Influence of thermochemical sulfate reduction on oxygen isotopic composition of calcite cements in carbonates of the Triassic Feixianguan and Permian Changxing formations in the Sichuan Basin, China

Calcite cement is a common diagenetic mineral in carbonate rocks and plays an important role on rock quality as hydrocarbon reservoirs. Traditionally, oxygen isotopic compositions (δ 18 O) of the calcites tend to decrease with increasing depths due temperature-dependent isotope fractionation. In this study, stable calcite cements Changxing Feixianguan formations from Puguang, Yuanba, Jiannan Fuling fields Sichuan Basin were analyzed. The results show that some have δ O values similar those their host carbonates, despite fact these formed at elevated temperatures (>∼100°C). Based petrographic geochemical analyses, O-enriched commonly occur solid bitumens lower 13 C compared rocks, suggesting thermochemical sulfate reduction (TSR) provided organic carbon for precipitation. During TSR, thermal oxidation hydrocarbons generated light carbon, simultaneously reduced ions oxygen. Comparison our study TSR worldwide, model behavior during was established. Oxygen TSR-related are function amounts initial anhydrite pore waters. shows two opposing effects calcites, depending ratios anhydrite. relatively low causes than theoretical directly precipitated heavy not only attributed water resulting extensive water-rock interaction, but also probably involvement high values.