Subsurface Controls on Historical Subsidence Rates and Associated Wetland Loss in Southeastern Louisiana

By

MORTON, ROBERT A., KROHN, M. DENNIS, and BUSTER, NOREEN A.

U.S. Geological Survey, St. Petersburg, FL

Two regional dip-oriented first-order leveling profiles, surveyed by the National Geodetic Survey, cross the structural grain of southeastern Louisiana along the levee roads of Bayou Lafourche and Bayou Petit Caillou. Temporal and spatial analyses of the profiles demonstrate strong correlations among highest historical rates of subsidence, recent wetland losses, deep subsurface faults, and large oil and gas fields that produce primarily from middle/upper Miocene reservoirs.

Other researchers have demonstrated that the highest geological subsidence rates of the Mississippi delta plain correspond closely with the thickest Holocene sediments that fill the underlying incised valley. Within that overall geologic framework, highest historical subsidence rates correspond with surface projections of the reactivated Lake Hatch and Golden Meadow fault zones, and individual oil and gas fields. For example, the Madison Bay “hot spot” of recent wetland loss in Terrebonne Parish correlates with fault movement and gas extraction at the adjacent Lapeyrouse field, which has produced more than 600 Bcf of gas as reservoir pressure gradients declined as much as 0.2 to 0.4 psi/ft. Additional evidence of induced subsidence comes from the releveling profiles at two benchmarks over the Valentine field in Lafourche Parish. Those measurements show that subsidence accelerated between the first (1955-1965) and second (1965-1982) survey periods, at a time of maximum hydrocarbon production rates. Subsidence rates associated with natural compaction and dewatering of Holocene deltaic sediments should decrease with time; therefore historical subsidence rates that accelerate and typically exceed geological subsidence rates are probably caused by anthropogenic activities, such as hydrocarbon production.