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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. Ortoleva
Authors:
H. P. Heasler, Ronald C. Surdam, and J. H. GeorgeMethodology 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 16
*
Pressure Compartments
in the Powder River Basin, Wyoming and Montana,
as Determined from Drill-Stem
Test DataH. P. Heasler
Ronald C. Surdam
University of Wyoming
Laramie, Wyoming, U.S.A.
J. H. George
Embry-Riddle Aeronautical
University
Daytona, Florida, U.S.A.
*
ABSTRACT
Drill-stem test (DST) pressures from oil
and gas wells were analyzed in an attempt to determine the existence of
pressure compartments in the Powder River basin. DST data for the entire
basin were first sorted by geologic unit for the Mesaverde Formation (984
data values), Sussex Formation (1041 data values), Frontier Formation (821
data values), Muddy Formation (3888 data values), Dakota Formation (1157
data values), and Minnelusa Formation (4470 data values). Initial and final
shut-in pressures (ISIP and FSIP) were graphed versus each other and versus
depth and elevation to display functional relationships. Potentiometric
surfaces were then constructed using the maximum of the ISIP and FSIP.
The pressure-elevation plots and potentiometric
surfaces clearly show the existence of anomalously pressured zones in the
Frontier, Muddy, and Dakota formations. The anomalously pressured zones
as determined from the potentiometric surfaces are discrete areas on the
scale of individual oil fields. The boundaries of the anomalously pressured
areas as shown on the potentiometric surfaces are characterized by steep
hydraulic head gradients of up to 12,000 ft (3600 m) of head difference
across small horizontal distances of less than 1 mile. These gradients
are interpreted as discontinuities in the fluid-flow regime of the Powder
River basin. The internal shape of the anomalies is difficult to determine
because data are sparse. However, piecewise continuous  least -squares analyses
indicate that many of the anomalies contain a nearly horizontal internal
potentiometric surface.
Given the discontinuous nature of the constructed
potentiometric surfaces and the shape of the pressure anomalies, we conclude
that oil-field-size pressure compartments exist in the Powder River basin
in the Frontier, Muddy, and Dakota formations. |
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