ELECTROFACIES CHARACTERIZATION OF THE UPPER CRETACEOUS MCQUEEN BRANCH AQUIFER OF THE SOUTH CAROLINA COASTAL PLAIN

Interpreting the stratigraphy, lithology, and hydrogeology of subsurface units has historically depended, in large part, on core samples. However, cores may be prohibitively expensive or difficult to access, whereas wireline logs can be obtained for the entire interval of interest under varied environmental conditions and at great detail. One approach integrating log-derived information on stratigraphy, lithology, and hydrogeology in a single study is through electrofacies analysis.

The McQueen Branch aquifer is ideal for electrofacies studies because its lithologic and stratigraphic characteristics change vertically and laterally, and it is present throughout the Coastal Plain of South Carolina. Considerable information on the aquifer has been derived from studies of core taken at the Savannah River Site in Aiken County, South Carolina, but few studies have addressed the regional aspects of the aquifer. Because wireline logs contain information for estimating porosity and permeability, electrofacies analysis may provide an efficient and cost-effective method for characterizing and mapping the regional distribution of these parameters.

Ten wells throughout the Coastal Plain of South Carolina were selected for the study, and five geophysical logs from each well were obtained: gamma-ray, single point resistance, long and short normal resistivity, and spontaneous potential. Each log was normalized to eliminate the influence of both scale and units of measure among the logs.

Because each log contains a large array of normalized log data, the data were compressed by blocking. Each well was blocked (or zoned) in such a way that dispersion of the data was minimized within the blocks, and boundaries were placed at those horizons where dispersion was maximized. Electrofacies were delineated from the blocked logs through cluster analysis, and each electrofacies defined in terms of the median grain size and sorting of the sediments representative of that electrofacies. Such studies may be applied to porosity and permeability predictions for aquifers and confining units in the Coastal Plain.