ABSTRACT

Growth faulting, rapid deposition, and deep burial of sediments in south Louisiana have resulted in the abnormal pressurization of pore fluids within some sedimentary units. Fluid production from geopressured sandstones may result in pore-pressure reductions, clay compaction, and land-surface subsidence

Subsidence is being monitored at the U.S. Department of Energy's Sweet Lake, Parcperdue, and Rockefeller Refuge geopressured-geothermal prospects in southwestern Louisiana. Reservoir-defining growth faults were extrapolated to the land surface, surface lineations were mapped from aerial photographs, and historical elevation surveys were compared to obtain base-line subsidence rates.

Data from the Sweet Lake prospect indicate that no correlations exist between subsidence anomalies and lineations or fault projections. Subsidence here is probably a result of normal sediment compaction, salt domerelated land-surface adjustments, and/or historical subsurface fluid production. Conversely, a good correlation exists between the Parcperdue subsidence profile and the extrapolated updip extensions of mapped subsurface faults. Historical land-surface subsidence north of the Parcperdue prospect may have resulted from down-to-the-coast slumping along these growth faults which define the prospect. Subsidence at the Rockefeller Refuge prospect ranges from virtually zero on the eastern end of the prospect to 50 mm on its western end. The gradation may be due to oil- and gas-associated fluid withdrawals or land management variations.

A subsidence analysis near Lake Charles, Louisiana, suggests that a more extensive historical record of leveling data could provide a more qualifying subsidence record. Repeated leveling surveys at the geopressured-geothermal prospects should aid in determining the causes of subsidence before and after depressurization of the geopressured aquifers.