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A large portion of the sediments within the northern Gulf of Mexico contain pore fluid pressures in excess of hydrostatic. Development of geopressure is generally attributed to compaction disequilibrium caused by rapid deposition of low‐permeability sediments in the Miocene and Plio‐Pleistocene. Numerous studies have examined the formation of overpressures and/or expulsion of geopressured fluids into overlying hydropressured strata. However, very little attention has been given to fluid flow within the geopressured zone itself. Movement of oils from Cretaceous or older source rocks into Plio‐Pleistocene reservoirs in the Gulf Basin requires as much as 10 km of vertical migration in a few million years. Precipitation of cements in some geopressured sediments also implies large‐scale fluid flow. New evidence from a deep well in the Eugene Island area, offshore Louisiana, indicates that geopressured sediments are mechanically very weak with a Poisson's ratio greater than 0.4 and a shear modulus or rigidity less than 1 GPa. In addition, large‐scale fluid flow either through interconnected pores or fractures is not occurring in this location, at least at present. An alternative hypothesis is that upward fluid transport in geopressured sediments is caused by buoyancy‐driven propagation of isolated fluid‐filled fractures. Using linear fracture mechanics, I show that vertical fractures with lengths of a few meters can propagate at velocities of 1000 m/yr. Mass flux rates (∼100 kg/m2/yr) are significant assuming a mechanism for formation of fluid‐filled fractures exists, such as hydrofracturing when fluid pressures exceeded the minimum confining stress. Fracture propagation velocity and mass flux rate are strongly dependent on the shear modulus of geopressured sediments.

Groundwater flow near salt domes is complex because groundwater is subject to a variety of driving forces including the release of geopressured fluids, large lateral density gradients, and regional hydraulic head gradients. The complexity of this environment is born out by recent geochemical and geophysical observations that indicate the occurrence of upward groundwater flow near some salt domes. In order to evaluate the relative importance of different mechanisms driving groundwater flow near salt domes, we have developed a numerical model that couples groundwater flow, heat transport, and transport of dissolved salt, and accounts for salt diapirism. Our calculations indicate that upward groundwater flow can occur as the result of thermal convection when the regional background salinity is greater than 15 weight percent, a value typical of many areas of the south Louisiana salt dome province. For lower background salinities, dissolution causes salt‐laden groundwater near the dome to sink, leading to depressed isotherms. While the release of geopressured fluids is difficult to quantify, it remains a likely mechanism for driving upward groundwater flow near some salt domes.

Complex groundwater convection patterns are possible near salt domes because groundwater is subject to both lateral heat and salinity gradients. In order to assess the mechanisms responsible for driving convection near salt domes we use dimensional analysis and numerical simulations to investigate coupled heat and salt transport in homogeneous sediments surrounding a cylindrical salt column. The dimensional analysis does not require the Boussinesq assumption. The coupled heat, solute, and groundwater transport equations are controlled by three dimensionless parameters: the Rayleigh number, the Lewis number, and the buoyancy ratio. The buoyancy ratio is the ratio of salinity to temperature effects on groundwater density, and it directly affects the groundwater flow equation. A finite difference numerical multigridding algorithm is used to iteratively solve the coupled transport equations. The multigridding technique was about 3 times faster than a point‐wise successive overrelaxation solution. Boundary conditions for the numerical simulations were adjusted to represent different contrasts in the thermal gradient between the salt and the overlying sediments. The contrast in thermal gradient is parameterized by the thermal conductivity ratio and is responsible for isotherms being elevated near the salt. The analysis suggests that a wide range of convective flow patterns are possible, with flow occurring either up or down along the salt flank. The sense of convection is dependent mainly on the value of the buoyancy ratio and how sharply isotherms are pulled up near the salt. These factors in turn depend on the regional salinity variation, the time since diapirism, and the thermal conductivity of water saturated sediments. While this analysis provides useful insight into the mechanisms driving free convection near salt domes, the assumptions about medium and fluid properties may limit the applicability of dimensional analysis in simulating flow in specific geologic settings.

including crustal stretching and emplacement of dense rocks into the crust, is necessary to explain the net subsidence of the basin from the time before the heating event until the lithosphere beneath the basin cooled. Without this driving load, the thermal expansion would produce uplift, but the surface would subside only back to sea-level after the lithosphere cooled. Theoretical 5 88 J. A. Nunn and N. H. Sleepgravity results indicate that the driving load is centred at a depth of approximately 15 km.Deviations in subsidence curves from exponentials associated with thermal contraction can be explained by changes in sediment supply. Spatial variations in sediment load, caused by regional facies changes, produce migration of the centre of maximum deposition. Water and basement depths are determined for sequential time intervals during basin development. The predicted depositional environments are consistent with lithofacies maps of the Middle Devonian. J. A . Nunnand N. H. Sleep 33 I 31 -30 -29 -28 -27 -26 -l-v this paper log,, t(year4