Effects of mineralogy on pore structure and fluid flow capacity of deeply buried sandstone reservoirs with a case study in the Junggar Basin

Integrated experiments including porosity, permeability, casting thin section (CTS), scanning electron microscopy (SEM), pressure-controlled mercury porosimetry (PCP), rate-controlled mercury porosimetry (RCP), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) are conducted to investigate the impacts of mineralogy on the pore structure and fluid flow capacity of the deeply buried sandstone in the Jurassic Sangonghe Formation of the Junggar Basin. The results indicate that the heterogeneous deeply buried sandstone is rich in quartz (54.2%), feldspar (25.1%), and clay (14.2%), with dominant kaolinite and chlorite cementation. The reservoir has a variable overall pore-throat diameter distribution with three peaks in the ranges 0.01-1, 10-80, and 200-1000 mu m. Pores are tri-fractal and can be divided into micropores, mesopores, and macropores. The movable porosity is primarily contributed by intergranular and intragranular pores and widely ranges from 1.75 to 8.24%. Most of the fluids are movable in intergranular pores but are irreducible in intragranular pores. Correlation analyses indicate that quartz benefits the intergranular porosity preservation, increasing pore size and macropores porosity, and reducing heterogeneities of the macropores, micropores, and pore system. Feldspar exhibits poorly opposite influences due to the simultaneous clay precipitation and complex roles of feldspar dissolution in the microporosity. All of the clay minerals act as destructions to intergranular porosity, enhancing the mesopores and micropores porosities, reducing the pore size, and increasing the heterogeneities of the macropores, micropores, and pore system. Quartz is favorable for the fluid flow capacity of deeply buried sandstone, but feldspar and clay play negative roles. The reversed impacts of quartz and feldspar lay in their opposite controls on pore size. Both pore size and hydrophilia should be taken into account when considering the effects of clay. This negative effect is associated with types, contents, hydrophilic degrees of clay minerals, in which I/S and illite exhibit the strongest influences. The fluid flow in the intergranular and intragranular pores is generally enhanced by higher quartz content, and reduced by higher clay content.