Deposition of Woodbine-Eagle Ford Sandstones, Aggieland Field, Brazos County, Texas

Joseph R. Dedominic

Sandstones of the Upper Cretaceous Woodbine-Eagleford interval produce oil and gas from a stratigraphic trap at Aggieland field. The reservoir has an overall north-south-trending, ovate morphology, and is composed of two elongate bodies 3 mi (4.8 km) long by 1.5 mi (2.4 km) wide. The sandstones are thin and have an average net thickness of 16 ft (4.9 m). Grain size ranges from 0.15 mm (fine grained) to 0.30 mm (medium grained). The sandstones are composed of 48% quartz, 42% matrix, and are classified as a lithic graywacke. Due to the high percentage of matrix and pore filling shale clasts, permeability values are low, averaging 0.10 md. Porosity ranges from 2.5 to 22%. Based on sedimentary structures, texture, and composition, two main facies can be defined: (1) a lowermo t massive clast facies, which contains numerous shale clasts and oolites, (2) a sandstone facies, which exhibits bioturbated, rippled, and cross-bedded intervals and contains mainly monocrystalline quartz and a clay matrix.

The sandstones are encased within marine shales and were transported to a middle to outer-shelf location by a combination of tidal and storm-driven marine currents. The eastern edge of the reservoir is truncated and unconformably overlain by the Austin Chalk. This truncation resulted from submarine erosion over a deep-seated salt uplift, which also influenced the paleotopography and deposition of the sandstones. The lowermost massive clast facies was deposited in topographic lows by debris and gravity flows, and the upper sandstone facies was deposited across the top of the clast facies.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.