Lower Ordovician Ellenburger Group Collapsed Paleocave Facies, Associated Pore Networks, and Stratigraphy at Barnhart Field, Reagan County, Texas

COMBS, D.M., R.G. LOUCKS, and S.C. RUPPEL, Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, 78713

The Barnhart Ellenburger field, located in the southeast corner of Reagan County, Texas, is an excellent example of a carbonate reservoir that contains a complex stratigraphy, and a pore network that is a function of collapsed paleocave facies. Barnhart field is a Lower Ordovician dolomite reservoir which covers an area of approximately 36 square miles and is overlain by Wolfcampian shale. The contact between the Ellenburger dolomite and the overlying shale is a composite unconformity. As is evident from the Goldrus Unit #3 core, the Ellenburger at Barnhart field has undergone a complex history of fracturing and karsting associated with several unconformities. Through the use of SP and resistivity wireline logs, zones of porosity, karst and collapse, and fault offset have been correlated throughout the field.

Paleocave facies in the 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. 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 study of Barnhart field is an excellent example of how the creative use of old wireline logs can be a valuable tool in determining karst/collapse and fault features.