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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
.
Block
330 Field, Offshore Louisiana
1Manuscript received June 24, 1996; revised manuscript received
June 16, 1998; final acceptance July 14, 1998.
2Department. of Geological Sciences, Cornell University,
Ithaca, New York 14853; e-mail: [email protected]
3Department of Marine Chemistry and Geochemistry, Woods
Hole Oceanographic Institution, Woods Hole, Massachusetts 02543.
4Chevron Overseas Petroleum, Inc., San Ramon, California
94583.
5Michigan Technological University, Houghton, Michigan 49931.
ABSTRACT
Simple modeling of coupled heat and mass flux indicates the paleothermal
anomaly in the fault zone intersected by A6ST well was short-lived, having
a duration less than 150 yr. The anomaly could have been produced by a
2 ´ 106 m3 pulse
of fluid ascending the fault at an actual velocity of over 1 km/yr (Darcy
flux of 330 m/yr) from 3 km deeper in the basin. Simple Darcy law computation
indicates a transient fault permeability on the order of 110 md during
this flow. Pulsing of fluid up the fault was probably the norm, although
most flow did not produce such strong thermal anomalies as the one detected
in the A6ST well.
Analysis of fluid pressures shows that the main fault is a profound
lateral permeability barrier having up to 1800 psi of water pressure differential
across it. The hydrocarbon sealing capacity of the fault depends on the
pressure difference across the fault. Fault permeability is best understood
in terms of effective stress. Under ambient conditions, the fault is at
high pressure relative to downthrown reservoirs. A pulse of high-pressure
fluid ascending
End page 244----------------
the fault lowers effective stress in the fault zone sufficiently to
produce a significant transient increase in permeability. If the fluid
is in an area of the fault adjacent to downthrown, relatively low pressure
reservoir sands, the fluid will discharge into them. Permeability in and
adjacent to the fault then decreases, such that fluid cannot reenter the
fault zone and escape from the reservoir. Block
330 field, offshore Louisiana, indicate
that the fault has acted as a conduit for fluids whose flux has varied
in space and time. Core and cuttings samples from two wells that penetrated
the same fault about 300 m apart show markedly different thermal histories
and evidence for mass flux. Sediments within and adjacent to the fault
zone in the U.S. Department of Energy-Pennzoil Pathfinder well at about
2200 m SSTVD (subsea true vertical depth) showed little paleothermal or
geochemical evidence for throughgoing fluid flow. The sediments were characterized
by low vitrinite reflectances (Ro), averaging 0.3% Ro,
moderate to high d18O and d13C
values, and little difference in major or trace element composition between
deformed and undeformed sediments. In contrast, faulted sediments from
the A6ST well, which intersects the A fault at 1993 m SSTVD, show evidence
for a paleothermal anomaly (0.55% Ro) and depleted d18O
and d13C values. Sodium is depleted
and calcium is enriched in a mudstone gouge zone at the top of the fault
cut in the well; this effect diminishes with distance from this gouge zone.
Cuttings from other wells in South Eugene Island
Block
330 show slightly
elevated vitrinite reflectance in fault intercepts relative to sediments
outside the fault zone. Overall, indicators of mass and heat flux indicate
the main growth fault zone in South Eugene Island
Block
330 has acted as
a conduit for ascending fluids, although the cumulative fluxes vary along
strike. This conclusion is corroborated by oil and gas distribution in
downthrown sands in Blocks 330 and 331, which identify the fault system
in northwestern
Block
330 as a major feeder.
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