Porosity data from 15 boreholes (0 to 5,500 m) in the South Florida basin show a trend of steadily decreasing porosity with depth (Fig. 1A). This trend is interpreted to result from “compaction” of carbonates in response to overburden pressure. Compaction is used here in a broad sense to include mechanical and chemical compaction, the latter encompassing carbonate dissolution and reprecipitation as burial cement (solution-transfer of Bathurst). Factors which contribute to the scatter about the trend include variations in depositional environment, diagenetic history, pore-fluid composition, pressure, age, geothermal gradient, and experimental error.
The compaction curve for south Florida (Fig. 1A) represents a composite of curves for different carbonate lithologies, including platform-interior limestone and dolomite. Curves for limestone and dolomite (Fig. 1B) illustrate that dolomite, although less porous than limestone at shallow depths, retains more porosity than limestone during burial, and is more porous than limestone below 2,000 m. Below 4,500 m, porosity greater than 5% occurs primarily in dolomites, an observation commonly made for the deeper parts of Paleozoic sedimentary basins.
If porosity reduction were due to cement derived from dissolution at the surface or from outside the basin, the curve (Fig. 1A) would simply be a record of increasing cementation with depth. This would require the removal of between 1,000 and 1,500 m of carbonate rock from somewhere within the section or from carbonates outside the basin. It seems more likely that the cement is locally derived. If mass transfer of carbonate is limited to a local scale, then the curve is a true compaction curve.