成岩过程中碳酸盐-二氧化碳平衡体系的热力学模拟

Thermodynamic simulation of carbonate-carbon dioxide equilibrium system during diagenetic processes.

碳酸盐矿物的溶解与沉淀在沉积岩成岩作用中具有非常重要的意义,在与之有关的地层流体中,CO_2组分受到了广泛关注。本文选取CaCO_3(方解石)-H_2O-CO_2与CaMg(CO_3)_2(白云石)-H_2O-CO_2体系作为研究对象,利用热力学方法在假设体系达到平衡的前提下对100～7000m深度范围的温度压力条件下的CaCO_3-H_2O-CO_2与CaMg(CO_3)_2-H_2O-CO_2体系中各组分的平衡浓度([H^+]、[Ca^(2+)]、[Mg^(2+)]……)进行了数值模拟,并对不同埋藏深度的温度压力下方解石、白云石的溶解与沉淀,流体对CO_2的吸收与释放,pH变化等相关地质过程进行了讨论。研究表明:随埋深增加,体系的pH值越来越趋于酸性,但碳酸盐并没有持续溶解,而是呈先溶解后沉淀的趋势。对比CaCO_3-H_2O-CO_2体系中碳酸盐的溶解量和沉淀量,可将小于700m的深度范围定义为碳酸盐溶解带,700～2500m的深度范围定义为补偿带,大于2500m的深度范围定义为沉淀带。对比两个体系的固体相体积变化发现,含CO_2的溶蚀性流体虽然不能在白云岩地层中溶出较灰岩地层更多的阳离子,但会产生更高的孔隙度。

Dissolution and precipitation of carbonates tire important processes during burial diagenesis. CO(2)-bearing fluids always play art important role in these geological processes. Therefore, in this paper, we established the thermodynamic model of pure CaCO(3)-H(2)O-CO and CaMg(CO(3))(2)-H(2)O-CO(2) Systems theoretically. Moreover, based on an assumption that these two systems have reached equilibrium and have buried over a range of 100 to 7000m depth, the concentrations of all dissolved species ([H(+)], [Ca(2+)], [Mg(2+)] center dot center dot center dot) in the aqueous solution from these systems were calculated. By doing so, we simulated the solubility of calcite/dolomite, the concentration of CO(2) absorbed/released and pH evolution in the CO(2)-H(2)O solutions at different burial depth. The results show that, although the solution tends to be more acidic with depth increasing, it does not lead carbonates to dissolve continuously: the [Ca(2+)] increases at shallower depths and decreases at deeper depths. According to the dissolved and precipitated amounts of CaCO(3), the burial depths shallower than 700m, between 700 and 2500m and deeper than 2500m, can be zoned as dissolution range, compensation range and precipitation range, respectively, for CaCO(3)-H(2)O-CO(2) system. By comparing the dissolved solid volume in two systems, we found that. although the number of dissolved moles of dolomite in CO(2)-bearing fluid is not more than that of calcite, but higher porosity can be formed within the dolomite formation.