Interstitial material control of sandstone weathering fracture modes under FT–WD cycling: Wangshi group of Laiyang Sag and implications for buried-hill reservoirs

Weathering fractures constitute significant reservoir spaces within weathering crusts. To investigate the controls on their development within clastic weathering crusts of the Wangshi Group in the Laiyang Sag, field observations were integrated with laboratory analyses, including thin-section and scanning electron microscopy (SEM), X-ray diffraction (XRD), petrological analysis, and statistical analysis of fractures. The results demonstrate that: (1) Interstitial material dictates fracture type and intensity. Matrix sandstones (MSs) predominantly develop weathering macroscopic fractures (WMAFs), whereas calcareous-cemented sandstones (CCSs) favor weathering microfractures (WMIFs). The latter are dominant, exhibiting fracture intensities inversely correlated with interstitial material content. (2) The lack of dissolution features in initial state implies a genesis primarily driven by physical weathering. In the local monsoon climate, freeze-thaw (FT) and wet-dry (WD) cycles are identified as the dominant and secondary mechanisms, respectively, acting synergistically. (3) The coupling between interstitial material and physical weathering governs sandstone mechanical properties and hydric behavior, leading to differential stress responses during weathering. This coupling ultimately controls the patterns of rock fragmentation and fracture formation, thereby influencing reservoir space in buried-hill weathering crusts. This research highlights the previously underappreciated role of physical weathering in fracture development and provides a new model for predicting weathering-fracture reservoirs in clastic protolith.