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Subsurface Facies and Non-Matrix Features Characterization of Sandy Point, San Salvador Island, Bahamas: Impact on Porosity, Permeability, and Fluid Flow

Abstract

The Pleistocene island of San Salvador, Bahamas is a natural laboratory that provides a unique opportunity to study isolated carbonate platforms, their sequence stratigraphic framework and facies associations, diagenetic overprinting, and structural evolution through time. This increase in understanding provides an opportunity to better assess the factors that affect how the carbonate pore system evolves, and allows interpreters to be predictive about flow behavior through these systems. This research aims to address three main questions: (1) is there a relationship between porosity and permeability by depositional facies, (2) is there a relationship between the development of non-matrix features and depositional facies and, (3) is flow concentrated in a particular zone and/or depositional facies type? A drilling campaign was launched in the Sandy Point area (south-west portion of the island) in an effort to capture and characterize depositional environment associations and heterogeneity through time. To date, 21 boreholes have been drilled, totaling over 490 meters of core, with core recovery between 85-98%. Each core was logged on a centimeter scale resolution for grain type, grain size and sorting, cement percentage (when applicable), diagenetic overprinting, and pore type, and ultimately classified using a modified Dunham’s classification scheme. Non-matrix features including vugs (separate and touching), fractures, and caves were also documented in each core. Initial findings suggest that there is a relationship between (1) matrix properties and depositional facies at the scale of a core plug (1” diameter), (2) the type and number of non-matrix features by depositional facies at the scale of a whole core (2.25” diameter), and (3) an unclear relationship between depositional facies and measured fluid flow at the interwell scale (5 meters) within the system. These results confirm the complexity of the total pore system and further demonstrate that fluid flow in carbonate settings is influenced by the interplay between matrix and non-matrix features, facies types and distributions, age and diagenetic overprinting and the scale at which these phenomena are observed. These results have implications for improved reservoir characterization, performance prediction, and field development optimization in settings where non-matrix features contribute to flow.