Hydrodynamics of Geopressure in the Northern Gulf of Mexico Basin

Abstract

The behavior of water in geopressured systems of the northern Gulf of Mexico basin is non-Newtonian; hydrodynamic forces are gravity related, but the dominating forces such as osmosis, diagenesis, and terrestrial heat flow are molecular. Introduction The term "geopressure" was first used by Charles Stewart of the Shell Oil Co. to describe abnormally high subsurface fluid pressure, defined by Dickinson as "any pressure which exceeds the hydrostatic pressure of a column of water [extending from the stratum pressure of a column of water [extending from the stratum tapped by the well to the land surface] containing 80,000 mg/1 total solids." A pressure of approximately 0.465 psi is exerted by each foot of such a water column. Geopressure may be expressed in terms of the geostatic ratio, which is the ratio of the observed fluid pressure in an aquifer to the pressure due to the weight pressure in an aquifer to the pressure due to the weight of overlying deposits, computed for the depth at which the aquifer occurs. The geostatic load at any given depth, based upon observed bulk densities of sediments, is generally very close to the following ratio: load (lb/sq in.) depth (ft) equals 1.0, to depths of more than 20,000 ft. This is because the average density of all rocks in the stratigraphic column changes but slowly with depth. Any observed subsurface fluid pressure for which the geostatic ratio is between 0.465 and 1.0 psi i, by this definition, a geopressure. Geologic Occurrence The structural and stratigraphic environments of geopressure in the northern Gulf basin are well known. Three types of reservoir seals necessary to preserve geopressure are illustrated by Dickinson, and described as "small reservoir sealed by pinchout,large reservoir sealed updip by faulting down against thick shale series, and sealed downdip by regional facies change, andrelative position of fault seals in upthrown and downthrown blocks." The depth distribution, range of observed pressures, and hydrodynamic features of geopressured reservoirs then known in the Gulf Coast are reviewed in detail; and the belt they underlie, "35 to 75 miles wide along the coast from the Rio Grande in the southwest to the Mississippi Delta in the east, a distance of approximately 800 miles", is said to coincide approximately with the area of Pleistocene and Holocene formations. As of 1968 it was known that this belt extends far out beneath the Gulf Continental Shelf, and that geopressure occurs at some depth beneath the main del talc sand series in Miocene and Pliocene (Neogen) sediments everywhere Gulfward from the innermost zone of growth faults. (See Table 1.) The role of growth faults in the structural deformation of geopressured sediments is defined by Ocamb and Thorsen. Shelton explains growth faults as "contemporaneous faults" that occurred when "salt and some thick shale units [had] been deformed by uniform flow which, in turn, apparently caused failure by faulting in the overlying paralic sediments." Dickey et al. suggest that the areal continuity and depth of occurrence of geopressure are due to reversal of dip and landward thickening of stratigraphic units associated with growth faults. JPT P. 803