Differential Enrichment of Deep Shale Gas in Southeastern Sichuan Basin: Dynamic Evolution and Preservation Mechanisms

Abstract

Significant heterogeneity in deep shale gas enrichment is observed within the Wufeng-Longmaxi formations in southeastern Sichuan, yet the dynamic evolution processes and mechanisms driving differential enrichment remain unclear. This study focuses on the complex marginal structural belt (Dongxi, Dingshan, Xinchang) and the steep intra-basin structural belt (Shilongxia, Lizi), and investigates the mechanisms of deep shale gas enrichment based on analyses of vein types, carbon and oxygen isotopes, rare earth elements, and fluid inclusion homogenization temperatures, integrated with basin modeling. The results show that: (1) Vein characteristics reflect differences in gas preservation conditions. Fracture-filling veins are mainly composed of calcite and quartz, formed during the Late Cretaceous to Miocene (10-95 Ma). In the intra-basin high-steep anticlines, veins are associated with hydrothermal and meteoric water origins, and exhibit wide homogenization temperature ranges and three phases of fluid activity, indicating poor gas preservation. In contrast, the marginal zones are dominated by veins derived from in situ formation fluids, show narrower homogenization temperature ranges, and experienced fewer fluid events (1–2 phases), suggesting favorable preservation conditions. (2) Fracture-fluid interactions dominate reservoir evolution. At maximum burial depth, deep shale reservoirs in the Wufeng-Longmaxi formations were strongly overpressured (pressure coefficient >2.0) and had high gas contents (>6 m³/t). In the intra-basin high-steep structures, repeated opening and closure of high-angle fractures during the Yanshanian–Himalayan uplift led to rapid gas loss from the Late Cretaceous to Oligocene, resulting in present-day gas content below 2 m3/t, whereas marginal fault-bend anticlines with weak high-angle fracture development retained overpressure (pressure coefficient > 1.8), and current gas contents remain above 6 m3/t. (3) Structural style controls differential enrichment. In areas of structural superposition between the Luzhou paleo-uplift and the southern Sichuan finger-like structures (Lizi, Shilongxia), significant stratigraphic shortening and the development of back-thrust and strike-slip faults led to severe reservoir damage. In contrast, marginal fault-bend anticlines experienced less shortening and weaker deformation; regions without back-thrust faults retained higher gas contents. Favorable enrichment zones are identified in fault-bend anticlines without back-thrusts and in slope or syncline areas far from high-steep or strike-slip fault zones. These findings provide theoretical support for targeted exploration and efficient development of deep shale gas resources in the Sichuan Basin.

Keywords

shale gas / Wufeng Formation-Longmaxi Formation / fractures / calcite vein / fluid inclusions / dynamic evolution / differential enrichment

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