Wells of the Elk City field of Washita and Beckham Counties, Oklahoma, are completed from Hoxbar "granite wash" sediments. Granite wash is a term used to designate deposits of granite conglomerate, arkose, and a conglomeration of decomposed and disintegrated acidic and basic igneous rocks. Limestone, dolomite and chert detritus are frequently included as pebbles or fragments. Grain size of the clastics varies from 0.1 mm. (silt size) to 50 mm. (pebble size). These clastics may be relatively unconsolidated or in an argillite or limestone matrix.

Facies changes occur so rapidly that reliable correlations are sparse. (To be illustrated by electric log cross sections.) The average total depth of wells in the Elk City field is 10,500 feet, which is adequate to penetrate the entire Permian, Cisco and most of the Hoxbar section. Granite wash sediments, that compose approximately 60 per cent of this interval, were derived from the Wichita Mountains to the south. Conditions under which deposition occurred were probably varied, but the presence of some marine limestones in the section suggests that some of the clastics also may be marine deposits.

Average porosity of the pay zones is approximately 18 per cent, and average permeability is 200 millidarcys. In general, there is a demarcation in permeability and porosity between the central and eastern portions of the field. The critical physical characters which control porosity and permeability are the type and amount of materials which fill the interstices. Small lime fragments, crystalline calcium carbonate, and materials of mud or silt size deposited along with granitic materials fill voids between grains and reduce interstitial space.

Lantern slides of core chips and thin sections will be used to illustrate the mineralogical composition and variations in the porosity and permeability of the Elk City pay zones.

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