ABSTRACT: Horizontal Drilling in the Austin Chalk: Stratigraphic Factors

C. O. Durham Jr., R. A. Bobigian

Horizontal drilling has renewed interest in the Austin chalk in south-central Texas. Large fields on opposite sides of the San Marcos arch, Giddings to the northeast and Pearsall to the southwest, were active with vertical drilling 10 years ago. Giddings' 4500 Austin wells produced 209 million BO and 934 bcfg of gas through 1988; Pearsall's 1440 wells produced 57 million BO abd 35 bcfg of gas.

Most vertical wells were completed, 20% were economic successes, 40% were marginal, 40% were uneconomic due to uneven areal distribution of near-vertical fractures and small faults, which provide reservoirs in otherwise tight chalk. Horizontal drilling, led by Amoco in Giddings and Oryx in Pearsall, enhances the chances of encountering the fractures by drilling perpendicular to the fracture trend.

Horizontal drilling requires preselection of the stratigraphic horizon to be penetrated. One must understand the variable Austin stratigraphy to choose the zone with the most brittle character and best matrix porosity, both reduced by increased clay content. Chalk 130 ft thick on the San Marcos arch thickens to 600 to 800 ft in central Giddings field where middle marl separates lower and upper chalk. Northeastward, only lower chalk is preserved beneath a post-Austin submarine channel.

The Austin thickens to 300-500 ft in Pearsall field where middle-member ash beds separate lower and upper chalk inhibiting vertical reservoir communication. Locally, on the Pearsall arch, ash is missing, lower chalk thickens, and upper chalk thins.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990