Physical Properties, Pore-Throat Structure, Fractal Characteristics and Their Effects on the Gas-Bearing Capacity of Tight Sandstone: A Case Study from the Northern Tianhuan Depression, Ordos Basin, China

In this study, various testing methods were used to analyze the mineral characteristics, reservoir storage types and pore-throat network characteristics of the Permian tight sandstone in the Upper Paleozoic of the Ordos Basin. A fractal method was applied to evaluate the reservoirs’ microscopic heterogeneity. According to the results, the storage space of the sandstone reservoir is predominantly intra-granular dissolution pores and kaolinite inter-crystalline pores, while primary pores and inter-granular dissolution pores form secondary spaces with low concentration and poor homogeneity of pore-throat size distribution. All samples were divided into four categories on account of porosity (ϕ), pore-throat size distribution and multifractal dimensions (D): Type A (ϕ > 10%) consisted mainly of macropores; Type B (ϕ < 6%, consisting mainly of transitional pores and micropores); Type C1 (6% < ϕ < 10%, consisting mainly of mesopores, D > 2.45); and Type C2 (6% < ϕ < 10%, consisting mainly of mesopores, D < 2.45). In general, the piecewise fractal dimension was associated with porosity, different pore size ranges and the pore-throat parameters. Between Types A and B samples, porosity and pore size distribution were the major controlling factors on gas-bearing capacity, while the microscopic pore-throat heterogeneity had little influence. However, the microscopic pore-throat heterogeneity had a significant effect on gas-bearing capacity in Types C1 and C2 samples. This study discusses the applicable pore size range for fractal analysis of tight sandstone based on fine reservoir division and provides a novel idea for the control of the microscopic pore-throat heterogeneity on gas-bearing capacity in tight sandstone.