Eolianite versus subtidal, microbially encrusted coral samples: comparative porosity and permeability analyses of Pleistocene carbonate rocks from San Salvador Island, the Bahamas

This study documents differences in reservoir properties between two contrasting carbonate lithologies from a core collected on San Salvador’s south coast, and among results of multiple analytical techniques. Laminated, peloidal–ooid–skeletal eolian grainstone with abundant small interparticle pores from the upper MIS 5 part of the core (-2.8 m depth), was compared to shallow-marine Acropora cervicornis coral from older MIS 9 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 6900 vs. 47 mD, respectively, indicating the role of well-lithified micrite in reducing permeability of porous coral-bearing subtidal deposits. Blue-epoxy thin sections made from the plugs were analyzed using point-counting and ImageJ. Both methods yielded lower porosity than plug analysis and demonstrate the difference between surface area vs pore volume determinations. ImageJ analysis included the entire thin-section area and yielded higher porosity (23.2% eolianite; 15.7% coral) than point counting (18–22.8% eolianite; 8.6–12.5% coral). This range of values represents the differences in counting 200, 400 and 800 points, as well as the grid placement, with no clear trends within this small dataset.

Eolianite thin-section scan revealed more common air bubbles which were manually painted blue, and the difference between ImageJ porosity measurements of painted vs unpainted scans was greater for eolianite (1.8%) than coral (0.3%). The results illustrate: 1) important differences between the two samples; 2) impact of pore size, shape and distribution on porosity determination; and 3) systematic decrease in porosity values determined from plug volume measurements to ImageJ analysis to point-counting of thin sections.