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Stratigraphic Controls on Diagenesis in the Paleogene-Neogene Strata on Grand Cayman, British West Indies


Reservoirs hosted in carbonate sedimentary rocks have long confounded geologists due to their inherent diagenetic heterogeneity. The limestone and dolostone bedrock succession on Grand Cayman (Caribbean Sea) developed on isolated banks in response to eustatic changes in sea level and tectonic activity. Accordingly, the carbonate sedimentary succession on the island is ideally suited for assessing the stratigraphic controls on diagenesis in carbonate reservoirs. The Paleogene – Neogene succession that forms the cores of the Cayman Islands belongs to the Bluff Group (Brac Formation, Cayman Formation, and Pedro Castle Formation), which is onlapped and overlain by the Ironshore Formation (Pleistocene). This study will focus on the Miocene Cayman Formation (100 – 120 m) on the western and central parts of Grand Cayman, which is formed largely of fabric-retentive microcrystalline dolostones. Diagenetic heterogeneities in the Cayman Formation parallel depositional features, which governed sediment permeability, post-depositional fluid flow, and the position of the water table. The lower “porous unit” of the Cayman Formation (0 – 80 m thick), which filled paleotopographic lows on the underlying Brac Unconformity, consists of porous (>30%) Amphistegina, coralline red algae, and bivalve grain- to packstones with scattered rhodoliths, Halimeda, and Porites. In contrast, the upper “caprock” of the Cayman Formation (25 – 35 m thick) consists of tightly cemented (<10% porosity) Stylophora and bivalve floatstones with red algae-foraminifera wackestone matrices. This study has implications for the exploitation of petroleum and groundwater, as well as the development of reverse osmosis facilities, wastewater injection, and CO2 sequestration in carbonate reservoirs.