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The origin and geochemical evolution of formation water trapped in continental shale of the Dameigou Formation in the Northern Qaidam Basin have been investigated for selecting unique signatures of formation water compared with shallow groundwater and conventional oilfield brines (COB). The formation water, which is trapped in a closed shale with no halite and sylvite in it, has a higher Cl/Br ratio (1183.43) than that of seawater (657.89). Furthermore, δ2H (−67‰) and δ18O (−8.1‰) values of the formation water lies near the local evaporation line. These suggest that the formation water originated from evaporated rainwater. The temperature (>120°C) of the Dameigou Formation and the vitrinite reflectance values (1.0-1.3%) of the shale indicate that thermochemical sulfate reduction (TSR) was responsible for the depletion of SO4 in the formation water. The increased CO2 concentration and Mg/Ca ratio in formation water leading by TSR caused kaolinization of plagioclase feldspar and dolomitization, respectively. The formation water depletion in Ca and HCO3 was caused mainly by calcitization. The Na/Ba ion exchange and dissolution of continental borate occurring in the shale-formation water system were vital to the geochemistry of trace elements in formation water. All these geochemical processes have given the formation water distinctive δS-H2S (25.9‰), 87Sr/86Sr ratio (0.713842), Ba/Ca ratio (2.60×10-2) and δ11B (1.1‰) signatures compared with COB in shallow

Origin and Geochemical Evolution of Formation Water in the Dameigou Shale Gas Reservoir in Northern Qaidam Basin