Water geochemistry in the Bozhi-Dabei area, Tarim Basin and its implications for natural gas accumulation in deep basin

Recently, numerous gas fields have been discovered in deep basins worldwide. However, the geochemistry of the produced water has been poorly studied. Here, we present the chemical and isotopic compositions of water produced from the deep (4–7 km) Bozhi–Dabei (BD) gas field, Tarim Basin, China. The data indicate that the produced water is a mixture of formation water and water condensed from gas. The latter of which has contents of lower total dissolved solids (TDS), different ionic compositions and more negative δD and δ18O values than that of formation water. The formation water in the Lower Cretaceous sandstone reservoirs consists mainly of meteoric water that was strongly affected by the dissolution of Paleogene halite, forming a regional seal. Meteoric water infiltrated the Kumugeliemu halite during the Kangcun–Kuqa stage (16.3–1.64 Ma, Middle Miocene–Late Pliocene interval). A smaller portion of the formation water consists of iodine-rich evaporated seawater that migrated together with gas from transgression-influenced layers in Triassic–Jurassic source rocks. Using the 129I dating model, different episodes of this migration in the Dabei (DB) East District were recognized, although the dating results were not precise. This study enhances our understanding of the migration and accumulation processes of formation water and natural gas in deep basins and suggests that the chemical and isotopic composition of formation water can be used together with that of gas to constrain the sources and accumulation processes of deeply buried gas.