Formation and enrichment mode of Jiaoshiba shale gas field, Sichuan Basin

TLDR

The Jiaoshiba Structure is a faulted anticline that has undergone multiphase tectonic movements, and the Longmaxi Formation contains high‑quality, thermally mature shale (Ro > 2.2 %, TOC > 2 %) in its lower part, conditions typical of high‑yield shale plays. The study investigates the key factors controlling shale gas enrichment in the Jiaoshiba Longmaxi play, proposing a model of ladder migration, anticline accumulation, fault‑slip fracture control, and box‑shaped reservoiring. The authors model shale gas enrichment as ladder migration, anticline accumulation, fault‑slip fracture control, and box‑shaped reservoiring. The Jiaoshiba Longmaxi reservoir is overpressured (pressure coefficient 1.55) with stable production, and its high yield results from fault‑controlled reticular cracks, overpressure preservation, and a sealed box‑shaped system that maintains dynamic balance.

Abstract

The Silurian Longmaxi shale gas play in Jiaoshiba Structure in the southeast margin of the Sichuan Basin is studied to discuss the key controlling factors of shale gas enrichment in complex tectonic zone with high evolution. Jiaoshiba Structure is a faulted anticline which experienced multiphase tectonic movements. The Longmaxi Formation has high thermal evolution degree with Ro more than 2.2%, 35-45 meters thick high-quality shale (TOC > 2%) in its lower part. The reservoir is overpressure with a pressure coefficient of 1.55, and the shale gas production and pressure are stable. Structure type, evolution and geochemical analyses show that there are several stages of hydrocarbon generation, migration and accumulation in the Longmaxi Formation. The joint action of two groups (two stages) of fault systems and the detachment surface at the bottom of the Longmaxi Formation control the development of reticular cracks and overpressure preservation, and it is the key to the shale gas accumulation and high yield. The sealed box-like system in the Longmaxi Formation ensures the gas reservoir dynamic balance. The model of high yield and enrichment of Jiaoshiba shale gas play is "ladder migration, anticline accumulation, fault–slip plane controlling fractures, and box shape reservoiring". Like in conventional gas plays, good preservation and tectonic conditions are also required to form high yield shale plays in areas which have complex structures, multi-stage tectonic movements, and have high evolution shale.