Stratiform Flow in Aquifers: Implications of Thalassinidean-Generated Ichnofabrics in Lower Cretaceous to Pleistocene Carbonates
Cunningham, Kevin J.
Thalassinideans and thalassinidean-like crustaceans have produced megaporous and high-permeability ichnofabrics in shallow-marine Cretaceous to Pleistocene carbonate rocks that represent an important contributor to concentrated flow in aquifers. This ichnogenic megaporosity can develop into a highly transmissive, fabric-selective, stratiform, dual- or triple-pore system. Outcrop and subsurface examples in Texas, Florida, the Bahamas, and possibly Spain suggest an increasingly unified model for the characteristics and occurrence of ichnogenic megaporosity.
Ichnogenic megaporosity has been identified in carbonate rocks of Ophiomorpha-dominated Skolithos Ichnofacies (OSI), Thalassinoides-dominated Cruziana Ichnofacies (TCI), and Glossifungites Ichnofacies (GI). Megaporous examples of these ichnofacies have been recognized in five ideal cycle types: aggradational subtidal cycles; upward-shallowing subtidal, peritidal, and paralic cycles; and upward-deepening subtidal cycles. Megaporous OSIs can occur throughout the entire thickness of aggradational cycles and may extend for tens of miles across a paleoshelf. In upward-shallowing subtidal, peritidal, and paralic cycles, megaporous OSIs mainly occur in the middle to lower part of cycles and form stacked, stratiform flow zones. These upward-shallowing shelf cycles are usually capped by offshore to tidal-flat or paralic depositional facies. Megaporous TCIs are mostly associated with middle to lower parts of restricted-marine shelf subtidal cycle types. Megaporous GIs are typically observed in substrates that were firm during burrowing at subtidal cycle tops. In some cases, extreme dissolution of Ophiomorpha- or Thalassinoides-dominated ichnofabrics has a major impact on karst development, producing karst features, such as collapse structures.
Centimeter-scale ichnogenic megaporosity, associated with thalassinidean-generated ichnofabrics, measured using CT scans in Lower Cretaceous (Texas) and Pleistocene (Florida) rock samples are as high as 78%, and permeabilities calculated by Lattice Boltzmann methods exceed the upper range measured with laboratory permeameter. Borehole flowmeter data from Lower Cretaceous carbonate aquifers in Texas, and Oligocene, Pliocene, and Pleistocene aquifers in Florida indicate that thalassinidean-linked megaporosity produces concentrated groundwater flow. These examples suggest similar widespread ichnogenic megaporous stratiform flow occurs in carbonate reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013