Hydrodynamics of gravity-driven flow systems in sedimentary basins : example of the Palo Duro Basin, Texas




Senger, Rainer K.

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The hydrodynamics of a mature sedimentary basin affected by uplift, deposition, and erosion is examined in the Palo Duro Basin, Texas, with a cross-sectional ground-water flow model. Transient simulations of ground-water flow assuming uniform fluid density are used to describe hydrodynamic conditions arising from changes in topography. Modeling shows that uplift and tilting of the basin (10 to 15 m.y. ago) caused significant increase in ground-water flow rates. Erosion of the Pecos River valley and the more recent westward retreat of the Caprock Escarpment cause significant hydraulic-head changes in deep aquifer units within the last few million years. Buoyancy effects on regional ground-water flow through the Palo Duro Basin where densities range between 1.00 and 1.15 g/cm³ are studied using a new modeling technique, based on equivalent fresh-water heads and stream functions, incorporating fluid densities that vary in space but are assumed to be time invariant. Flow velocities based on the stream-function solution were found to be more accurate than those based on the head solution, allowing a more accurate interpretation of local flow patterns. Simulated streamlines show that the regional groundwater flow in the vertical plane is not significantly affected by density variations, indicating that the topographically-driven flow component dominates potential buoyancy phenomena. However, equivalent freshwater heads differ from those obtained assuming fresh-water density, which has important implications on model calibration. Paleohydrologic modeling indicates that prior to basin uplift, when the topographically induced head gradients were smaller than at present, buoyancy phenomena can produce stationary convection cells in the western part of the basin, characterized by stratified density distribution. Another convection cell is simulated in the eastern part, where density within the deep aquifer decreases with depth creating thermohaline convection. Potential effects of regional ground-water flow on thermal regime, suggested by the thermal gradient pattern, were studied by simulating energy transport associated with heat flow from the basement and advection through ground water. Simulation results suggest that the thermal regime is dominated by conduction. Advection is probably insignificant due to large anisotropy of the hydrostratigraphic units, relatively shallow depth of the flow system, and large ratio of basin width to basin depth.