Temperature and origin of fluid inclusions in shale veins of Wufeng?Longmaxi Formations, Sichuan Basin, south China: Implications for shale gas preservation and enrichment

In the southeastern Sichuan Basin, the Wufeng–Longmaxi Formations are major strata used for shale gas exploration and development. Although the Fuling shale gas field—the largest Lower Paleozoic shale gas field in the world—has already been developed, shale gas wells in other parts of this area apart from the Jiaoshiba anticline vary greatly in production capacity. Microstructures and inclusion homogenization temperatures were analyzed for venous fluid inclusions in samples taken from the main shale gas wells in the Wufeng–Longmaxi Formations and were combined with simulated burial–thermal histories, hydrocarbon generation and expulsion histories, and fracture formation–closure times and stages to determine how fracture and fluid activities influenced shale gas preservation/destruction conditions. Veins were composed of quartz, calcite, and quartz–calcite and contained gas–liquid two-phase brine and high-density methane inclusions. Combined with simulated burial–thermal histories, the results showed that fracture-fluid activity time could be inferred from inclusion homogenization temperatures and was consistent with gas content and output of shale gas wells. The deeper and earlier the vein formation, the more favorable the preservation conditions of the shale gas reservoir. Furthermore, the more the fracture formation–closure stages and the lower the inclusion homogenization temperature, the more severe damage to the shale gas reservoir has happened. Clearly, in areas from Jiaoshiba to Pingqiao, Nanchuan, and Dingshan in the southwest, increasing the number of paleo-fluid activity and vein formation stages, as well as later paleo-fluid activity time, worsened shale gas preservation conditions. The results can act as a reference for studying fracture-fluid activity, enrichment, and preservation of shale gas in the southeastern Sichuan Basin and other parts of the world. © 2020 Elsevier B.V.