Application of Carbonate Cyclostratigraphy and Borehole Geophysics to Delineate Porosity and Preferential Flow in the Karst Limestone of the Biscayne Aquifer, SE Florida

Cunningham, Kevin; Renken, Robert; Wacker, Michael; Zygnerski, Michael; Robinson, Edward; Shapiro, Allen; Wingard, G. Lynn

Combined analyses of cores, borehole geophysical logs, and cyclostratigraphy produced a new conceptual hydrogeologic framework for the triple-porosity (matrix, touching-vug, and conduit porosity) karst limestone of the Biscayne aquifer in a 0.65 km2 study area, SE Florida. Vertical lithofacies assemblages, which have recun·ent stacking patterns, fit within highfrequency cycles. Three ideal high-frequency cycles are defined as: (i) upward-shallowing subtidal cycles, (ii) upward-shallowing paralic cycles, and (iii) aggradational subtidal cycles. Digital optical borehole images, tracers, and flowmeters indicate that there is a predictable vertical pattern of porosity and permeability within the three ideal cycles, because the distribution of porosity and permeability is related to lithofacies. Stratiform zones of high permeability commonly occur just above flooding surfaces in the lower part of upwardshallowing subtidal and paralic cycles, forming preferential ground-water flow zones. Aggradational subtidal cycles are either mostly high-permeability zones or leaky, lowpermeability units. In the study area, ground-water flow within stratiform high-permeability zones is through a secondary pore system of touching-vug porosity principally related to molds of burrows and pelecypods, and to interburrow vugs. A conservative tracer test infers heterogeneous permeability based on movement of a dye-tracer pulse observed using a borehole fluid temperature tool during a conservative tracer test. Advective movement of the tracer appears to have been most concentrated within a thin stratiform flow zone contained within the lower part of a high-frequency cycle, indicating a distinctly high relative permeability for this zone. Borehole flowmeter measurements corroborate the relatively high permeability of the flow zone. Identification and mapping of such high-pe1meability flow zones is key to conceptualization of karst ground-water flow within a cyclostratigraphic framework. Many karst aquifers are included in cyclic platform carbonates. Clearly, a cyclostratigraphic approach that translates carbonateaquifer heterogeneity into a consistent framework of correlable units will improve simulation of karst ground-water flow.

Application of Carbonate Cyclostratigraphy and Borehole Geophysics to Delineate Porosity and Preferential Flow in the Karst Limestone of the Biscayne Aquifer, SE Florida

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