Shale gas accumulation mechanism of deep-buried marine shale of Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the southeast Sichuan Basin

Deep shale gas has become an important frontier for future shale gas exploration and development. The Wufeng-Longmaxi formations in southern China have undergone complex tectonic and transformation through multi-stage tectonic movements. Deep shale gas enrichment conditions are complex, which greatly restricts the exploration and development of deep shale gas. In this study, based on systematic analysis of basic geological characteristics and gas reservoir characteristics of deep shales, the main factors controlling deep shale gas enrichment in southern China were investigated, and enrichment modes were established. The results show that high-quality shales were developed in the deep-water continental shelf facies, characterized by moderate thermal maturity, high silica content, and abundant organic matter. These characteristics provide a good basis for the formation and enrichment of shale gas. The deep shale gas reservoir is featured by overpressure, high porosity and high gas content. The development and maintenance of high porosity, favorable roof and floor sealing conditions, and weak tectonic activity during uplift stage are the main factors to control deep shale gas enrichment. Based on a comprehensive analysis, the enrichment modes of deep shale gas under three different tectonic patterns were established, namely overpressure enrichment within the basin, overpressure enrichment in the faulted nose or slope of the margin, and overpressure enrichment in the remnant syncline outside the basin. This study provides a reference for exploration and development of deep shale gas in Sichuan Basin and other areas.