ABSTRACT

Analyses of core samples obtained from oil and gas tests in the South Florida basin indicate that the Cretaceous carbonates of Comanchean age are relatively richer in average organic carbon (0.41%) than those of Coahuilan age (0.20%) and Gulfian age (0.18%), and richer than those of Paleocene age (0.20%). They are also nearly twice as rich as the average for carbonates worldwide (0.24%). The majority of carbonates have organic carbon values less than 0.30 percent, but the presence of many beds relatively rich in organic matter composed of highly bituminous, argillaceous, highly stylolitic, and algal- or sapropel-bearing limestone and dolomite accounts for the higher percentage of organic carbon in some of the stratigraphic units.

Carbonate rocks that contain greater than 0.4 percent organic carbon and that might be considered as possible petroleum source rocks were noted in almost every subdivision of the Lower Cretaceous section, and particularly in units of Fredericksburg `B' of Exxon (Dollar Bay Formation of Winston), Trinity `A' and `F'(of Exxon) and Sunniland Limestone. Rocks of Gulfian and Paleocene age are generally relatively low in organic carbon; however, bituminous carbonates having as much as 3.2 percent or more of organic carbon were observed in the lowermost Gulfian Series. Oil stains and "dead" and "live" oil have been noted by others in the uppermost Gulfian and uppermost Paleocene.

The South Florida basin is sparsely tested and, to date, produces hydrocarbons only from the Sunniland Limestone at an average depth of 11,500 ft. (3,500 m). Because the Sunniland contains good reservoir rocks and apparently adequate source rocks, and because the rate of new oil-field discoveries has increased in recent years, the chances of finding additional oil reserves in the Sunniland seem promising. Furthermore, the presence of reservoir rocks and possible source rocks in many additional stratigraphic units, such as those of Fredericksburg age, should provide incentive for exploring these additional units.