Geochemical characteristics and origin of the formation water of the Saline Lake Basin: a case study of the Quaternary Qigequan Formation in the Sanhu Depression, Qaidam Basin

Clarifying the geochemical characteristics of formation water and its origin is conducive to clarifying the gas migration path, elaborating the law of gas migration and accumulation, and further predicting the favourable area of gas accumulation. Taking Quaternary formation water from the Tainan-Sebei area of the Sanhu Depression as the research object, the chemical characteristics and origin of the region are clarified using anion analysis, cation analysis, hydrogen isotope analysis, oxygen isotope analysis and so on. The results are as follows. (1) The formation water in the study area has a high total dissolved solids (TDS) content and is mainly type IV and V of CaCl2. (2) Low r(Na+)/r(Cl−), low desulfurization coefficient, high r(Ca2+)/r(Mg2+) and high indices of base exchange indicate that the Qigequan Formation is in a stagnant zone, which is beneficial for the accumulation and preservation of biogenic gas. (3) Due to albitization and water–rock reactions, the formation water is rich in Ca2+, poor in Na+ and poor in Mg2+. (4) The formation water in the study area originates from the glacial meltwater of the Kunlun Mountains, which converts into groundwater and seeps into the formation along the piedmont slope zone. In the process of groundwater infiltration and convergence, many salt substances in the formation are dissolved, resulting in a gradual increase in TDS. Then, the formation water with a high TDS migrates to the anticline in the northern part of the depression and is finally trapped in the formation.