Print this pageMacroporosity Related to Bioturbation in the Karst Carbonate Biscayne Aquifer, Southeastern Florida: Stratiform “Super-K” Zones
A combination of cyclostratigraphic, ichnologic, and borehole geophysical analyses of coreholes; tracer-test analyses; and lattice-Boltzmann methods (LBMs) were used to characterize and quantify biogenic macroporosity and permeability of the karst carbonate Biscayne aquifer. Biogenic macroporosity largely manifests as: (1) ichnogenic macroporosity primarily related to post-depositional bioturbation by callianassid shrimp and fossilization of components of their complex burrow systems (Ophiomorpha); and (2) biomoldic macroporosity originating by dissolution of fossil hard parts, principally mollusk shells. Ophiomorpha-dominated ichnofabric contributes most to shape the hydrologic characteristics of the Biscayne aquifer in a 345-km2 study area. Stratiform tabular-shaped units of thalassinidean-associated macroporosity are commonly confined to the lower part of upward-shallowing high-frequency cycles of the Fort Thompson Formation. In an aggradational cycle of the Miami Limestone intense bioturbation by deep-tier callianassids played a major role in development of widespread zones of relatively high-permeability macroporosity. Broad continuity of many of the macroporous units concentrates groundwater flow in extremely permeable passageways. Ichnogenic macroporosity represents an alternative pathway for concentrated groundwater flow that differs considerably from standard karst flow-system paradigms used to describe groundwater movement through fractures and cavernous dissolution features.
Permeabilities were calculated using LBMs applied to computer renderings assembled from X-ray computer tomography scans of various biogenic macroporous limestone samples. The highest simulated LBM permeabilities are about 5 orders of magnitude greater than standard laboratory measurements using air-permeability methods, which are limited in their application to extremely permeable macroporous rock samples. Based on their close conformance to analytical solutions for pipe flow, LBMs offer a new means of obtaining accurate permeability values for such materials.
We suggest that the stratiform ichnogenic groundwater flow zones have permeabilities even more extreme (about 2-5 orders of magnitude higher) than the Jurassic “super-K” zones of the giant Ghawar oil field. The flow zones of the Pleistocene Biscayne aquifer provide examples of ichnogenic macroporosity for comparative analysis of its origin and evolution in other carbonate aquifers, as well as petroleum reservoirs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009