43 Sedimentary Geology (Posters)

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

Comparative porosity and permeability analysis of carbonate eolianite versus subtidal, microbially encrusted coral samples: Pleistocene of San Salvador Island, The Bahamas

Paper number: 43-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Two samples from a core collected in 2019 at The Gulf on San Salvador’s south coast were examined to document differences in reservoir properties between contrasting carbonate lithologies and among results of multiple analytical techniques. Laminated, peloidal-ooid-skeletal eolian grainstone with abundant small interparticle pores from the upper MIS5 part of the core (-2.8 m depth), was compared to shallow-marine Acropora cervicornis coral from older MIS9 or 11 deposits (-31.3 m). The coral has common larger intraskeletal pores and endolithic bore-holes partially occluded by micrite, and is encrusted by thin coralline red algae and up to 2 cm-thick micritic microbialite, surrounded by coarse skeletal wackestone to packstone.

Conventional core plug analyses revealed similar porosity of 40.8% for eolianite and 36.6% for coral, but drastically different permeability of 6,900 vs 47 mD, respectively, indicating the role of well-lithified micrite in reducing permeability of porous coral-bearing subtidal carbonate deposits. Blue-epoxy thin sections made from the plugs were analyzed using point-counting and ImageJ. Both methods yielded substantially lower porosity than plug analysis and demonstrate the difference between surface area vs pore volume determinations. As expected, ImageJ analysis yielded higher porosity (23.2 % eolianite; 15.7% coral) than point counting (18-22.8 % eolianite; 8.6-12.5 % coral), because the former method considers the entire thin-section area, even though inclusion of pores smaller than 30 μm is not straightforward and contributes to porosity underestimation. Analysis of cross-polarized light images resulted in even lower porosity of 16.5 and 6.7 % for the two samples. The reported ranges of point-counting values reflect the differences in counting 200, 400 and 800 points as well as the grid placement, with no clear trends within this small dataset.

More common air bubbles in thin-sections of eolianite represent an example of the impact of pore size, shape and distribution on porosity determination. These air bubbles were manually painted blue, and the difference between ImageJ porosity measurements of painted vs unpainted slides was greater for eolianite (1.8 %) than coral (0.3 %). The results indicate important differences between the two samples, and the systematic decrease in the determined porosity values from plug volume analyses to ImageJ and point-counting of thin sections. It is important to consider and understand these differences in comparative analyses of carbonate rocks for their reservoir characterization.

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

doi: 10.1130/abs/2026NE-411391

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