Keywords: 
• NE Sichuan basin

Abstract: Sulfate reduction has been recognized recently as a process of geologic significance, and is common and widespread in the Feixianguan of Triassic, NE Sichuan Basin of China. The widespread distribution of calcite cements with extremely high strontium concentrations (more than 3 000×10-6) indicate that the digenesis of Feixianguan carbonate have been strongly influenced by an unconventional strontium-rich pore fluids. The interstitial fluid with strontium concentration much higher than normally encountered is resulted by bacterial and (or) thermochemical sulfate reduction which removed the SO42- from the system and therefore, changed the solubility of celestite (SrSO4). The strontium in diagenetic pore fluids is original from the recrystallization of metastable aragonite and highMg calcite to diagenetic lowmagnesium calcite and dolomite. This mechanism must be related to the large and superlarge scale celestite deposits of Triassic, NE Sichuan Basin of China. Both H2S and CO2 are the common products of bacterial and thermochemical sulfate reduction. The increment of Gibbs free energy (ΔG) and equilibrium constants (K) for the reactions of
H2S(aq)H2S(g) and CO2(aq)CO2(g) were calculated here. The results indicate that, in the temperature interval of 25℃ to 220℃, the equilibrium constants (K) for the two reactions increase from about 10 to 240 and from about 20 to 500, respectively, and always more than 1. Both H2S and CO2 are more preferable to gas than aqueous solution, and the solubility of H2S and CO2 drop down with the increase of temperature, so the lower ability of dissolution for carbonate of H2S and CO2 in the deeper burial environment should be expected. The lower temperature diagenetic setting, hydrothermal migration upward, the mixing of oxygenated groundwater and aqueous solution of H2S to form sulfuric acid，and metal sulfide precipitation are the four ways to enhance the ability of dissolution of CO2 and (or) H2S for carbonate. The carbonate dissolution by H2S and CO2 during earlier diagenesis is closer relation to bacterial sulfate reduction rather than thermochemical sulfate reduction. The presence of faults is an important condition for the path of H2S and CO2 fluidflow upward. The facies such as bankmargin lime grainstones or reef limestones with more primary porosity and permeability are the preferential sites for lateral fluid flow and carbonate leaching. The uplift of the crust and karstification can provide a geologic setting for the oxidation of H2S in order to enhance the leaching ability. We hope that the research conclusion here is helpful for the prediction of good quality reservoir in the Feixianguan carbonate of Triassic, NE Sichuan Basin of China.