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Reservoir Implications of Facies and Diagenetic Variability in an Oolitic Grainstone - Pleistocene Miami Oolite


The Miami oolite (MO) of South Florida is representative of a grainstone-rich reservoir layer (high-frequency sequence) that has been surficially karsted (eogenetic karst), and therefore may be considered an analog for subsurface examples with “high” matrix porosity-permeability and localized touching-vug porosity. The deposit can potentially serve to illustrate heterogeneity in this type of reservoir, as imparted by facies changes and early meteoric diagenesis.

The MO displays the preserved morphology of a fossilized ooid sand body, even though it has been subaerially exposed in a tropical climate since its deposition during the last interglacial highstand - Marine Isotope Stage 5e. The depositional motif is one of a dip-oriented tidal bar belt of shoals and shallow channels fronted by a strike-oriented barrier bar. The barrier bar comprises cross-stratified grstns and locally bioturbated gr/pkstns, whereas the tidal shoals and channels are more commonly bioturbated pk/grstns. Surficial karst features (dolines and stratiform caves) have been added during the ~120 ky of subaerial exposure.

Since the MO is the uppermost portion of the Biscayne Aquifer, a rich understanding of fluid flow through the deposit exists and sheds valuable insight to the larger-scale permeability patterns and reservoir implications of facies and diagentic overprint. The pore system comprises matrix porosity (interparticle and separate vugs) and touching-vug macroporosity that is commonly ichnologically influenced (associated with burrowed [Ophiomorpha] intervals). GPR, well, and flow-test data indicate that matrix porosity provides most of the groundwater storage, whereas the various types of touching vug macroporosity account for the majority of flow. The dolines and shallow caves seem to be sufficiently spaced to prevent direct connection, with the result that they are less important in terms of regional flow than the prevailing pore system.

An important “So What” from the observations of the MO reported here is that a depositional facies (burrowed intervals) has directed early-stage dissolution (creating touching-vug macroporosity) to produce the stratiform high-permeability zones that dominate flow at the larger scale. Thus, a profound implication for analogous grainy, karsted reservoirs is that a fundamental understanding of depositional facies variation remains critical for characterizing reservoir quality and performance, even in cases of substantial diagenetic overprint.