Compaction of Carbonate Grainstones and the Destruction of Permeability
a Simple View of the Hole Story
Dr. David Budd
Professor of Geological Sciences.
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.