鄂尔多斯盆地延安气田山西组致密砂岩气有效储层发育模式

Effective reservoir development model of tight sandstone gas in Shanxi Formation of Yan’an gas field,Ordos Basin

通过大量岩石薄片显微观察、扫描电镜分析及流体包裹体均一温度测定,研究了鄂尔多斯盆地东南部延安气田山西组致密储层岩石学组成,划分了岩石相类型,并将不同类型岩石的成岩演化过程与埋藏史、热史、烃类充注过程相结合,分析关键油气充注期与储层致密化的时间匹配关系。研究表明:纯石英砂岩和富石英低塑性岩屑砂岩主要发育机械压实、次生溶蚀和高岭石沉淀,在二次关键烃类充注之前,孔隙度为15.8%~31.5%,属于中高渗储集岩石。而高凝灰质杂基石英砂岩、富塑性颗粒岩屑砂岩及碳酸盐致密胶结砂岩在第一次关键烃类充注之前,孔隙度为4.6%~10.8%,属于特低孔—低孔储集岩石,后期烃类充注难度较大。因此,早期油气充注时保持较高孔渗的纯石英砂岩和富石英低塑性颗粒岩屑砂岩,其成岩作用受到抑制,储层物性相对较好,是晚期天然气优势运移通道和聚集空间,进而构成了致密砂岩气储层中的甜点。研究成果对于深入理解储层致密化过程与成藏耦合关系,厘清有效储层岩石形成机理及甜点分布预测具有重要意义。

Through a large number of rock thin section microscopic observations, scanning electron microscopy analysis and fluid inclusion homogenization temperature measurement, the lithological composition of tight reservoirs in the Shanxi Formation of Yan’an gas field in the southeast of Ordos Basin was studied, lithofacies types were classified, and the diagenetic evolution of different types of rocks is combined with the burial history,thermal history, and hydrocarbon charging process to analyze the time matching relationship between the key oil and gas charging period and reservoir densification. Studies have shown that pure quartz sandstone and quartzrich low-plasticity lithic sandstone mainly develop mechanical compaction, secondary dissolution and kaolinite precipitation. Before the two key hydrocarbons are charged, the porosity is 15.8%-31.5%, which belongs to medium and high permeability reservoir rocks. The high tuffaceous heterogeneous quartz sandstone, plastic-rich granular lithic sandstone, and carbonate tight cemented sandstone have a porosity of 4.6% to 10.8% before the first key hydrocarbon charge, which belongs to ultra-low porosity-low porosity reservoirs. It is difficult to charge the hydrocarbons in the later stage. Therefore, the diagenesis of pure quartz sandstone and quartz-rich low-plastic granular lithic sandstone that maintained high porosity and permeability during early oil and gas charging has been inhibited, and the physical properties of the reservoir are relatively good. They are the dominant migration channels and accumulation spaces for late natural gas. This constitutes a sweet spot in tight sandstone gas reservoirs. The research results are of great significance for understanding the coupling relationship between reservoir densification process and accumulation, clarifying the formation mechanism of effective reservoir rocks and predicting the distribution of sweet spots.