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Assessing Flow Potential in a Fringing Reef Matrix-Vug Dual Pore System


Accurately predicting flow in both carbonate aquifers and reservoirs can be extremely challenging due to the complex porosity and permeability distribution. This complexity stems from both primary matrix porosity related to depositional facies and secondary porosity resulting from diagenetic alteration and fracturing. Integration of lithology, diagenesis, and sequence stratigraphy constitutes a powerful tool for predicting the spatial and temporal distribution of reservoir properties. The prograding Pleistocene reefs of the southern Dominican Republic provide an opportunity to understand the temporal development of a matrix-vug dual porosity system within a reefal margin exposed to varying degrees of both marine and meteoric diagenesis. Integration of borehole permeability measurements (straddle packer), whole core porosity estimates (core bulk density), and small sample estimates of porosity and permeability (standard 2.54 cm plug), provide a means of assessing the variability within, and primary factors that control flow within this complex carbonate pore system. Vertical profiles of porosity, derived from bulk density of the whole core, were developed for a six core transect. The average porosity for all six cores ranges between 35.5-40.2%. Porosity trends can be correlated to lithologic and diagenetic changes in the core. Whereas the plugs represent the matrix component, vuggy porosity was quantified by subtraction of the plug-based “matrix” porosity from the “total” porosity derived from whole-core bulk density values. Trends of larger vuggy and moldic porosity are associated with dissolution of the aragonitic macrofauna. In addition, borehole permeability values (converted from hydraulic conductivity) measured in the upper section of three wells, show a strong correlation to this larger-scale vuggy porosity, and no correlation to the plug-based porosity. These borehole permeability values based on in situ injection tests ranged between 5-25 Darcy, up to three orders-of-magnitude higher than associated plug permeability values (0.0001-19 Darcy). Increased underestimation in permeability from plug data is enhanced with time and diagenetic overprint, as cementation occludes matrix porosity and dissolution opens up larger vugs especially corals and other large aragonitic grains.