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








U.S. Geological Survey
Denver, Colorado
Many oil and gas reservoirs in Tertiary strata of southern Louisiana are located close to the interface between a sandrich, normally pressured sequence and an underlying sand-poor, overpressured sequence. The transition from normally pressured to highly overpressured sediments is documented by conversion of mud weights to pressure, plotting of all pressure data from an individual field as a function of depth (Figure 1), and identification of a top and base of the pressure transition zone. 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. Display of pressure transition zones on geologic cross sections illustrates the relative independence of the depth of the pressure transition zone and geologic age.
Comparison of the depth -
distribution
of pressure transition
zones with production intervals
confirms previous findings that
production intervals generally
overlap the pressure transition zone
in depth, and that the median
production depth lays 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 (Figure 2).
Figure 2: Pressure transition zones in 10 fields are shown on a north-south cross section in southern Louisiana. Vertical blue bars show top and base
of pressure transition zone in each field. Note the gulfward progression of
overpressure
from older to younger strata, as originally pointed out by
Dickinson (1953).
Mud weight data in seven fields show that “local” pressure gradients range from 0.91 to 1.26 psi/ft below the base of the pressure transition zone. Pressure gradients are higher, and computed effective stress gradients are negative in younger strata in coastal areas. This finding indicates that a greater potential for fluid and sediment movement exists there than in older Tertiary strata.