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

This study examines the porosity of limestones and dolomites in the South Florida basin using data derived from borehole-gravity measurements and from suites of acoustic, neutron, and density logs. These wire-line measurements are averaged into intervals of relatively uniform log character with vertical scales approaching those of aquifers and hydrocarbon reservoirs, and are combined into regional data sets that are further smoothed by regression-line representations. Thus, the results presented here are based on a large sampling scale that is uncommon in carbonate studies.

Results are summarized as follows:

1. The decrease of carbonate porosity with depth for a composite section representing a wide range of depositional environments and diagenetic histories can be characterized by an exponential curve (equation 1).
2. Average porosity of the composite carbonate section is reduced by a factor of two in a depth interval of about 5,700 ft (1,740 m); published curves for deep-water carbonates show a greater rate of porosity decrease with depth.
3. Although the evidence is not conclusive because of limited age differences, the decrease of carbonate porosity appears to be primarily a function of depth rather than of time.
4. Porosity loss in the shallow-water carbonate rocks of south Florida appears inversely related to the magnesium content of pore waters.
5. Dolomite porosity is lower than limestone porosity in the near surface, but does not decrease as rapidly with depth. Below about 5,600 ft (1,700 m), dolomite is, on the average, more porous than limestone.

The data developed here indicate that regional-porosity relations in the South Florida basin are not primarily dependent upon the imprint of the highly variable surface and near-surface environment, but relate instead to more predictable parameters of burial. Although the particular numerical results presented here should be applied with considerable caution to areas other than south Florida, the form of the porosity-depth relations may result from basic properties of carbonate rocks, and thus represent a more general case.

A direct implication of the data presented here is that the porosity of Cenozoic and Mesozoic carbonate rocks in a given exploration province may be empirically predictable on a regional scale as a function of depth. A less direct implication of the same data is that most shallow-water carbonate sediments enter the subsurface with high porosities relative to the requirements of hydrocarbon reservoirs (Figs. 5, 9), regardless of early depositional and diagenetic history. A change in geologic emphasis, from the establishment of correlations between early history and porosity evolution in the near surface to the establishment of correlations between early history and porosity preservation in the subsurface, could prove to be of significant economic value.

A next step in this study (and one being pursued) is to examine regional carbonate-porosity trends in other basins, using similar large-scale measurement methods. Such studies, whether confirming or refuting the hypotheses presented here, should contribute to the understanding of porosity in carbonate rocks.