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Reservoir Description of H. O. Mahoney Lease, Wasson (San Andres) Field, Yoakum County, Texas

Patricia D. Ballard

Wasson field, a broad, domal structure, is located on the southeastern part of the Northwest shelf. Mobil Producing Texas & New Mexico, Inc., owns and operates the 640-ac H. O. Mahoney lease on the northeast flank of Wasson field.

The lease produces from the Permian San Andres, Clear Fork, and Wichita-Albany Formations. The San Andres is a dolomitized carbonate deposited in an arid, tidal-flat environment with subtidal through supra-tidal facies in a regressive, shoaling-upward sequence.

Production occurs from two intervals, the First Porosity and the Main Pay. Average reservoir parameters are 193 net ft, 13% porosity, and 6.2-md permeability. The First Porosity interval, consisting of zones F1-F5, is 400 ft below the San Andres top and is comprised of porous supratidal and intertidal dolomites. These zones have a discontinuous, stringerlike geometry and display poorer reservoir development. The principal reservoir is the continuous Main Pay interval, which consists of zones M1-M8 and underlies the First Porosity at an average depth of 5,100 ft. The best porosity and permeability are developed within dolomitized pelloidal packstones of zones M1-M3 and F4. These packstones, deposited subparallel to the San Andres shoreline, exhibit excellent intercrystalline and leache -vuggy porosity with good permeability and lateral continuity. Understanding the depositional history allowed the identification of northeast to southwest porosity and permeability trends at Mahoney. Interbedded within the more porous zones are dolomitized mudstones displaying poor porosity and permeability. These laterally continuous layers restrict vertical fluid movement.

Estimated original oil in place is 75,150 Mstb. About 80% of this volume is contained in zones F-4 and M1-M3 and represents most of the flow capacity.

Carbon dioxide injection began October 17, 1985. A 40% hydrocarbon pore-volume CO2 slug injection using a 1:1 water alternating gas ratio is planned for the project.

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