Discussion on the characteristics and controlling factors of differential enrichment of shale gas in the Wufeng-Longmaxi formations in south China

Abstract

The annual output of shale gas from the Wufeng-Longmaxi formations in South China reached 15.4 × 109 m3 in 2019, which makes Sichuan basin the second-largest shale gas producing area in the world. Based on the data of more than 1000 shale samples of the Wufeng-Longmaxi formations taken from typical drilling and outcrop profiles in the Weiyuan, Changning, Fuling, and Wuxi areas in South China, the characteristics of differential enrichment of shale gas are analyzed and the main controlling factors are discussed. The study shows that the shale gas from the Wufeng-Longmaxi formations in South China has some differential enrichment characteristics in both vertical and lateral orientations. In vertical orientation, sweet-spot intervals of shale gas developed concentratedly sweet-spot. The key parameters (e.g., thickness, gas content, TOC content) in the sweet-spot intervals vary greatly in regional aspect, among which they are better in the Changning and Fuling areas. The gas contents and TOC contents of the Wufeng-Longmaxi shale in different areas have good positive correlations, indicating that the abundance of organic matters (TOC content) is a key factor that affects the degree of shale gas enrichment (gas content). The high primary productivity of marine surface water during the shale deposition period of the Wufeng-Longmaxi formations is an important prerequisite for the generation of large amounts of organic matters. However, in relatively stable tectonic zones with weak faults development, euxinic bottom water condition is the key factor controlling the formation of sweet-spot intervals of shale gas vertically and the distribution of sweet-spot area laterally, that is the key factor for the differential enrichment of Wufeng-Longmaxi shale gas.