Quantitative characterization of complex multi-scale fractures in low-permeable sandstone reservoir: Insights from geological and mathematical approach

Natural fractures are regarded as important reservoir space and seepage channels in low-permeable sandstone reservoirs. Taking the complex multi-scale fractures of the Xujiahe Formation in the YB area as a case study, the characteristics and controlling factors are analyzed using data from multi-scale observations, including core samples, thin sections, and field outcrops. Multi-scale fracture distribution is predicted through a combination of geological analysis, fractal theory, and rock fracture theory. Specifically: (1) It is dominated by tectonic shear fractures of high angle (45°–75°) and vertical (75°–90°), the fracture density (D) in sandstone is higher than mudstone, negatively correlated with the thickness of the rock mechanical layer (RML). (2) A comprehensive fracture prediction method based on weight theory is proposed, where gray correlation analysis is used to determine the weight coefficients of different methods’ prediction results. The comprehensive fracture development index (IF) accurately delineates the fracture distribution range. The greater the IF value, the greater the fracture D. (3) The fracture development area is classified into three grades (Grade I, II, and III). A strong positive correlation exists between the IF value and fracture D. Grade I areas, with IF ≥ 2.40, are mainly distributed near faults and the high parts of nose structures. The comprehensive fracture evaluation method overcomes the limitations of individual methods and provides a more precise characterization of fracture development.