The Role of Flexure Margins in Controlling Open Fracture Distribution: Insights from Analogue Modeling of Orthogonal Rift and Pull-Apart Systems

Hydrocarbon exploration in Basement Fracture Reservoirs faces significant challenges in predicting fracture connectivity below seismic resolution. This study utilizes analogue sandbox modeling to compare the spatial distribution of open fractures in orthogonal normal fault systems and pull-apart basins. Methodological innovation is implemented through the use of a 1 mm thick gypsum layer atop 8 cm of Ngrayong Formation sand, serving as a brittle rheological interface to simulate the mechanical response of the basement top. Experimental results demonstrate that open fractures are consistently localized within the flexure margin zone. In the orthogonal system, fractures exhibit a broad and elongated distribution parallel to the master fault, governed by the fault's total length. Conversely, the pull-apart system yields a narrower and more localized fracture network, predominantly concentrated within the inter-fault transfer zones at the master fault tips, with additional margin-parallel fractures and an intra-basin damage zone developing at high cumulative strains. The direct inter-fault connectivity observed in the pull-apart transfer zones is geometrically consistent with a higher proportion of X and Y-node configurations relative to the orthogonal system, suggesting superior fracture network connectivity in pull-apart settings. This study proposes an evolved paradigm in fracture prediction, suggesting that traditional distance-to-fault methods can be significantly enhanced by incorporating the estimation of cumulative extensional strain via streamlined 3D palinspastic restoration. The findings confirm that topographic curvature is only valid as a fracture indicator when genetically associated with active flexural bending mechanisms. For exploration practitioners, this study provides a direct, tectonic-regime-specific framework for delineating basement reservoir sweet spots and reducing dry-hole risk in fractured basement plays.