Making sense of micro-fractures to the Longmaxi shale reservoir quality in the Jiaoshiba area, Sichuan Basin, China: Implications for the accumulation of shale gas

Differences in the Nanopore Structure of Organic-Rich Shales with Distinct Sedimentary Environments and Mineral Compositions

“…However, the work of Guo and Zhang has pointed out that the Wufeng–Longmaxi organic-rich shales in the well JiaoyeA of the Jiaoshiba area remain a closed system. Moreover, the overpressure with a pressure coefficient of 1.55 in this shale system also indicates that the loss of gas is small. , Due to the closure of the fluid system, most residual gas can be preserved, and the organic-rich shales have high free gas contents at present (Figure J). ,,, Furthermore, the high values of brittle minerals within the marine shales are beneficial to the fracturing of shale reservoirs. ,, It is worth pointing out that the storage space of marine shales is dominated by OM pores with large sizes (Figure ). Thus, the shale gas stored in these well-connected hydrocarbon pore networks has a high methane diffusion rate during production stages (Figure G and Table ).…”

“…For the marine shales, the quartz is mainly from biological sources, whose content has a positive correlation with TOC (Figure A). On the one hand, quartz can form a rigid framework to prevent the pores from being compressed. , On the other hand, the relatively high content of OM means that more OM pores can be formed and further accumulated to form pore networks in the overmature stage. , Thus, the quartz content increases, the pore volume and specific surface area of marine shales gradually increase, while the fractal dimension is decreased (Figure A–C). Similarly, this may explain why the relationship between clay minerals and pore parameters is opposite to that between quartz and pore parameters (Figure D–F).…”

“…Thus, a slightly positive correlation between quartz and fractal dimension is observed (Figure C). Also, the increase of clay minerals can bring more OM and improve the specific surface area, but also reduce the compaction resistance ability of shales. , Accordingly, the complicated role of clay minerals in lacustrine shales results in a low correlation between clay minerals, the specific surface area, and the fractal dimension (Figure E,F).…”

“…After satisfying the adsorption of clay minerals and OM pores, most gas will enter the well-developed macropores and microfractures associated with clay in the form of a free state (Figure C,F,I). The high clay content leads to strong toughness of the continental shale, which is difficult to form large faults and fractures during the uplift and erosion stage. ,,, This situation is conducive to the storage of free gas in the shale system. ,, In such cases, the proportion of free gas in lacustrine shales is superior (Figure L) . This result is favorable for fracturing to obtain higher initial production .…”

“…On the one hand, quartz can form a rigid framework to prevent the pores from being compressed. 51,54 On the other hand, the relatively high content of OM means that more OM pores can be formed and further accumulated to form pore networks in the overmature stage. 55, 56 Thus, the quartz content increases, the pore volume and specific surface area of marine shales gradually increase, while the fractal dimension is decreased (Figure 11A−C).…”