Successive pore fluid generations in a Lower Permian brine aquifer, Palo Duro Basin, Texas Panhandle, U.S.A.

Abstract

The successive presence of four compositionally distinct pore fluids in Lower Permian carbonate strata of the Palo Duro Basin, Texas Panhandle, is interpreted from (1) lithological, minerlogical, and petrographic evidence for depositional conditions; (2) isotopic compositions of C, O, and Sr in limestone and dolomite; (3) limited data on fluid inclusions in sphalerite; and (4) chemical and isotopic analyses of formation water. Wolfcamp carbonate mudstone containing normal marine fauna was initially deposited on an open, shallow shelf. The C and O isotopic compositions indicate precipitation from Permian seawater with minor terrestrial and meteoric influence; Sr isotopic compositions document a Leonardian seawatersource of material. Conditions gradually became more restricted and seawater was concentrated by evaporation, resulting in deposition of penecontemporaneous dolomite and anhydrite in overlying Wichita strata. Reflux of Wichita brine dolomitized underlying Wolfcamp carbonate mudstone and generated the upper Wolfcamp aquifer. Isotopic compositions of Sr in Wolfcamp dolomite suggest that the reflux brine was evaporatively concentrated Leonardian seawater. Expulsion of warm, saline formation water from deeper in the basin through the upper Wolfcamp aquifer is recorded in fluid inclusions in sphalerite. This change in pore fluid composition occured before and during regional Tertiary uplift and tilting of the Texas Panhandle. More recently meteoric water has recharged at least the western one-third of the upper Wolfcamp aquifer. This modern flow regime was established within the past 10–15 Ma.