Abstract

The Barnhart Field is an excellent example of a carbonate reservoir in which part of the pore network is controlled by collapsed paleocave facies. The Ellenburger Group at Barnhart Field has undergone a complex history of fracturing and karsting associated with multiple uplift and exposure events. This is evident from the Goldrus Unit #3 well core. However, the extent of these karst-related breccias and fractures in the field is difficult to determine because of the poor quality of wireline logs available and the lack of other cored wells. Nevertheless, we used log correlations to define the probable extent of karsting in the field. Areas of the field with non-correlative wells are interpreted to correspond to zones of brecciation produced by cave formation and later collapse. Log-based correlation discontinuities also suggest the presence of west-trending faults. Recognition that apparent breaks in correlations may define collapsed brecciated bodies provides a potential basis for the mapping of the paleocave breccias.

The paleocave facies present in the Goldrus Unit #3 core include: (1) collapsed-cave ceiling facies, (2) coarse-clast collapsed-cavern chaotic breccia facies, (3) fine-clast transported chaotic-breccia facies, (4) cave-sediment fill facies, and (5) speleothem. The collapsed slabs, blocks, and clasts show a strong overprint of crackle brecciation. Some crackle breccias formed early in 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 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 regional fracture pores. Some chaotic breccia and crackle breccia pores are partly to fully occluded with baroque dolomite or coarse-crystalline calcite. The baroque dolomite is interpreted to be related to hydrothermal fluids associated with the Ouachita Orogeny that occurred during Pennsylvanian time. These fluids had little effect on creating pores but had a great affect on occluding pores.