NEW INFORMATION FROM CORES PROVIDES UPDATES TO KNOWLEDGE OF THE ELLENBURGER GROUP IN CENTRAL TEXAS AND COMPARISONS WITH PRODUCTIVE WEST TEXAS ELLENBURGER DEPOSITS

The Lower Ordovician Ellenburger is a very prolific hydrocarbon reservoir in parts of the Permian Basin. In the subsurface of west Texas it is completely dolomitized or silicified, hence most of its original depositional textures and grain types are obscured or obliterated. The Ellenburger Group of central Texas is equivalent to the productive Ellenburger of the Permian Basin and nonproductive El Paso group in far west Texas. The central Texas Ellenburger rocks are not completely dolomitized and hence can provide more information about the original depositional environments, cyclicity and controls on reservoir development.

Some mineral exploration cores taken in shallow wells on the north and west sides of the Llano uplift cut the entire Ellenburger Group. They are currently being studied at The University of Texas of the Permian Basin as part of an ongoing Cambrian/Ordovician research project. Results from the first two cores (the Johanson in McCullogh County and the Glaze in San Saba County) are incorporated with the classic Ellenburger outcrop work published in the late 1940’s.

The cores confirm Cloud and Barnes’ (1948) conclusions that the Ellenburger Group thins from northeast to west due to post-depositional erosion of the upper Honeycut and middle Gorman Formations. Despite a strong diagenetic overprint in portions of both of the cored wells, several types of depositional cycles were identified. They include: 1) tidal flat/exposure capped cycles, 2) high energy grainstone shoal capped cycles, 3) rip-up clast capped cycles and 4) open marine, fossiliferous cycles.

The Johanson and Glaze cores have been described and incorporated into a cross section hung on the Cambrian San Saba/Ordovician Ellenburger contact. Several of Cloud and Barnes’ (1948) outcrop measured sections have been recreated at the same scale as the cores and correlated with them for comparison.

There are several Ellenburger producing fields on the southern and southwestern flanks of the Llano Uplift. While these fields are not large enough to be of interest to major oil companies, they show the potential for central Texas Ellenburger production. They were mostly discovered by accident, and the possibility of additional, targeted Ellenburger discoveries should be of interest to mid-sized independents and smaller operators.