Abstract Introduction Geologic Setting Stratigraphic Setting Porosity Determination Data Set Porosity vs. Depth Depth vs. Age Effect of Pore- Water Composition Limestone & Dolomite Porosity Summary References

The wire-line data of this report are from wells within, and along the northern shelf edge, of the South Florida basin (Fig. 1). The South Florida basin comprises an area of about 77,000 sq mi (200,000 sq km) in southern peninsular Florida and adjacent waters to the west (Oglesby, 1965). The basin contains up to 20,000 ft (6,100 m) of Cenozoic and Mesozoic rocks, most of which are limestones, dolomites, and anhydrites (Pressler, 1947).

General depositional conditions in the basin continued more or less unchanged from the Early Cretaceous through much of the Tertiary, with carbonates and evaporates deposited by shallow seas upon a slowly subsiding landmass (Palacas, 1978). Within this framework, however, there were repeated cycles of transgression and regression (Banks, 1960; Oglesby, 1965; Winston, 1972; Puri and Winston, 1974, p. 9; Randazzo and Hickey, 1978).

Fig. 1 - Map of peninsular Florida showing locations of wells where borehole-gravity (squares) and conventional-porosity-log (circles) data were obtained. Also shown are the approximate boundaries of the South Florida basin and the Sunniland producing trend. Wells are identified in Table 1. [larger image]

The cyclic transgressions and regressions typical of the basin throughout its history produced lateral shifts of shallow-shelf, subtidal, intertidal, and supratidal environments. It follows that the wire-line data of this report, which sample the complex carbonate assemblage of the South Florida basin both laterally and vertically, represent most types of shallow-water carbonate depositional environments.

The present temperature gradient in the South Florida basin, based on corrected onshore bottom-hole well temperatures, averages about 1.0^o F/100 ft (1.8^o C/100 m) (Geothermal Survey of North America Subcommittee, 1976). In the upper part of the section, the geothermal gradient is locally affected by the circulation of fresh and salt water (Kohout, 1967). Zones of geopressure, to our knowledge, were not encountered in wells drilled to date.

Commercial hydrocarbon production in the South Florida basin occurs only along a northwest-southeast-trending fairway known as the Sunniland producing trend (Fig. 1). Here, 10 fields with total recoverable reserves of about 100 million barrels of oil produce at an average depth of 11,500 ft (3,500 m) from the Sunniland Limestone of Early Cretaceous (Trinity) age (Pontigo et al, 1979). Production is from low-relief depositional and compactional structures which, in Sunniland time, bordered the northeastern rim of a Cretaceous basin (Applin, 1960; Feitz, 1976). The likely source of the oil is organic matter within the Sunniland Limestone (Palacas, 1978).

The South Florida basin is particularly well-suited for the study of regional carbonate porosity because (1) it contains a nearly continuous sequence of carbonate rocks ranging in depth from the surface to about 20,000 ft (6,100 m) and in age from Pleistocene through Cretaceous; (2) it contains little clastic material, either disseminated in carbonate rocks or as separate facies; anhydrite is the only non-carbonate rock present in significant amounts; (3) present depths of burial are about equal to maximum depths of burial, so complicating effects of subsidence and subsequent uplift upon porosity are limited; and (4) the influence of tectonism, geopressures, and hydrocarbon-containing pore fluids has been minimal.

The South Florida basin is thus a setting where carbonate porosity can be determined on a regional basis over a wide range of ages and depths, with porosity evolution affected by a minimum of complicating factors relative to many other basins.