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 δ³⁴S-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 groundwater.