碳酸盐倒退溶解模式的化学热力学基础——与H_2S有关的溶解介质及其与CO_2的对比

Chemical Thermodynamics Foundation of Retrograde Solubility for Carbonate:Solution Media Related to H_2S and Comparing to CO_2

以化学热力学中的吉布斯自由能增量为基础,计算了与H2S(g)/H2S(aq)/HS-/H+/S2-系统有关反应在不同温度下的平衡常数,同时根据方解石和白云石在酸性条件下的溶解过程,获得了这两种碳酸盐矿物溶解过程中地层中流体pH值与PCO2、地层压力和埋藏深度的关系。在此基础上,对比了从地表到深埋藏的温度和压力条件下,与CO2和H2S有关的酸性介质对流体[H+]贡献的差异性,以及对于碳酸盐溶解作用的差异性。计算结果表明:1)无论以CO2还是以H2S作为溶解介质,温度增加和(或)PCO2增加,都会造成方解石和白云石溶解所需要的[H+]增加,在深埋藏相对高温和高压条件下,高PCO2条件会使得碳酸盐矿物的溶解更加困难,如果[H+]受到缓冲,CO2的增加不仅不能造成碳酸盐矿物的溶解,反而会造成碳酸盐矿物的沉淀;2)无论在何种酸性介质中,碳酸盐矿物的倒退溶解模式在化学上都是成立的,低温的成岩环境,深部地层中高温流体的向上运移、构造抬升所造成的温度降低(-ΔT)都会提高H2S和(或)CO2流体(也包括其它酸性介质)对碳酸盐矿物溶解能力;3)在地表和近地表条件下(几百米深度范围内),在同时存在CO2和H2S的环境中,CO2对应酸的电离提供的[H+]略多于H2S对应酸的电离,与CO2有关的酸性流体对碳酸盐矿物溶解相对重要,而在深埋藏条件下、尤其是深度大于4000m的深埋藏地层中,H2S对应酸的电离提供的[H+]显著大于CO2,其对碳酸盐矿物的溶解更为重要,在存在硫酸盐还原作用的深埋藏地层中,碳酸盐的深部溶解作用会更为发育,这可能是川东北地区深埋藏条件下次生孔隙发育的重要原因之一。

In this paper,the equilibrium constant of the reactions related to the H2S （ g） /H2S （ aq） / HS-/ H ＋ / S2-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 CO2 and H2S 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） PCO2 increasing will lead to the elevation of[H ＋ ]that need to dissolve the calcite and dolomite whether CO2 or H2S as a dis-solvent. At relatively deep burial conditions,high PCO2 led by high fluid pressure would make the carbonate dissolution more difficult. CO2 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 （ -ΔT） and improve the solution ability of H2S and CO2（ 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 CO2 is slightly higher than that contributed by dissolved H2S when CO2 and H2S coexist. Acid fluid related to CO2 is relatively important for the dissolution of carbonate minerals,however,the [H ＋ ]contributed by dissolved H2S is much greater than that contributed by dissolved CO2 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.