Abstract: Development of Reservoir Facies and Porosity-Permeability Compartmentalization in Middle Mississippian Salem Limestone, Eastern Margin of the Illinois Basin

Nadeem Ahmad, Brian D. Keith, Todd A. Thompson

Shoal facies of the Salem Limestone in south-central Indiana are composed of 1) bryozoan- and echinoderm-rich (85-95%) grainstones with sparse amounts of other skeletal and nonskeletal grains, 2) fossiliferous oolitic grainstones, and 3) foraminiferal-echinodermal grainstones. Microfacies and bedform analyses indicate that deposition took place under tidal conditions where sporadic storm events mobilized the entire sand body. Strong, primarily unidirectional currents, abundant sediment supply, and rapid burial resulted in thick, massive deposits in the Salem shoal.

In this hydraulic regime, shape and specific gravity determined the distribution of echinoderm grains relative to heavier bryozoan grains. The relative amount of echinoderms controlled the development of early syntaxial rim cement, which preserved porosity during early compaction. A significant amount of calcium carbonate (~58%) filled the micropores within the echinoderm grains before the intergranular cement could form. Areas such as toes of avalanche foresets, where intermediate amounts of echinoderms are mixed with other grains, have greater porosity and permeability. Numerous stylolites also occur throughout the Salem grainstones. Variations in microfacies composition and bedforms are grouped into architectural packages that compartmentalize the porosity and permeability and form micro to macroscale heterogeneities.

The basal channel architectural package studied exhibits porosity-permeability compartments orthogonal to the overlying channel-fill sandwave package. In an analogous petroleum reservoir this can result in poor reservoir performance during primary and enhanced recovery leaving behind significant amounts of unrecovered mobile oil. Higher porosity and permeability at the base of a package due to the presence of toesets of a certain grain composition can result in poor sweep efficiency for enhanced recovery.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York