Gas preservation conditions of Cambrian Niutitang formation shale in western margin of Xuefeng uplift, SE China-based on the analysis of paleofluid

Introduction The lower Cambrian Niutitang Formation shale in the western margin of the Xuefeng uplift (WMXU), South China, possesses favorable material conditions including significant thickness, high organic carbon content, and high thermal maturity. However, industrial shale gas has not been discovered in this region, suggesting that preservation conditions may be the key limiting factor. Methods In this study, we comprehensively analyzed the preservation conditions of shale gas in the Niutitang Formation using paleofluid analysis, including calcite carbon–oxygen isotope analysis, fluid inclusion analysis, and in situ calcite LA-ICP-MS U–Pb dating. Results The results show that calcite veins in WMXU exhibit negative carbon and oxygen isotopes, low Z values, and low-salinity brine inclusions, indicating exogenous fluid infiltration and poor sealing conditions. Calcite U–Pb dating reveals one reliable paleofluid episode at 84.4 ± 2.9 Ma, corresponding to the middle–late Yanshanian strike-slip movement. A Neogene episode at ca. 17 Ma is inferred from apatite fission-track thermochronology and fluid inclusion data. Discussion We propose that large-scale fractures and detachment surfaces at the base of the Niutitang Formation provided pathways for gas escape, with the Yanshanian and Himalayan tectonic movements being the critical periods for shale gas loss. These findings provide important insights for shale gas exploration in structurally complex areas.