The Wufeng-Longmaxi marine shale of the Sichuan Basin is characterized by a high degree of thermal evolution and good gas bearing properties. The evolution mechanism of organic matter and organic matter porosity is coupled with the increase of the degree of evolution, which has a controlling effect on the gas content of shale gas reservoirs in different areas of the Sichuan Basin. In order to reveal the controlling effect of the co-evolution of organic matter and organic matter porosity at different evolutionary stages on the shale gas enrichment and accumulation, a high-temperature and high-pressure thermal simulation experiment was carried out on samples of the Xiaming marine shale due to its similar geological and geochemical characteristics to the Wufeng-Longmaxi shale in the study area. The experiment was targeted at simulating the hydrocarbon generation process of marine shale. Based on the hydrocarbon gas productivity and characteristics of carbon isotope sequences of marine shale in the hydrocarbon-generation thermal simulation experiment, the initial maturity of cracked gas in retained oil has been determined as 1.5%, and at the peak stage it was 2.5%~3.4%. We have carried out scanning electron microscopy (SEM) examination of samples of the Xiamaling and Wufeng-Longmaxi marine shales, based on organic matter qualitative identification and organic matter porosity quantitative extraction. The results of parameter analysis including organic matter morphological parameters and pore
structure parameters (equivalent circle diameter and perimeter area ratio) show that original kerogen dominates the major part of the marine shale organic matter in the low maturity stage. This has the morphology of isolated, micropores mainly developed on the organic matter with poor pore connectivity and inorganic mineral intergranular porosity is usually remnant; At the highly mature evolutionary stage, the proportion of original kerogen in all organic matter is gradually reduced, while the percentage of migrated organic matter formed by crude oil filling in remaining intergranular pores tends to increase, up to 80%. Asphalt porosity is widely developed in migrated organic matter, the equivalent diameter and perimeter area ratio parameter of pore structure shows that asphalt porosity is well developed with relatively large pore diameters and round to long-oval pore morphology as well as good pore connectivity. The coupling evolution of organic matter and organic matter porosity ensures the enrichment and accumulation of marine shale gas at the high maturity evolutionary stage. Original kerogen provides the material basis and the residual intergranular pores provide the filling space for the liquid hydrocarbon at the low evolution stage. The migrated organic matter provides materials for gas generation and organic matter pores provide the storage space at the high evolution stage, which
YANG Wei, CAI Jianfeng, WANG Qianyou, CUI Zhengjie, CUI Zhe, XU Liang, LI Lan, GU Xiaoming, WANG Jingling. The controlling effect of organic matter coupling with organic matter porosity on shale gas enrichment of the Wufeng-Longmaxi marine shale. Petroleum Science Bulletin, 2020, 02: 148-160.
