白云岩溶蚀的热力学分析与次生孔隙带预测--以三塘湖盆地二叠系芦草沟组二段致密凝灰质白云岩为例

Thermodynamic analyses of dolomite dissolution and prediction of the secondary porosity zones: A case study of tight tuffaceous dolomites of the second member, Permian Lucaogou Formation reservoir, Santanghu Basin, NW China

针对三塘湖盆地二叠系芦草沟组二段致密凝灰质白云岩,研究其次生孔隙发育带的纵向分布规律与地质成因,利用热力学方法预测次生孔隙发育带横向展布。三塘湖盆地马朗—条湖凹陷纵向上存在3个次生孔隙发育带,主要是干酪根脱羧形成的有机酸和黏土矿物转化产生的无机酸共同溶蚀储集层而成。通过计算盆地内不同温度和压力条件下各种矿物溶蚀反应的自由能增量,并结合芦二段岩相分布,预测芦二段白云岩次生孔隙发育带的横向展布。计算结果表明,马朗—条湖凹陷深层白云石最易溶蚀,芦二段凝灰质白云岩溶蚀形成次生孔隙,其中马朗—条湖凹陷中央为有利溶蚀带,与半深湖—深湖相优质烃源岩重叠,有利于致密油聚集。

The vertical distribution and geological origin of secondary porosity zones have been studied in the tight tuffaceous dolomites of the second member of Permian Lucaogou Formation, Santanghu Basin, Xinjiang, China, and the lateral distribution of secondary porosity zones is predicted using the thermodynamic method. There are three secondary porosity zones in Malang-Tiaohu Sag, formed by reservoir dissolution by the acids including the organic acids generated from decarboxylation of kerogen and the inorganic acids generated from the clay mineral transformations. Gibbs free energy increments of dissolution reactions for different minerals are calculated under various pressures and temperatures to investigate the lateral distribution of secondary porosity zones, combined with litho-facies distribution of the second member of the Lucaogou Formation. Calculation result shows deeply buried dolomite strata are most prone to be dissolved and secondary pores in the second member of the Lucaogou Formation have been formed by tuffaceous dolomites. In general, the most developed secondary porosity zones with favorable tight oil reservoir potentials are located in the central Malang-Tiaohu Sag, overlapped with the high-quality source rocks that are semi-deep to deep lacustrine facies in origin.