Processes, patterns and petrophysical heterogeneity of grainstone shoals at Ocean Cay, western Great Bahama Bank.

摘要

Holocene and Pleistocene deposits at Ocean Cay provide key evidence to comprehend patterns, processes and petrophysical heterogeneity of carbonate grainstone shoals. New datasets of high-resolution satellite images, acoustic Doppler measurements, sub-bottom profiles, and sedimentological and petrophysical analyses elucidate various factors in the deposition of a shoal complex and assess how much the sedimentary fabric and early diagenesis influence the petrophysics of similar ancient deposits.;The Holocene deposits investigated here include the Cat Cay ooid shoal and the Ocean Cay tidal deltas, which collectively form a 1-3 km wide, 35 km long sand belt. Controls on distribution, preservation and modification of these sediments are the antecedent Pleistocene topography, bathymetry, and hydrodynamics. Seismic data reveal that the Cat Cay shoal north of Ocean Cay is situated over a flat Pleistocene surface and platformward of a bedrock ridge. This finding challenges assumptions of previous studies that an antecedent high is needed for ooid shoal initiation. In contrast, south of Ocean Cay, skeletal-rich tidal deltas occur over a slightly shallower and irregular surface than the surface north of Ocean Cay.;In addition, shoal morphology and sediment attributes vary between north and south of Ocean Cay because of linkages between hydrology, shoal geometry and granulometry. Hydrodynamic data document that tidal flows can modify bars creating sinuous and parabolic forms. Spatial distribution of grain size and sorting is affected by flow velocities (up to 100 cm sec-1) inside tidal channels and inlets that remobilize and mix skeletal grains, peloids and ooids. Tidal channels can focus flow during flood tide creating lobes platformward instead of previously assumptions on the effect of storm forming spillover lobes.;The subsurface Pleistocene grainstone at Ocean Cay shows similar facies architecture to the Holocene shoals with bars, channels, and stabilized areas. Cross-bedded oolitic/peloidal and bioturbated skeletal/peloidal facies exhibit petrophysical heterogeneity, and reveal early diagenetic controls on rock properties. Porosity and permeability are up to 47% and up to 11500 mD, respectively. Early diagenesis modifies pore geometry, thus reducing permeability. This study integrates both modern and ancient carbonate systems to offer a better understanding of the factors controlling shoal morphology, architecture, and petrophysics.