The Austin chalk and its petroleum potential : South-Central Texas
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Thornhill, Stephen Alan
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The Austin Chalk Formation (Upper Cretaceous) consists of uniformly bedded lithologies throughout south-central Texas. The lithologies found within the subsurface are technically wackestones to mudstones. These general textures may be divided into three specific facies: 1) Chalk Marl facies (1,000-7,500 feet), 2) Micritic Limestone (7,500-12,000 feet), and 3) Black calcarous shale facies (12,000 feet). The Chalk has been divided into four specific mappable units: 1) Lower Atco, 2) Upper Atco, 3) Bruceville Chalk Marl, and 4) Futchins Calk. The thickening trends of three units along with the total Austin Chalk interval suggest a uniform period of deposition during the Upper Cretaceous. The sediments deposited during this period were formed along a carbonate ramp-platform depositional environment. Structuring in south-central Texas is primarily faulting and fracturing trends and the homoclinal dip of Chalk to the southeast. The faulting planes are near vertical (65-70 degrees) and strike NE-SW. Fractures are oriented perpendicular to one another and are positioned NE-SW and NW-SE. Production potential of hydrocarbons within the Austin Chalk is controlled mainly by fracture development. Therefore, zones which intersect highly fractured areas are generally good producing wells. Giddings field and various fields within Gonzales County possess the largest amount of hydrocarbon production throughout the study area. Production is greatest in Lee County -- 27 million barrels (bbls) of oil and 75.5 billion cubic feet (BCF) of gas. The cumulative production along the productive chalk trend decreases in a southwesterly direction. This situation implies that fracture development will also show a decrease in this direction. Future development of the Austin Chalk is controlled by economic conditions. However, the best potential zones for future drilling sites may be indicated by "sweet spots" located on cumulative production maps.