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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 491

Last Page: 491

Title: Facies and Reservoir Characteristics of Shelf Sandstone, Hartzog Draw Field, Powder River Basin, Wyoming: ABSTRACT

Author(s): Randi S. Martinsen, Rod W. Tillman

Article Type: Meeting abstract

Abstract:

Hartzog Draw field is a stratigraphically controlled oil reservoir which produces from the Upper Cretaceous Shannon Sandstone at depths from 9,000 to 9,600 ft (2,700 to 2,880 m). The producing interval consists of a large, midshelf sand-bar complex deposited below effective normal wave base more than 100 mi (160 km) from shore. The productive interval in the bar complex has a maximum thickness of 65 ft (19.5 m), is over 21 mi (34 km) long, and up to 3.5 mi (5.6 km) wide. Over 170 wells have been completed on 160-acre (64 ha.) spacing since its discovery in 1975, and ultimate oil recovery may exceed 100 million bbl.

The reservoir is completely enveloped in shale, has a solution-gas drive, no water table, and no produced formation water. Even zones calculated from logs to have water saturations of over 65% do not produce water. Net pay is primarily a product of porosity, permeability, and thickness of the sandstone, and is directly related to sedimentary facies. Of six facies observed in cores, only the central bar facies--a high angle, trough-cross-bedded, glauconitic quartz sandstone--is a consistently high-quality reservoir. Two others, the bar-margin facies, a ripple to trough cross-bedded sandstone with abundant shale and siderite clasts, and the interbar facies, a rippled, interbedded sandstone and shale, generally are marginal-quality reservoirs.

Data from three cores indicate the central bar facies to have a significantly better average porosity and permeability (12.7%, 6.5 md) than either the bar-margin facies (8.1%, 3.7 md) or interbar facies (6.2%, 2.1 md). In addition, wells with a thick central bar facies appear to maintain higher reservoir pressures. Recognition of the facies, and understanding their distribution and interrelations are prerequisites to developing a program which will maximize oil recovery from the field.

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