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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
Jean K. Whelan, Lorraine Buxton, and Lawrence M. Cathles
III 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 7
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Pressure Seals--Interactions
with Organic Matter, Experimental Observations, and Relation to a "Hydrocarbon
Plugging" Hypothesis for Pressure  Seal FormationJean K. Whelan
Lorraine Buxton Eglinton
Woods Hole Oceanographic
Institution
Woods Hole, Massachusetts,
U.S.A.
Lawrence M. Cathles III
Cornell University
Ithaca, New York, U.S.A.
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ABSTRACT
Organic geochemical characteristics diagnostic
of pressure seals have been determined for two wells in the Moore-Sams
field of the Tuscaloosa trend, Louisiana Gulf Coast (Mix and Bizette wells)
and one well penetrating a much weaker pressure transition zone of the
Anadarko basin, Oklahoma (Weaver well). Preliminary data suggest these
characteristics of organic matter in zones of pressure seals: a rapid increase
in vitrinite reflectance near the top of the pressure seal; fractionation
of bitumens through the pressure seal with a gradual change from lighter
to heavier n-alkanes with increasing depth in the pressure seal; a buildup
of hydrocarbons just beneath the pressure seal; and an enhancement of asphalt
(or asphaltene) throughout the general zone of the pressure seal. For all
three wells, very tight associations of carbonate cements, fine pyrite,
asphaltenes, and micrinite (generally considered to be a residual product
of hydrocarbon generation) were observed in the general zone of pressure
seals, suggesting that interactions of organic and inorganic materials
may be required for pressure seal formation and maintenance, even in fairly
organic lean wells such as Weaver. A sharp jump in thermal maturity, as
measured by vitrinite reflectance, occurs at the top of the Mix pressure
transition zone. Maturity levels below the seal reach gas thermal window
levels, suggesting that gas formation within and below the (seal) zone
is contributing both to overpressuring and sealing of pressure seals investigated
here. It is proposed that all these observations can be accommodated if
the pressure drop across the seal pressure transition zone causes separation
of oil and gas and deposition of asphalt from the upward-streaming |
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