Why Won't it Fracture? Investigating the Low Success Rate in the Tigh Gas Formation, North of Oman

Abstract The Amin Formation in northern Oman spans two structurally and depositional distinct basins—the Fahud Salt Basin (FSB) and the Ghaba Salt Basin (GSB) with the Triplet Fields forming a notable cluster of fields within the GSB. Despite generally similar reservoir properties and mineralogical makeup, hydraulic fracturing success across the Amin Formation has shown substantial spatial and vertical variability. Stratigraphically, the Amin Formation is subdivided into three principal units arranged from top to bottom as S30, S20, and S10. Particularly, the S30 zone consistently delivers better frac performance than the overlying S20 and S10 intervals. This study investigates the reasons behind such variation by analyzing structural heterogeneity, stress regimes, and sedimentary architecture, with a focus on the geomechanical role of cross-bedding and bedding-parallel anisotropy in enhancing fracture development and propagation. To understand these variations, an integrated workflow was followed that included gamma-ray profiling of Amin outcrops to delineate stratigraphic subunits, borehole image log interpretation to identify breakout zones and induced fractures, and cross-bedding feature density analysis across wells in both basins. Geomechanical modeling using Kirsch equations was conducted under various stress regimes normal, strike-slip, and reverse faulting to evaluate the impact of stress anisotropy on hoop and radial stress magnitudes. Laboratory and log-derived mechanical properties such as unconfined compressive strength (UCS), cohesion, and friction angle were integrated with structural features to assess failure thresholds. Additionally, radioactive tracer data was cross-referenced with gamma- ray log signatures to validate the relationship between cross-bedding prevalence and effective fracture propagation. The analysis revealed a clear stratigraphic trend, with cross-bedding density increasing significantly from S10 to S30. These cross- bedded intervals, particularly in S30, showed higher correlation with induced fractures and successful frac stages, as confirmed by radioactive tracer distribution and field image logs. While breakouts were primarily associated with flat, cemented sandstone— indicating compressive stress regimes induced fractures preferentially formed in cross-bedded zones, where mechanical anisotropy likely promotes tensile failure at lower effective stress differentials. Stress modeling demonstrated that under high anisotropy, hoop stress increases while radial stress decreases, setting conditions conducive to fracture initiation, especially in zones with structural heterogeneity. The ability of cross-bedded units to redistribute stress and facilitate fracture containment was further supported by the observed alignment between image-log fractures and stratigraphic fabric. This study provides a novel, data-driven interpretation linking sedimentary architecture particularly cross-bedding with hydraulic fracture success in deep, tight formations. By quantifying cross-bedding as a proxy for fracability, this work offers a practical screening tool for future well placement and stimulation design. The findings challenge conventional assumptions that treat homogeneous intervals as mechanically ideal, highlighting instead the beneficial role of heterogeneity in stress perturbation and fracture containment. Operationally, these insights allow for better targeting of fracable intervals, reduced stimulation risk, and improved return on investment in tight gas reservoirs across North Oman