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Sensitivity of Steep-Rimmed Carbonate Platforms to Early Deformation With Respect to Changes in Mechanical Rock Properties, Slope Angle, and Sea Level

Abstract

The development of syndepositional faults and fractures is ubiquitous in steep-rimmed carbonate shelf systems and recognized worldwide (e.g. Devonian of Western Australia, Permian of West Texas, and the Carboniferous of Kazakhstan). Steep-rimmed carbonate margins represent a unique depositional setting in which biological reef growth and early marine cementation create sub-vertical shelf margins and high angle upper slopes. In addition to steep geometries, these environments have drastic heterogeneities in mechanical rock properties that are prone to fracturing and faulting in areas with high mechanical contrast. These deformation features are prone to subsequent deformation or enhanced subsurface fluid flow and thus are a critical component of reservoir models in these settings. Despite the recognition of syndepositional deformation and its importance to subsurface fluid flow, very little is known about the controls and timing behind its development. Forward structural modeling is an often overlooked approach, yet is key a component to understanding early deformation features in carbonates. This study aims to investigate the controls behind syndepositional deformation in steep-rimmed carbonate margins by developing geomechanical finite element models. A series of models investigate the critical mechanisms and sensitivity of a carbonate platform profile to deformation via variations in (1) mechanical rock properties, (2) angle of the wall and slope, and (3) sea level. To investigate the sensitivity of the platform to deformation, a carbonate platform profile is constructed representing the western margin of West Caicos Island, B.W.I. Models are populated with mechanical rock properties collected from modern outcrop exposures located on West Caicos Island and San Salvador Island, Bahamas. Additional rock properties come from laboratory testing of a cored interval from the Great Bahama Bank. Slope angle and its variations have been simulated after field observations and published examples. This study models sea level variations after the icehouse conditions of the Pleistocene Epoch, where high frequency high amplitude oscillations subaerially expose and subsequently drown the platform top. Each variable is isolated to test its effect on deformation, which allows for variables to be ranked based on their impact and importance to the development of early deformation in steep-rimmed carbonate platform systems.