AQUEOUS RARE EARTH ELEMENT PATTERNS AND CONCENTRATION IN CO-PRODUCED BRINES AND INDUSTRIAL PONDS, WYOMING

Aqueous Rare Earth Elements (REEs), stable isotopes, and geochemistry where quantified in 29 Wyoming samples spanning two basins and four industrial sites. Measurement of REEs at the ppt level in oily brines was possible thanks to the methods developed by INL co-authors. Isotope, geochemical, and trace analyses were performed at UW and by commercial laboratories.

Basin samples contain elevated Eu and industrial sites contain elevated HREEs (Tb-Lu). In the sampled oil and gas co-produced waters Eu averages 18ppt and can exceed 70ppt. Respectively, these concentrations are 80 and 300 times greater than found in ocean waters (North Pacific Deep Water). In industrial power station ponds, total HREEs average 58ppt and can exceed 190ppt. The HREE enrichment at station ponds is also evident from their Yb_NASC:La_NASC ratio, which averages 16:1 for the sampled station ponds. In contrast, the ocean's Yb_NASC:La_NASC ratio is 1:2. Isotopic ratios range from -16.8 to 9.8‰ for δ¹³C, -119 to -33‰ for δD, and -14 to +2‰ for δ¹⁸O.

To a first order approximation, aqueous REEs combine signatures for the geologic basin they sample, and also the field, but show little variation between formations within the same field. The basin signature for the Powder River Basin (PRB) is a Gd anomaly nearly as large as Eu after normalization, which distinguishes them from the Wind River Basin (WRB) where a similar Gd anomaly is absent. Fields also have a signature: In the PRB, field H has slight HREE enrichment over NPDW, but field P has HREE contents far greater than the NPDW. In the WRB, field M has slight HREE depletion, but field L has HREE enrichment. Although the PRB samples include common formations between fields H and P there is no detectable formation signature in the REEs independent of field signature.

A formation signature is available in isotope and trace geochemistry. δ¹³C isotopes in some formations show evidence of biogenic natural gas, and industrial site isotopes conform to the superficial water δ¹³C range. All water samples are displaced to the right of the δD-δ¹⁸O GMWL. Although all waters were sodium-chloride to sodium-sulfate type, trace geochemistry also includes information on source formation. REEs, isotopes, and trace elements offer enough information to identify the basin, field, and formation a water came from.