Origin and evolution of formation water from the Ordovician carbonate reservoir in the Tazhong area, Tarim Basin, NW China

Abstract

Chemistry, H, O and Sr isotopes of formation waters were determined from the Ordovician carbonate reservoirs in the Tazhong area, Tarim basin. The aim is to elucidate their origin and migration and their alteration by thermochemical sulfate reduction (TSR) and water-rock interactions. The waters were originated from evaporated seawater, and are enriched in Ca and Sr, and depleted in Mg and SO4 compared with the seawater evaporation trajectory (SET), thus are considered to result from dolomitization, anhydrite dissolution and in situ TSR. δ18O values show positive shift due to water-rock interactions, and δD values show negative shift with increasing TSR extents or total thiaadamantanes concentrations of the associated oils, likely resulting from hydrogen isotope exchange with TSR-H2S. However, the δD-δ18O relationship was mainly controlled by the mixing with paleo-meteoric water with δD of −50‰ and δ18O of −7.5‰. Paleo-meteoric water may have leached the Silurian siliciclastic rocks and thus has 87Sr/86Sr up to 0.715. Upper Ordovician (O3l) formation water shows more negative δD and δ18O values, higher 87Sr /86Sr ratios and lower total dissolved solids (TDS) than the O1–2y water, which may have resulted from mixing of paleo-meteoric water. Lower Ordovician (O1–2y) formation water shows 87Sr /86Sr ratio of 0.7095–0.7105, TDS from 160 to 240g/l, and Sr concentrations from 300 to 1000mg/L, which may have resulted from mixing with hydrothermal fluid up-migrated from Ediacaran and Lower Cambrian siliciclastic rocks. Significant mixings with paleo-meteoric water and hydrothermal fluid occurred at paleo-highland at the east of No.1 Fault-Slope Zone and active faulting zone near No.10 Structural Belt, respectively. Hydrothermal fluid and oils probably shared similar migration pathways in No.10 Structural Belt. Formation water geochemistry may thus provide clues to constraint on petroleum migration.