Compaction of Carbonate Grainstones and the Destruction of Permeability —
a Simple View of the Hole Story

Dr. David Budd

Associate Professor of Geological Sciences.
Fellow, Energy and Minerals Applied Research Center
Ph.D., 1984, University of Texas, Austin

The relationship between matrix permeability and depth in shallow-buried (<500 m) Cenozoic limestones was investigated using minipermeameter measurements on 1,210 meters of slim-hole core. Only packstones and grainstones exhibit a systematic reduction in permeability with depth, and depth only explains 22% to 31% of the permeability variance in those rocks. The balance of the variability is attributed to textural parameters and pre-burial cementation. Extrapolation of the depth trends suggests that (1) permeability is more sensitive to burial compaction than porosity, and (2) reservoir-grade limestones in the deeper subsurface were most likely those grain-supported limestones that had higher than average matrix permeabilities in the near surface.

Petrographic observations indicate that mechanical compaction drives permeability reduction above 335 m (~500 md). Linear grain contacts and embayed grains become more abundant and close packing textures become more homogeneously distributed as permeability decreases. However, there is no concurrent change in the amount of pore space accessed by large or small throats, indicating that mechanical compaction does not uniformly constrict or seal pore throats. Mechanical compaction apparently reduces permeability by reducing porosity, lengthening pore throats, and increasing tortuosity.

Below 335 m (~500 md), grain-to-grain pressure solution dominates. This constricts and eliminates pore throats and isolates pore space, resulting in an increase in the pore volume accessed by small throats. However, the most effective pore radii are still >1 mmm at tens of millidarcies of permeability. Such low amounts of permeability can be reached even when the pressure solution is not pervasively distributed throughout a sample.