Origin, migration and mixing of oilfield brines: Stable isotopic evidence from Kuqa Foreland Basin

Abstract

87Sr/86Sr, δD and δ18O together with chemistry of oilfield brines, were measured from Cambrian to Neogene strata in the Kuqa Foreland Basin, northwest China. The brines have87Sr/86Sr ratios ranging from 0.70944 to 0.71716, δD values from −33‰ to −67‰ and δ18O from −9.0‰ to 3.9‰. The most radiogenic87Sr brines have relatively heavy δ18O in the Cambro-Ordovician carbonates, and are suggested to originate from crystalline basement. δD and δ18O systematics show that all the brines slope to Global Meteoric Water Line, and intersect at δD=−60‰ and δ18O=−8.8‰, close to that of present-day local meteoric water, suggesting that meteoric water has mixed with evaporated ancient meteoric water. More saline samples have the lowest δD and δ18O contents and molar Na/Cl ratios of more than 0.9, resulting from salt dissolution.