Evolution History of Overpressured and Normally Pressured Shale Gas Reservoirs in Wufeng Formation-Longmaxi Formation, Sichuan Basin, China: An Analysis from the Perspective of Source and Seal Coupling Mechanism

Among the shale gas fields in the Sichuan Basin, the Fuling, Changning, and Weiyuan fields with Wufeng FormationLongmaxi Formation as the target formation have made significant contributions to the shale gas industry in China. However, several other areas in the basin which have witnessed gas flow from wildcat wells have not realized commercial development. In this study, we analyzed the Wufeng Formation-Longmaxi Formation shale gas reservoirs from the perspective of a coupling mechanism of source and seal, by considering three aspects, including the characteristics of source and cap rocks, evolution history of the source rock-reservoir caprock, and the differential enrichment of shale gas. The results indicate the following: (1) The occurrence of the regional caprock (Triassic gypsum-salt) determines the distribution of overpressured and normally pressured shale gas reservoirs. In addition, the erosion of the caprock and its integrity control the gas content and cost-efficiency of the normally pressured shale gas reservoirs. (2) For the overpressured reservoirs, the thickness of the siliceous and carbonate-bearing siliceous shales in the WF2-LM4 graptolite zone determines the gas content and producibility of the lower shale gas layer. In terms of the upper layers, the structural form plays a critical role. Generally, gas reservoirs in the upper layers that occupy the higher part of an anticline are more favorable, whereas those in a syncline or monocline exhibit poor productivity. (3) For the normally pressured shale gas reservoirs with poor caprock integrity, the enrichment and the commercial value of the gas depend mostly on the thickness of the WF2-LM4 graptolite zone. Therefore, we conclude that the areas near the core of synclines are more favorable and structurally stable, with relatively high formation pressure and moderate burial depths. Our findings can improve sweet spot prediction and offer valuable guidance for shale gas development in China and globally.

Automated summary

This study analyzed the shale gas reservoirs in the Sichuan Basin and explored the factors influencing the source and cap rocks, evolution history, and differential enrichment. The research revealed the characteristics of different types of reservoirs and provided valuable guidance for shale gas development.