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3-D Forward Geomechanical Modeling of Differential Compaction Fracturing in Carbonate Reservoirs: Insights From Outcrop- and Platform-Scale Models


Differential compaction driven by contrast in early cementation in carbonate systems can cause syndepositional fracturing and faulting. Such early fractures can persist to be major permeability corridors in hydrocarbon reservoirs. We forward model the differential compaction process using combined finite-discrete element code to predict potential locations and patterns of syndepositional fractures in the Late Pennsylvanian Cisco reservoir of the SACROC field, Midland Basin, West Texas. Models were constructed on two scales: carbonate platform (10s km) and individual build-up (10s m). The platform model is based on key surfaces interpreted on 3D seismic data of the SACROC field. The geometry of individual build-up models were based on 3D outcrop model of analogous carbonate build-ups. A simplified mechanical stratigraphy scheme was implemented based on susceptibility to early cementation. Strata likely to undergo early cementation (e.g., build-ups) were prescribed brittle properties and failure criteria. Strata likely to undergo compaction (e.g., mud-rich basinal sediments) were decompacted and prescribed elasto-plastic failure criteria. Both models were only subjected to gravitational load under hydrostatic conditions. Modeling results showed that differential compaction subjects the entire top of the carbonate platform to increased tensile stress state. Areas of greatest tensile stresses (i.e., highest fracture potential) are located primarily at the windward margin, where carbonate build-ups are also present. At the center of the field the windward-leeward asymmetry is minimal and areas of greatest fracture potential are located at the center of the platform. These predictions are consistent with vertical fracture intensity from image logs and mud loss data within the Cisco. Outcrop-scale modeling results revealed that build-up shape dictates the resulting fracture pattern in overlying strata. Circular build-ups produced a radial fracture pattern and elongate build-ups produced an approximately linear fracture pattern. The observed fracture orientations were more consistent with build-up shapes (observed from coherence attribute) than the predicted platform-scale principal stress orientations. This suggests that mapping carbonate build-ups and their geometry can be useful in predicting related syndepositional fracture patterns. This study showcases how a process-based approach to fracture modeling can contribute to enhanced fracture prediction.