Facies Belts, Microfacies, and Karst Features of the Ellenburger Group, Kerr Basin, Texas: Observations Based on Cores

Geesaman, Richard C.; Wilson, James L.

Carbonate-dominated strata of the Lower Ordovician Ellenburger Group have potential for significant reserves of hydrocarbons in the sparsely explored Kerr Basin of southwest Texas. The Kerr Basin, located east of the gas-rich Val Verde Basin, is characterized by similar tectonic history and resultant structural style.

We described and interpreted 1200' of core from a total of ten wells. Analysis was restricted to macroscopic description including lithology, sedimentary structures, pore types, and allochems. Emphasis was placed on determining the depositional sequences and lithofacies, and characterizing the nature and extent of the karst overprint.

Wilson (1975) defined nine Standard Facies Belts "across a gently sloping shelf atop a platform with an abrupt shelf margin". This model with slight modifications is applicable to the Ellenburger of the Kerr Basin. We apply Wilson's facies belts to a ramp model, and not a composite ramp/platform model.

Ellenburger strata of the Kerr Basin have been deposited in five of the Standard Facies Belts, reflecting deposition in a broad low-relief low energy shelf or ramp setting. The facies belts include:

SFB 2: Open shelf facies
SFB 5: Organic buildup
SFB 6: Winnowed shelf carbonate sands
SFB 7: Restricted shelf/lagoon
SFB 8: Restricted shelf/tidal flat

An extended period of subaerial exposure affected the upper strata of the Ellenburger Group. This lengthy hiatus or "composite unconformity" produced the extensive karst collapse features oftentimes characteristic of the Ellenburger. Karst-related dissolution and collapse overprints much of the Ellenburger strata, locally resulting in breccias, cave fill, and/or fractured cave roofs..

Ellenburger strata of the Kerr Basin were deposited in similar environments as equivalent strata in the Val Verde Basin, are comprised of the same lithofacies, and were exposed to similar diagenetic processes. It is probable that reservoir-quality porosity and permeability will be preserved, and these reservoirs could be hydrocarbon productive in structural closures in this under-explored frontier basin.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013