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Two types of organic-rich shale, i.e. marine-continental transitional shale and deep-water shelf shale, are well developed in the Upper Permian of the Sichuan Basin. The coal-bearing shale of the marine-continental transitional facies is primarily distributed in the Longtan Formation to the south of Chengdu-Nanchong-Guang'an-Shizhu-Fuling, which has thin individual layers but is cumulatively thick. It is dominated by type Ⅲ kerogen in the high-maturity to over-mature stage. The shale reservoir is primarily composed of clay-mineral-hosted pores, intergranular pores and micro fractures, with a small amount of organic porosity. However, the high clay mineral content and low brittle mineral content result in poor fracturing performance, requiring further engineering work. The organic-rich shale of the deep-water shelf facies is mainly distributed in the Wujiaping and Dalong formations along Guangyuan-Bazhong-Xuanhan-Yunyang-Shizhu in the northern part of the Sichuan Basin, where the Wujiaping and Dalong shales are 20-80 m and 10-40 m thick, respectively. These two shales are rich in organic matter with TOC contents higher than 2%, which are dominated by type Ⅱ1 kerogen at the high-maturity to over-mature stage. The storage space is mainly contributed by organic pores and intragranular dissolution porosity with good storage performance. High siliceous mineral content and low clay mineral content give rise to good fracturing performance, indicating for a good shale gas exploration potential in the Sichuan Basin. |
