柴达木盆地英西地区渐新统下干柴沟组上段储层微观孔隙结构特征

Microscopic pore structure characteristics of the upper member of Oligocene Xiaganchaigou Formation in Yingxi area,Qaidam Basin

柴达木盆地英西地区渐新统下干柴沟组上段为“自生自储”型油气藏。利用X射线衍射全岩矿物含量分析、薄片鉴定、扫描电镜观察、二氧化碳吸附、氮气吸附/脱附、高压压汞、核磁共振、自发渗吸等实验手段,系统分析柴达木盆地英西地区渐新统下干柴沟组上段储层特征,精细表征了其微观孔隙结构特征。研究结果表明:(1)英西地区渐新统下干柴沟组上段储层为细粒沉积岩,其矿物成分主要为白云石和铁白云石等碳酸盐,含有长石、石英等陆源碎屑,伊蒙混层和伊利石等黏土矿物。(2)研究区储集岩整体较为致密,孔隙度为1.0%~14.5%,平均为4.0%,渗透率为0.011~6.146 mD,平均为0.125 mD,具有低孔特低渗特征。(3)研究区储集岩的孔隙类型以晶间孔、溶蚀孔、溶洞和裂缝为主,储层孔隙形态为平行狭缝型。(4)研究区微孔的质量体积平均为0.005 6 mL/g,占总孔的质量体积的34.70%;中孔的质量体积平均为0.006 6 mL/g,占总孔体积的40.84%,宏孔的质量体积相对较小,为0.003 9 mL/g,占总孔体积的24.46%,因此,对岩石储集性贡献最大的为中孔,其次是微孔。

The upper member of Oligocene Xiaganchaigou Formation(E3~2)in Yingxi area of Qaidam Basin is a self-generation and self-storage type oil and gas reservoir. By using a series of experimental means such as X-ray diffraction analysis of whole rock mineral content,thin section identification,scanning electron microscope observation,carbon dioxide adsorption,nitrogen adsorption/desorption,high pressure mercury injection,nuclear magnetic resonance and spontaneous absorption,the reservoir characteristics of E3~2 in Yingxi area were systematically analyzed and the microscopic pore structure characteristics were finely characterized. The results show that:(1)The reservoirs of E3~2 in Yingxi area are dominated by fine-grained sedimentary rocks. Its mineral composition is mainly carbonate such as dolomite and ankerite,containing terrigenous clasts such as feldspar and quartz,and clay minerals such as illite-montmorillonite mixed-layer and illite.(2)The reservoir rocks in the study area are relatively dense as a whole. The porosity is 1.0%-14.5%,with an average of 4.0%,while the permeability is 0.011-6.146 mD,with an average of 0.125 mD,which is characterized by low porosity and ultra-low permeability.(3)The pore types of the reservoir rocks in the study area are mainly intercrystalline pores,dissolved pores,caves and fractures,and the pores are parallel slit-shaped.(4)The average pore volume of micropores in the study area is 0.005 6 mL/g,accounting for 34.70% of the total pore volume. The average pore volume of mesopores is 0.006 6 mL/g,accounting for 40.84% of the total pore volume. The pore volume of macropores is 0.0039 mL/g,relatively small,accounting for 24.46% of the total pore volume. Therefore,mesopores contribute the most to rock reservoirs,followed by micropore.