Experimental investigation of the petrophysical properties, minerals, elements and pore structures in tight sandstones

Geological conditions on the eastern margin of the Ordos Basin are complex, and the amount of oil and gas exploration is low. In this paper, taking the S block in the eastern Ordos Basin as an example, we investigated the petrophysical properties, minerals, elements and pore structures of the middle Permian Shihezi Formation tight sandstones. The results show that the main lithologies of the target layer are lithic feldspar sandstone, lithic quartz sandstone, feldspar lithic sandstone and lithic sandstone. The pore types include residual intergranular pores, intergranular dissolution pores, intragranular dissolution pores, intercrystalline pores and microfractures. The porosity of the rock samples ranges between 1.4% and 19.5%, with an average of 9.95%; the permeability varies between 0.030 mD and 1.941 mD, with an average of 0.342 mD; and the water saturation is distributed between 1.0% and 69.5%, with an average of 30.7%. The water saturation of rock samples has negative correlations with porosity and permeability. Both displacement pressure (P-d) and median pressure (P-m) have good negative correlations with rock permeability. The median pore throat radius (r(m)) has a good positive correlation with rock permeability. Large pores with a throat radius greater than 0.1 mu m are most favorable for rock permeability. The Al/(Al + Fe + Mn) values of the rock samples range from 0.67 to 0.80, indicating that the silica in the rock was not affected by hydrothermal fluid during the formation process. The Si/(Si + Al + Fe) values of the rock samples range from 0.69 to 0.78, with an average of 0.73. Si has good positive correlations with Si/Al, Al/(Al + Fe + Mn) and Si/(Si + Al + Fe). The sedimentary environment of the Shihezi Formation sandstone was characterized by fresh water, a dry-hot climate and oxidizing conditions. The major elements correlate well with the pore structure parameters: as the debris particles or quartz grains increase, the petrophysical properties gradually improve. Finally, based on the petrophysical properties, minerals, elements and pore structure parameters, classification criteria for the pore structure of the Shihezi Formation tight sandstones in the study area are proposed.