Differences of Main Enrichment Factors of S1l11-1 Sublayer Shale Gas in Southern Sichuan Basin

In this study, shale cores from 20 wells in the S1l(1-1)(1) sublayer of Longmaxi Formation buried in shallow shale (<3500 m) and deep shale (>3500 m) in the southern Sichuan Basin, China were collected to compare their pore structures and gas-bearing properties using multiple experiments. Results showed that the deep layer has relatively lower brittle mineral content, which is disadvantageous in terms of the higher requirements it imposes on hydraulic fracturing. Results also showed that the most important factor controlling the differential enrichment of S1l(1-1)(1) shale gas in southern Sichuan Basin is porosity. Moreover, the porosity composition of shallow shale and deep shale has significant differences: the porosity of shallow shale is dominated by organic pores, while for deep shale, both organic and inorganic pores are important. The inorganic pores provide significant storage space for free gas in deep shale; their contribution warrants more attention. We also found that the difference in organic porosity of the shallow and deep shale samples resulted from large differences in pore development ability, while the highest inorganic porosity was concentrated near the optimal mineral composition when the content of quartz plus feldspar plus pyrite was about 70%. This study revealed the primary factor controlling the difference in gas content between shallow and deep shale and detailed the characteristics of microscopic pore structure, providing a basis for the exploration and development of deep shale gas in the Wufeng-Longmaxi Formation in the southern Sichuan Basin.

Automated summary

This study compared the pore structures and gas-bearing properties of shallow and deep shale in the southern Sichuan Basin, China, revealing that deep shale has lower porosity and brittle mineral content, requiring higher hydraulic fracturing demands. The primary factor controlling shale gas enrichment in the region is porosity, with deep shale having both organic and inorganic pores providing significant gas storage space. Additionally, differences in organic and inorganic pore compositions between shallow and deep shale were found, with the highest inorganic porosity concentrated near optimal mineral composition.