Evolution of overpressure in the Wufeng-Longmaxi shale reservoirs of the Jiaoshiba area, Sichuan Basin, southwestern China: Implications for shale gas preservation

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Overpressure development in shale gas reservoirs is complicated by a variety of factors, including reservoir heterogeneity, fluid dynamics, and external influences. Currently, no comprehensive framework exists for evaluating the development of overpressure in shale gas reservoirs. In this study, a workflow comprising multiple approaches was used to investigate overpressure evolution and its implications for shale gas enrichment and yield in the Jiaoshiba area, Sichuan Basin, southwestern China. The influence of diverse reservoir types, fluid dynamics, and other natural factors on the reservoir framework was assessed using fluid inclusion, microthermometry, laser Raman spectroscopy, and thermodynamic modeling. The Wufeng-Longmaxi shale reservoir in the study area shows high variability in hydraulic fracturing productivity of shale gas and fracture development, which is attributed to the generation of overpressure during three stages of tectonic evolution and activity, namely 160–85, 85–60, and 60 Ma to present day. Pressure coefficients increased from 1.49 to 2.09 during the first stage due to favorable preservation conditions that promoted gas generation, but dropped from 2.09 to 1.53 during the second stage due to the poor sealing capacity of shale reservoirs. The third stage was characterized by a pressure drop from overpressure to normal pressure, possibly because of the development of faults and fractures, as well as gas leakage, resulting in poor shale gas preservation conditions and low hydraulic fracturing productivity in the Zilichang and Wujiang fault peripheral zones. Importantly, our findings show that overpressure is a critical factor in the enrichment and yield of shale gas. In particular, the shale reservoirs in the Pingqiao zone were found to have an elevated level of enrichment and productivity due to favorable preservation conditions and overpressure preservation during the Himalayan Orogeny, providing greater insight into the enrichment and yield of shale gas in other tectonic settings around the world.