ABSTRACT

Red-bed sandstones of the Abo Formation (Lower Permian) currently produce natural gas from the Pecos Slope Abo field in northern Chaves County, New Mexico. The Pecos Slope Abo field is on the northwest shelf of the Permian basin. Production currently comes from an approximately 700 mi2 (1,813 km2) area. The Abo has been designated a “tight gas sand,” thus Abo gas can be sold for as much as $5.41 per mcf, $2.60 more than the regulated ceiling price of gas produced from formations not designated as tight. The tight-sand designation greatly stimulated drilling and over 250 wells have been drilled since field discovery in 1977. Because of low permeability, wells must be artificially fractured to obtain economic production. Initial production rises from A few tens of MCFGD before fracturing to an average of about 2,200 MCFGD after fracturing.

The Abo red beds are subdivided vertically into three lithologic intervals on the northwest shelf of the Permian basin: a lower “granite wash” interval, a middle mudstone interval, and an upper interval of interbedded sandstone and mudstone.

The lower “granite wash” interval is more than 800 ft (248 m) thick in some places and is composed of interbedded coarse-grained arkosic sandstones and arkosic conglomerates. It rings Abo-age uplifts of granitic Precambrian basement and inter-tongues shelfward with marine limestones of the Hueco Formation.

The middle interval is about 100 ft (30 m) thick and conformably overlies the lower interval and the Hueco limestones. It is composed of calcareous, sparsely fossiliferous, argillaceous mudstone and minor fine-grained sandstone. It is a marine shelf deposit.

The upper interval is about 600 ft (183 m) thick and is composed of interbedded mudstones and lenticular sandstones. It conformably overlies the middle interval and is disconformably overlain by the dolostones, anhydrites, and fine-grained sandstones of the Yeso Formation. The upper interval intertongues southward with dolostones of the shelf-margin Abo reef facies. Sandstone lenses are generally 10 to 20 ft (3 to 61 m) thick. Sandstones are very fine-grained, arkosic, and hematitic. The upper interval was deposited as a fluvial-deltaic system which prograded south over the marine mudstones of the middle interval.

Gas is produced from sandstones in the upper interval. Primary porosity has been reduced by compaction to values near zero and the average in-situ matrix permeability of Abo sandstones is only 0.0067 md. Such small amounts of matrix porosity and permeability do not contribute greatly to production. The Abo wells tap a gas-filled natural fracture system. Mudstones seal the fractured sandstone reservoirs. Because fluvial-deltaic deposits extend almost 100 mi (161 km) north of present production, the area underlain by potential, fractured, Abo sandstone reservoirs is at least five times greater than the area which is currently productive.

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