Regional Hydrodynamics of Variable-Density Flow Systems, Palo Duro Basin, Texas

In mature sedimentary basins, where mechanical compaction is negligible, the hydrodynamics is typically described by steady-state flow driven by potential energy represented by the water table, which generally follows the topography. In regional flow systems consisting of both shallow fresh-water aquifers and deep saline aquifers, however, ground-water flow can be greatly affected where buoyancy forces associated with high salinities dominate the flow component caused· by topographic relief.Effects of significant fluid-density variation on regional ground-water flow through the Palo Duro Basin. Texas, where densities are between 1.0 and 1.15 g/cm3, were studied using a regional cross-sectional model. Steady-state flow of variable-density ground water was simulated by computation of equivalent fresh-water heads and stream functions incorporating fluid densities that vary in space but are assumed tobe time invariant. Modeling indicates that overall ground-water circulation in the basin is not significantly affected by regional fluid-density variations, except near the discharge boundary, where high fluid densities cause increased downward flow. However, simulated equivalent fresh-water heads differ from those obtained assuming fresh-water density, which is important for model calibration. The hydrodynamics associated with the present topography is characterized by relatively steep hydraulic gradients across the basin, thereby dominating potential buoyancy-flow phenomena.The relation between the two driving forces, buoyancy associated with variable-density ground water and the head gradient arising from topographic relief, was studied using a generic flow and solute-transport scenario in the context of basin hydrology affected by uplift, deposition, or erosion. Results from the flow and solute-transport simulations indicate that the dissolved mass, represented by high-density fluids, responds to changes in hydrologic boundary conditions over a much longer time than do fluid pressures, thus changing the relative magnitude of the two driving forces. Considering the hydrodynamic changes associated with Cenozoic basin uplift, deposition, and erosion, the observed salinity distribution in the Palo Duro Basin may represent some past state of the hydrodynamic system. Paleohydrologic modeling indicates that before maximum basin uplift, when the topographically induced hydraulic gradient across the basin was smaller than at present, buoyancy-dominated flow patterns occurred, characterized by a continuous overturn of fluids, which restrict recharge to the upper part of the deep aquifers. With continued uplift, buoyancy-dominated flow phenomena diminish, and the flow component arising from topographic relief becomes more important. The presence of biodegraded hydrocarbons in the Triassic Dockum Group overlying the evaporite confining unit in eastern New Mexico can be related to secondary migration of hydrocarbons associated with the paleoflow system that existed before Pecos River valley erosion, when basin uplift was most advanced.
Rainer K. Senger

Senger, R. K., 1991, Regional Hydrodynamics of Variable-Density Flow Systems, Palo Duro Basin Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 202, 54 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation