Geounion-AAPG Petroleum Geoscientist Series
Speaker: Paul M.(Mitch) Harris ( Adjunct at Rice University, Ex – Chevron)
Talk Title – Morphometric Comparison of the Pleistocene Miami Oolite and Modern High-Energy Sand Bodies of Great Bahama Bank
Paul (Mitch) Harris and Sam J. Purkis, CSL – Center for Carbonate Research, University of Miami, Department of Marine Geosciences
Continued interest in modern and outcrop analogs for carbonate sand reservoirs is warranted based on the substantial number of these types of reservoirs. The spatial variability of depositional environments and early diagenetic overprint that potentially creates reservoir heterogeneity within a fossilized carbonate sand system can be observed in outcrops of the oolitic facies of the Pleistocene Miami Limestone. Particularly informative are bare-earth airborne LiDAR (Light Detection And Ranging) data acquired over the breadth of the deposit to create more accurate elevation maps for flood assessment. We use the LiDAR to interrogate bar and channel patterns of the Miami oolite and compare results with those from our previous studies of modern counterparts on Great Bahama Bank.
The Miami oolite was deposited as mainly marine sand bars (or shoals), tidal channels, and a barrier bar during the last interglacial highstand – Marine Isotope Stage 5e – when sea level was ~7 m higher than today. The oolitic facies is a wedge-shaped unit reaching its maximum thickness in the barrier bar along the seaward edge of the deposit, whereas a widespread platform interior bryozoan-rich facies occurs to the west. The spatial dimensions of the Miami oolite are comparable to modern ooid shoal systems; the bar and channel portion, 95 km long and 15 km wide, consists of tidal bars and numerous tidal channels. Within the sand body, tidal bars cover approximately 324 km2, or 32% of the area, whereas the tidal channels form the remainder.
The dip and strike extents, as well as shoal morphology, of the Miami oolite and modern Exumas Cays sand body, are similar. The shoals from both display similar diversity in shape, whereas the modern shoals of the Schooner Cays and Tongue of the Ocean adopt a much broader span of shapes. These results confirm that in terms of shoal morphology, the Exumas Cays offers the best analogue to the Miami oolite. Channel patterns show the cumulative lengths of the channels of the Exumas Cays and Miami oolite are conspicuously short (198 km and 229 km, respectively) and these two sites also have the fewest channels (62 and 43). Channels in the southern reaches of the outcrop belt have low offset angles to the platform margin, reflecting either the N-S drainage and a karst origin or depositional origin related to the barrier bar which fronts the deposit.