Preservation condition evaluation of normal pressure shale gas in the Wufeng and Longmaxi Formations of basin margin transition zone, Southeast Chongqing

Abstract

Despite the huge gas resources in the shale of the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the Sichuan Basin and its periphery, commercial breakthroughs are mainly located inside the basin.The Southeast Chongqing, located in the basin margin transition zone, was intensively deformed by tectonic activities. As compared to the gas enrichment model of the Wufeng-Longmaxi Shale inside the basin, the basin margin transition zone requires better preservation conditions.According to the complicated structure patterns of the Wufeng and Longmaxi Formations in Southeast Chongqing, the preservation conditions are analyzed by the super position of structure evolution, uplift and exhumation, fault development, structural style, roof and floor conditions, and formation pressure. The results show that the basin margin is seriously uplifted and exhumated, with well-developed faults, forming a series of residual structural styles; natural gas leakage in the area close to the exhumation boundary and major faults contributed to the low formation pressure coefficient (less than 1.2). Sangtuoping syncline and Wulong syncline show good preservation conditions of shale gas because of the relatively less large faults and the far away from the exhumation area. Therefore, the late uplift, exhumation, and fault development are the main reasons for the gas leakage and the change of formation pressure from overpressure to normal pressure across the basin margin. Tectonic deformation is the fundament of current shale gas preservation in the structures of narrow and steep residual anticlines, broad and gentle residual synclines, and residual slopes. Finally, an evaluation standard was established for the preservation conditions of normal-pressure shale gas. According to this standard, favorable target areas are divided for normal shale gas in the study area.

Keywords

• southeast chongqing
• basin margin transition zone
• normal pressure shale gas
• preservation conditions
• wufeng and longmaxi formations