ABSTRACT

The Fredericksburg of South Florida has been overlooked as a primary oil target. Previous exploration in the area has emphasized the Lower Cretaceous oil-producing Sunniland formation which is lower in the stratigraphic section. Many wells in the Fredericksburg Dollar Bay formation yield hydrocarbon shows in limestone biohermal deposits and in an upper dolomitic section. The Dollar Bay formation is part of the Big Cypress Group of the Comanchean Series of the Lower Cretaceous. The Lower Cretaceous is composed of evaporite-carbonate cycles consisting of anhydrites, dolomites, and limestones. Also present are calcareous shales, mudstones, salt, lignitic material, and carbonaceous material, especially in the anhydrite and limestone intervals. The limestones are usually micritic, chalky, calcarenitic, sometimes argillaceous, and contain skeletal particles of gastropods, pelecypods, ostracods, algae, and forams, along with vadoids, ooids, oncolites, grapestones, and fecal pellets. The color of the limestone ranges from white to gray to tan to dark brown. The anhydrite is nodular, microcrystalline, or crystalline and is usually bedded. The dolomite crystals are euhedral to sucrosic and occur in about 30% of the formation. The expected production in the Dollar Bay formation will be from a leached calcarenite or a dolomite. Examination of cores and cuttings suggests a trapping mechanism and reservoir quality and content similar to the oil pay-zone of the Sunniland formation in Felda field. In the wells examined, the reservoir porosities range from 13.6% to 28.7%, with permeabilities ranging from 4.9 md to 119.0 md in a dolomite section, while in the limestone the porosities range from 9.2% to 32.7% with permeabilities from 5.2 md to 59.0 md. More than twenty wells have been evaluated for lithology, porosity, diagenesis, cementation, and for evidence of the direction of hydrocarbon migration. All indicators suggest that the Dollar Bay formation should produce oil similar to that of the Sunniland formation but from an area updip and northeast of the existing oil producing trend.