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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
Zung Sheng Jiao and Ronald C. Surdam 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 19
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Stratigraphic/Diagenetic
Pressure Seals in the Muddy Sandstone, Powder River Basin, Wyoming
Zun Sheng Jiao
Ronald C. Surdam
University of Wyoming
Laramie, Wyoming, U.S.A.
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ABSTRACT
Sandstones in the Rozet unconformity
zone in the Muddy Sandstone are characterized by abundant clay matrix (up
to 55% of the rock volume), absence of intergranular pores, and very low
permeability. The diagenesis of clay minerals in the Rozet unconformity
sandstone and overlying Mowry Shale includes smectite altering to illite
in mixed-layer smectite/illite clays (I/S) and kaolinite reacting to chlorite.
The I/S composition changes with progressive burial from approximately
20% illite in the mixed-layer smectite/illite clays at 900 m (3000 ft)
to 85% illite at 4200 m (13,500 ft).
High-pressure mercury injection tests were
performed on the sandstone samples from the Rozet unconformity zone. Pore
throats for those samples are primarily in the subnano and nano categories
(<0.01 to 0.05 µm), and permeabilities are from 0.02 to 0.08 md.
Such sandstones can hold a differential pressure of 1800 psi, which is
the same as the differential pressure in the Amos Draw overpressured compartment
from which the samples were taken.
There is a direct correlation between the
diagenesis of clay minerals in the Rozet unconformity zone and the maturation
of the Mowry Shale, and the sealing capacity or displacement pressure of
the pedogenic units. The sealing capacity of the sandstone associated with
the unconformity is derived from primary pedogenic processes and from diagenetic
enhancement during progressive burial. The diagenetic processes can increase
the sealing capacity of sandstone along the unconformity by an order of
magnitude, or from a type C seal to a type A seal. The transition of the
fluid-flow system from single phase to multiphase results in converting
the low-permeability rocks along the unconformity to fluid/pressure seals
capable of withstanding >1800 psi pressure differentials.
The recognition of the presence of widespread
subaerial unconformities in the Muddy Sandstone is important in understanding
abnormally-pressured compartments within the reservoir facies. |
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