Processes, Patterns and Petrophysical Heterogeneity of Grainstone Shoals at Ocean Cay, Western Great Bahama Bank

Holocene and Pleistocene grainstone deposits surrounding Ocean Cay located on the western margin of Great Bahama Bank provide key evidence for the comprehension of patterns, processes and petrophysical heterogeneity of carbonate grainstone shoals. New datasets consisting of high-resolution remote sensing data, acoustic Doppler current measurements, sub-bottom profiles, and sedimentological and petrophysical analyses offer an opportunity to elucidate the various factors in the deposition of a grainstone shoal complex and assess of how much of the sedimentary fabric and early diagenetic overprint influences the petrophysical characteristics of similar ancient deposits. The Holocene shoal complex investigated here includes the Cat Cay ooid shoal and the Ocean Cay tidal deltas, which collectively form a 1-3 km wide, 35 km long sand belt around Ocean Cay. These factors controlling the distribution, preservation and modification of these sediments are the antecedent Pleistocene topography, bathymetry, and hydrodynamics at this margin. High-resolution seismic data reveal that the laterally continuous and thick Cat Cay ooid shoal north of Ocean Cay is situated on top of a flat Pleistocene surface and located platformward of a Pleistocene rock ridge. This finding challenges the assumption of previous studies that an antecedent high is needed for ooid shoal initiation. In contrast, south of Ocean Cay, skeletal-rich tidal deltas occur east of rocky Pleistocene islands and formed over an irregular Pleistocene surface that is slightly shallower than the flat surface north of Ocean Cay. In addition to the antecedent topography, differences in shoal morphology and sediment attributes between the north and south areas around Ocean Cay are related to linkages among fluid flow patterns, shoal morphology and granulometry. The hydrodynamic data document the influence of tidal flows in modifying the shape of bars creating sinuous and parabolic forms during flood and ebb reversing flows. Spatial distribution of grain size and sorting is affected because high flow velocities (up to 100 cm sec-1) inside tidal channels and inlets can erode and remobilized sediments mixing skeletal grains, peloids and ooids. Dominance of flood tide across this Holocene shoal complex allowed tidal deltas to form bankward of inlets between rock islands. Tidal channels and inter-bar troughs can focus tidal flow during flood tide creating lobes platformward instead of previously assumptions on the effect of storm and formation of spillover lobes bankward. Cores from the subsurface at Ocean Cay show that the architecture of the Pleistocene grainstone facies is similar to the Holocene shoal configuration of bars, channels, and bioturbated stabilized areas. Cross-bedded oolitic/peloidal and bioturbated skeletal/peloidal facies exhibit facies-dependent petrophysical heterogeneity, and reveal depositional and early diagenetic controls on petrophysical properties. Porosity and permeability in the grainstones at Ocean Cay are high, up to 47% and up to 11500 mD, respectively. Early diagenesis modifies the pore geometry of the rock, thus reducing permeability. A comparison of petrographic and petrophysical properties of the Pleistocene shoal with those from the Pennsylvanian ooid shoals reveals that were strongly influenced by the original fabric and early near-surface diagenesis. The integration of data from both the modern and ancient carbonate systems provides a better understanding of the factors controlling shoal morphology, facies architecture, and rock properties. The results of this study can be used as a guide for interpreting heterogeneity and reservoir properties of analogous facies within ancient ooid shoals