Downdip Mesozoic Rocks of South Texas
The Downdip Cretaceous Province includes approximately 16,500 square miles, located downdip from the Luling fault zone, underlying the Wilcox to Claiborne outcrop, and extending southwest from the Brazos River to approximately the western boundaries of Zavala and Dimmit counties (Fig. 97).
Regionally, the structure is a southeastward dipping homocline interrupted by local faulting or folding (Fig. 98).
The rocks of Cretaceous age range in thickness from 4,000 to 15,000 feet and have an average thickness of approximately 7,500 feet. Sediments of Upper Cretaceous age are primarily marine shale and sandstone, being nearly 60 per cent shale and 25 per cent sand. The Lower Cretaceous sediments are about 75 per cent limestone and 25 per cent sandstone and shale.
The Cretaceous rocks have a volume of about 25,000 cubic miles. They have been sparsely explored with a density of 1 well per 27 square miles and only 10 exploratory wells have been drilled to depths greater than 10,000 feet. The only important producing fields in the area are: Pearsall, which produces from the Upper and Lower Cretaceous rocks; and Jourdanton and Imogene, which produce from Lower Cretaceous rocks.
The deepest production is from the Lower Cretaceous (Edwards) at 9,019 feet and the shallowest is from the Upper Cretaceous (Navarro) at 425 feet.
Showings of oil and gas have been encountered in wildcat wells in all formations of the Cretaceous. The Navarro-Taylor sands (Upper Cretaceous) and the Edwards limestone (Lower Cretaceous) produce oil most prolifically in adjacent areas.
Exploration in the updip