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

Abstract

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