Abstract
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.