ABSTRACT

Knowledge of in-situ physical and chemical properties of pore fluids in sedimentary basins is essential in understanding problems of fluid migration and diagenetic reaction. We have completed a study of the regional variations in pore fluid temperature, pressure, and salinity for the entire Tertiary section of South Louisiana to depths of 4 to 6 km. From this information it is possible to calculate values for hydraulic head and in-situ pore fluid densities and viscosities, parameters necessary in quantifying rates and directions of subsurface fluid migration. It is also possible to determine such chemical properties as methane solubility.

Maximum salinities, typically in the range of 120-150 g/1, are found in the lower part of the hydropressured zone, where salt is introduced by dissolution of salt diapirs. Hydraulic head increases from essentially zero in the hydropressured zone to values exceeding 3 km above mean sea level deep in the geopressured section. Fluid densities increase with depth to maximum values approaching 1.07 g/cm at depths of approximately 2 km. Densities decrease with further depth as a result of increasing temperature and decreasing pore water salinity. The entire lower portion of the pore fluid column is thus gravitationally unstable, a factor favoring fluid overturn. Fluid viscosities systematically decrease from 1 cp at the surface to less than 0.2 cp at depth. As a result of these variations in density and viscosity, pore waters will flow five times more efficiently at depth. Methane solubility decreases upward from over 50 SCF/bbl in deep overpressured sediments to 10 SCF/bbl at the base of the hydropressured zone to less than 5 SCF/bbl at shallow depth. Significant exsolution of methane should thus occur as a result of upward migration of pore fluid or molecular diffusion of dissolved methane.