PREDICTION OF FRACTURED DOLOMITE IN A CARBONATE AQUIFER SYSTEM, OCALA LIMESTONE, WEST-CENTRAL FLORIDA

The Floridan Aquifer System is the most extensive carbonate aquifer in the United States and a major source of groundwater for the southeastern U.S. The aquifer is a continuous succession of Paleocene to Early Miocene carbonates underlying parts of Alabama, Georgia, and South Carolina and all of Florida. It is subdivided into the Upper and Lower Floridan Aquifers by a middle confining unit. The Ocala Limestone is part of the Upper Floridan Aquifer, and is dolomitized in its lower half in west-central Florida. South of Tampa, FL, the Ocala limestones and dolomites exhibit relatively low matrix permeabilities and low transmissivity in comparison to the overlying and underlying limestones of the Suwannee and Avon Park Formations, respectively. In this area, the Ocala thus forms a subregional semi-confining unit that separates the two transmissive zones, the upper of which contains freshwater and the lower of which contains more saline fluids and may be used for waste water injection.

Two distinct dolomite endmembers are recognized in the Ocala: a vertically restricted, poorly cemented, friable sucrosic dolomite with high porosity and permeability and a tightly cemented, indurated dolomite with low porosity and permeability. Within this dolomitized section, tensile fractures are observed in cores. These fractures vary in scale from a few centimeters up to a foot in length and form due to overburden pressure, which is approximately 6 to 8 Mpa in the study area. Flow meter logs indicate that fractures locally enhance the hydraulic conductivity of the dolomite, and could enhance vertical leakage through the semi-confining dolomite bodies.

In order to understand the propensity for the formation of these fractures, seventeen triaxial strength compression tests were conducted on a suite of 1-inch diameter dolomite core plugs from five boreholes. Samples were texturally subdivided on the basis of degree of induration. Results from the triaxial tests indicate elevated cohesion magnitude and tensile strength as the degree of induration increases in the dolomites. Combining strength data with known stratigraphic patterns in the dolomite bodies allows for prediction of fractured horizons in the dolomites of the upper Floridan aquifer, and provides insight into regions of potentially increased vertical hydraulic conductivity.