致密砂岩储层自生矿物特征及其对储集性能的影响——以鄂尔多斯盆地陇东地区长3油组为例

Authigenic minerals and their influences on the physical properties of tight sandstone reservoirs: An example from the Chang-3 oil reservoirs in the Longdong region,Ordos basin

鄂尔多斯盆地陇东地区长3油组是近年来该区油藏评价新层系,分析砂岩中自生矿物特征对储层致密化机理研究有重要的意义。长3油组主要发育岩屑长石砂岩和长石岩屑砂岩,平均孔隙度为13.35%,渗透率为4.90×10-3μm2,为低孔低渗透致密储层,目前砂岩处于中成岩阶段A期。通过铸体薄片、扫描电镜、阴极发光、电子探针、X射线衍射等技术和手段,研究自生矿物的含量、类型、分布及成岩共生序列等特征。研究结果表明,早期绿泥石包膜有利于原生孔隙的保存,后期孔隙式充填降低了岩石孔隙度和渗透率;伊利石易堵塞孔隙喉道,导致储层物性变差;高岭石沉淀往往与长石溶解伴生,局部溶蚀作用强烈;晚期含铁碳酸盐胶结作用发育,镜下溶蚀现象少见,对储层的发育有破坏作用;石英次生加大边和自生晶粒占据了大量孔隙空间,同时石英也增强了岩石的抗压能力,对孔渗影响不大。总体上,长3油组自生矿物胶结对储集物性起负面效应,是砂岩储层致密化的主要因素之一。

The Chang-3 oil reservoirs in the Longdong region are recently selected as the important oil reservoirs in the Ordos basin. The study of the authigenic minerals is significant to the study of the densification mechanism of the oil reservoirs. The Chang-3 oil reservoirs consist of lithic arkose and feldspathic lithoarenite,and are interpreted to be the low-porosity and low-permeability reservoirs with an average porosity of 13. 35% and average permeability of 4. 90 × 10-3μm2,belonging to the A stage of the middle diagenesis. The contents,types,distribution and diagenetic sequences are discussed in the present paper on the basis of cast section,SEM,CL images and X-ray diffraction analysis. The early chlorite coatings facilitate the preservation of the primary porosity,and late pore filling may reduce the porosity and permeability of the sandstones. The mineral illite is prone to block the pores and throats,resulting in poor reservoir properties. The precipitation of kaolinite is often associated with the dissolution of feldspar, and partial strong dissolution can be seen under the microscope. The iron-bearing carbonate cementation is developed in the later stages,and destructive to the development of the sandstone reservoirs. The quartz overgrowths and authigenic grains may take up much of the pore spaces,and in turn,strengthen the pressure resistance. On the whole,the cementation of the authigenic minerals in the Chang-3 oil reservoirs has the negative effects on the reservoir properties,and also is one of the controlling factors of densification mechanism of the oil reservoirs.