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Depositional Environment and Reservoir Characteristics of Middle Pennsylvanian Granite Wash, Northern Palo Duro Basin, Oldham County, Texas

Amy L. Vanderhill, Robert R. Berg

The Lambert 1, Hryhor, and Sundance fields in Oldham County, Texas, produce oil from the Middle Pennsylvanian Canyon granite wash. Canyon granite wash conglomerates and sandstones have a maximum thickness of approximately 450 ft (140 m) and were derived from granitic rocks of the Bravo dome. The sediment was transported across carbonate platforms by streams and deposited into the Oldham trough as fan deltas.

The granite wash deposits consist primarily of imbricated gravels and cross-stratified sands. Debris-flow deposits are also present. The sediment is dominantly feldspar, granitic rock fragments, and quartz. Carbonate cement averages 5% of the bulk composition.

The overall geometry of the granite wash deposits is lobate. Parts of the reservoir show high lateral variability with no continuity between wells on 40-ac spacing. These reservoir facies are thus limited in areal extent and drainage area. However, some intervals can be correlated between wells, which suggests that part of the reservoir may be simulated as layered with no crossflow.

The reservoir conglomerates and sandstones have an average porosity of 18% and an average permeability of 75 md/ft. Reasonable net-pay cutoff values in these granite wash reservoirs are 9.5% for porosity and 1.5 md for permeability, and were established by crossplots and cumulative volume capacity.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.