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

Sr-87/Sr-86, deltaD and delta O-18 together with chemistry of oilfield brin es, were measured from Cambrian to Neogene strata in the Kuqa Foreland Basi n, northwest China. The brines have Sr-87/Sr-86 ratios ranging from 0.70944 to 0.71716, deltaD values from -33 parts per thousand. to -67 parts per th ousand. and delta O-18 from -9.0 parts per thousand. to 3.9%parts per thous and. The most radiogenic Sr-87 brines have relatively heavy delta O-18 in t he Cambro- Ordovician carbonates, and are suggested to originate from cryst alline basement. deltaD and delta O-18 systematics show that all the brines slope to Global Meteoric Water Line, and intersect at deltaD = -60 parts p er thousand. and delta O-18 = -8.8 parts per thousand, close to that of pre sent-day local meteoric water, suggesting that meteoric water has mixed wit h evaporated ancient meteoric water. More saline samples have the lowest de ltaD and delta O-18 contents and molar Na/Cl ratios of more than 0.9, resul ting from salt dissolution.