Free Convection, Mass Transport and Diagenesis in Thick Heterogeneous Sediment Sequences: Implications for the Gulf Basin Geopressured Zone

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9316627 Nunn A large portion of the sediments within the northern Gulf Basin are over pressured (i.e., contain pore pressures in excess of hydrostatic). In some areas, pore pressures may be more than 80% of lithostatic pressure. Development of geopressure is permeability sediments in the miocene and particularly the Plio- Pleistocene. Water and carbon dioxide generated by thermobaric chemical reactions, such as clay dehydration and hydrocarbon maturation, may also contribute to the formation of excess pore pressure. Numerous studies have examined the formation of geopressure in sedimentary basins and/or expulsion of geopressured fluids into the overlying into the overlying hydropressured zone. However, very little attention has been given to fluid flow within the geopressured zone itself. A number of priori considerations make it likely that regional scale thermohaline convection may be occurring within geopressured zones: 1) sediments are undercompacted and therefore porous and permeable; 2) undercompacted sediments have low thermal conductivities which increases the conductive geothermal gradient; and 3( in the Gulf Basin, dissolution of stratigraphically high salt in the form of domes or allochthonous sheets produces dense saline brines which further destabilizes the water column. Movement of oils from source rocks to reservoirs in the Gulf Basin requires as much as 10 km of vertical migration in a few million years. Precipitation of large amounts of quartz and calcite cements in some geopressured sediments also implies fluid flow on a massive scale. We propose to carry out a two year study on the following aspects of thermohaline conviction and theoretical diagenesis in heterogeneous sediment sequences: 1) modify our existing numerical models to simulate thermohaline convection beneath allochthonous salt sheets: 2) examine the effects of heterogeneous sediments (e.g., growth faults and is continuous sand/shale layers) on fluid flow patterns; 3) dev elop and couple the physical models for thermohaline convection to numerical models for quartz and calcite diagenesis assuming that solubility is a function of temperature and salinity only; and 4) compare results from the numerical models to the pattern of diagenesis in the Gulf Basin including the formation of vertical or horizontal chemical seals. ***