Geochemistry and hydrodynamics of deep formation brines in the Palo Duro and Dalhart basins, Texas, U.S.A.

Abstract

Geological characterization of evaporite deposits as potential host rocks for radioactive waste burial must include hydrogeological investigations on both local and regional scales. The Palo Duro and Dalhart basins of Texas contain candidate salt deposits which 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. Salinities computed from geophysical logs or obtained from chemical analyses indicate only small aberrations in the regional salinity profile for brines in carbonate rocks and sandstones of Late Pennsylvanian-Early permian age. Brines reflect reactivity with the host rock deriving salinity primarily from evaporite facies and at present, apparently follow the anhydrite and calcite phase boundaries. Substantial outgassing of CO2 and oxidation of ferrous iron appear to have occurred during collection of the samples during wildcat drilling by industry. Mass-transfer computer programs have been used to determine the most probable in situ brine composition. Additional support for the computed equilibrium state is the correlation between computed PCO2 in the brines and observed PCO2 in adjacent natural gas reservoirs. © 1982.