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.
|Number of pages||6|
|Journal||Acta Petrologica Sinica|
|Publication status||Published - 2009|
- Hydrothermal fluid
- Strontium isotopes
- Sulphur isotopes
- Tarim basin