Massive dolomitization driven by MgSO₄-rich seawater and its effects on thermochemical sulfate reduction, Upper Permian Changxing Formation, northeastern Sichuan, China

Massive dolostones replacing build-ups and containing H2S of thermochemical sulfate reduction (TSR) origin occur in the Upper Permian reef bank deposits, northeastern Sichuan, China, despite absence gypsum or anhydrite which are usually present TSR cases. Fluid chemistry from fluid incluions δ13C, δ18O 87Sr/86Sr values different diagenetic phases were measured to determine dolomitization regime, assess relationships between occurrence accumulation. Dolomitization was initiated by seawater with slightly increased salinities (penesaline) at shallow depths prior chemical compaction. The micro- fine-crystalline cloudy dolomite formed relatively high Na Sr contents, (2.2‰∼4.8‰, −3.9‰∼-5.0‰ 0.70724∼0.70746, respectively) inherited seawater. evaporation back-reef inter-reef lagoons during sea-level fall subsequent seepage reflux into reef-beach bodies led greater Mg 2+ SO 4 2− concentrations higher-salinity pore waters. Further massive promoted compactional flow hotter residual intermediate resulted formation fine- medium-crystalline clean lower more depleted δ 18 O (-5.1‰∼-6.0 ‰), andδ 13 C (2.5‰∼4.8 ‰) 87 Sr/ 86 (0.70726∼0.70741) similar those coeval closed hydrodynamic system burial facilitated preservation. whole process enriched porewater , have been almost exhausted TSR, accounting for present-day H 2 S. output this study shows that scenarios involving driven condensed MgSO S accumulations can evaporite-free settings across a broad range deserves special attention deep oil/gas exploitation.