191-5 No More ‘Pore’ Decisions! Understanding Controls on Porosity and Permeability in Pleistocene Carbonate Rocks from San Salvador Island, The Bahamas

Session: Aspects of Carbonates (Posters)

Poster Booth No.: 148

Presenting Author:

Han (Ashley) Zhang

Authors:

Zhang, Han (Ashley)¹, Jensen, Nikola², Wheeler, Hanna³, Thieler, Sarah⁴, Anton, Claire⁵, Smith, Dillon⁶, Stephens, Bethany⁷, Corcoran, Erin⁸, Espinosa, Isabel⁹, Wenick, Milla¹⁰, Plevyak, Raina¹¹, Gu, Yilin¹², Glumac, Bosiljka¹³, Curran, H. Allen¹⁴, Griffing, David H.¹⁵

(1) Smith College, Department of Geosciences, Northampton, MA, USA, (2) Smith College, Department of Geosciences, Northampton, MA, USA, (3) Smith College, Department of Geosciences, Northampton, MA, USA, (4) Smith College, Department of Geosciences, Northampton, MA, USA, (5) Smith College, Department of Geosciences, Northampton, MA, USA, (6) Smith College, Department of Geosciences, Northampton, MA, USA, (7) Smith College, Department of Geosciences, Northampton, MA, USA, (8) Smith College, Department of Geosciences, Northampton, MA, USA, (9) Smith College, Department of Geosciences, Northampton, MA, USA, (10) Smith College, Department of Geosciences, Northampton, MA, USA, (11) Smith College, Department of Geosciences, Northampton, MA, USA, (12) Smith College, Department of Geosciences, Northampton, MA, USA, (13) Smith College, Department of Geosciences, Northampton, MA, USA, (14) Smith College, Department of Geosciences, Northampton, MA, USA, (15) Hartwick College, Department of Geology & Environmental Sciences, Oneonta, NY, USA,

Abstract:

Porosity and permeability of sedimentary rocks provide key insights for the discovery and responsible use of natural resources (groundwater, oil, gas) and services (carbon storage). Detailed understanding of controls on porosity and permeability distribution supports effective resource management. This study focuses on Late Pleistocene limestones in two 54 mm-diameter cores from San Salvador Island, The Bahamas. The 15 m-deep CT (Cockburn Town) core was drilled in 1997 on the west coast, and the 34 m TG (The Gulf) core was produced in 2019 on the south shore. TG core plug samples were analyzed for porosity and permeability using standard methods. Petrographic thin sections were made with blue epoxy for porosity determinations using Fiji/ImageJ software and point-counting. X-Ray Computed Tomography (XCT) scans of six core segments were analyzed with Dragonfly software. 

Plug samples of friable eolian and beach grainstones from the uppermost 7.5 m of TG core have high intergranular porosity (40-50%) and permeability (4,500-19,500 mD). In contrast, the underlying shallow-marine, coral-rich deposits have variable and lower porosity and permeability (15-48% and 2-1,700 mD, respectively). The highest values are associated with skeletal-ooid-peloidal grainstones or peri-reefal sands separating four coral-rich TG-core intervals. The uppermost subtidal intervals in both cores belong to the Cockburn Town Member of the Grotto Beach Formation (MIS 5e, Eemian or Last Interglacial), and are dominated by Acropora cervicornis corals encrusted by up to ~15 cm thick microbialites. XCT imaging of these corals revealed the lowest porosity of 3-4%. Even though these values were somewhat underestimated due to XCT resolution limits, they illustrate the role of well-lithified micritic microbialites in porosity occlusion of coral-rich carbonate rocks. 

Informed by TG core data and in the absence of plug samples, the analysis of CT core samples had to take into account that: 1) porosity values determined from image analysis and point-counting of thin sections were lower than standard plug measurements, demonstrating differences between pore volume and surface area determinations; and 2) the presence of micrite with isolated fenestral, intraskeletal (corals and skeletal encrusters) and endolithic borehole pores also contributed to permeability reduction of heterogeneous shallow-marine carbonate deposits. This study illustrates the value of comparative porosity and permeability studies, and the need to understand differences and limitations imposed by variable analytical techniques.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9874

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