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/

Mississippi delta plain area of detailed investigations

~ 4000 km2 land loss since the 1930s

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

Continuous Interior Wetland

Sub

surf

ace

dept

h m

eter

s X

103

BeforeInducedSubsidence

FluidProduction

Sub

surf

ace

dept

h m

eter

s X

103

AfterInducedSubsidence

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)

Integrated Datasets

Representative Field Production Volumes

Representative Annual Fluid Production

Lapeyrouse Field

Lapeyrouse FieldExposito ReservoirMultiple Wells

Representative Pressure Decline

DeLargeField

1968

Continuouswetlands

DeLargeField

1973

Example of RapidInteriorWetlandLoss

VibracoringMadison Bay, LA

MadisonBaycore

NGS Relevel Lines

NGS Subsidence Rates La 1 1965-1993

Historical Subsidence Rates in Coastal Louisiana

Tide Gauges

Relative sea-level rise mm/yr 1960s -1980s

Grand Isle Annual Mean Sea Level 1947-2006

NGS GPS CORSSubsidence Rates 2002-07

4.4

6.3

Boothville 3.5

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

Delta Plain Fluid Production vs. Wetland Loss

• 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

• 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