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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
Jean M. Bahr, Gerilynn R. Moline, and Gregory C. Nadon Methodology and Concepts
Published 1994 as
part of Memoir 61
Copyright © 1994 The American Association of Petroleum
Geologists. All Rights Reserved.
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Chapter 11
*
Anomalous Pressures
in the
Deep Michigan BasinJean M. Bahr
Gerilynn R. Moline
Gregory C. Nadon
University of Wisconsin--Madison
Madison, Wisconsin, U.S.A.
*
ABSTRACT
In this chapter we examine pressures in
the St. Peter Sandstone and associated formations of the deep Michigan
basin. A comparison of computed brine heads to surface elevations reveals
a large area of overpressures within the St. Peter Sandstone and the Glenwood
Formation to the west and north of Saginaw Bay. Contrary to the patterns
expected for a steady-state, topographically driven flow system, heads
in these formations are highest in the regional discharge area. Vertical
gradients between the Glenwood and the St. Peter and between the St. Peter
and the Prairie du Chien Group would generate downward flow in the regional
discharge area, which is also inconsistent with a steady-state flow system.
Low-permeability zones must exist within the Glenwood and St. Peter to
inhibit equilibration with  normal pressures in the underlying and overlying
units. Overpressures appear to be dissipating by both upward and downward
leakage through high-permeability zones located in anticlinal structures
and possibly related to basement faults. Within the larger area of anomalous
pressures, repeat formation test data from selected wells reveal even greater
overpressures locally within the St. Peter. These vertical variations in
pressures are associated with vertical variations in permeability, suggesting
a stacked system of compartments separated by low-permeability zones of
diagenetic origin.
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