Unveiling the Oldest Industrial Shale Gas Reservoir: Insights for the Enrichment Pattern and Exploration Direction of Lower Cambrian Shale Gas in the Sichuan Basin

Abstract

The lower Cambrian Qiongzhusi (Є1q) shale in the Sichuan Basin, formerly considered a source rock, recently achieved high gas production (7.388 × 105 m3·d−1) from well Z201 in the Deyang–Anyue rift trough (DART), marking an exploration breakthrough of the world’s oldest industrial shale gas reservoir. However, the shale gas enrichment mechanism within the DART is not fully understood. This study reviews the formation of the Qiongzhusi shale gas reservoirs within the DART by comparing them with cotemporaneous deposits outside the DART, and several findings are presented. The gas production interval was correlated with the main phase of the Cambrian explosion (lower Cambrian stage 3). In the early Cambrian ecosystem, dominant animals likely accelerated the settling rates of organic matter (OM) in the upper 1st member of Є1q (Є1q12) by feeding on small planktonic organisms and producing larger organic fragments and fecal pellets. High primary productivity and euxinic conditions contributed to OM enrichment in the lower 1st member of Є1q (Є1q11). Additionally, shale reservoirs inside the DART demonstrated better properties than those outside in terms of thickness, brittle minerals, gas content, and porosity. In particular, the abundant OM pores inside the DART facilitated shale gas enrichment, whereas the higher thermal maturity of the shales outside the DART possibly led to the graphitization and collapse of some OM pores. Meanwhile, the overpressure of high-production wells inside the DART generally reflects better shale gas preservation, benefiting from the shale’s self-sealing nature, “upper capping and lower plugging” configuration, and limited faults and microfractures. Considering these insights, we introduced a “ternary enrichment” model for the Qiongzhusi shale gas. Although the current high gas production of Z201 was found at the reservoir 3, two additional reservoirs were identified with significant potential, thus suggesting a “multilayer stereoscopic development” strategy in future shale gas exploration within the DART.