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Abstract


 
Chapter from: M 61: Basin Compartments and Seals 
Edited by 
Peter J. Ortoleva

Author:
Randi S. Martinsen

Methodology and Concepts

Published 1994 as part of Memoir 61
Copyright © 1994 The American Association of Petroleum Geologists.  All Rights Reserved
 
Chapter 18

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Stratigraphic Compartmentation of Reservoir Sandstones: Examples from the Muddy Sandstone, Powder River Basin, Wyoming

Randi S. Martinsen
University of Wyoming
Laramie, Wyoming, U.S.A.


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ABSTRACT


The Lower Cretaceous Muddy Sandstone (Viking Formation equivalent) is a thin but complex stratigraphic unit that contains a variety of anomalously pressured compartments. One or more lowstand surfaces of subaerial exposure and erosion (LSEs), numerous transgressive surfaces of submarine erosion (TSEs), and varying lithofacies compartmentalize the Muddy Sandstone stratigraphically on at least three levels. The first level of compartmentation is defined by the relief along the LSE surface(s), which is highly variable and physically divides the Muddy, both vertically and laterally, into older and younger sequences. The second level is defined by the intersection of shales above the TSEs with the LSE (either by onlap or truncation). The third level results from variations in lithofacies. Whereas many of the compartments comprise classic stratigraphic traps consisting of shale (seal) encompassing sandstone (reservoir/compartment), compartments exist wherein sand is juxtaposed against sand without benefit of intervening shales to serve as a seal. In these situations, the seal appears to consist of a paleosol developed beneath the LSE. The distribution and geometries of pressure compartments in the Muddy have a high degree of correspondence to the various scales of stratigraphic compartmentation observed. In all probability, similar levels of stratigraphic complexity characterize many basins. Any analysis of the controls on pressure compartment formation and distribution therefore should incorporate these stratigraphic complexities and not assume that stratigraphic systems are characteristically simple.

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