Genesis of formation water in the northern sedimentary basin of South China Sea: Clues from hydrochemistry and stable isotopes (D, 18O, 37Cl and 81Br)

Formation water is an important fluid in sedimentary basins, hydrogeochemical characteristics of which has important instructions to oil and gas generation and preservation. The South China Sea (SCS) oilfield is a large oil field in China, but research on origin and evolution of formation waters in SCS oilfield has been poorly documented. In this study, major ions, as well as δ2H, δ18O, δ37Cl and δ81Br in formation waters of two basins (BBWB and ZJKB) of SCS were analyzed to fill the gap. The results showed that the chemical types of formation waters in both basins are ClNa. The relationship between Br vs major ions and 2H/18O compositions showed the origin of formation water are mainly ancient seawater. The formation waters are enriched in Ca and Sr, and depleted of Na and Mg compared with seawater mainly owing to albitization of plagioclase, whereas the dolomitization is minor. Comparing to the BBWB, the formation waters in ZJKB were more enriched in 2H and 81Br, and more depleted in 37Cl. The formation waters in ZJKB were stored in a relatively more closed environment and might have been affected more by evaporation, whereas those in BBWB might experience more influence from halite dissolution by ancient precipitation through open fracture structures.