致密砂岩有效储层形成的控制因素

在大面积分布的致密砂岩气藏中，通常局部发育相对优质的有效储层。在对比分析四川盆地广安须家河组气藏及鄂尔多斯盆地苏里格石盒子组八段和山西组一段气藏基础上，主要从沉积分异和成岩作用2方面研究了有效储层形成的控制因素，得出以下认识：物源体系和水动力环境差异造成的沉积非均质性强烈影响成岩作用，是有效储层形成的基础，高能复合水道中的“粗岩相”利于有效储层的形成;压实作用和胶结作用是砂岩致密化的主要机制，以粗粒级的石英颗粒为主的砂（砾）岩，利于原生孔隙的保存，并作为后期成岩流体运移的通道，但在煤系地层酸性成岩环境中，较纯的石英砂岩容易发生石英次生加大，从而显著降低了砂（砾）岩的孔隙度;自生黏土矿物可能会降低储层渗透率，但自生黏土矿物环边能阻止石英次生加大而保存原生孔隙;溶蚀作用是改善储层质量的主要机制，受长石等易溶矿物的分布、成岩流体性质和流体通道的共同控制。

Control factors on the formation of effective reservoirs in tight sands:examples from Guang'an and Sulige gasfields

Among widely-distributed low-permeability and low-porosity tight sand gas reservoirs, there usually develop some local reservoirs with relatively higher qualities. In order to understand control factors on the genesis of favorable tight gas reservoirs, we comparatively studied tight gas reservoirs in two intracratonic basins（Sichuan and Ordos Basin) in the sense of depositional fractionation and diagenesis. Sedimentary and diagenetic characteristics of the Xujiahe Formation in Guang’an, Sichuan Basin and the He-8 and Shan-1 members in Sulige, Ordos Basin were examined in detail. The results showed that:（1) the sedimentary heterogeneity determined by differences in provenance and hydrodynamic environments affects diagenesis significantly, which forms a basis for the formation of effective reservoirs. “Coarse-lithofacies” sandstones in stacked high-energy channels are thought to be favorable to forming high-quality reservoirs;（2) compaction and cementation are the main causes of reservoir quality reduction;（3) rigid and coarse grains such as quartz may contribute to retaining more initial intergranular pores by resisting compaction during early diagenesis, which can be pathways for diagenetic fluids in late diagenesis, but quartz overgrowth in a coal-measure acidic diagenetic environment may largely reduce the reservoir quality during late diagenesis;（4) authigenic clay minerals may reduce the permeability of reservoirs, while clay coatings may help retain initial intergranular pores by preventing quartz overgrowth; and（5) secondary dissolution controlled by dissolvable minerals, diagenetic fluids and fluid pathways is the main cause for enhancing reservoir quality. As for the studied tight gas reservoirs, sandstone diagenesis is strongly controlled by the initial heterogeneity derived from sedimentation process, which consequently controls the formation of favorable reservoirs as well. In order to predict the distribution of favorable reservoirs, sedimentary characteristics should be carefully studied.