NEW METHOD FOR QUANTIFICATION OF VUGGY POROSITY FROM DIGITAL OPTICAL BOREHOLE IMAGES AS APPLIED TO CENOZOIC CARBONATES OF THE BISCAYNE AQUIFER AND FLORIDAN AQUIFER SYSTEM, SOUTHERN FLORIDA

Well-connected vugs can form major conduits for flow of ground water, especially in carbonate rocks. A new method is presented for quantification of vuggy porosity calculated from digital borehole images for Cenozoic carbonate rocks from the southern Florida Platform. The method interprets vugs and background based on the grayscale color of each in digital borehole images and calculates a percentage of vuggy porosity. Development of the method was complicated because borehole conditions created an uneven grayscale contrast in the digital images that made it difficult to distinguish vugs from background. The irregular contrast was produced by unbalanced illumination of the borehole wall, which is attributed to eccentering of the borehole-image logging tool. Experimentation showed that a simple, single grayscale threshold fails to differentiate between the grayscale contrast of vugs and background. Therefore, an equation was developed for an effective subtraction of the changing grayscale contrast, due to uneven illumination, to produce a grayscale threshold that successfully identifies vugs. In the equation, a moving average calculated around the circumference of the borehole and expressed as the background grayscale intensity is defined as a baseline from which to identify a grayscale threshold for vugs. An empirically derived constant was determined by calibration with vuggy porosity values derived from digital images of slabbed-core samples. This constant is used to make the subtraction from the background baseline to derive the vug grayscale threshold as a function of azimuth.

Examples of application of the method are shown for cyclic platform carbonates from the Pleistocene Biscayne aquifer and the Eocene portion of the Floridan aquifer system. The method should be effective in estimating vuggy porosity in any carbonate aquifer. It has proven useful in assembling a high-resolution hydrogeologic framework and in selection of preferential ground-water flow zones within the Biscayne aquifer and Floridan aquifer system.