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Seismic Analysis and Characterization of Gas Hydrates in the Northern Deepwater Gulf of Mexico*
By
Fred Snyder1, Nader Dutta1, Deborah Hutchinson2, Patrick Hart3, Myung Woong Lee4, Brandon Dugan2, Carolyn Ruppel5, Warren Wood6, Richard Coffin7, Robert Evans8, and Emrys Jones9
Search and Discovery Article #40137 (2004)
*Adapted from expanded abstract prepared for presentation at AAPG Annual Convention, Dallas, Texas, April 18-21, 2004.
1Schlumberger Reservoir Services / Data & Consulting Services, Houston, TX ([email protected]); 2USGS, Woods Hole, MA; 3USGS, Menlo Park ([email protected]), CA; 4USGS, Denver, CO; 5Georgia Tech, Atlanta, Georgia; 6NRL, Stennis Space Center, MS; 7NRL, Washington, DC; 8WHOI, Woods Hole, MA; and 9ChevronTexaco, Houston, TX.
Abstract
The deepwater Gulf
of Mexico contains many known hydrate environments. Complex lithostratigraphy
and active salt movement create complicated thermal regimes and fluid chemistry.
These in turn affect hydrate formation and distribution. In an effort to study
and eventually test different hydrate settings, two separate areas were selected
for review by a multi-disciplinary team. Reprocessed 3D and high-resolution 2D
multichannel seismic data were analyzed over Keathley Canyon block 195 and
Atwater Valley block 14, both of which are in water depths of approximately 1300
meters. Keathley Canyon exhibits a deep (250 to 500 meters sub-seafloor),
pronounced regional bottom
simulating
reflector
(
BSR
), a notable geologic and
geophysical barrier between free gas and solid hydrate. The
BSR
is bounded to
the east by a salt-produced fault ridge, which is also a probable fluid
migration pathway. The
BSR
has reverse polarity relative to the water bottom
interface and obliquely cuts stratigraphic reflections. In some areas the
BSR
is
also defined by periodic, high amplitude terminations of free gas in the
coarser-grained, interbedded sands below. The Atwater Valley study area is
located in the middle of the Mississippi Canyon and contains numerous hydrate
mound features. Most mounds show strong seismic evidence of hydrates including
gas chimneys, amplitude blanking, and near-seafloor BSRs. Beginning in 2004,
drilling through the gas hydrate zone within these two areas, for research
purposes, will test these ideas of hydrate occurrences. The complexity and
diversity of all these hydrate occurrences clearly drives the need for a
cross-disciplinary approach.
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General StatementThe northern deepwater Gulf of Mexico (GOM) contains many naturally occurring gas hydrate environments (Figure 1). Here and in other areas, hydrate concentrations, once viewed as a drilling safety hazard, are fast gaining attention as potential energy sources. The reason is simple: natural gas hydrates, 99% of which exist in deep offshore marine basins (Makogan, 1997) are estimated to account for more than half of the total carbon content of the world’s known hydrocarbon resources (Kvenvolden, 1993), with 1 m3 of natural gas hydrate yielding approximately 164 m3 of CH4 methane (Collett, 1993). Finding and
assessing areas of gas hydrate occurrence is therefore important,
although not always straightforward. Traditionally, a In an effort to study and eventually drill different hydrate settings, two separate GOM deepwater areas were selected for review by a multidisciplinary team. Reprocessed 3D prestack time migrated data (PSTM) and high-resolution 2D multichannel seismic data (MCS) were analyzed over one Keathley Canyon (KC) block (195) and one Atwater Valley (AV) block (14), both of which are in water depths of approximately 1300 m.
Keathley CanyonA dominant
feature in the KC area is a pronounced regional
Atwater ValleyThe Atwater
Valley study area is located in the middle of the Mississippi Canyon
paleo-channel system and contains numerous features interpreted to be
hydrate mounds (Figure 4). Although no evidence of a regional
Concluding RemarksNatural gas
hydrate occurrences within northern GOM deepwater areas are often
complex and diverse. Differences in lithologic, thermal, and geochemical
regimes can create variations in hydrate formation and concentration
levels. In addition, the presence of low permeability, fine-grained
sediments in the shallow section reduces gas migration except in
proximity to fault or fracture systems. These and other factors serve to
make seismic detection of hydrates challenging. However, these two study
areas have shown that high quality seismic data and detailed
stratigraphic interpretation can be used to find and delineate gas
hydrate concentrations, whether with the aid of a prominent
AcknowledgementsWe thank Diana Gillespie, Adam Koesoemadinata, Lecia Muller, Dianna Shelander, Randy Utech, and Gary Wool for help with the project; we also thank WesternGeco and the USGS for the use of seismic data.
Collett, T.S., 1993, Natural gas hydrates of the Prudhoe Bay-Kuparuk River area, North Slope, Alaska: AAPG Bulletin, 77, p. 793-812. Cook, D., and D’Onfro, P., 1991, Jolliet Field thrust fault structure and stratigraphy Green Canyon Block 184, offshore Louisiana: Transactions – Gulf Coast Association of Geological Societies, v. XLI, p. 100-121. Cooper, A., and Hart, P., 2002, Seismic studies of the gas-hydrate stability zone, northern Gulf of Mexico: Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19-23, p. 115-123. Kvenvolden, K.A., 1993, Gas hydrates as a potential energy resource: a review of their methane content: Geological Survey Professional Paper U.S., 1570, p. 555-561. Makogan, Y.F., 1997, Hydrates of hydrocarbons: Pennwell Books, USA. McConnell, D., and Kendall, B., 2003, Images of the base of gas hydrate stability in the deepwater Gulf of Mexico: Examples of gas hydrate traps in northwest Walker Ridge and implications for successful well planning: The Leading Edge, April. Milkov, A., 2000, Worldwide distribution of submarine mud volcanoes and associated gas hydrates: Marine Geology, v.167, p. 29-42. Milkov, A., Sassen, R., 2002, Resources and economic potential of individual gas hydrate accumulations in the Gulf of Mexico continental slope: Marine and Petroleum Geology, November review, 45 p. Weimer, P., and Dixon, B., 1994, Regional sequence stratigraphic setting of the Mississippi fan complex, northern deep Gulf of Mexico: Implications for evolution of the northern gulf basin margin: GCSSEPM Foundation 15th Annual Research Conference Submarine Fans and Turbidite Systems, December, p. 373-381. Wood, W., Gettrust, J., Chapman, N., Spence, G., and Hyndman, R., 2002, Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness: Nature, v. 420, p. 656-660. Wood, W. T., Stoffa, P.L., and Shipley, T.H., 1994, Quantitative detection of methane hydrate through high-resolution seismic velocity analysis: Journal of Geophysical Research, 99, p. 9681-9695.
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