ABSTRACT

The characterization of geopressured sediments in the Gulf Coast is commonly made on the basis of increases in pressure gradient or geostatic ratio, that is, in situ fluid pressure divided by total depth. Fluids and sediments having geostatic ratios in excess of 0.47 psi/ft are usually considered 'geopressured', those with geostatic ratios greater than 0.70 psi/ft, 'hard-geopressured'.

An alternative and more informative way of characterizing geopressured sediments is by use of the parameter hydraulic head. Hydraulic head can be used not only to distinguish between hydropressured and geopressured sediments, but also to provide potentially quantitative information on the direction and rate of subsurface fluid flow. Hydraulic head, h (feet), is defined as h = Z + Pg/, where z is depth below a reference datum, P is measured or calculated fluid pressure, g is acceleration due to gravity, and is fluid density. In the absence of osmotic effects, the hydraulic gradient, dh/dl (dimensionless), where l is path length, can be used in conjunction with hydraulic conductivity in Darcy's Law to predict the direction and rate of fluid migration.

Variations in calculated hydraulic head have been correlated along several regional cross-sections through the Tertiary sedimentary sequence of South Louisiana. Four distinct hydrodynamic zones can be distinguished with depth in the region studied:

1. Near surface to 7000 feet in depth. Hydropressured fresh to saline waters having hydraulic heads near zero and vertical hydraulic gradients of 0.01 or less.
2. 7000 feet to 10,000-14,000 feet in depth. A transition zone of weakly geopressured sediments with hydraulic heads ranging from 0 to 3000 feet above sea level. Hydraulic gradients range from 0.1 to 4 and show many reversals with depth. Many commerically important hydrocarbon plays occur within this zone:
3. 10,000-14,000 feet to 12,000-18,000+ feet. The hard geopressured zone in which hydraulic head progressively increases with depth from values of 3000 to 16,000+ feet of head above sea level. Gradients of 4 to 5 are typical but locally can exceed 25. There are few reversals in gradient with depth.
4. Depths below Zone 3. Suggestion, based on sparse data, that hydraulic gradients begin to decrease with depth.

In contrast to the hard geopressured zone, where hydraulic flow appears to consist of a broad, generally upward-moving front, hydraulic flow in the transition zone appears to be more strongly focused through preferred channel-ways.