David A. Budd1
(1) University of Colorado, Boulder, CO
Abstract: Compaction, cementation, and permeability loss with depth in carbonate rocks
It has been argued that porosity loss in carbonates is a function of initial porosity (zero-depth) and chemical compaction, with reservoir-quality porosity representing conditions of unique porosity preservation. Unclear is whether similar generalities can be made about permeability. This question was investigated using shallow-buried (0-500 m) Cenozoic grainstones and dolomites of Florida. Measured permeabilities range from <4 to 6,450 md with grainstones, sucrosic dolomites, and non-sucrosic dolomites having mean permeabilities of 420, 1000, and 23 md, respectively. All three lithofacies show a wide variation in permeability at any one depth, and grainstones show a systematic decrease in permeability with depth.
Petrographic analyses indicate that dolostone permeability was determined by near-surface dolomitization and has been unaffected by burial. Permeability variations at any one depth in grainstones result from early near-surface cementation, but the depth signal is driven by mechanical and chemical compaction. Permeability loss with depth in the grainstones has a half-depth of 130 m, which is significantly less than the reported half-depth of 1,200 m for porosity loss in the same units. Petrographic point-count data indicate that a small increment of pore-reducing near-surface cement destroys more permeability than an equivalent amount of pore-reducing burial compaction.
Collectively, the data indicate that permeability loss is similar to porosity loss in that both are dependent on zero-depth values and burial history. They differ in that permeability loss is clearly impacted by early near-surface cementation, and exhibits a more sensitive response to burial processes.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana