43 Sedimentary Geology (Posters)

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Ballroom C

X-Ray Micro-Computed Tomography (µCT) in Comparative Porosity and Permeability Analyses of Pleistocene Carbonate Rocks from San Salvador Island, The Bahamas

Paper number: 43-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Pore networks of five small cylindrical plug samples (~21 mm in diameter and height), collected from two cores in Late Pleistocene limestone from Cockburn Town (CT) and The Gulf (TG) on San Salvador Island, The Bahamas, were examined in µCT scans (14-16 µm resolution) for comparison with other analytical methods for determining porosity and permeability. The research objective was to investigate these reservoir properties in limestones from different depositional environments, including eolian and beach versus shallow-marine subtidal deposits that include 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 (1,500-13,900 mD) values were determined by conventional plug analyses (using helium expansion methods under confining pressures of 1,000 and 1,500 psi) 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 the two 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 porosity and permeability reduction of coral-rich carbonate rocks. In comparison, porosity analysis of µCT scans using Dragonfly and Fiji software resulted in lower porosity 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 of such scans, porosity values derived from them are still underestimated due to resolution limits.

Ongoing work includes comparisons of plug pore volumes with generally lower surface area porosity determined by point-counting and ImageJ analysis of blue-epoxy petrographic thin sections. Future research will also utilize mercury injection capillary pressure (MICP) method in porosity analysis, and fluid flow simulations to further investigate pore connectivity. This study demonstrates the value of comparative porosity and permeability analyses and the need to better understand variabilities among different research methodologies.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411226

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