基于热力学与动力学方法预测碎屑岩的次生孔隙发育带

摘要

The fourth member of Quantou Formation is composed of K-rich feldspathic litharenite and lithic feldspar sandstones in northern Songliao Basin. As the hydrocarbon accumulation sites, high-quality reservoirs occur in the secondary porosity which mainly caused by the dissolution of K-feldspar. In order to predict the distribution of secondary porosity zones, the burial, thermodynamics and organic matter thermal evolutionary histories were restored through basin modeling based on the formation mechanism of the secondary porosity. On the basis of the diagenetic environment and fluid properties,the influences of Gibbs free energy (ΔG) and dissolution velocity (v) on the secondary porosity were analyzed, and the dissolution-precipitation state and development degree of secondary porosity were discussed applying the temperature, pressure, potassium concentrations and pH of the porewater. The results show that the secondary pores are formed in the Nenjiang and Mingshui stage, and dissolution still occurs in most areas. The secondary pores are formed in the reservoir with ΔG<0 kJ/mol and high K-feldspar dissolution velocity. Moreover, the secondary pore developmental zones are mainly located in the northwest and southeast of Central Depression with ΔG<−38 kJ/mol. However, the secondary pore relatively developmental zones with low dissolution velocity are mainly located in the middle(A2)and southern of Central Depression with −38≤ΔG<0 kJ/mol.%松辽盆地北部下白垩系泉四段为富钾长石碎屑岩，油气主要聚集在以钾长石溶蚀孔为主的次生孔隙发育带中。为了预测其次生孔隙发育带的分布，从次生孔隙的形成机制出发，应用盆地模拟技术，恢复泉四段埋藏史、热史和有机质热演化史等。以成岩环境和流体性质为基础，应用次生孔隙形成时期的古地温和古压力，结合地层水中K+和H+质量浓度，计算钾长石溶蚀反应的Gibbs自由能增量(ΔG)和反应速率(v)，讨论钾长石溶解—沉淀状态以及次生孔隙发育程度。研究结果表明：泉四段的次生孔隙主要形成于嫩江期和明水期，且现今大部分区域仍发生溶蚀作用。钾长石自发反应(ΔG＜0 kJ/mol)且溶解速率快的砂体易发育次生孔隙。松辽盆地北部泉四段储层的次生孔隙发育带主要位于中央坳陷区的西北和东南部，处于ΔG＜−38 kJ/mol的砂体发育区。然而，位于中央坳陷的中部和南部，发育中成岩阶段A2亚期及−38≤ΔG＜0 kJ/mol的砂体，钾长石溶蚀反应速率低，次生孔隙发育较好。