Differential compaction fractures in carbonate mound complexes : pioneering numerical models applied to outcrops and subsurface reservoirs

Differential compaction is thought to be a primary driver for syndepositional fracture development in carbonate platforms. Outcrop and subsurface observations of syndepositional fractures in carbonate mound complexes and platforms cannot be used to directly identify the mechanism or controlling factors behind their formation, because these observations represents the end state of potentially long and complex stress and diagenetic history. The limitations of outcrop observations are overcome by using a finite-element and combined finite-discrete forward models to simulate differential compaction and subsequent fracture development in carbonate mound complexes. Differential compaction deformation is modeled at the mound scale (tens of meters) and at an isolated platform-scale (kilometers). Numerical models are used to (1) quantify amount of differential subsidence required to develop fractures, (2) predict areas susceptible to fracture development, and (3) identify the most critical factors controlling differential compaction fracturing.

2D and 3D models are constructed based on classic outcrops of Late Pennsylvanian carbonate mounds in the Sacramento Mountains and age-equivalent Canyon and Cisco formations in the Midland Basin, West Texas. Modeling results are consistent with fracture observations in outcrops and the subsurface. Geometry of lithified antecedent topography and the overlying strata controls the location of differential compaction fractures. Fractures develop in strata overlying antecedent topography in transitional crest-to-off-mound/platform areas. Another location for fracture development corresponds to strata overlying the mound/platform slope-to-off-mound/basinal setting transition.

Modeling results demonstrate that only a minor amount (cm -10s cm scale) of differential subsidence is required to develop fractures in early lithified carbonates. This suggests that differential compaction fractures in carbonate systems may be generally underestimated. Fracture intensity is found to be proportional to the amount of differential subsidence. A greater control on fracture intensity is the bedding contact nature. Fracture development in strata with bedding contacts that are resistant to layer-parallel slip display almost double the fracture intensity of strata with contacts favoring slip. Layer-parallel slip is concluded to be a major mechanism for dissipating stress during compaction-driven folding. The process-based modeling approach applied by this work provides fundamental understanding of differential compaction fracture development in carbonate mound complexes, which is valuable to prediction of fractures in subsurface reservoirs.