Present-day stress control on natural fracture effectiveness: quantitative evidence from borehole image logs in the Fengcheng Formation shales, Mahu Sag, Junggar Basin, NW China

Shale oil reservoirs are typically characterized by ultra-low porosity and permeability, in which natural fractures provide key pathways for hydrocarbon migration from the matrix to the wellbore. These fractures significantly influence production performance. In the Mahu Sag of the Junggar Basin (NW China), the Permian Fengcheng Formation comprises saline lake–facies mixed shales with limited primary porosity, where natural fractures and dissolution pores dominate the available storage space. The sustained high single-well production (exceeding 100 t/day in parts of the sag) underscores the importance of understanding fracture occurrence and effectiveness for efficient reservoir development. In this study, we interpret Full-bore Micro-scanner Imager (FMI) borehole image logs using Schlumberger Techlog to identify and quantify both drilling-induced and natural fractures. The results show that drilling-induced fractures, which appear as short vertical features with symmetric “feather” or en-echelon patterns, are used to infer the orientation of the current maximum horizontal stress (SHmax). SHmax varies across structural domains: it trends near E–W to ENE–WSW adjacent to the Wuxia fault belt, shifts locally toward NE–SW at the junction of the Wuxia and Kebai fault belts, and transitions back to ENE–WSW to E–W toward the southwestern and southernmost regions. Natural fractures are abundant, predominantly striking NE–SW and near E–W (40°–100° and 220°–280°, accounting for 51% of fractures), with a secondary set trending NNW–SSE (140°–160° and 320°–350°, accounting for 22%). These orientations largely align with major fault trends. Fracture dip distributions vary significantly between wells and are primarily controlled by bedding attitude, with the apparent dip deflection closely mirroring the formation dip. In proximity to faults, tectonic fractures tend to exhibit lower dips. Aperture statistics reveal that fracture effectiveness is strongly stress-dependent: fractures more closely aligned with SHmax exhibit larger apertures and higher inferred effectiveness, while aperture size decreases with increasing misalignment angle. In a representative well, ENE–WSW fractures exhibit the largest mean apertures (tens of micrometers) compared to other fracture sets. Overall, SHmax) in the Fengcheng Formation shale is predominantly oriented E–W to ENE–WSW, and natural-fracture trends broadly match the strikes of major faults. Fracture dip angles are largely governed by bedding attitude, whereas fracture effectiveness is strongly stress-dependent. These results provide a direct basis for sweet-spot evaluation (targeting intervals with larger apertures under more favorable stress conditions) and for optimizing stimulation orientation and treatment design.