Date of Award

1988

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Jeffrey S. Hanor

Abstract

This dissertation consists of three papers which quantitatively model subsurface brine formation in passive, continental margin evaporite basins. In the first paper, a one dimensional finite difference model is developed to evaluate the brine-forming role of molecular diffusion of dissolved NaCl in the subsidence history of a hypothetical sedimentary basin containing a thick basal halite bed. It is shown that diffusion of dissolved NaCl from evaporites may be an important brine-forming process in young passive margin evaporite basins where evaporites are flat lying or gently dipping and that vertical fluxes of dissolved NaCl are great for basins with slow subsidence rates and sandy sediments. Whereas diffusion may be an important process in areas with a thick, relatively horizontal halite bed, solute-induced fluid density variations near salt domes may give rise to density-driven groundwater flow. In the second paper, it is shown that solute-induced fluid density gradients can induce convective flows with maximum velocities of 10$\sp{-2}$ to 1 m/yr even in low-permeability sediments, brine plumes can form very rapidly by solute-induced convection, and up to 2 km$\sp3$ halite can be dissolved in 1 Ma. In the third paper, the combined effects of geopressure and solute-induced fluid density gradients are modeled around a hypothetical salt dome bearing some resemblance to Welsh Dome in South Louisiana where a perched brine plume has been shown to occur above the dome, and near a fault. It is shown that geopressured fluids escaping up a fault zone on the flank of the 'Welsh-type' salt dome may create a time-transient perched brine plume above the dome in very short times, 10$\sp{-1}$ Ma, provided that permeability of the geopressured zone is sufficiently high. However, enormous volumes of water must be sourced from the geopressured zone to form the perched brine plumes, volumes which appear to be very large in comparison to estimates of water available upon compaction of geopressured sediment.

Pages

194

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