Reservoir Characterization of Tight Sandstones Using Nuclear Magnetic Resonance and Incremental Pressure Mercury Injection Experiments: Implication for Tight Sand Gas Reservoir Quality

A series of experiments including porosity and permeability measurements, thin section and scanning electron microscopy (SEM) observations, incremental pressure mercury injection (IPMI), and nuclear magnetic resonance (NMR) were conducted to systematically characterize the pore structure of tight sandstone from the Lower Shihezi Formation of Permian (P₂x) in the northeastern Ordos Basin, China. The influences of pore types, pore size distribution, and fractal characteristics on reservoir quality of tight sandstones are also investigated. Results show that the studied tight sandstones generally possess poor quality and complex pore structure. The porosity and permeability range from 4.08% to 17.56% (average 9.22%) and from 0.05 to 16.66 mD (average 2.49 mD), respectively. Five pore types were observed in thin section and SEM images: primary intergranular pores, intergranular dissolution pores, intragranular dissolution pores, micropores within clay aggregates, and microfractures. The pore throats are mainly hairy/fibrous, inhibiting the connectivity between pores. Three types of pore structures were identified in the mercury-injection curves and pore size distribution curves from the IPMI experiment and in the T₂ relaxation time spectrum obtained by NMR. Both experiments yielded consistent classifications, and their combination was necessary to analyze the pore structure effectively. In general, permeability and porosity are positively related and depend on pore types. Large numbers of small pores confer high storage capacity, whereas small numbers of larger pores improve the flow capability. In the high porosity–permeability zone, larger pores also determine the storage capacity. The P₂x tight sandstone is fractal, and macropores are more heterogeneous while micropores are more homogeneous. The fractal dimensions of macropores are good indicators of the reservoir quality of the P₂x tight sandstone as larger fractal dimension values of macropores reflect poor reservoir quality.