Enrichment of shale gas in different strata in Sichuan Basin and its periphery-The examples of the Cambrian Qiongzhusi Formation and the Silurian Longmaxi Formation

This paper discusses the main factors affecting generation and accumulation of shale gas in the Qiongzhusi and Longmaxi formations in and around the Sichuan Basin, China, based on case studies and comparative analysis of sedimentary environments, lithological associations, basic parameters of shale gas, roof and floor conditions and carrier systems. Both the Qiongzhusi and Longmaxi Formations contain good-quality thick shale strata with high TOC, brittle minerals contents, and gas contents. The accumulation of shale gas is affected by the transport systems and the preservation conditions, especially the roof and floor lithology. The overlying strata of the Longmaxi Formation is tight and thick mudstone and muddy siltstone. The underlying strata of the Longmaxi Formation is tight and thick limestone. Both overlying and underlying strata all have low porosity and low permeability, which acts as a good roof and floor for shale gas preservation and subsequent mass fracturing reformation. Overlying strata of Qiongzhusi formation and Longmaxi is similar and served as a good roof. But the underlying stratum of the Qiongzhusi Formation is a regional reservoir that usually contains aquifers; theredore, it isn't a good floor for shale gas preservation. As a result of these conditions, the Longmaxi Formation has better shale gas accumulation conditions than that of the Qiongzhusi Formation.