Melzer, Susan E.1, David A. Budd1, Anthony J. Park2
(1) University of Colorado, Boulder, CO
(2) Laboratory for Computational Geodynamics, Indiana University, Bloomington, IN
ABSTRACT: Retention of High Matrix Permeability in Carbonates During Progressive Burial
The study of how pore systems evolve in carbonates has traditionally focused on porosity changes. Consequently, we do not fully understand how surface variations in permeability are translated to the deep subsurface, a statement made by Scholle & Halley (1985) and still true today. Accordingly, Paleogene grainstones of west-central Florida with permeabilities of 20 to 5000 md and buried below 300 m were examined to quantitatively decipher the roles of depositional and diagenetic factors in the preservation of the higher permeabilities.
The samples are skeletal grainstones dominated by foraminifera, micritized bioclasts, and green algae. Mean grain sizes are 260 to 630 um (medium- to lower coarse-grained sand), and size positively covaries with sorting (R=0.78). Multivariate regression reveals that 64% of the variance in permeability is attributed to variations in grain size and the abundance of micritized grains and intraclasts. Sorting and abundance of other grain types have no statistically significant impact on permeability.
The remaining variance in permeability relates to how each grainstone responded to preburial cementation and burial-related compaction. Low permeability occurs in grainstones that experienced either significant early cementation or little early cementation but significant burial-related compaction. High permeability is preserved in samples with no appreciable cementation and minimal compaction. Reaction-transport-mechanical process simulations indicate that increasing amounts of early cement retards compaction. Whether there is an optimum amount of early cement that inhibits compaction, but is not so great as to have destroyed high matrix permeability, is suggested but not clearly resolved by the petrographic data and simulations.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.