Origin and evolution of oilfield brines from Terti- ary strata in western Qaidam Basin: Constraints

cated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ 18 O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line (QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from meteoric water, and then might be affected by water–rock metamorphose, because most oilfield brines distribute in the range of metamorphosing water. The 87 Sr/ 86 Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition, all oilfield brines have obviously much positive δ 34 S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting in positive shift of δ 34 S value and depleteed SO4 2- in oilfield brines. Combined with water chemical data and δD, δ 18 O, 87 Sr/ 86 Sr, δ 34 S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected by salt dissolution, water–rock metamorphose, sulfate reduction and dolomitization during the process of migration. These processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so on) for sustainable utilization of salt lake resources in the Qaidam Basin.