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AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 6. (June)

First Page: 689

Last Page: 707

Title: Depositional and Statistical Exploration Models, Upper Cretaceous Offshore Sandstone Complex, Sussex Member, House Creek Field, Wyoming

Author(s): John P. Hobson, Jr. (2), Michael L. Fowler (2), Edward A. Beaumont (3)

Abstract:

The Upper Cretaceous Sussex sandstone reservoir at House Creek field, Powder River basin, Wyoming, is part of an offshore bar complex deposited many tens of miles from the contemporaneous shoreline and a few tens of miles from the shelf-slope break. Deposition occurred in a narrow Campanian seaway that extended from the Arctic to the Gulf of Mexico.

The sandstone complex, which overlies a thick progradational sequence of silt and clay, may represent a delayed response to events at the shoreline. That is, sand escaped the "littoral energy fence" during initial phases of the rapid transgression that deposited the overlying shale and equivalents, including the Claggett Shale of central Montana. The resulting shoreface retreat left residual sand sheets on the shelf where the sand was further transported, structured, and shaped into broad elongate sand complexes (offshore bars) by storm-related and other marine currents. Within the study area, at least, sand trends are about N40°W. A few tens of miles to the southeast, the general trends of shelf-slope breaks are N-S to N45°E.

The sandstone complex at House Creek is divided into six, generally gradational facies ranging from sandstone through interbedded sandstone and claystone to silty claystone. Thickness and lateral facies distribution are asymmetrical and reflect relatively sustained higher depositional energies and a steeper slope along the northeast basinward flank of the complex. The "shoestring" appearance of producing sandstone along the northeast flank does not reflect the overall geometry of the bar complex. The complex was an extremely broad, low-relief feature, with a few tens of feet of relief over a distance of tens of miles.

Asymmetry of thickness and facies made possible an exploration-development model that uses discriminant analysis with data taken from wireline logs of 78 wells. Dry holes with sandstone southwest and northeast of the productive trend were successfully reclassified, with one exception, into their proper categories by a discriminant function. An additional model, which used producing wells as a third category, also yielded excellent results with the same variables. Discrimination in both models may be accomplished with significantly less than the full complement of variables used.

The general approach, using discriminant function analysis with relatively standard mapping techniques, should be considered more frequently by explorationists. Exploration and development of other sandstones, even of the offshore bar type, would probably require unique models.

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