Control mechanism of tectonic action on continuous accumulation and coupling of conventional-unconventional oil and gas reservoirs: a case study of Pingqiao area, southeastern Sichuan Basin

Abstract

This study explored the dynamic control and differences of tectonic action on the formation of conventional and unconventional gas reservoirs in Pingqiao area, southeastern Sichuan Basin. Based on seismic, geolo-gical, inclusion, and other relevant data, together with fault-related fold theory, this study systematically analyzed thestructural characteristics and evolution processes of the area, as well as the dynamic accumulation processes of conventional and unconventional gas reservoirs and the differences in structural impacts on both. A coupled accumulation model of typical conventional and unconventional oil and gas continuous aggregation was established. The Pingqiao area, located in the SE-NW foreland progressive deformation zone of southeastern Sichuan, is controlled by multiple detachment layers and the Jiangnan-Xuefeng orogenic belt, developing fault extension structures and back-thrust structures. The Pingqiao anticline structure was formed during the Yanshanian period. During the middle Yanshanian period, influenced by the foreland expansion of the Jiangnan-Xuefeng tectonic system, the study area experienced intense NE-directed fault-folding. From the late Yanshanian to the Himalayan period, the uplift of central Sichuan and the rise of the Qinghai-Tibet Plateau caused the Pingqiao anticline to continuously uplift. The Cambrian Qiongzhusi Formation and Ordovician Wufeng Formation to Silurian Longmaxi Formation source rocks mainly experienced long-term burial and hydrocarbon generation in the early Yanshanian period and before. In the middle Yanshanian period, tectonic deformation extended to the study area, affecting the conventional gas reservoir (Xixiangchi Group), cap rocks, hydrocarbon traps, and migration, resulting in poor preservation conditions of gas reservoirs. Unconventional gas layers were transformed into anticline structures, with shale gas accumulating towards the core of the anticline, resulting in better overall preservation conditions. From the late Yanshanian to the Himalayan period, strata uplift and depressurization continued to deteriorate the preservation conditions of both types of gas reservoirs. Therefore, this study suggests that the difference in tectonic control on conventional and unconventional gas reservoirs is reflected in the control methods and transformation timing: the lateral hydrocarbon supply during the middle Yanshanian period is the key for conventional gas reservoir formation; tectonic activity in the Late Cretaceous and tectonic uplift from the late Yanshanian to the Himalayan period affect the preservation of shale gas.

Keywords

• tectonic action
• conventional gas reservoir
• unconventional gas reservoir
• difference control
• xixiangchi group
• wufeng formation - longmaxi formation