POROSITY AND PERMEABILITY TRENDS FROM RESERVOIR ANALOG MIOCENE (PUERTO RICO), PENNSYLVANIAN (KANSAS), AND MISSISSIPPIAN (ALABAMA) CARBONATE ROCKS

Outcrops of the Ponce Limestone of Puerto Rico (Miocene), Westerville Limestone of Kansas and Missouri (Pennsylvanian), and Tuscumbia Limestone of Alabama (Mississippian) were sampled to quantify porosity and permeability and determine controls on reservoir character (e.g., age, depositional facies, diagenetic history, paleogeography, tectonic setting). All three units preserve carbonate rocks that were deposited in a variety of shallow-water settings. Mississippian facies are mostly characterized by silica-bearing heterozoan (mainly crinoidal)-dominated packstones and grainstones. Pennsylvanian facies are mostly grained supported (heterozoans, oolitic, peloidal, and oncolitic) packstones and grainstones. Miocene facies are mainly composed of heterozoan -large benthic foraminifera-dominated packstones and grainstones facies. Photozoan coral (Montastraea and Porites) red algae packstones and grainstones facies also occur. Facies and porosity types were determined from outcrop and petrographic analysis and by computer-based image analysis. Permeability was initially estimated using the capillary tube model of effective porosity and the general Kozeny-Carman equation. Analysis of Tuscumbia core plug samples aided in refining estimated porosity and permeability results. Preliminary results from this ongoing study show Miocene limestones with the highest porosity values (up to 30%) and Mississippian and Pennsylvanian limestones with the lowest porosity values (up to 5%). Plots organized by facies show well sorted bioclastic fine-grained packstones and grainstones with the most porosity (up to 30%), whereas mudstones and some wackestones have the lowest porosity values (less than 1%). Porosity values appear to be mostly controlled by a combination of grain size, sorting, and diagenesis associated with subaerial exposure and burial history. Porosity and permeability trends from this outcrop-based study of Miocene (Puerto Rico), Pennsylvanian (Kansas), and Mississippian (Alabama) carbonate rocks can be used as analogs for modeling reservoir properties of similar age systems in the subsurface (ex. Miocene Perla Gas Field offshore Venezuela, Pennsylvanian Lansing-Kansas City Group in Kansas, and Mississippian Black Warrior Basin in Alabama).