Favorable tectonic patterns for shale gas preservation and enrichment in lower Paleozoic, southeastern Chongqing, China

Marine shale reservoirs have experienced intense tectonic movements in southeastern Chongqing, resulting in widely developed folds, faults and bedding slip deformation. The gas content is of a wide range while the organic matter content and thickness are sufficient for shale gas accumulation. Here, we review previous work and present outcrop and core data from 11 shale wells around southeastern Chongqing to analyze and summarize tectonic types and their combinations, as well as their influence on shale gas preservation and enrichment. Fault and bedding slips are prevalent in shale reservoirs, and they apparently influence shale gas content. Bedding slip crush is usually formed at the bottom interval where the total organic content is high from intense horizontal compression. Bedding slip crushing and well-developed high-angle natural fractures form a net fracture system where shale gas is enriched in an anticline structure or lost to air in a syncline structure laterally. Strata dip is the key geological factor that controls bedding slip deformation in shale reservoirs in certain areas. According to the established bedding slip geological model, when bedding slip occurred, the critical strata dips of the lower Silurian Longmaxi Formation were 13.34°, respectively. Bedding slip crushed anticlines without faults and synclines without faults and bedding slip crushing are the two tectonic patterns that are favorable for shale gas preservation and enrichment, which should be the focus of further explorations of lower Paleozoic shale gas in southeastern Chongqing.