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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Subsurface Vents Under the Gulf of Mexico Shelf:
Characteristics and Significance for
Hydrocarbon
Migration
and Trapping
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By
Stone Energy Corporation
Houston, TX
Mud volcanoes throughout the world erupt with mixtures of
mud, brine and hydrocarbons. As the apex of expulsion
systems, these features give insight into
the process of
hydrocarbon
expulsion
and
shale dewatering at depth. Many eruptions
are believed to be sourced from
overpressured shales lying at great depths.
Compact structures observed on seismic
data near the top of geopressure appear to
be at the root of expulsion
systems and
are interpreted as “subsurface vents” where fluids are expelled
from overpressured shales into the transitional and normally
pressured section above. The structures are located downthrown
on deeply rooted faults. Collapse topographies surrounding the
vents appear to have been created by fluid withdrawal from geopressured
shales subjacent and upthrown to the vents. As fluid
expulsion
is often linked with fault movement, the vertical reach
of conducting faults above these vents may be governed by the
effective fountainhead of the ascending pressured fluids. Bright
spots occasionally stream from these faults and may be evidence
of actively migrating hydrocarbons.
The significance of subsurface vents related to petroleum exploration
is two fold. First, subsurface vents appear to be almost
always charged. In fact, this author has yet to document a
case that lacks hydrocarbon
accumulations. Additionally, these
structures are often filled to the spill point with reserves in the
range of 5 to 50 BCF. Second, subsurface vents may be important
point sources of
hydrocarbon
migration
into larger fields nearby. A better understanding
of these structures and their
evolution may aid in predicting
hydrocarbon
accumulations in neighboring
structures and lead to a knowledge of
specific
migration
pathways within a
basin.
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