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Prominence of ichnologically influenced macroporosity in the karst Biscayne aquifer: Stratiform 'super-K' zones

Kevin J. Cunningham^1,*, Michael C. Sukop², Haibo Huang², Pedro F. Alvarez^2,, H. Allen Curran³, Robert A. Renken⁴ and Joann F. Dixon⁴

¹ U.S. Geological Survey, 3110 SW 9th Avenue, Fort Lauderdale, Florida 33315, USA
² Department of Earth Sciences, Florida International University, Miami, Florida 33199, USA
³ Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
⁴ U.S. Geological Survey, 3110 SW 9th Avenue, Fort Lauderdale, Florida 33315, USA

Correspondence: *E-mail: kcunning{at}usgs.gov

A combination of cyclostratigraphic, ichnologic, and borehole geophysical analyses of continuous core holes; tracer-test analyses; and lattice Boltzmann flow simulations was used to quantify biogenic macroporosity and permeability of the Biscayne aquifer, southeastern Florida. Biogenic macroporosity largely manifests as: (1) ichnogenic macroporosity primarily related to postdepositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems (Ophiomorpha); and (2) biomoldic macroporosity originating from dissolution of fossil hard parts, principally mollusk shells. Ophiomorpha-dominated ichnofabric provides the greatest contribution to hydrologic characteristics in 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, throughout aggradational cycles, and, in one case, they stack vertically within the lower part of a high-frequency cycle set. Broad continuity of many of the macroporous units concentrates groundwater flow in extremely permeable passageways, thus making the aquifer vulnerable to long-distance transport of contaminants. Ichnogenic macroporosity represents an alternative pathway for concentrated groundwater flow that differs considerably from standard karst flow-system paradigms, which describe groundwater movement through fractures and cavernous dissolution features.

Permeabilities were calculated using lattice Boltzmann methods (LBMs) applied to computer renderings assembled from X-ray computed tomography scans of various biogenic macroporous limestone samples. The highest simulated LBM permeabilities were about five 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 (~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 origin and evolution in other carbonate aquifers, as well as petroleum reservoirs.

Key Words: carbonate sedimentology • ichnology • karst • lattice Boltzmann methods • aquifers • petroleum reservoirs