Peng Yang

and 6 more

Reconstructing fluid evolution history in old sedimentary basins is extremely challenging due to the prolonged evolution, lack of exact age constraints and absence of preserved fluids. By integrating in-situ calcite U-Pb dating, fluid inclusion analysis, and high-resolution measurements of carbon-oxygen-strontium isotopes and rare earth elements (REEs), we have reconstructed the fluid evolution history in an Ordovician deeply-buried carbonate reservoir in the Tarim Basin, China with unprecedented temporal resolution. Five generations of calcite cementation spanning over 37 m.y. are dated in a single calcite vein with U-Pb ages ranging from ~353 Ma to ~316 Ma. The initial cement was associated with an intrusion of hyper-saline brine enriched REEs. This was followed by influx of meteoric water to precipitate the second and third generations of cement, characterized by a reduced salinity, anomalously high Sr isotopic ratios and extremely low concentrations of REEs. The fourth generation of cement was formed in a deep-fluid system accompanied by oil charge as indicated by the presence of bitumen, oil inclusions and lighter C and Sr isotope ratios. The fifth generation of cement was precipitated from an equilibrated connate water, having the lowest salinity and a comparable REE signature with the host rock. Oil charge was dated to be around 326 Ma. This study demonstrates the effectiveness in combining in-situ calcite U-Pb geochronology, fluid inclusion analysis and other geochemical analyses to elucidate high-resolution temporal fluid evolution in an old sedimentary basin with a complex tectonic history, providing a new paradigm for studying geofluid evolution.