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.

Figure 1: A field with a thin pressure transition zone whose top is at 11,500 feet depth, as revealed by mud weights. Most of the oil and gas produced from this field has been produced from depths above the pressure transition zone (blue rectangle). Other fields differ in the thickness of the pressure transition zone and the relative depth range of productive intervals.

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.