Chemical Thermodynamics Foundation of Retrograde Solubility for Carbonate:Solution Media Related to H2S and Comparing to CO2

Key words: 
• retrograde solubility

Abstract: In this paper, the equilibrium constant of the reactions related to the H2S (g) /H2S (aq) / HS / H+ / S2 system at various temperatures has been calculated as a foundation of increment of Gibbs free energy. The relationship between pH, fluid pressure and buried depth during the dissolution of calcite and dolomite in acetic medium are attained. The different contributions of CO2 and H2S to \[H+\] and different dissolution ability to carbonate of the two media under the conditions from surface to deep buried are described as follow: 1) Temperature and (or) PCO2 increasing will lead to the elevation of \[H+\] that need to dissolve the calcite and dolomite whether CO2 or H2S as a dissolvent. At relatively deep burial conditions, high PCO2 led by high fluid pressure would make the carbonate dissolution more difficult. CO2 increasing may cause the precipitation of carbonate minerals instead of the dissolution if \[H+\] was buffered; 2) The retrograde solubility model of carbonate minerals is tenable on chemistry no matter what type of acid medium. Low temperature diagenesis environment, upward migration of deep hydrothermal and the tectonic uplift would result in the temperature decreasing (ΔT) and improve the solution ability of H2S and CO2 (including other acid medium) to carbonate minerals; 3) In surface and near surface (the depth range of several hundred meters), the \[H+\] contributed by dissolved CO2 is slightly higher than that contributed by dissolved H2S when CO2 and H2S coexist. Acid fluid related to CO2 is relatively important for the dissolution of carbonate minerals, however, the \[H+\] contributed by dissolved H2S is much greater than that contributed by dissolved CO2 in deep burial, especially when the depth is more than 4000 m. Deep dissolution of carbonate would be more developed in deep buried condition where sulfate reduction exists, which could be one of the important reasons why secondary porosity develop in deep burial, NE Sichuan.