Comparative porosity and permeability analyses of Pleistocene carbonate rocks from San Salvador Island, the Bahamas: insights from x-ray micro-computed tomography (μCT)

Pore networks of five small plug samples (20–21 mm diameter; 20–25 mm height) from two cores in Late Pleistocene limestone from Cockburn Town (CT) and The Gulf (TG) on San Salvador Island, were examined in μCT scans (14–16 μm resolution) for comparison with other methods for determining porosity and permeability. The objective was to investigate the reservoir properties in limestones from various settings, including eolian and beach versus shallow-marine subtidal deposits that contain clotted and laminated microbial encrusters on corals from the Cockburn Town Member of the Grotto Beach Formation (MIS 5e, Eemian or Last Interglacial).

The highest porosity (41–48%) and permeability (1500–13,900 mD) values were determined by conventional plug analyses (using helium expansion methods) of friable skeletal–ooid–peloidal eolian and beach grainstones from the uppermost 7.5 m of TG core, and subtidal grainstone from ~15 m core depth. Their abundant intergranular pores as well as fenestrae in beach deposits are primary contributors to pore connectivity. Plug analyses of microbialites from CT core revealed much lower permeability (0.5–200 mD), and somewhat lower, but still relatively high porosity (31–39%) due to common fenestrae and intraskeletal pores within associated encrusting foraminifera. These differences illustrate the role of well-lithified micritic microbialites in reducing permeability of porous coral-rich carbonate rocks. In comparison, analysis of μCT scans using Dragonfly and Fiji software yielded lower porosities of 24–33% for eolian/beach/subtidal grainstones, and 1–13% for clotted and laminated microbialites. Such comparisons with conventional plug measurements provide a valuable reference for setting parameters for μCT analyses and interpreting their results because they indicate that despite high resolution, porosity values derived from μCT scans are still underestimated due to resolution limits. This study demonstrates the value of comparative porosity and permeability analyses and the need to understand variabilities among different research methodologies.