Extent of water-rock interactions in Lower Permian Wolfcamp carbonates, Palo Duro Basin, Texas

A laterally-extensive permeable zone in upper Wolfcampian carbonate strata constitutes the first important transmissive unit below a thick evaporite sequence that is being considered for nuclear waste isolation. The extent of water-rock interaction was evaluated by comparing the chemical and isotopic composition of formation water and core collected at four DOE test wells. Wolfcamp mineralogy is dominantly calcite and dolomite with minor anhydrite at each of the four sites despite minor variations in depositional environment, diagenetic alteration, and abundance of clastic material. Isotopic compositions of calcite and dolomite and of anhydrite are within the ranges expected for Permian marine sediments altered by early diagenesis. Wolfcamp formation water compositions are more variable than host rock compositions, and are not completely controlled by local water-rock equilibria. Wolfcamp brines from two wells in the western part of the basin have depleted isotopic compositions relative to the two eastern wells and have not equilibrated with Wolfcamp carbonates, whereas the eastern fluids have. Strontium in the western samples is more radiogenic than that of the eastern samples, indicating a greater influence of clays or feldspars on the western fluids. Comparison of water and rock compositions suggests: (1) the western brines have interacted less with the carbonate host than the eastern brines due to shorter residence times or greater water: rock ratios, and (2) the minerals encountered along flow paths prior to entering the Wolfcamp are different for the western and eastern Wolfcamp brines