed gas. • Preservation conditions mainly controls shale pore volume and free gas. [ABSTRACT FROM AUTHOR]