Abstract

Lower Permian (Wolfcampian and Leonardian) shelfal carbonate deposits are widespread around the margins of the Midland Basin and within the “Horseshoe Atoll” trend that runs east-west through the northern portion of the basin itself. These “insitu” packstones, grainstones, and organic buildups host numerous oil and gas deposits, although the Wolfcamp and lower Leonard producing fields are dwarfed by the overlying giants of the upper Leonardian and Guadalupian. Geometry of the slopes that rimmed the lower Permian shelf margins varied around the basin, but regional seismic lines indicate that the slope profile was locally quite steep. At times, basinal water depths approached 1,000 ft. (330 m) below sea level. Shallow water carbonate components (both individual grains and lithified clasts) were transported into the Midland Basin as debris and grain flows during times of shelf margin instability. These “resedimented” carbonates constitute anomalous deposits, shallow water materials encased in basinal black shales. In some cases, they have been interpreted as in-place accumulations that formed downslope during times of sea-level lowstand, but examination of cores from several units indicates that many of them contain transported clasts that originated on the shelf and were carried into the basin.

Two carbonate debris flow examples are found in the Texaco Glasscock “N” Fee #3 and the Texaco Glasscock “K” Fee #4 wells, both located in the western part of Glasscock County, Texas. These wells cored approximately 100′(33 m) and 150′(50 m) of carbonate and shale from depths of 8000-8100′ and 8400-8500′ respectively. The cored carbonates are predominately limestone (virtually no dolomite) with up to 15% silica replacement in the Glasscock “N” Fee #3 core. The limestone intervals are composed of mud-rich to grain-rich packstones containing angular clasts of a variety of lithofacies in a matrix of discrete carbonate skeletal grains, pellets, and micrite. Clasts as large as 4” in diameter are observed. Lithologically the clast types include Archaeolithoporella/Tubiphytes boundstones, sponge boundstones, and a variety of pelletal/skeletal wackestones, packstones & grainstones. Discrete particles between the clasts are rich in individual pelmatozoan fragments, fusulinids, other benthic foraminifera, brachiopods, bryozoans, gastropods, sponges, and broken pieces of Tubiphytes, as well as pellets and micrite. Both cored wells display a 10′ (3.3 m) zone of Tubiphytes boundstone that is probably a very large block of debris that has been transported downslope. Additionally, the Texaco Glasscock “K” Fee #4 well contains two intervals of ooid/pelletal, fusulinid bearing grain-rich packstone to grainstone. These units are distinctly different from the debris flow units and are interpreted as grain flows with a timing and/or source distinct from the debris flow units.

Both Texaco Glasscock wells are productive. The “K” Fee #4 had an initial IP of 433 BOPD and 691 MCFD when it was discovered in November, 1989, and the “N” Fee #3 had an initial IP of 1,038 MCFD upon discovery in June, 1991. Current production (August 1998) is 1 BOPD and 15 MCFD for the “K” Fee #4 and 34 MCFD for the “N” Fee #3. Porosity ranges from <1% to 18% and permeabilities from < 1 md to 75 md in the “K” Fee #4 well. Porosity is highest in the leached moldic pores of the grain flow units, but much of it is poorly connected, resulting in lower overall permeability in these units. Porosity ranges from 2% to 15.9% and permeabilities from <1 md to >100 md in the “N” Fee #3 well. This well produces mostly gas, although abundant oil stain is present in its pores. Possibly much of the original oil was drained by surrounding wells that were drilled prior to the completion of the “N” Fee #3.