Deep brine aquifers in the Palo Duro Basin : regional flow and geochemical constraints

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Bassett, R. L. (Randy L.), 1948-

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University of Texas at Austin. Bureau of Economic Geology


Geologic characterization of evaporite deposits as potential host rocks for burial of radioactive waste must include hydrogeologic investigations at both local and regional scales. The Palo Duro and Dalhart Basins of Texas contain candidate salt deposits that are underlain by shelf carbonates and fan-delta sandstones. These basins are ancient intracratonic elements exhibiting regional eastward flow in the deep brine aquifers. Pressures in these aquifers are "subnormal"; however, the major component of flow appears to be parallel to bedding, owing to the low permeability of the overlying evaporite strata in the central part of the basin. Salinity values computed from geophysical logs or obtained from chemical analyses indicate only small aberrations from a regional average salinity for brines in carbonate rocks and sandstones of Late Pennsylvanian and Early Permian age. Brine composition is derived by reaction with the host rock, obtaining salinity primarily from evaporite facies and, at present, apparently follows the calcite phase boundary. Brines may also be near equilibrium with anhydrite except in regions where sulfate reduction has generated hydrogen sulfide. Evidence of ion exchange is tenuous; however, clastic sediments predominate in the western part of the basin, early in the flow path, and a significant reduction in the molar ratio of sodium to chloride is observed in many samples. Substantial outgassing of carbon dioxide (CO2) and oxidation of ferrous iron appear to have occurred as the samples were collected by industry during wildcat drilling. Mass transfer computer programs have been used to determine the most probable in situ brine composition. Support for the validity of the computed equilibrium state is the correlation between the values of partial pressure of carbon dioxide (Pco,) calculated for the brines and the PCO2 observed in adjacent natural gas reservoirs.


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