Geochemical characteristics and origins of fracture- and vug-fillings of the Ordovician in Tahe oilfield, Tarim basin

Fracture-filling calcite and huge crystalline calcite from Ordovician carbonate reservoirs of Tahe Oilfield were measured to have ⁸⁷Sr/ ⁸⁶Sr mainly higher than bulk limestone and contemporaneous seawater, indicative of a significant contribution from external ⁸⁷Sr-rich fluid in this area. ⁸⁷Sr-rich fluid may have derived from paleo-freshwater flowing through detrital strata during Hercynian Orogeny or from deep hydrothermal fluid. Paleo-freshwater is indicated by low Sr concentrations and the relationship between λD and λ ¹⁸O. Infiltration of paleo-freshwater resulted in biodegradation of early emplaced oil and generation of organic acids and anions (OAAs) and pyrite with λ³⁴ down to -25‰. Hydrothermal fluid is indicated by relatively high Sr concentrations and significantly higher homogenization temperatures of fluid inclusions (up to 150 ̃ 190°C ) than what ambient limestone experienced, suggesting that radiogenic Sr may have derived from deeper pre-Cambrian detrital strata. Mixing of hydrothermal, basinal and likely fresh fluids led to dissolution of calcite, thus improvement of porosity in local areas.