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

Richard C. Geesaman¹, John V. Leone², and James L. Wilson³

¹Indian Peaks Exploration, Boulder, CO
²Whiting Oil and Gas Corporation, Midland, TX
³University of Michigan, Ann Arbor, MI

Abstract

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 is adjacent to and east of the gas-rich Val Verde Basin, and is characterized by similar tectonic framework and structural style. Though commercial production has yet to be established, significant and enigmatic shows were encountered in several wildcat wells, including the Whiting #1 Tar Cedar in Bandera County.

Wilson (1975) defined nine Standard Facies Belts to characterized the depositional facies of carbonate strata. This model with slight modification is applicable to Ellenburger Groupin the Kerr Basin, which were deposited on a broad low-energy ramp setting. Here deposition occurred in five of the Standard Facies Belts, which include:
SFB 2: Open shelf faces
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 overprinted upper strata of the Ellenburger Group. This hiatus or "composite unconformity" resulted in the extensive suite of karst features evident in cores. More than 35% of the core footage described in this study is characterized by a pronounced karst overprint, such as various types of breccias, cave fill, infiltrated sediment, and/or fractured cave roofs. These karsted carbonates are key to the development of hydrocarbon reservoirs, as there is typically limited primary porosity in the Ellenburger.

Ellenburger strata of the Kerr Basin were deposited in similar environments as age-equivalent strata in the adjacent Val Verde Basin. Moreover, these rocks are comprised of the same lithofacies and were exposed to similar diagenetic processes, with an emphasis on subaerial exposure leading to karst development. However, to date there have been no major hydrocarbon discoveries despite the drilling of a number of wildcat wells, some with impressive shows. The Shell #1 Signal-Tarbutton wildcat was drilled in 1971, and tested up to 240 BOPD from the Ellenburger with indications of significant overpressure. This well was ultimately abandoned due to low product prices of the period. In 2011-2012, Whiting drilled a 10,000' twin to the original Shell well, the #1 Tar Cedar (< 75' from surface location). This well encountered a hydrocarbon saturated column throughout the Ellenburger which extended into the Cambrian section. Oil, with no or limited formation water was swabbed in three separately tested intervals in the Ellenburger, following acid stimulation. The oil rates from the swab tests, however, were not commercial. Significant lateral heterogeneity, resulting in poor reservoir quality is attributed to extensive karst development.

AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014