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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthor:
Randi S. Martinsen 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 2
*
Summary of
Published Literature on Anomalous Pressures: Implications for the Study
of Pressure CompartmentsRandi S. Martinsen
University of Wyoming
Laramie, Wyoming, U.S.A.
*
ABSTRACT
The phenomenon of anomalous geopressure
has been a focus of study for almost 40 years. Until recently, most studies
were concerned with either the detection of overpressure or with the processes
by which anomalous pressure is generated. Two processes, disequilibrium
compaction and hydrocarbon generation/maturation, probably account for
the majority of overpressured rocks observed worldwide. Because anomalous
pressure is often associated with undercompacted rocks, various techniques
based on detecting undercompaction (in addition to formation pressure tests)
are commonly used to detect anomalous pressure indirectly. Whereas formation
pressure tests can only be obtained in porous and permeable reservoir-type
rocks, indirect techniques can only be applied in thick shale sequences.
Therefore, data sets obtained by these two different approaches are mutually
exclusive.
There has been a great deal of uncertainty
with regard to the existence and nature of pressure seals. Overall, researchers
studying anomalous pressure fall into one of two categories: those who
accept the existence of effective seals and those who do not. Researchers
who do not believe seals exist support classic hydrologic interpretations
that suggest anomalous pressures are bounded by low-hydraulic- conductivity
rocks and are, therefore, geologically ephemeral. Those who do believe
seals exist, for the most part, do not present specific evidence of what
comprises a seal . Although the sealing capacity of various rocks for hydrocarbon
accumulations has been investigated, very little research has been conducted
specifically concerning the seals that contain anomalous pressures. The
influence of capillary forces on hydrocarbon entrapment is widely recognized,
but capillary phenomena have largely been ignored in studies dealing with
geopressures, even though hydrocarbon generation is commonly cited as the
mechanism by which overpressure is generated. The delineation of pressure
compartments, especially those of known geologic duration, is an important
tool by which potential seals can be mapped and investigated. Without specific
knowledge of seal characteristics, well-founded interpretations of seal
effectiveness and pressure compartment longevity cannot be made. |
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