From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

2003 West Texas Geological Society Fall Symposium, Midland, Texas, October 8–9, 2003


Lower Ordovician Ellenburger Group Collapsed Paleocave Facies and Associated Pore Network in the Barnhart Field, Texas

D. M. Combs, R. G. Loucks, and S. C. Ruppel

Abstract

The Barnhart Ellenburger Field, discovered in 1941, is an excellent example of a carbonate reservoir where part of the pore network is controlled by collapsed paleocave facies. This Lower Ordovician karsted, fractured dolomite reservoir, which is located in the southeast corner of Reagan County, Texas, is currently undergoing a tertiary high-pressure-air-injection program in order to recover the large volume of oil that remains in the reservoir. The field is approximately 36-square miles in size and is a structural trap with four-way closure. The Wolfcampian shale is the top seal. A composite unconformity forms the contact between the Lower Ordovician Ellenburger dolomite and the overlying Permian Wolfcampian shale.

The Ellenburger at Barnhart Field has undergone a complex history of fracturing and karsting associated with several composite unconformities. This is evident from the description of the Goldrus Unit #3 well core, but the extent of these karst-related breccias in the field is not obvious because of the poor quality of wireline logs and the lack of cored wells. Accordingly, stratigraphic correlations by wireline logs alone are difficult. However, we successfully used SP and resistivity log signatures from 105 wells in the field to correlate several zones of moderate porosity. Areas of the field in which correlations break down are interpreted to correspond to zones of brecciation produced by cave formation and latter collapse. Recognition that apparent breaks in correlations may define collapsed breccias provides a potential basis for the mapping of the buried paleocave system. Therefore, old resistivity logs can be a valuable tool when attempting to characterize a reservoir with zones of paleocave-collapsed breccias.

The paleocave facies present in the Goldrus Unit #3 core include zones of collapsed slabs and blocks, transported chaotic breccias, debris flow and suspension cave-fill deposits, and speleothems. The collapsed slabs, blocks, and clasts show a strong overprint of crackle brecciation. Some of the crackle brecciation formed during early the cave history as indicated by geopetal sediment in the fractures. Other crackle breccias formed during burial by mechanical compaction and contain no cave-sediment fill.

The average porosity and permeability within the field are 3.2% and 7 millidarcys (md), respectively. Matrix porosity ranges from 1.2 - 19.7 % and matrix permeability ranges from 0.02 - 48.2 md in the areas unaffected by cave development. The pore network in the dolomite matrix consists of intercrystalline pores.

The pore network in the collapsed paleocave facies consists of several pore types: (1) fine interclast pores, (2) large solution vugs, (3) crackle breccia fractures in blocks, slabs, and smaller clasts, (4) interparticle pores in the detrital carbonate matrix, and (5) possible fracture pores. The average porosity and permeability values in the Goldrus Unit #3 core, within the collapsed paleocave facies, are 4.8 % and 3.4 md, respectively. Some chaotic breccia and crackle breccia pores are partly to totally occlude with baroque dolomite or coarse-crystalline calcite. The baroque dolomite appears to be related to hydrothermal fluids associated with the Ouathica orogeny that occurred during Pennsylvanian time. These fluids had little effect on creating pores but had a great affect on occluding pores.
Reservoirs containing collapsed paleocave reservoirs produce complicated pore networks with strong heterogeneity. Cores and image logs are necessary to describe the reservoirs in detail. In the absence of cores and image logs, creative use of old wireline logs is necessary.