As a kind of chemical rock,the porosity and permeability of carbonate rock is influenced obviously by diagenesis.Dissolution is directly related to the quality of carbonate reservoir.The solubility of carbonate minerals in the buried environments is closely controlled by the chemistry of groundwater,the temperature,pH value and so on.According to the chemical thermodynamics theory,Gibbs free-energy change ΔG of the equilibrium reaction of calcite and dolomite can be calculated to judge the dissolution trend of calcite and dolomite.Thermodynamic models of the reactions between the minerals and groundwater were established in a closed system.This theoretical model is used in the northern part of Tarim Basin to evaluate the dissolution trend of the Cambrian-Ordovician under the buried condition.The calculated results of ΔG from the reactions between carbonate rock and formation water are almost negative in the study area,which indicates that the buried environment is favorite for the dissolution of carbonate rock in the Cambrian-Ordovician.Gibbs free-energy of Cambrian dolomite is less than Ordovician limestone,and that means dolomite is easier to dissolve than limestone in this deeply buried condition.This conclusion is also supported by cores and previous experimental simulation.Calculation results in Yuqi area are also negative,which implies the strong precipitation of calcite in the caves in this region does not happen in present condition.This conclusion also can be proved from the cores of the Cambrian-Ordovician.%碳酸盐岩作为一种化学岩类其物性受成岩作用的影响十分明显,溶解作用是否发育直接关系到碳酸盐岩储层质量的好坏。碳酸盐矿物在埋藏环境中的溶解度与地层水的化学成分、环境的温度、pH等有着密切的关系。利用地层水化学资料,根据化学热力学的理论,通过方解石及白云石与地层水作用的平衡反应,计算吉布斯自由能变化ΔG,以此来判断方解石及白云石的溶解趋势,并建立了封闭系统中这两种矿物与水作用的热力学模型。根据这个理论模型,对塔北地区159口井奥陶系及6口井的寒武系地层水进行了ΔG的计算,结果显示,塔北地区碳酸盐岩与地层水反应的ΔG基本为负值,表明该地区总体处于碳酸盐岩溶解的环境,寒武系白云岩反应的自由能小于奥陶系灰岩,即在这种深埋条件下白云岩比灰岩更易溶解,这一点在实际岩芯资料上及前人的实验模拟中均有显示。塔北于奇地区自由能的计算结果也为负值,说明该地区强烈的充填作用并非发生在现今条件下,于奇地区现今的埋藏环境是有利于碳酸盐矿物溶解的,这一结论也在该地区奥陶系及寒武系岩芯上得到了验证。
展开▼
