We have used two dimensional numerical modeling of groundwater movement to examine the controls on geopressure evolution and the interactions between the compactional and meteoric hydrologic regimes in the Gulf of Mexico basin. Our analysis examines the paleohydrologic history of the Gulf from Jurassic to present time and demonstrates that geopressures in the Gulf basin can be accounted for largely by compaction disequilibrium. The requirements for compactional geopressures to be developed are: (1) low permeability sediments, (2) thick shale sequences, (3) sedimentation rates of > 1 mm/yr for pressures approaching lithostatic gradients and > 0.1 mm/yr for moderate overpressure developments. Aquathermal pressuring contributes less than 1 percent to the total pore pressures in these geologic settings. The role of dehydration water released during the conversion of the clay mineral smectite to the clay mineral illite is not crucial to geopressure development in the Gulf basin. We have defined three stages in the paleohydrologic evolution of the Gulf basin. In the first stage from Jurassic to early Tertiary times, the basin was characterized by circulating waters of meteoric origin, driven into the basin by topographic elevation episodically enhanced by eustatic falls in sealevel. Abnormal pressures were restricted to the very deepest parts of the basin beyond the shelf margin. In the second stage, during much of the Tertiary, geopressures developed beneath the major sedimentary depocenters related to the positions of the ancestral Mississippi and Rio Grande river systems. The circulation of meteoric water became restricted by the opposing compactional hydraulic heads. The third stage in the development of Gulf basin hydrology began in late Miocene times and continues to the present day. Rapid increases in sedimentation rates in the area around the Mississippi delta have resulted in the widespread distribution of geopressured sediments that have pressure gradients approaching lithostatic. These pressures have been superimposed on sediments that were normally pressured for most of their burial history. Differences in the positions of the depocenters along the Gulf coast cause the duration of these three paleohydrologic stages to vary. For example, abnormal pressures developed as early as Eocene times in south Texas and are currently declining in response to reduced recent sediment accumulation rates. Eustatic rises and falls in sealevel have been important in shifting the interface between the meteoric regime and the compactional regime during Mesozoic and early Tertiary times before the onset of strong opposing geopressures. In Plio-Pleistocene times, relatively short lowstand durations may have prevented extensive diagenetic reactions in response to moving the freshwater interface but may have been long enough to allow bacterial degradation of hydrocarbons in reservoirs now buried to depths within the compactional regime. Our paleohydrologic analysis is supported by numerous observations of diagenetic reactions and geochemical measurements. Late sediment geopressuring is supported in particular by stable oxygen isotopic compositions of authigenic quartz. These data can be interpreted to show evidence for a progressive decrease in the amount of meteoric groundwater invading the basin from Mesozoic to present times.
