Stephen O. Sears
The Sunniland Limestone – Why is there oil below the Everglades and not Orlando?
Updated: Dec 31, 2019
The occurrence of oil in peninsular Florida, at least as far as discoveries to date, is restricted to fields in the southernmost part of the peninsula. Unfortunately, this is in the most environmentally sensitive portion of the state, below the beautiful wetlands of the Florida Everglades. The technology of “fracking has made new hydrocarbon development of this area economically feasible. For those interested in why the oil occurs where it does, this article is an explanation of the geology that controls the location of the oil accumlations. Eventual development, or the lack of it, will depend on societal forces and political will. The geological facts, however, were put in place during the Cretaceous period, over 100 million years ago.
The only geologic formation known to contain oil in the Florida peninsula is the Sunniland Limestone (also referred to as the Sunniland Formation and the Sunniland Shale, although it is not a true shale). When the Sunniland Limestone was deposited, the area south of Lake Okeechobee resembled the Florida Keys and Bahamas of today. Clear water varying in depth from shallow flats to a deep basin to the west resulted in a variety of sediments being deposited. These included isolated patch reefs, similar to the smaller reefs found in the Florida Keys today. An example is the famous Hens and Chickens snorkeling site offshore from Islamorada. These reefs are not continuous, but have a limited extent of several hundred to over one thousand acres. They formed at the edge of the South Florida Shelf, where the shallow waters of the shelf transitioned rapidly into the deep water of the South Florida Basin. In addition to the coral that we see today, the reefs were built by a now extinct species of large clams, call rudists. These clams grew as vertical cones, reaching over two feet in height. Over millions of years, these reefs were buried to a depth of about two miles, and form the oil fields found to date in the Sunniland.
The oil was not formed in the reefs, however, but in a lower part of the Sunniland, a fractured dark limestone. This part of the Sunniland was likely deposited in deeper water to the west of the shelf edge. When these rocks were buried and exposed to a temperature of about 200 degrees F, fossil remains of algae incorporated into this part of the Sunniland rock was transformed into crude oil. Some of the oil migrated upward into the overlying reefs, which trapped the oil and formed today’s oilfields.
With the advent of fracking, oil can potentially be produced directly from the dark, fractured limestone part of the Sunniland. This would be a new phase of development of the Sunniland, in addition to wells drilled into the reefs that can be produced without fracking. This is the same situation that occurs in the Permian Basin in West Texas, where fracking has allowed production from wells that would not produce otherwise, and started a new phase of drilling activity.
The map above, from Sears (1974) , shows a curving line marking the shelf edge of the South Florida Basin. This transition from deep to shallow water is where the patch reefs that form the oil fields found to date are located. It is likely that more undiscovered reefs containing oil exist below the surface along this line.
The dark limestone exists below the reefs and to the west of the line showing the shelf edge, but in the same general area. This means that any future oil discoveries, either in reefs or in the dark limestone, are likely to be in the southwest part of the Florida Peninsula, approximately south of Fort Myers. The formation is not found north of the line on the map showing the approximate updip limit, approximately at the northern edge of Lake Okeechobee. This line represents the shoreline during the Cretaceous period when the Sunniland Limestone was deposited, and dry land existed to the north.
In summary, the likelihood of the existence of undiscovered oil is highest south of Fort Myers. It is essentially zero north of Lake Okeechobee. A justified debate is taking place over future oil exploration in South Florida, especially in the context of global warming and the preservation of the Everglades. An understanding of the geology is required for a meaningful discussion, and the intent of this article is to provide that.
The initial discovery of oil in the Sunniland Limestone during WWII is setting for my recently published novel, Sunniland.