Multiple Geochemical And Isotopic (Boron, Strontium, Carbon) Indicators For Reconstruction Of The Origin And Evolution Of Oilfield Water From Jiuquan Basin, Northwestern China

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 (818O, 82H), the isotopes of carbon (813C-DIC), boron, (811B), 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 818O, 813C-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 811B. 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.