Multiple geochemical and isotopic (Boron, Strontium, Carbon) indicators for reconstruction of the origin and evolution of oilfield water from Jiuquan Basin, Northwestern China

Highlights: • Reconstruction of the origin of oilfield water from Jiuquan Basin, China. • Geochemical evidence for geothermal water origin in the western Jiuquan Basin. • Data suggest eastward migration of the oilfield water across the basin. • Water-rock interactions along fluids migration modified the original composition. • Distinction between salinization and freshening process of oil field water. Oilfield water contains valuable information on the origin, migration, and geochemical evolution of fluids in sedimentary basins. Jiuquan Basin is one of the richest oil basins in China and holds large potential for future tight oil exploration. We use a wide range of geochemical and isotopic tracers to evaluate the origin and reconstruct the migration of oilfield water across Jiuquan Basin, including major (Ca, Mg, Na, K, NH₄, Cl, SO₄, Br, HCO₃) and minor (B, Li, Ba, Sr, Rb) elements, water isotopes (δ¹⁸O, δ²H), the isotopes of carbon (δ¹³C-DIC), boron, (δ¹¹B), and strontium (⁸⁷Sr/⁸⁶Sr). We show that the oilfield water was co-generated with the original hydrocarbons in the deep Qingxi sub-basin in the western part of the basin, and were derived from blending of two distinctive sources (1) deep-source brine that originated from relicts of evaporated seawater; and (2) geothermal water that underwent intensive water-rock interactions with the Lower Cretaceous Xiagou Formation, characterized by high DIC, Li⁺ , B, and SO₄ concentrations and distinctive δ¹⁸O, δ¹³C-DIC, and ⁸⁷Sr/⁸⁶Sr, which are consistent with the composition of the source rocks of Xiagou Formation. The distinctive geothermal signature was detected in the Yaerxia oilfield water in the eastern side of the Qingxi sub-basin, suggesting eastward co-migration of the geothermal water and crude oil to the shallow geological trap. Further eastward migration of the saline formation water into less saline environment in the central (Laojunmiao) and eastern (Shiyougou) fields caused base-exchange reactions, adsorption, and sulfate reduction that resulted in a progressive reduction in the overall salinity, Na⁺, NH₄⁺, Li⁺, B, SO₄²⁻, and ⁸⁷Sr/⁸⁶Sr, coupled with increasing Ca²⁺, Mg²⁺, Sr²⁺, and δ¹¹B. Later dilution of Laojunmiao oil field caused B desorption, oxidation of organic matter, and secondary methanogenesis. The integration of multiple geochemical tracers provides systematic geochemical criteria's for reconstructing the origin and evolution of the oilfield water in Jiuquan Basin and the ability to distinguish between the original composition and secondary modification of the geochemistry of the oilfield water.