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Abstract

Hartzog Draw field is a stratigraphically controlled oil reservoir which produces from the Upper Cretaceous Shannon Sand¬stone at depths from 9,000 to 9,600 ft (2,727 to 2,910 m). The producing interval consists of a large mid- to outer shelf sand-ridge complex deposited well below effective normal wave base more than 100 mi (160 km) from shore. The productive interval in the shelf-ridge complex has a maximum thickness of 60 ft (18 m), and averages 20 ft (6 m) in thickness. The field is 22 mi (35 km) long and is as much as 3-1/2 mi (5.8 km) wide. Since its discovery in 1975, over 175 primary production wells were completed on 160-acre spacing. Initial oil-in-place was estimated to be 350 million barrels. Secondary waterflood was initiated in 1981 and 115 additional infield wells were to be drilled by the end of 1985.

The reservoir is completely enveloped in shale, has a solution gas drive, no water table and no produced formation water. Net pay is primarily a product of porosity, permeability and thickness of the sandstone, and is directly related to sedimentary facies. Detailed studies of five cores located in the northern, eastern, and central portions of the field allow definition of nine facies. Three of the facies are primarily high-angle crossbedded sandstones; the other facies show a variety of low-energy features including ripples and abundant burrowing. The Central Ridge Facies, a high-angle trough crossbedded slightly glauconitic quartz sandstone, is a consistently high quality reservoir. The High-Energy Ridge-Margin Facies, a crossbedded highly glauconitic sandstone containing siderite and clay rip-up clasts is also a relatively high quality reservoir; the Low-Energy Ridge-Margin Facies, which consists of interbedded ripples and troughs, and the Inter-Ridge Facies (Shaly), a rippled interbedded sandstone and shale, generally are marginal quality to non-reservoirs.

The average porosity for the field is 12% and the average permeability is 12 md. Higher mean values are recorded in the producing intervals of the five cores studied. Values for the Central Ridge Facies are 15% and 15 md and for the High-Energy Ridge-Margin Facies are 14% and 19 md.

Sandstone isopach maps and cross sections perpendicular to the field elongation show that the field is asymmetrical and considerably steeper on the northeast flank. Paleocurrent flow directions inferred from oriented cores indicate a southerly flow of the currents responsible for deposition of the Hartzog Draw shelf-ridge complex.

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