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Subsidence, Wetland Loss, and Hydrocarbon Production in the
Mississippi Delta Plain
Robert A. Morton1, Julie C. Bernier2, John A. Barras3
1U.S. Geological Survey, FISC, Austin, TX2U.S. Geological Survey, FISC, St. Petersburg, FL
3U.S. Geological Survey, NWRC, Baton Rouge, LA
http://coastal.er.usgs.gov/gc-subsidence/
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Mississippi delta plain area of detailed investigations
~ 4000 km2 land loss since the 1930s
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PRIMARY CAUSES OF WETLAND LOSS
CATEGORY PROCESS
Delta cycleConstruction and
destruction
Sediment compaction
Erosion
Biogeochemical Saltwater intrusion
Waterlogging
Sulfide concentration
Herbivory
Human activities Levee construction
Canal construction
Failed reclamation
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Continuous Interior Wetland
Sub
surf
ace
dept
h m
eter
s X
103
BeforeInducedSubsidence
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FluidProduction
Sub
surf
ace
dept
h m
eter
s X
103
AfterInducedSubsidence
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EVIDENCE OF INDUCED SUBSIDENCE AND FAULT REACTIVATION
• Surface changes occur at the same time and place as hydrocarbon production
• Large or rapid decreases in subsurface pressure (regional depressurization)
• Surface and subsurface fault traces have the same orientation and direction of displacement
• Historical subsidence rates were significantly greater than geological subsidence rates
• Preservation of marsh sediments beneath open water (historical wetland loss)
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Integrated Datasets
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Representative Field Production Volumes
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Representative Annual Fluid Production
Lapeyrouse Field
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Lapeyrouse FieldExposito ReservoirMultiple Wells
Representative Pressure Decline
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DeLargeField
1968
Continuouswetlands
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DeLargeField
1973
Example of RapidInteriorWetlandLoss
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VibracoringMadison Bay, LA
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MadisonBaycore
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NGS Relevel Lines
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NGS Subsidence Rates La 1 1965-1993
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Historical Subsidence Rates in Coastal Louisiana
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Tide Gauges
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Relative sea-level rise mm/yr 1960s -1980s
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Grand Isle Annual Mean Sea Level 1947-2006
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NGS GPS CORSSubsidence Rates 2002-07
4.4
6.3
Boothville 3.5
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Delta Plain Subsidence Rates
Source Period Rate mm/yr
Reference
Radiocarbon dates
Holocene 1-5 Penland et al, 1988; Roberts et al, 1994; Morton et al,
2006
Numerical model Holocene <5 Meckel et al, 2006
NGS Relevel La 1
1965-82 7.6 Shinkle & Dokka, 2004
NGS Relevel La 1
1982-1993 12.1 Shinkle & Dokka, 2004
NOS GI tide gauge
1947-65 3.3 Morton & Bernier
NOS GI tide gauge
1965-93 10.7 Morton & Bernier
NOS GI tide gauge
1993-06 4.1 Morton & Bernier
NGS GPS CORS 2002-07 3.5-6.3 Dokka et al, 2006
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Delta Plain Fluid Production vs. Wetland Loss
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• Subsidence associated with deep-basin processes (salt migration, gravity gliding) should be slow, continuous, and decrease with geologic time
• Historical delta-plain subsidence rates accelerated and were greater than geological subsidence rates, then they recently decelerated to near geological rates
• Close correlations among regional wetland loss rates, historical subsidence rates, rates of fluid extraction and pressure reduction, and locations of reactivated faults
IMPORTANT CONSIDERATIONS
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• Prior explanations of regional wetland loss failed to explain the rapid increase and decrease in rates of wetland loss
• Marsh sediments are preserved where subsidence was rapid
• Interior wetland subsidence rates were substantially higher than subsidence rates measured along the natural levees
• Although measured rates of induced subsidence in the Miss. delta are low compared to other areas, they were sufficient enough to cause widespread wetland loss