Influence of Organic and Inorganic Compositions on the Porosity of the Deep Qiongzhusi Shales on the Margin of the Deyang–Anyue Aulacogen, Sichuan Basin: Implications from the Shale Samples of Well Z204

The deep Qiongzhusi (QZS) shales in the Deyang–Anyue Aulacogen, Sichuan Basin, are important shale gas targets in China. However, the differences in pore development of the shales in the marginal areas of the aulacogen and its main controlling factors remain unclear. In this study, the organic and inorganic compositions, water-bearing characteristics, and porosities of the sublayer 7 (SL7) and sublayer 5 (SL5) deep QZS shales collected from Well Z204 were systematically investigated. The results indicate that, compared with the SL5 shales, the SL7 shales have higher total organic carbon (TOC) and clay mineral contents, lower brittle mineral contents, greater porosities, and water content and saturation. The TOC content of the deep QZS shale has a strong positive correlation with porosity, and a negative correlation with water saturation. Therefore, the TOC content is the main controlling factor for the development of effective porosity. The TOC content of the SL7 shale is higher than that of the SL5 shale, so the SL7 shale has a larger effective porosity. Therefore, the margin of the Deyang–Anyue Aulacogen is a new exploration area for the deep QZS shales, and the shale gas accumulate conditions of the SL7 shales are better than that of the SL5 shales, especially in the lower section of the SL7 shales. This new understanding provides an important geological basis for the next exploration of the Deyang–Anyue Aulacogen.