致密砂岩储层特征及质量影响因素——以鄂尔多斯盆地西北部二叠系下石盒子组为例

Characteristics of tight sandstone reservoir and influencing factors of reservoir quality:a case study in Lower Shihezi Formation of Permian,Northwestern Ordos Basin

以鄂尔多斯盆地西北部二叠系下石盒子组致密砂岩为研究对象,基于铸体薄片、扫描电镜观测结果,采用核磁共振和氮气吸附孔隙度、渗透率测试等方法,分析下石盒子组致密砂岩储层特征及质量影响因素。结果表明:鄂尔多斯盆地西北部二叠系下石盒子组储层主要由长石岩屑砂岩及岩屑砂岩组成,属于典型的致密砂岩。快速埋藏是储层致密化的最主要原因,压实作用是造成储层质量降低的最主要因素,原生孔隙度下降率为58%;胶结作用进一步使砂岩储层致密,是导致储层质量降低的次要原因,原生孔隙度下降率为24%;成岩作用形成方解石和铁方解石并充填粒间孔隙,储层质量降低。长石的溶蚀提供广泛粒间孔隙,以及刚性石英颗粒的抵抗压实保护部分原生孔隙,改善储层质量。该结果为其他河流相致密砂岩盆地的储层研究提供参考。

The tight sandstone of Lower Shihezi Formation of Permian in Northwestern Ordos Basin is taken as the research object,based on the observations of cast thin slice,scanning electron microscopy,nuclear magnetic resonance and nitrogen adsorption porosity permeability test,the characteristics of tight sandstone reservoirs in Lower Permian Shihezi Formation and the factors affecting reservoir quality were analyzed.The findings indicate that the Lower Shihezi Formation reservoir of Permian in the study area predominantly comprises feldspar lithic sandstone and lithic sandstone,representing a typical example of tight sandstone.Rapid burial is the primary cause of reservoir densification,while compaction plays a crucial role in reducing reservoir quality,resulting in a significant loss of 58%in primary porosity.Cementation further compacts the sandstone reservoir,resulting in a secondary reduction of reservoir physical properties and loss of 24%in primary porosity.Other factors diminishing reservoir quality include the occlusion of intergranular pores through diagenetic processes such as calcite and iron calcite formation.The factors influencing the improvement of reservoir quality include extensive intergranular porosity provided by the dissolution of feldspar and the protection of some primary pores by the resistance of rigid quartz particles to compaction.These phenomena can serve as a valuable reference for reservoir research in other fluvial tight sandstone basins.