砂岩成分对储层孔隙结构及天然气富集程度的影响——以苏里格气田西区二叠系石盒子组8段为例

鄂尔多斯盆地苏里格气田西区二叠系石盒子组8段是典型的致密砂岩储层,同时又具有低效充注、普遍产水的特点,其天然气有利勘探区的寻找不仅依赖于优质储层的预测,还需要对储层的含气性进行分析。为此,利用扫描电镜、CT扫描、覆压孔渗、激光拉曼等技术手段,对该区不同类型砂岩的微观特征差异及其对天然气富集程度的影响进行了研究。结果表明:(1)该区盒8段发育岩屑石英砂岩与岩屑砂岩2种类型储层,由于岩屑颗粒密度小且易破碎,在强水动力作用下不易富集,因此岩屑砂岩以细粒沉积为主;(2)岩屑石英砂岩中主要发育残余粒间孔与铸模孔,其喉道为石英颗粒间的孔隙,渗透率在覆压下损失小,而岩屑砂岩主要发育微孔,喉道以杂基内束状喉道为主,渗透率在覆压下损失大;(3)天然气充注时,岩屑砂岩经过压实作用已完成致密化,不利于天然气充注,而相反,因石英次生加大而致密的岩屑石英砂岩,天然气充注时其致密化过程尚未完成,物性与孔隙结构优于现今,是天然气的优势富集区。

Influence of sandstone composition on pore structures and gas enrichment degree: A case study on the 8th member of Permian Shihezi Fm in the western Sulige Gas Field,Ordos Basin

The 8^th member of Permian Shihezi Fm ("He 8 Member" for short) in the western Sulige Gas Field, Ordos Basin, is a typical tight sandstone reservoir with the characteristics of low-efficiency charging and extensive water production. In order to search for favorable gas exploration zones, therefore, it is necessary to predict high-quality reservoirs and analyze their gas-bearing potentials. In this paper, the microscopic features of various sandstones and their effect on natural gas enrichment degree were analyzed by means of scanning electron microscopy (SEM), CT scan, permeability overburden pressure and laser Raman technologies. It is shown that two types of reservoirs are developed in He 8 Member in this area, namely lithic quartzose sandstones and lithic sandstones. Lithic grains have low density and can be broken easily, so they may not accumulate under the action of strong hydrodynamic force. Therefore, lithic sandstones are deposited mainly in the form of fine grains. In lithic quartzose sandstones, residual intergranular pores and moldic pores are mainly developed with the pores between quartz grains as the throats and its permeability is less lost under an overburden pressure. In lithic sandstones, micropores are mainly developed with the intra-matrix fasciculate throats as the dominant, and its permeability is more lost under an overburden pressure. Lithic sandstones could not be charged with natural gas since they were completely tight under the compaction during gas charging. In lithic quartzose sandstones which got tight due to quartz overgrowth, however, gas charging could be fulfilled since the tightness process was ongoing, so their physical properties and pore structures were better at that time than nowadays and they were predominant gas accumulation zones.