Abstract

Relationships between matrix permeability and depth in shallow-buried (< 500 m) Cenozoic limestones of Florida were investigated using more than 12,000 minipermeameter measurements on 1210 m of slim-hole core. These data are considered representative of the matrix permeability reduction that accompanies early burial of shallow-water, subtidal to peritidal, nonreefal carbonates of sta ble cratonic margins, platforms, shelves, and ramps.

Measured matrix permeabilities range from less than 4.0 to 6450 md for both limestones and dolostones, although the median permeability of limestones (88 md) exceeds that for dolostones (15 md). Sucrosic dolostones, which have high intercrystalline po rosities, have median matrix permeabilities of 1000 md. Non sucrosic dolostones, which are much less porous, have median matrix permeabilities of only 13 md. Neither population of dolostones exhibits a permeability-with-depth trend.

Low-permeability limestones consist of mudstones, wackestones, (tidal) laminites, pedogenetically altered limestone, lime mud-rich breccia, and well-cemented grain-supported rocks. These low-permeability limestones all exhibit median matrix permeabilities less than 35 md, and none exhibit permeability-with-depth trends. In contrast, high-mud packstones, low-mud packstones, very low mud packstones, and grainstones exhibit larger median matrix permeabilities (69, 129, 181, and 418 md, respectively) and a systematic reduction with depth in their mean and maximum permeabilities. Least-squares exponential regressions indicate that burial depth explains 22-31% of the permeability variance in these grain-supported rocks; the balance of the variability within each textural class is attributed to variation in facies (grain types, grain sizes, sorting, etc.) and the amount of early, preburial cementation. In the absence of permeability enhancement by deep subsurface diagenesis or fracturing, extrapolation of the permeability-with-depth trends suggests that the limestones that have the best reservoir potential in the deeper subsurface are grain-supported limestones that have higher than average matrix permeabilities in the near-surface realm.

Petrographic analyses and consideration of diagenetic histories suggest that matrix permeability of the dolostones and low-permeability limestones was determined by early near-surface dolomitization and cementation, respectively, and has yet to be affected by burial processes. Petrographic observations also suggest that the depth signal in grain-supported limestones results from mechanical compaction and the onset of grain-to-grain pressure solution.

Extrapolation of the matrix permeability data indicates that permeability loss with burial occurs faster than the documented burial-related porosity loss for the same group of rocks. This indicates that limestone permeability is more sensitive to burial compaction than limestone porosity. Thus, preservation of reservoir-quality rock during burial is more dependent on permeability preservation than on porosity preservation.

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