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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
Y. Chen, W. Chen, A. Park, and Peter J. Ortoleva 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 27
*
Role of Pressure-Sensitive
Reactions
in Seal Formation and
Healing:
Application of the CIRF.A
Reaction-Transport CodeY. Chen
W. Chen
A. Park
Peter J. Ortoleva
Indiana University
Bloomington, Indiana,
U.S.A.
*
ABSTRACT
Pressure seals exist at depth that have
impressively low permeability and are robust, despite evidence of tectonic
or other fracturing. We suggest that there likely is some mechanism whereby
the quality of a seal improves with increasing pressure gradient to which
the seal is subjected. Thus, through diagenesis pressure seals self-enhance
and heal, if breached.
A generic mechanism for the development
of quality seals is proposed that involves a positive feedback as follows.
The pressure dependence of diagenetic reactions can lead to precipitation
from a fluid as it moves down a pressure gradient. Precipitation in a given
zone will decrease permeability and tend to focus the overall pressure
gradient in the zone. However, the augmented local pressure gradient promotes
an even greater rate of precipitation in the zone of the original precipitation.
Thus there is the tendency toward increased local pressure gradient development.
In the context of compartments, there is
an even greater tendency toward seal self-enhancement. As the seal develops
about an overpressuring compartment interior, the overall pressure drop
across the developing seal increases--i.e., fluid escape is increasingly
retarded as the seal develops. Thus there is a greater pressure head localized
to the region where the pressure gradient-induced precipitation is taking
place.
It is shown that one way this may occur
is via the pressure dependence of equilibria of aqueous and mineral reactions.
As an illustrative example, we consider the carbonate-quartz system, as
layers of such cements constitute an important contribution to the banded
seals in sandstone observed in a number of basins. We find that this system
allows for the rather efficient development or healing of seals. This phenomenon
is evaluated by using the
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