Stress states of isolated carbonate platforms: Implications for development and reactivation of natural fractures post burial

Abstract

We use 2D finite element numerical geomechanical modeling to interrogate how different scenarios of far field stress regimes, rock mechanical properties and margin morphology affect stresses post burial within a generic isolated carbonate platform. We test a range of stress scenarios that include normal and strike-slip faulting conditions, under low and high differential far field stress regimes. Rock mechanical properties are representative of both young (weak) and ancient (strong) end-member reservoir rocks and distributed within two distinct domains (rimming reef and platform interior). Results show that platform geometry is a first order control on the magnitude and orientation of stresses. Characteristic features of carbonate margins like scallops or promontories tend to preferentially concentrate stresses, and can become sweet spots for fracture development. In low differential far field stress scenarios, the maximum horizontal stress rotates to mimic the margin orientation, while no rotations are observed under high differential stress scenarios. The stiffer nature of the carbonate rocks, compared to the surrounding siliciclastic basin infill, amplifies the far field differential stresses within the carbonate platform. In high differential far field stress scenarios, the local differential stresses can increase to the point that shear failure can be achieved within the platform interior, resulting in the potential development of fracture corridors. Some margin parallel pre-existing fractures show a high likelihood of reactivation. We postulate that some early margin syndepositional fractures are long lived conduits for fluid flow as they are prone to undergo multiple reactivation phases during and post burial, thus potentially controlling diagenetic overprints and present-day fluid flow in hydrocarbon reservoirs. Results from this type of modeling can be used to predict areas of increased fracture density within a carbonate reservoir (i.e. reservoir characterization), as well as to optimize trajectories of highly deviated wellbores (i.e. wellbore stability).