Origin of Paleofluids in Dabashan Foreland Thrust Belt: Geochemical Evidence of 13C, 18O and 87Sr/86Sr in Veins and Host Rocks

Abstract:In the last ten years, with important discoveries from oil and gas exploration in the Dabashan foreland depression belt in the borderland between Shanxi and Sichuan provinces, the relationship between the formation and evolution of, and hydrocarbon accumulation in, this foreland thrust belt from the viewpoint of basin and oil and gas exploration has been studied. At the same time, there has been little research on the origin of fluids within the belt. Based on geochemical system analysis including Z values denoting salinity and research on δ13C, δ18O and 87Sr/86Sr isotopes in the host rocks and veins, the origin of paleofluids in the foreland thrust belt is considered. There are four principal kinds of paleofluid, including deep mantle‐derived, sedimentary, mixed and meteoric. For the deep mantle‐derived fluid, the δ13C is generally less than –5.0‰PDB, δ18O less than –10.0‰PDB, Z value less than 110 and 87Sr/86Sr less than 0.70600; the sedimentary fluid is mainly marine carbonate‐derived, with the δ13C generally more than –2.0‰PDB, δ18O less than –10.0‰PDB, Z value more than 120 and 87Sr/86Sr ranging from 0.70800 to 0.71000; the mixed fluid consists mainly of marine carbonate fluid (including possibly a little mantle‐derived fluid or meteoric water), with the δ13C generally ranging from –2.0‰ to –8.0‰PDB, δ18O from –10.0‰ to –18.0‰ PDB, Z value from 105 to 120 and 87Sr/86Sr from 0.70800 to 0.71000; the atmospheric fluid consists mainly of meteoric water, with the δ13C generally ranging from 0.0‰ to –10.0‰PDB, δ18O less than –8.0%cPDB, Z value less than 110 and 87Sr/86Sr more than 0.71000. The Chengkou fault belt encompasses the most complex origins, including all four types of paleofluid; the Zhenba and Pingba fault belts and stable areas contain a simple paleofluid mainly of sedimentary type; the Jimingsi fault belt contains mainly sedimentary and mixed fluids, both consisting of sedimentary fluid and meteoric water. Jurassic rocks of the foreland depression belt contain mainly meteoric fluid.