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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
Lisa D. Shepherd, Peter A. Drzewiecki, Jean M. Bahr, and
J. Antonio Simo 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 24
*
Silica Budget
for a Diagenetic SealLisa D. Shepherd*
Peter A. Drzewiecki
Jean M. Bahr
J. Antonio Simo
University of Wisconsin
-- Madison
Madison, Wisconsin, U.S.A.
*
ABSTRACT
Diagenetic banding commonly
occurs in association with zones of abnormal fluid pressures that have
been identified as pressure compartments. Although diagenetically banded
intervals may contain layers of moderately high porosity, the bands act
collectively as a low-permeability unit and are therefore important as
potential low-permeability seals for pressure compartments. This study
focuses on a diagenetically banded interval in the Middle Ordovician St.
Peter Sandstone of the Michigan basin. This interval is located within
a large area of anomalous pressures identified by Bahr et al. (this volume)
in the deep Michigan basin and is composed of a seal-forming lithology.
The banded interval is characterized by millimeter- and centimeter-scale
diagenetic banding, with alternating quartz-cemented bands, pressure solution-dominated
bands, and porous bands.
Point counting techniques and an image
analysis system were used to quantify porosity, textural properties, and
quartz cement. A theoretical model was used in conjunction with these data
to estimate the amount of silica dissolved by intergranular pressure solution.
Porosity variations in the St. Peter Sandstone are controlled by the combined
effects of quartz cementation and intergranular pressure solution. A silica
budget calculated for the banded interval indicates that more silica was
dissolved by intergranular pressure solution than is present as quartz
cement, suggesting that pressure solution alone could have produced enough
silica to account for the banded quartz cement. On a local scale, the banded
interval served as an exporter of silica. However, a larger-scale silica
budget analysis computed for another well in the same region of the basin
indicates that the St. Peter may actually be balanced on a regional scale.
Results of this study were used to investigate
the controls on diagenetic band formation. No significant correlation exists
between porosity and grain size or porosity and sorting in the banded interval,
suggesting that depositional textural parameters are not important in controlling
the distribution of porosity and cement within the banded interval itself.
However, original
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