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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

The Middle Amazon Basin is a large Paleozoic sedimentary basin on the Amazonian craton in South America. It contains up to 7 km of mainly shallow water sediments. A chain of Bouguer gravity highs of approximately +40 to +90 mGals transects the basin roughly coincident with the axis of maximum thickness of sediment. The gravity highs are flanked on either side by gravity lows of approximately -40 mGals. The observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide. Within this region, the continental crust has been intruded/replaced by more dense material to more than half its original thickness of 45-50 km. The much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere. The observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15-20 kin. Although this value is lower than expected for a stable cratonic region of Early Proterozoic age, it is within the accepted range of effective elastic thicknesses for the earth. Rapid subsidence during the late Paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere. The high-density zone in the lower crust, as delineated by gravity and flexural modeling, has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin. The linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust. sedimentary rocks that definitely record basin subsidence are of Permian age (245 Ma). A thin sequence of late Mesozoic/early Cenozoic age sedimentary rocks overlies Permian rocks. trusion of basaltic rocks associated with opening of the North and South Atlantic in the Mesozoic. Ordovician and Silurian strata are predominantly sandstone, siltstone, and shale, whereas late Carboniferous to Permian sedimentary rocks are primarily evaporites and red beds [Caputo et al., 1972]. Depositional environments range from lagoonal-marine in the Ordovician and Silurian to shallow marine, coastal deltaic sedimentation in the Devonian and early Carboniferous. The late Carboniferous and Permian also contain extensive marine, lacustrine, and lagoonal evaporites and limestone. Bouguer gravity anomalies in the Middle Amazon Basin are shown in Figure 4. A chain of gravity highs of +40 to •-90 mGals transects the basin roughly coincident with the axis of maximum thickness of sedimentary rocks. The gravity highs are flanked by gravity lows of -40 +_ 20 mGals. The basin also contains some high-amplitude, short-wavelength gravity anomalies caused by shallow ultrabasic intrusions.

The migration of abnormally warm, saline water through the Appalachian basin and North American midcontinent in Paleozoic time has been inferred from fluid inclusion studies, remagnetizations, and widespread potassic alteration. A time‐dependent numerical model of fluid, heat and solute transport is used to evaluate the viability of topographically driven recharge as a mechanism for brine migration. The model represents a wedge‐shaped sedimentary basin 400 km long by 6 km deep (maximum) with a basal aquifer 500 to 750 m thick overlain by a homogeneous aquitard. Temperature predicted by model simulations is found to be inconsistent with constraints inferred from fluid inclusion studies, unless average heat flow values greater than about 100 mW/m2 are used. Model simulations also lead to predictions of low heat flow and subsurface temperature in recharge zones that are generally not observed in modern orogenic zones. The initial solute content of pore waters in the model basin is flushed out by fresh water entering in the recharge zone before fluid velocities high enough to produce significant warming of the discharge zone can develop. Model simulations with source terms reveal that basin sediments can provide enough solute to maintain hot, hypersaline brine migration for about 1 m.y., at most. High fluid velocity in the basal aquifer is required to carry heat to the basin margins, but the higher the fluid velocity, the more quickly the basin's supply of solute is exhausted. Consideration of these constraints implies that topographically driven recharge may be an effective mechanism to explain regional brine migration only if flow is focused from regional scale recharge zones into more spatially restricted discharge zones.

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