江汉盆地白垩系渔洋组砂岩的成岩作用及其热力学分析

Diagenesis of the Cretaceous sandstone in Yuyang Formation of Jianghan basin and its thermodynamic analyse.

江汉盆地白垩系渔洋组砂岩存在五种类型的成岩作用。其中,与油气运移有关并对储层孔隙演化产生重要影响的是硬石膏的胶结与交代作用和后期方解石再次胶结作用。为了更好的研究成岩作用发生的控制因素以及成岩环境的变化,我们通过细致的热力学推导,绘制出包含方解石、白云石和硬石膏等矿物相在内的五组分相图。通过相图分析可以看出,pH 值在成岩作用过程中有着重要的作用。方解石和白云石的胶结及交代作用代表了偏中性或碱性的成岩环境;而硬石膏的胶结及交代作用则表明成岩环境由中性或碱性向酸性转变,例如酸性的油气前锋流体的注入。另外,成岩环境中阳离子(Ca^(2+)、Mg^(2+))和阴离子(CO_3^(2-)、SO_4^(2-))的浓度对方解石、白云石和硬石膏的胶结、溶解及交代作用也有重要影响。

Sandstones diagenesis in the Cretaceous Yuyang Formation, Jianghan Basin, China is charactered by cementation and replacement. Optical microscopy and other petrological studies have revealed four main diagenetic events. They include early carbonate cementation （mainly dolomite） , secondary growth of Fe-dolomite, anhydrite cementation and replacement and a phase of late carbonate （ mainly calcite） cementation and replacement. Thermodynamic modeling has been used to analyse the evolution of the geochemical environment during diagenesis. CaO, MgO, H2O, SO2 and CO2 were considered as major components in a closed diagenetic system. Thermodynamic phase diagrams of the five-component system were calculated, in order to understand the controlling factors and processes. Under certain restricted conditions extracted from measured data, assemblages of mineral phases appearing in each diagenetic event were compared with the phase diagrams. It could be shown that the pH of the solution plays an important role in the diagenesis process. Cementation and replacement of carbonates （ both, calcite and dolomite ） were caused by a higher pH, while the cementation and replacement of anhydrite occurred at lower pH conditions. For example, acidification of pore fluids by intrusion of organic acids as a result of oil migration leads to a cementation and replacement of anhydrite. The influence of the concentrations of related ions （ Ca^2＋ , Mg^2＋ , CO3^2-, SO4^2- ） in the solution on the diagenetic processes, will also be discussed.