conditions and for planning wetland

restoration projects.

Hydrocarbon production is interpreted to cause movement along faults and

wetland subsidence (fig. 1) if the following situations co-occur:

1. Large areas of wetland are lost at the

same time and in the same places as

hydrocarbons are produced

2. The cumulative volumes of fluids

produced are so large that they lead

to rapid declines in subsurface

pressures

3. Subsurface faults near the producing

reservoirs and surface faults activated after initial production have the

same orientation and direction of

displacement

4. Subsidence rates measured near the

oil and gas wells during production

are substantially higher than geological rates of subsidence

Each condition by itself does not directly link wetland loss, subsidence, and

hydrocarbon production, but together

they are compelling indicators of

causality.

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sh

S

Figure 1. Possible effects of petroleum production. Prolonged or rapid production 

and formation water (2) causes subsurface formation pressures to decline (3). The 

pressures (3) increase the effective stress of the overburden (4), which causes co

the reservoir rocks and may cause formerly active faults (1) to be reactivated (5). E

paction of the strata or downward displacement along faults can cause land-surfa

dence (6). Where subsidence and fault reactivation occur in wetland areas, the we

cally are submerged and changed to open water (7). Figure is not to scale. D, dow
Induced subsidence cannot be sustained indefinitely. Instead, the duration

of surface adjustment is related to the

history of production. As shown in figure

3, there was a time gap between the

onset of production and the first visible

evidence of surface disturbance and wetland loss. Whatever losses are occurring

today, they are occurring at a much slower rate than when the wetlands deteriorated between 1956 and 1978. This

reduction in rates of subsidence corresponds to the rapid decline in hydrocarbon production.

Regional Depressurization

When large volumes of oil, gas, and

associated formation water are extracted

from the subsurface, the natural pressures in the reservoirs are reduced (fig.

4) and stresses around the reservoir

increase (fig. 1). The increased stresses

cause reservoir compaction, which, in

places, leads to surface subsidence.

Nearly 20 billion barrels of oil and more

than 150 trillion cubic feet (4.2 trillion

cubic meters) of gas have been produced

from coastal Texas and Louisiana since

the 1920’s. Although the fluid production is concentrated within the field

areas, the effect of the pressure decline

extends far beyond the individual fields.

Where multiple fields are producing

from the same strata, regional depressurization can cause subsidence and wetland losses in the areas between the

fields (Kreitler and others, 1988). Consequently, induced subsidence can be

either near the fields (fig. 2) or away

from the fields.

Coincidence of Subsurface and

Surface Faults

Throughout the Gulf Coast region

there are many deep faults that serve as

structural traps for the hydrocarbons.

Some of the primary faults that trap

hydrocarbons also extend upward to

shallow depths near the surface (fig. 1).

If the pressure drop in the producing formation is large, faults that are near the

threshold of failure may be reactivated,

and rocks along them may mo

the fault is active, the land are

on the downthrown side of the 

the fault plane. Depending on 

and angle of the fault, the indu

dence may occur several kilom

away from the producing well

rather than directly above the 

reservoirs (fig. 2).

Releveling Surveys

Subsidence and fault disp

also can be inferred from benc

releveling surveys. The elevati

benchmarks in the Gulf Coast 

periodically resurveyed by the 

Geodetic Survey. Comparing l

surveys provides a basis for m

rates of subsidence for the per

between the dates of the surve

example, releveling surveys in 

1982 along Louisiana Highwa

between Valentine and Leevill

showed that subsidence was g

hydrocarbon-producing fields 

between the fields (fig. 5).

Figure 2. Air photograph of the Port Neches oil and gas field in Texas in 1978 and maps showing that wetlands above the field were h

continuous in 1956 but deteriorated and were converted to open water by 1978. The change was caused by induced subsidence and 

vation resulting from hydrocarbon production. From White and Morton (1997). Relative motion along faults: D, down; U, up.
4 0 4 8 km

-125

-100

-75

-50

-25

0

25

20 30 40 50 60 70 80 90 100

DISTANCE FROM REFERENCE BENCHMARK (km)

ELEVATION CHANGE (mm)

Interior Wetland 

Shoreline Loss

Oil and Gas Field

Water

Benchmarks

Valentine FieldCutoff FieldGolden Meadow FieldBayou Ferblanc FieldLeeville Field

EXPLANATION

Gulf of Mexi

Figure 5. Map along Louisiana Highway 1 between Valentine and Leeville, La., showing locations of benchmarks, oil and gas fields, a

and wetland losses and graph showing changes in surface elevation (in millimeters) at the benchmarks between 1965 and 1982. Subs

greatest near the oil and gas fields. Wetland losses from Britsch and Dunbar (1993). Elevation changes from National Geodetic Surve
Figure 6 shows that the correlated beds

have been displaced progressively downward 35–63 centimeters (cm) compared

to the adjacent marsh surface. Preservation of the marsh sediments beneath a

meter of water is clear evidence that the

marsh surface subsided first to form the

open water area. Subsequent erosion of

the marsh sediments was minor compared to subsidence. 

Subsidence Rates

One way of detecting artificially

induced subsidence around a producing

field is by comparing the measured rate

of subsidence there with natural subsidence rates in the same region. The wetland surface at Port Neches subsided

about 63 cm in 22 years (figs. 2 and 6).

These values yield a historical rate of

wetland subsidence of about 3 cm/yr.

sidence rate of 0.04 cm/yr in southwest

ern Louisiana, which is the closest area

having both a similar geologic setting

and estimated subsidence rates.

Environmental Implications

Subsidence of a meter or less

induced by oil and gas production is not

great, especially when compared to the

extreme examples of subsidence such as

9 m at Wilmington, Calif., or 4.5 m at

the Ekofisk Field in the North Sea. Nevertheless, the relatively small observed

reductions in elevation in the Gulf Coast

region are sufficient to cause dramatic

changes in the affected wetland ecosystems. This study demonstrates that even

minor lowering of the marsh surface can

translate to large wetland losses through

a combination of coastal plain subsidence and marsh sediment erosion.

—ROBERT A. MORTON

NOREEN A. PURCELL

White, W.A., and Morton, R.A

Wetland losses related to f

ment and hydrocarbon pro

southeastern Texas coast

of Coastal Research, v. 13

1305–1320. 

Williams, S.J., Penland, Shea, 

Roberts, H.H., 1994, Proce

affecting coastal wetland l

Louisiana deltaic plain, in 

S.J., and Cichon, H.A., ed

es of coastal wetlands loss 

Louisiana . . . as presented 

Zone ’93, New Orleans, L

U.S. Geological Survey O

Report 94–0275, p. 21–29

For more information, pleas

Robert A. Morton

U.S. Geological Survey

600 4th Street South

St. Petersburg, FL 33701

Telephone: 727–803–8747 x3

Fax: 727–803–2032

E-mail: rmorton@usgs.gov