Cross-Formational Flow in the Palo Duro Basin, Texas Panhandle

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Permian evaporite strata have been proposed as media for nuclear waste storage. Repository integrity is dependent on the extent of cross-formational flow, or leakage, through the Evaporite aquitard into the underlying Deep-Basin Brine aquifer. Pertinent data were reviewed and integrated to evaluate potential leakage. The Deep-Basin Brine aquifer is underpressured and has hydraulic heads hundreds of meters below those of the fresh-water Upper aquifer, indicating potential for downward flow between the Upper and Deep-Basin Brine aquifers. In the northwestern part of the basin, water in the Deep-Basin Brine aquifer has a meteoric isotopic signature, and its presence is cited as evidence for leakage from the Upper aquifer downward through the Evaporite aquitard. A plausible alternative source is lateral recharge from eastern New Mexico. Darcian leakage rates calculated from numerical models are very small (hundredths to thousandths of a μm/d) and depend primarily on the permeability of salt, which is stress dependent and overestimated in testing. If salt permeability is 10^-5 md, as measured recently in situ in competent New Mexico bedded salt, leakage through the Evaporite aquitard would be 6 x 10^-9 m/d and flow through an area of 25 km^2 (9 mi^2) approximately 0.15 m^3/d (40 gal/d). Though fractured, the Evaporite aquitard probably behaves regionally as an extremely low-permeability, low-flux porous medium. Basinal brines are compositionally stratified, ruling out substantial vertical mixing or mass transfer. Hydrologic isolation of the salt is further indicated by the inferred presence of Permian connate water in the aquitard, high bromide content of halite, and dominance of syndepositional and early diagenetic halite textures. Only the salt dissolution zone, atop the aquitard, has experienced post-Permian dissolution. Available geochemical and petrographic evidence shows that cross-formational flow, or leakage, through the Evaporite aquitard is very slight.


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