Facies and Reservoir Characteristics of a Shelf Sandstone: Hartzog Draw Field, Powder River Basin, Wyoming
R. S. Martinsen, R. W. Tillman, 1985. "Facies and Reservoir Characteristics of a Shelf Sandstone: Hartzog Draw Field, Powder River Basin, Wyoming", Shelf Sands and Sandstone Reservoirs, R. W. Tillman, D. J. P. Swift, R. G. Walker
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Hartzog Draw Field is a stratigraphically controlled oil reservoir which produces from the Upper Cretaceous Shannon Sandstone at depths from 9000 to 9600 ft. The producing interval consists of a large midshelf sand bar complex deposited below effective normal wave base more than lOO miles from shore. The productive interval in the bar complex has a maximum thickness of 65 ft, is over 21 miles long and is up to 3 1/2 miles wide. Over 170 wells have been completed on 160 acre spacing since its discovery in 1975, and ultimate oil recovery may exceed 100,000,000 barrels.
The reservoir is completely enveloped in shale, has a solution gas drive, no water table and no produced formation water. Even zones that calculate water saturations of over 65% from logs 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 one, 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 indicates the central bar facies to have a significantly better average porosity and permeability (12.7%, 6.4 md) than either the bar margin facies (8.1%, 3.7 md) or interbar facies (6.2%, 2.1 md). In addition
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Shelf sandstone reservoirs are becoming a more and more common exploration target. What they are, how they may be characterized, and how they differ from shoreline and deep-water deposits in the subject of this publication. Shelf sands and sandstone reservoirs are among the more poorly understood types of sandstones. Continental, shoreline and deep water sandstones have all been studied in much more depth than have shelf sands and sandstones. However, during the last fifteen years significant progress has been made in understanding shelf sands and sandstones. Studies of modern sediments have allowed us to understand many of the depositional processes active on the shelf. This book is intended to be an up-to-date summary of shelf processes and products. The papers are intended for those new to shelf sands and sandstones as well as the shelf specialist.