Date of Award

1989

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Jeffrey A. Nunn

Abstract

Groundwater near salt domes is an inherently complex hydrogeologic system because groundwater is subject to large lateral gradients in salinity and temperature. Moreover, groundwater flow is indirectly coupled to salt tectonics because diapirism alters the salinity and thermal conditions as a sedimentary basin evolves. In order to study this complex environment, mathematical and numerical models are developed which explicitly couple heat and dissolved salt transport to groundwater flow, and account for basin subsidence and salt diapirism. The groundwater flow field is described using a mass stream function which does not require the Boussinseq assumption. The numerical model uses a control volume finite difference scheme and resolves nonlinearities iteratively using under-relaxation. In order to assess the role of thermohaline convection near salt domes, dimensional analysis is used to simplify the transport equations and reduce the number of model parameters to three: the Rayleigh number, the Lewis number, and the buoyancy ratio. The buoyancy ratio is the ratio of salinity to temperature effects on pore water density, and it is the only dimensionless parameter that appears in the groundwater flow equation. The sense of convective circulation near a salt column depends primarily on the value of the buoyancy ratio and the thermal gradient contrast between the salt and overlying sediments. When the thermal gradient contrast is large or the buoyancy ratio is small, convective circulation drives groundwater up along the salt edge. These conditions arise over a limited range of geologic circumstances. A more comprehensive numerical model is used to investigate groundwater flow when salt dissolves preferentially at the crest of a salt dome during diapirism. Simulation results indicate that groundwater will flow down along the salt flank except when regional background salinities are high. Downward groundwater flow leads to low surface heat flow above the dome, in contrast to calculations based on heat conduction alone. Simulation results further indicate that groundwater flow is only weakly dependent on the hydrologic boundary conditions because groundwater density gradients are large. In the absence of high regional salinities, upward flow of groundwater near salt domes is probably driven by the release of geopressured fluids.

Pages

339

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