Characteristics and controlling factors of meso-macropore pore structure of Permian Lucaogou Formation reservoir in Jimsar Sag, Junggar Basin

Abstract

The shale oil resource potential of the Jimsar Lucaogou Formation is enormous. To investigate the reservoir characteristics and controlling factors of its meso-macroporous pore space, the drilling cores of the Lucaogou Formation in the study area were used to discuss its reservoir characteristics, pore structure and controlling factors using X-ray diffraction, cast body thin section, scanning electron microscopy, pore penetration test, high-pressure mercury pressure, nitrogen adsorption, fractal theory and correlation analysis. The study found that: 1）the reservoir of the Lucaogou Formation in the study area is a set of low-pore-extra-low-pore, extra-low-permeability reservoirs dominated by feldspathic-quartziferous carbonates （A）, feldspathic-quartziferous claystone （B）, calcitic/dolomitic fine-grained feldspar-quartz sedimentary rocks （C） and mixed sedimentary rocks （D）, with the main pore type being solution pores, in addition to the development of remaining intergranular pores, intergranular pores and microfractures. 2）The pore distribution of the reservoir of the Luchaogou Formation in the study area is complex, and the pore size distribution curve shows a multi-peaked shape, with mesopores providing most of the pore volume and specific surface area; the average values of the fractal dimension of macro-pores calculated by high-pressure mercury compression and the average values of the fractal dimension of mesopores calculated by nitrogen adsorption are 2.864 7 and 2.507 5, respectively, and the complexity of the mesopore pore structure is relatively low. 3）The abundance of organic matter favors the development of meso-macropores of A, meso-macropores of B and macropores of C, which leads to a reduction in pore volume and specific surface area of the mesopores of C and D rocks; the degree of organic matter evolution has a building effect on the mesopores of A, meso-macropores of B and mesopores of C, and a destructive effect on the macropores of C and meso-macropores of D. 4）The higher the content of brittle minerals the greater the specific surface area of the mesopores of C and the meso-macropores of D, while the pore volume and specific surface area of the meso-macropores of A, the macropores of B and the macropores of C decrease; clay minerals have a building effect on the mesopores of A, the mesopores of C and the meso-macropores of D, and a damaging effect on the macropores of B and the macropores of C. The development of pore structure is mainly influenced by organic matter（organic matter abundance, degree of organic matter evolution） and mineral composition （brittle mineral content, clay mineral content）, and the influencing factors of pore development vary from lithology to lithology and from scale to scale.