Retention of High Permeability During Shallow Burial (300 to 500 M) of Carbonate Grainstones

摘要

A progressive decrease in matrix permeability occurs with burial in the Paleogene limestones of west-central Florida, yet grainstones with permeabilities > 400 md persist below burial depths of 300 m. Why some grainstones retain high permeability during passage through the shallow-burial realm was investigated using 85 dominantly foraminifera and micritized-grain grainstones from the Eocene Avon Park Formation. Twenty-seven variables representing diagenesis, stratigraphic packaging, depositional energy, grain types, and pore-network attributes were quantified. Permeabilities range from 23 to 7,160 md and thin-section porosities range from 14% to 36%. Grain size (lower coarse to upper fine sand) covaries positively with sorting (poor to very good). The samples are from three different systems tracts within the Avon Park, and sampled grainstone bodies ranged in thickness from 0.5 to 3.5 m. Evidence of diagenesis include mechanical and chemical burial compaction, pre-compaction cements, and dissolution. Original interparticle porosity loss due to compaction (COPL) and cementation (CEPL) range from 0 to 35.3% and 1.6 to 26.2% respectively. Most original intraparticle porosity is occluded, but most moldic porosity, which ranges from 1.2% to 12%, remains open.