MULTIFACETED APPROACH FOR EVALUATING RESERVOIR PROPERTIES OF PLEISTOCENE CARBONATE ROCKS FROM THE BAHAMAS: PRELIMINARY INSIGHTS FROM A NEW CORE ON SAN SALVADOR ISLAND

This research focuses on deposits from a newly acquired core, 54 mm diameter, 34 m deep, and with an overall recovery rate of 86%, from The Gulf site on the south coast of San Salvador Island, Bahamas. Exposed at sea level in a wave-cut terrace at this site is the top of a Pleistocene (Eemian, MIS 5e) coral reef of the Cockburn Town Mbr., Grotto Beach Fm. In the core, the reef top is at depth of 8.4 m, and it underlies carbonate eolianites capped by a Holocene-Pleistocene boundary terra rossa paleosol.

Core plug samples were analyzed for porosity, permeability and grain density, and made into petrographic thin sections. Fiji/ImageJ software was used to analyze porosity in high-resolution scans of thin sections using blue epoxy, and selected slides were point-counted for porosity determinations. Radiographs of selected core segments were recorded on an ITRAX core scanner, and gamma-density data were collected on a GeoTek core logger to document relative variations in bulk density. A small subset of samples was also CT scanned.

Eolianites from the upper part of the core are finely-laminated grainstones with isolated rhizoliths and some fenestral porosity. Their plug samples have relatively high porosity and permeability ranging from 40-50% and ~4500-19000 mD, respectively. Underlying reef deposits are more heterogeneous and include multiple species of corals surrounded by micritic to coarse skeletal matrix. As a result, their porosity and permeability are variable and generally lower, ranging from 15.5-48.5% and ~2-1600 mD, respectively. Percentages of pore space determined from image analysis and point-counting of thin sections are on average lower by as many as 15 and 18 value points, respectively, and illustrate the difference between pore volume vs. surface area determinations. Core radiographs, gamma-density data and CT scans complement petrographic observations of the distribution of various pore types, ranging from small intergranular voids in sandy deposits and primary porosity within coral skeletons to larger, cm-scale secondary porosity created by bioerosion and carbonate dissolution. This approach also documented porosity occlusion by the processes of sediment infilling and encrusting of corals by algae, microbes and other organisms, that can substantially modify reservoir properties of carbonate reefs.