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Sequence Stratigraphy of the Upper Cretaceous Austin Chalk Group in South and Central Texas


Recent drilling activity has extended the downdip limits of production of the Austin Chalk, a major oil and gas reservoir in Texas. The best reservoirs for hydraulic fracturing have a high carbonate content and occur on the south side of the isolated Llano and Sabine uplifts. Chalk beds in the East Texas Basin are less favorable reservoirs due to more clay and different maturation and migration history. At least four unconformity-bounded shallowing upward sequences are recognized in the Austin Chalk in outcrop and cores in Texas. Sequences show facies transitions upward from laminated organic-rich or burrowed clay- rich chalks, to more pure chalks with more abundant whole and fragmental macrofossils. Sequence boundaries are characterized by hard grounds with Thallasinoides burrow networks, borings, lithoclasts, and glauconite-rich intervals with abundant oysters. The shallowest depositional setting and the most erosion occurs near San Antonio; outcrops to the north and west are thicker, and include a younger sequence, due to less erosion below the Taylor Group unconformity. Subsurface cross sections show 1) Eagle Ford truncation and onlap of basal Austin Chalk onto the San Marcos Arch 2) wedging and internal unconformities within the Austin Chalk 3) facies changes in the chalk, and 4) truncation at the top of the Austin Chalk and onlap of the overlying Taylor Group. The Austin Chalk varies from 100’ to 900’ thick. The total isochore pattern generally corresponds with the positions of arches and basins in south and central Texas, being thickest in the Maverick Basin in south Texas and thinning over the San Marcos Arch. However, the thinnest Austin Chalk is west of the previous Lower Cretaceous and Eagle Ford thins that define the Arch. The chalk forms a wedge, which thins basinward, toward the southeast. The basinward thin is updip of the underlying Edwards shelf edge on the Arch, but extends further downdip in south Texas. Isochores of sub-units in south Texas show distinct depocenters, indicating compensational stacking. The oldest unit (Pearsall D or Atco equivalent) is thickest to the north; it thins downdip, with the basal portion becoming similar to the upper Eagle Ford Formation. Younger Austin Chalk units prograde and downlap basinward. The B unit (possible Dessau equivalent) is thickest in Maverick County where it filled original paleotopographic relief, then became a relative high, based on overlying isochores.