Origin and evolution of oilfield brines from Tertiary strata in western Qaidam Basin: Constraints from 87Sr/86Sr, δD, δ18O, δ34S and water chemistry

Abstract

Chemistry of major and minor elements, 87 Sr/ 86 Sr, δD, δ 18 O and δ 34 S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry data show that all oilfield brines are CaCl 2 type. They were enriched in Ca 2+ , B 3+ , Li + , Sr 2+ , Br -, and were depleted in Mg 2+ , SO 4 2-, which indicated 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 SO 4 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.Key words 87 Sr/ 86 Sr, δD and δ 18 O, δ 34 S, origin and evolution of oilfield brines, western Qaidam Basin

The Source, Distribution, and Sedimentary Pattern of K-Rich Brines in the Qaidam Basin, Western China

“…The large reserves (~3.5 × 10 8 t, [13]) and simple chemical compositions (mainly KCl and NaCl) of the pore brines make it a promising K resource for the future. Meanwhile, the oilfield brines are also a very important K resource in the QB for their high K + concentrations (0.03-7.34 g/L, average 2.70 g/L; Table S2; Figure 3a) [14][15][16]19,20]. They mainly distributed in the Nanyishan and Shizigou anticline structure areas and occur in the Tertiary (Paleogene and Neogene) strata (Figures 1, 2 and 10a) [18].…”

“…They mainly distributed in the Nanyishan and Shizigou anticline structure areas and occur in the Tertiary (Paleogene and Neogene) strata (Figures 1, 2 and 10a) [18]. The lithology of the reservoir for oilfield brines is sandstone, mudstone, siltstone, and marlite ( Figure 2) [14][15][16]19,20]. Table S2); (b) the consistent variations of K + , Na + , Cl − , and TDS in each pore brine sample indicate that the K has the same source as Na, Cl, and TDS (data from Table S2); (c) the δD and δ 18 O values reveal that the oilfield brines were meteoric water origin, and experienced evaporation and water-rock interaction processes (global meteoric water line is from [56]; local meteoric water line is from [4]; the data of river waters are from [57,58]; data of surface and intercrystalline brines are from [4,15]; data of oilfield brines are from [15,16,19]; the local evaporation line is fitted in this study: δD = 3.5 × δ 18 O-21).…”

“…Previous studies [14][15][16][17][18][19][20][21] show that these two types of deep brines have different sources of K and experienced different evolution processes. The origin of the pore brines was interpreted by the fact that the buried pore water leached the overlying evaporite layer (mainly halite) and obtained solutes [17].…”

“…Therefore, the water-salt interaction (original pore water with the overlying evaporites containing potash salts or K-rich intercrystalline brines) is responsible for the formation of the K-rich pore brines. By contrast, the origin of the oilfield brines is used to interpreted as meteoric waters according to the D-O isotopic evidence [15,20]. The meteoric waters generally experience salt dissolution, water-rock interactions, and the mixing of hydrothermal fluids when circulating the strata, supported by the Na/Cl and Br/Cl ratios, and D-O-Sr isotopes of the oilfield brines.…”

“…An abundance of shallow, K-rich brines, including the surface brines and intercrystalline brines, occur in these salt lakes (Figures 1 and 2), making the QB the largest potash fertilizer production base in China at present. In recent years, geological explorations have also reported that deep K-rich brines, including pore brines and oilfield brines, exist in the western QB (Figures 1 and 2) [13][14][15][16][17][18][19][20][21], which are of great importance for the future supply of potash fertilizers in China. All these shallow (including surface brines and intercrystalline brines) and deep brines (including pore brines and oilfield brines) in the QB have reached up to the industrial requirements (K + > 1.5 g/L for brines) in China [22].…”