TSR-Derived Quthigenic Calcites in Triassic Dolomite, NE Sichuan Basin, China—A Case Study of Well HB-1 and Well L-2

It has been proven that thermochemical sulfate reduction (TSR) took place extensively in the Lower Triassic carbonate reservoirs in Northeast (NE) Sichuan (四川) basin. We have carried out analyses on bulk rock compositions and isotope ratios together with petrography and fluid inclusions to assess the impact of TSR on diagenetic process of Triassic dolomites. In this article, TSR-related burial diagenesis is characterized by precipitation of calcite cement with negative δ ¹³C values and high homogenization temperature. The light carbon isotopic compositions of this phase indicate that carbon incorporated in this cement was partly derived from oxidation of hydrocarbon. The high homogenization temperatures indicate that the thermochemical reduction of sulfates has been taking place in the deep part of NE Sichuan basin. Additional evidence supporting this interpretation is the high Sr values of this calcite cement. Moreover, the calcites have aδ ¹⁸O of −8.51‰ to −2.79‰ PDB and are interpreted to have precipitated from high salinity fluids with δ ¹⁸O of +5‰ to +13‰ SMOW. Under cathodoluminescence, these calcite cements appear dark brown or black, and both Mg concentrations and Mn/Sr ratios are low. It is therefore indicated that seawater was the principal agent of precipitation fluids. Finally, it should be noted that although H₂S and CO₂ increased as TSR continued, porosity has been ultimately destroyed by calcite cementation.