AAPG Bulletin, V. 83 (1999), No.
2 (February 1999), P. 244-276
.
Vertical and Lateral Fluid Flow Related to a
Large Growth Fault, South Eugene Island Block 330 Field, Offshore Louisiana1
Steven Losh,2 Lorraine Eglinton,3 Martin
Schoell,4 and James Wood5
©Copyright 1999. The American Association of
Petroleum Geologists. All Rights Reserved
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.
Funding for the research described here was provided by the
Global Basins Research Network and Department of Energy grant DC-FC22-93BC14961
to Roger Anderson, Lamont-Doherty Earth Observatory, with subcontracts
to Larry Cathles of Cornell University, Jean Whelan of Woods Hole Oceanographic
Institution (WHOI), and James Wood of Michigan Technological University
(MTU). Funding was also provided by Gas Research Institute grant GRI-5097-260-3787
to L. Cathles, Cornell University. We are deeply grateful to Pennzoil Exploration
and Production Company for their matchless cooperation and generous access
to samples and data; without their support, this entire project would not
have been possible. Losh also acknowledges the valuable help of Peter Vrolijk
and Michael Wooten, Exxon Production Research, and the participants in
the coring strategy meetings. MTU smectite-illite analyses were performed
by Ahn Tu (Pathfinder samples) and Kirsten Price (A6ST). Biostratigraphic
analysis was performed by Applied Biostratigraphix, Houston, and elucidated
by Denise Butler, Pennzoil. Chevron Oil Company is thanked for the loan
of a research microscope to WHOI for vitrinite analysis. Fault gouge capillary
entry pressure measurements were performed under the auspices of Exxon
Production Research Company; we thank them for release of these data. The
manuscript has benefited significantly from discussions with Doug McDowell
of MTU, Dave Pevear and Peter Vrolijk at Exxon Production Research, Larry
Cathles of Cornell University, and Jeff Seewald of WHOI, and reviews by
Jim Boles, University of California-Santa Barbara, Jean Whelan, WHOI, and
AAPG Bulletin reviewers Grant Skerlec, Lee Fairchild, and Dan Berg. Losh
thanks Laurel Alexander for providing a preprint of her fault plane paper
coauthored with Jim Handschy, and Peter Flemings for providing a manuscript
regarding fluid pressures. This is WHOI contribution No. 9829.
ABSTRACT
Data from sediments in and near a large growth fault adjacent
to the giant South Eugene Island 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.
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.
