Aulacogen-uplift enrichment pattern and exploration prospect of Cambrian Qiongzhusi Formation shale gas in Sichuan Basin, SW China

Abstract

The shale of the Cambrian Qiongzhusi Formation in the Sichuan Basin is characterized by large burial depth and high maturity, but the shale gas enrichment pattern is still unclear. Based on the detailed characterization of Deyang-Anyue aulacogen, analysis of its depositional environments, together with reconstruction of shale gas generation and enrichment evolution against the background of the Leshan-Longnüsi paleouplift, the aulacogen-uplift enrichment pattern was elucidated. It is revealed that the Deyang-Anyue aulacogen controls the depositional environment of the Qiongzhusi Formation, where high-quality sedimentary facies and thick strata are observed. Meanwhile, the Leshan-Longnüsi paleouplift controls the maturity evolution of the shale in the Qiongzhusi Formation, with the uplift located in a high position and exhibiting a moderate degree of thermal evolution and a high resistivity. The aulacogen-uplift overlap area is conducive to the enrichment of shale gas during the deposition, oil generation, gas generation, and oil-gas adjustment stage, which also has a joint control on the development of reservoirs, resulting in multiple reservoirs of high quality and large thickness. Based on the aulacogen-uplift enrichment pattern and combination, four types of shale gas play are identified, and the sweet spot evaluation criteria for the Qiongzhusi Formation is established. Accordingly, a sweet spot area of 8 200 km² in the aulacogen is determined, successfully guiding the deployment of Well Zi 201 with a high-yield industrial gas flow of 73.88×10⁴ m³/d. The new geological insights on the aulacogen-uplift enrichment pattern provide a significant theoretical basis for the exploration and breakthrough of deep to ultra-deep Cambrian shale gas, highlighting the promising exploration prospect in this domain.