Fine description of natural fractures in the tight sandstone reservoir of the Yanchang reservoirs in the southern Ordos Basin

The Ordos Basin in China hosts substantial tight sandstone gas resources, characterized by well-developed natural fractures resulting from multi-phase tectonic activities. These fractures significantly enhance porosity and serve as critical pathways for gas migration and storage. However, they also introduce complexities to hydraulic fracturing processes and influence numerical characteristics. To effectively explore and develop these reservoirs, it is imperative to conduct quantitative research on the fracture numerical characteristics of the Yanchang Formation tight sandstone reservoir. This study integrates field outcrop observations, core descriptions, thin section analyses, tectonic stress field studies, and other methods to characterize the quantitative features of the tight sandstone reservoir in the study area while elucidating fracture development characteristics and spatial distribution. Specifically, high-angle fractures predominantly exhibit dip angles exceeding 60° due to nearly vertical structures. Additionally, considering the varying influence of different lithologies on fracture development is essential. Quantifying the relationship between fracture spacing and rock thickness in the Yanchang Formation is crucial. At a microscopic level, shear-type and tensile-shear type fractures are relatively developed, with average micro-fracture densities of 0.62 and 1.59 mm⁻¹, respectively, and an average micro-length value of approximately 0.00065 m, with widths ranging from 2 to 20 μm. Understanding the formation and distribution characteristics of natural fractures is vital for optimizing the development of low-permeability oil and gas fields and enhancing production through hydraulic fracturing.