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Microfacies and Seismic Interpretation of Caddo Lime (Desmoinesian) in Chalky Mountain Field, Taylor County, Texas

Elizabeth G. Lewis

A detailed microfacies and seismic interpretation was conducted for the Caddo lime, a relatively unstudied Middle Pennsylvanian hydrocarbon reservoir in north-central Texas. Five cores from Chalky Mountain field in Taylor County were studied and found to contain six distinct but gradational facies: (1) shale, (2) dark, clay-rich limestone, (3) algal wackestone-boundstone, (4) algal grainstone, (5) skeletal packstone-grainstone, and (6) oolitic grainstone. The limestone facies form three cyclic packages, each of which contain shoaling-upward regressive facies followed by deepening-upward transgressive facies. The cycles are bounded by the transgressive, clay-rich limestone facies identified in the gamma-ray logs as positive radioactive kicks that can be correlated between ells.

Well logs, synthetic seismograms, and high-frequency seismic reflection profiles were used to map the subsurface structure of the Caddo lime in the Chalky Mountain field area. "Layercaking," a method which divides the seismic section into intervals of similar lithology and velocity, was used to correct for an overlying high-velocity wedge of Missourian Palo Pinto Limestone, found to cause erroneous pull-up and distortion of the underlying seismic signal of the Caddo lime. Structure mapping revealed a series of lobate features trending northeast-southwest. The combination of the residual topography and modeling of carbonate deposition indicates that the lobate highs are shoaling oolitic and skeletal carbonate sand bodies deposited in a high-energy, shallow marine environment. Algal bio erms and restricted lagoonal facies were deposited in a quieter water environment behind the shoals. An algal grainstone facies accumulated at the margins of the algal bioherms and now form the hydrocarbon reservoir in Chalky Mountain field.

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