ABSTRACT

Geopressures in the northern Gulf Coast basin occur primarily in basinward shaly-lithofacies equivalents of overlying sandy regressive sequences. The close regional lithostratigraphic control of the top of geopressures at the top of shaly magnafacies spanning a large chronostratigraphic range suggests that geopressures have either been preserved or replenished over geologic time intervals. If it is accepted that resistance to compactional fluid expulsion is the principal mechanism of geopressuring in Gulf Coast clastic sequences, and that this process began soon after deposition as in modern sediments, then it may be inferred that geopressures in different-aged strata developed sequentially in conjunction with successive episodes of progradational sand-shale deposition. The ages of Gulf Coast geopressures should then be about the same as the ages of the strata in which they occur, varying systematically and becoming progressively older updip.

These conditions have specific consequences for fluid flow. Geopressured, regressive sequences are characterized by convergent flow into sandy facies tongues, with consequent lateral flow away from sand pinch-outs and toward basin margins. Intertonguing of sands and shales at the major lithofacies boundary controlling the top of hard geopressures in the northern Gulf Coast basin is such that convergent-flow zones may be common near the boundary. Such flow may have combined with exsolutional effects to localize hydrocarbon accumulations near where the boundary crossees chronostratigraphic units. Long-term containment of early-formed geopressures suggests that present directions of hydraulic flow, which are definable using existing well information, may have persisted since before hydrocarbons underwent migration and entrapment.