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Abstract: South Texas Austin Chalk (Cretaceous) Play

Robert J. Scott, Jeff Prestridge

Austin Chalk production was firmly established at Pearsall in Frio County, Texas, in April 1936 by the Amerada and Rycade 1 Tract 6 McWilliams flowing oil at the rate of 2,249 b/d. This well has produced over 200,000 bbl of oil and in May 1976 produced 239 bbl. The high rate of initial production and long life of the well is attributed to intense fracturing of the Austin Chalk which is approximately 350 ft (107 m) thick on the Pearsall anticline.

Fracturing has created the reservoir for the oil that is believed to have migrated from the highly petroliferous Eagle Ford Shale just below the Austin Chalk. Primary porosity is 8%, or less, with permeabilities less than 0.1 md.

The Austin Chalk was deposited in Late Cretaceous seas that covered the Gulf Coast basin. Subsequent deposition of younger sediments caused structural downwarping that fractured the older sediments along the basin margin forming the trend now being drilled and explored.

Modern hydraulic fracturing techniques substantially should improve ultimate recoveries. Estimates of recoverable oil per well on an 80 acre unit range from 60,000 to 160,000 bbl. Average cost of a well completed into the tanks is about $250,000.

Drilling for the Austin Chalk has spread to the surrounding counties of Zavala, Dimmit, La Salle, Atascosa, Wilson, and Gonzales, an area 150 mi (240 km) long by 20 mi (32 km) wide. Leasing is heavy in Bastrop, Fayette, Lee, Burleson, Milam, and Robertson Counties. Several wells are to be drilled later this year in Lee, Burleson, and Robertson Counties and, if successful, could extend the trend another 150 mi.

150 mi.

AAPG Search and Discovery Article #90970©1977 AAPG Southwest Section Meeting, Abilene, Texas