Pore Structure and Fractal Characteristics of Tight Sandstones Based on Nuclear Magnetic Resonance: A Case Study in the Triassic Yanchang Formation of the Ordos Basin, China

[Image: see text] The characterization of the pore structure of tight sandstones is of great importance for the exploration and development of tight oil reservoirs. However, little attention has been given to the geometrical features of pores with various scales, which implies that the effect of pores on the fluid flow and storage capacity is still ambiguous and presents a significant challenge to the risk assessment of tight oil reservoirs. This study investigates the pore structure characteristics of tight sandstones by applying thin section petrography, scanning electron microscopy, nuclear magnetic resonance, fractal theory, and geometric analysis. The results indicate that the tight sandstones have a binary pore system, consisting of small pores and combine pores. A shuttlecock model expresses the shape of the small pore. The radius of the small pore is comparable to the throat radius, and the connectivity of the small pore is poor. A spiny spherical model describes the shape of the combine pore. The connectivity of the combine pore is good, and the pore radius is larger than the throat radius. The most significant contribution to the storage space of the tight sandstones is attributed to the small pores, while permeability is primarily controlled by the combine pores. The heterogeneity of the combine pore has a strong positive correlation with flow capacity, which is associated with the multiple throats of the combine pores that developed during diagenesis. Therefore, the sandstones that are dominated by combine pores and are located near the source rocks represent the most favorable area for the exploitation and development of tight sandstone reservoirs.