Depositional Porosity and Permeability Calculated from Modern Carbonate Sediments, Turks and Caicos Islands, British West Indies

Abstract Accurate estimates of depositional porosity and in particular permeability are critical initial inputs for process-based models used to predict diagenesis in carbonate reservoirs. Unfortunately, the depositional porosity and permeability of modern carbonate sediments have rarely been measured. In this study, porosity and permeability, using Kozeny-Carman and Lattice-Boltzman algorithms, were calculated on simulated grain packs built from grain-size analyses of carbonate sediments. Five depositional environments on the Caicos Platform were sampled: Tidal Flat, Lagoon, two different Shoals, and Beach. Sediments were analyzed for texture, composition, mineralogy, grain size, and sorting. The depositional environments sampled have a characteristic grain-size distribution and the mean grain size for individual sample locations was relatively uniform. Porosities calculated for grainstones (0.39 to 0.42) were lower than those measured in a separate study on the Great Bahama Bank (0.5 to 0.53). Porosity calculated for tidal-flat sediments (0.31) is significantly lower than measured values (0.61 to 0.78) because intragranular porosity is not calculated. Depositional permeability calculated for grainstones with Lattice-Boltzman (290 to 531 darcies) was greater than with Kozeny-Carman (137 to 285 darcies). Values of permeability for ooid grainstones measured on the Great Bahama Bank (16 to 57 darcies) are an order of magnitude lower, but those sediments are also finer grained (200 versus 700 microns mean grain size).