Overpressure, Stress Gradients, and Hydrocarbon Accumulations in Tertiary Strata, Gulf Coast of Louisiana

Philip H. Nelson
U.S. Geological Survey, Denver, CO

The association of oil and gas reservoirs with top of overpressure, recognized for many years by Gulf Coast explorationists, is revisited because of its relevance to a USGS assessment of undiscovered oil and gas potential of Tertiary strata in the onshore Gulf Coast of Louisiana. The top and base of the pressure transition zone, which spans the interval from normally pressured to highly overpressured strata, are selected from plots of all mud weights for a field. Vertical extents of pressure transition zones in 34 fields across southern onshore Louisiana range from 300 to 9,000 feet and are greatest in younger strata and in the larger fields. Progressing gulfward, the top of overpressure lies in progressively younger strata.

Comparison of the depth-distribution of pressure transition zones with production intervals confirms previous findings that (1) production intervals generally overlap pressure transition zones, and (2) the median production depth lies above the base of the pressure transition zone in most fields. However, in 11 of 55 fields with deep drilling, substantial amounts of oil and gas have been produced from depths deeper than 2,000 feet below the base of the pressure transition zone.

Below the base of the pressure transition zone, local pressure gradients range from 0.91 to 0.98 psi/ft in the Oligocene Frio and Anahuac Formations in three northwestern fields, and from 1.08 to 1.26 psi/ft in lower and middle Miocene strata in four southeastern fields. The higher pressure gradients in Miocene strata produce negative effective stress gradients in coastal areas, indicating greater potential for fluid flow and fracturing in overpressured Miocene strata than in older overpressured Tertiary strata.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas