SLIDE 1

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, TX
2U.S. Geological Survey, FISC, St. Petersburg, FL
3U.S. Geological Survey, NWRC, Baton Rouge, LA

http://coastal.er.usgs.gov/gc-subsidence/

SLIDE 2

Mississippi delta plain area of detailed investigations
~ 4000 km2 land loss since the 1930s

image: map of Louisiana showing 1932-2004 land loss

notes: The study area is part of the Mississippi River delta plain that has formed over the last 10-15 ky. The delta plain is a dynamic environment in which delta-lobe switching is common. This process results in:
   - seaward progradation and accumulation of sediment associated with active delta-lobe distributary
   - compaction, subsidence, and inundation (100s to 1000s of years) of land and wetland areas adjacent to abandoned distributaries
Accelerated historic wetland loss has resulted in submergence of ~ 4000 km2 formerly emergent delta-plain wetlands since 1930s (from Barras et al., 2004, with 1956-1978 LCA and 1932-1956 BTNEP data)

SLIDE 3

Primary Causes of Wetland Loss
Category: Delta cycle, Process: Construction and destruction, sediment compaction;
Category: Biogeochemical, Process: Saltwater intrusion, Waterlogging, Sulfide concentration, Herbivory;
Category: Human activities, Process: Levee construction, Canal construction, Failed reclamation

SLIDE 4

Before Induced Subsidence
figure: diagram showing wetland diagram before induced subsidence

SLIDE 5

After Induced Subsidence

figure: Diagram showing possible effects of petroleum production. Prolonged or rapid production of oil, gas, and formation water causes subsurface formation pressures to decline. The lowered pressures increase the effective stress of the overburden, which causes compaction of the reservoir rocks and may cause formerly active faults to be reactivated. Either compaction of the strata or downward displacement along faults can cause land-surface subsidence. Where subsidence and fault reactivation occur in wetland areas, the wetlands typically are submerged and changed to open water.

SLIDE 6 

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)

SLIDE 7

Integrated Datasets
image: Satellite image of the souther Mississippi delta plain showing locations of Bayou Petit Caillou Relevel Line, Bayou Lafourche Relevel Line, Tide Gauges, Louisiana DNR oil and gas Fields, Fault Lines, and areas of Historical Wetland Loss

SLIDE 8

Representative Field Production Volumes
Valentine Field - Discovery Year: 1936; Cumulative Gas: 920 Bcf; Cumulative Oil: 55 MMbbl; Cumulative Water: 87 MMbbl;
Houma Field - Discovery Year: 1945; Cumulative Gas: 851 Bcf; Cumulative Oil: 7 MMbbl; Cumulative Water 21: MMbbl;
Lirette Field - Discovery Year: 1937; Cumulative Gas: 1.3 Tcf; Cumulative Oil: 18 MMbbl; Cumulative Water: 59 MMbbl;
Lapeyrouse Field - Discovery Year: 1941; Cumulative Gas: 624 Bcf; Cumulative Oil: 18 MMbbl; Cumulative Water: 39 MMbbl

SLIDE 9

Representative Annual Fluid Production
image: graph showing annual fluid production at Lapeyrouse field between 1940 and 2005 and corresponding period of rapid wetland loss

SLIDE 10 

Representative Pressure Decline
image: graph showing decline in bottom hole pressures in the Exposito reservoir of the Lapeyrouse field between 1964 and 1978

SLIDE 11


image: aerial photo of DeLarge Field in 1968 showing continuous wetlands


SLIDE 12

image: aerial photo of DeLarge Field in 1973 showing Rapid Interior Wetland Loss


SLIDE 13

image: Vibracoring operation at Madison Bay, LA


SLIDE 14

Madison Bay sediment core
image: photo of Madison Bay core

SLIDE 15
image: diagram showing subsidence and erosion measurements derived from cores at Madison Bay

SLIDE 16
National Geodetic Survey Relevel Lines
image: satellite image showing locations of Bayou Petit Caillou Relevel Line, Bayou Lafourche Relevel Line, Tide Gauges, Louisiana DNR oil and gas Fields, Fault Lines, historical Wetland Loss

SLIDE 17

National Geodetic Survey Subsidence Rates along La 1 from 1965-1993
image: graph showing accelerated NGS Subsidence Rates from Raceland to Leeville for years 1965-1982 and 1982-1993

SLIDE 18

Historical Subsidence Rates in Coastal Louisiana
image: map of southern Mississippi delta showing subsidence rates for 1965-1993

SLIDE 19
Tide Gauges
image: satellite image showing locations of Bayou Petit Caillou Relevel Line, Bayou Lafourche Relevel Line, Tide Gauges, Louisiana DNR oil and gas Fields, Fault Lines, historical Wetland Loss

SLIDE 20

Relative sea-level rise mm/yr 1960s-1980s
image: map showing sea level rise contours for the 1960s-1980s for Cocodrie, Leeville, Bayou Blue, Golden Meadow, Valentine, and Houma tide gauges

SLIDE 21

Grand Isle Annual Mean Sea Level 1947-2006
image: graph showing relative rise in sea level for Grand Isle and Pensacola tide gauges
Grand Isle RSLR for 1947-2006: 9.3 mm/yr
Grand Isle RSLR for 1947-1965: 3.3 mm/yr
Grand Isle RSLR for 1965-1993: 10.7 mm/yr
Grand Isle RSLR for 1993-2006: 4.1 mm/yr
Pensacola RSLR for 1924-2006: 2.1 mm/yr

SLIDE 22

National Geodetic Survey GPS Continuously Operating Reference Stations (CORS) Subsidence Rates 2002-07
Houma: 4.4
Cocodrie: 6.3
Boothville: 3.5

SLIDE 23

Delta Plain Subsidence Rates

Source: Radiocarbon dates; period: Holocene; Rate (mm/yr) 1-5; reference: Penland et al, 1988; Roberts et al, 1994; Morton et al, 2006
Source: Numerical model; Period: Holocene; Rate (mm/yr): less than 5; Reference: Meckel et al, 2006
Source: NGS Relevel La 1; Period: 1965-82; Rate (mm/yr): 7.6; Reference: Shinkle & Dokka, 2004
Source: NGS Relevel La 1; Period: 1982-1993; Rate (mm/yr): 12.1; Reference: Shinkle & Dokka, 2004
Source: NOS GI tide gauge; Period: 1947-65; Rate (mm/yr): 3.3; Reference: Morton & Bernier
Source: NOS GI tide gauge; Period: 1965-93; Rate (mm/yr): 10.7; Reference: Morton & Bernier
Source: NOS GI tide gauge; Period: 1993-06; Rate (mm/yr): 4.1; Reference: Morton & Bernier
Source: NGS GPS CORS; Period: 2002-07; Rate (mm/yr): 3.5-6.3; Reference: Dokka et al, 2006

SLIDE 24

Delta Plain Fluid Production vs. Wetland Loss
image: graph showing annual production of oil, gas, and water in the Mississippi delta plain between 1940 and 2005 and associated land loss 

SLIDE 25

Conclusions
- 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

SLIDE 26

Conclusions continued
- 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