Abstract: Austin Chalk--Buda Trend of South Texas

Robert J. Scott

The completion of the Southland Royalty 1-A Harris in the Austin Chalk on November 7, 1974, touched off a massive exploration effort by the oil and gas industry in a trend approximately 250 mi (400 km) long and 20 mi (32 km) wide from the Rio Grande River to Robertson County, Texas. The 1-A Harris was drilled in the Pearsall field in Frio County, Texas, and an index of the activity that followed is the fact that, as of March 1977, 568 new wells had been completed, 101 more were being drilled or completed, and 70 new locations have been staked in Frio County, Texas, alone. 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 Limestone. 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 $260,000 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 production rates. Extensive acid treatments should be avoided, as they free argillaceous material in the limestone that can plug the fractures.

Economic production from the Austin Chalk-Buda Limestone was not generally possible until the price of oil passed $10/bbl and modern fracturing techniques were developed. On the basis of results of production to date, it is estimated that 50% of the wells in the trend will produce at a rate of 50 BOPD or more and will have average reserves of 120,000 bbl of oil. The remaining 50% will produce a lesser amount and their economic success will depend on future prices and improvement in recovery techniques.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas