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

Gareth D. Jones¹ and David N. Awwiller^2
1ExxonMobil Upstream Research Company, Houston, TX
²ExxonMobil Exploration Company, Houston, TX

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 sampled 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 are also finer grained (200 versus 700 microns mean grain size).

Preliminary results suggest that further testing and validation between calculated and measured values is warranted before grain size derived values of depositional porosity and permeability can be used to populate process-based predictive reservoir quality models.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas