ABSTRACT

The completion of the Southland Royalty No. 1-A Harris in the Austin Chalk formation on November 7, 1974, touched off a massive exploration effort by the oil and gas industry in a trend approximately 325 miles long and 20 miles wide from the Rio Grande River to Robertson County, Texas. The No. 1-A Harris was drilled in the Pearsall Field located in Frio County, Texas. An index of the activity that followed can be seen in the fact that as of February, 1977, 554 new wells were producing in this field alone. In addition, there are 182 Austin Chalk tests currently in progress or listed as a location in Frio County. The rest of the trend has been extensively leased and drilling activity is increasing rapidly.

The primary objectives of this exploration program are the Austin Chalk and the Buda formations. Both of these are dense limestones that owe their reservoir qualities to fracturing. The fracturing is due to tensional forces that developed as a result of subsidence in the Gulf Coast Basin.

The average Austin Chalk-Buda well costs $260 thousand to drill and complete. While drilling, mud weights should be kept at near balance to avoid losing circulation into the fracture system. Perforating and treating should be accomplished promptly after setting casing to prevent any mud in the fractures from gelling, thereby plugging the fractures. Most of the wells require fracture treating to attain the best possible production rates. Extensive acid treatments should be avoided as they free argillaceous material in the lime that can plug the fractures.

Economic production from the Austin Chalk-Buda was not generally possible until the price of oil approached $12 per barrel and modern fracturing techniques were developed. Based on the results of production to date, it is estimated that 50 percent of the wells in the trend will produce at a rate of 40 bbl/d or more and will have average reserves of 100,000 to 120,000 bbl. The remaining 50 percent will produce a lesser amount and their economic success will depend on future prices and improvement in recovery techniques.