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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 δ13C values and high ho-mogenization 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 homogeniza-tion 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 a δ18O of-8.51‰ to-2.79‰ PDB and are interpreted to have precipitated from high salinity fluids with δ18O of +5‰ to +13‰ SMOW.Under cathodolumi-nescence,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.Fi-nally,it should be noted that although H2S and CO2 increased as TSR continued,porosity has been ul-timately destroyed by calcite cementation.
It has been proven that thermochemical sulfate reduction (TSR) took place extensively in the Lower Triassic carbonate reservoirs in Northeast (NE) Sichuan (Sichuan) basin. We have carried out analyzes 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 δ13C values and high ho-mogenization temperature. The light carbon isotopic compositions of this phase that carbon incorporated in this cement was partly derived from oxidation of hydrocarbons. The high homogeniza-tion temperatures indicate that the thermochemical reduction of sulfates has been taking place in the deep part of NE Sichuan basin. Additional conditional supporting this interpretation is the high Sr values of this calcite cement.Moreover, the calcites have a δ18O of-8.51 ‰ to-2.79 ‰ PDB and are interprete d to have precipitated from high salinity fluids with δ18O of +5 ‰ to + 13 ‰ SMOW.Under cathodolumi-nescence, 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.Fi-nally, it should be noted that although H2S and CO2 increased as TSR continued, porosity has been ul-timately destroyed by calcite cementation.