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Overlooked Gas Exploration Play: The Deep Lake Formation of Southern Florida

Hugh J. Mitchell-Tapping
EBML Inc, Fort Myers, FL

In southern Florida, the Triassic Deep Lake Formation has been overlooked as a viable exploration target as shallower horizons produce oil. Only about 20 wells have been drilled into the formation, most of them near or on the Ocala Uplift in central Florida. The Deep Lake sandstones were deposited in a shallow marine environment atop a rhyolitic basement. Only 3 wells have detailed descriptions of the sandstones that indicate that they were deposited, such as an offshore bar or barrier island, in shallow-marine environment, confirmed by the presence of fish remains. Reservoir seals are the overlying Jurassic marine shales, dense carbonates, dense dolomites and anhydrites. Source rocks are the deep offshore shales that are thermally mature by geochemical analyses. Oil stain is common and significant gas shows have been reported in some intervals, both in the basal clastics as well as in the overlying Jurassic carbonates. The quartz grains are generally clean, unimodal, and fine-grained, with formation thicknesses ranging from 37m downdip to 180m updip. The sandstone usually has excellent porosity, greater than 20%, with high permeability. Any cement present is usually calcitic, and clay and anhydrite have filled some of the pore-spaces in downdip wells. The application of models, petrology, and stratigraphy will enhance success and reduce exploration risk. Estimates of reserves are about 75 BCFG for a field, which makes the Deep Lake sandstones an extremely attractive unexplored horizon.