THE FULL PICTURE OF REE MOBILIZATION IN LOW TEMPERATURE SEDIMENTARY BASINS: WATER, ROCK, AND OIL IN THE BIG HORN BASIN, WYOMING

Deposits of rare earth elements (REEs) are primarily attributed to high temperature, magmatic and hydrothermal systems, yet substantial amounts of these metals occur in low temperature sedimentary basins. To address this dichotomy, we evaluate fluids expelled from the Phosphoria Formation, an REE-enriched, hydrocarbon source rock in western Wyoming and eastern Idaho, to investigate movement and behavior of REEs in one low-temperature sedimentary basin in Wyoming. Samples of Phosphoria Formation sourced oils and produced waters were collected from 16 wellheads in three separate oil fields across the Bighorn Basin in northern Wyoming. The fluids were collected directly from the wellhead to avoid mixing, degassing, and contamination associated with sampling from separators. After separating the oil and water in the laboratory by gravity, oil samples were heated at 90°C for 10 hours on a hot plate to remove volatiles, ashed in a muffle furnace at 550 °C for 12 hours and at 600°C for 2 hours to remove organic matter, dissolved using ~1 molar HNO3, and analyzed for REE using inductively coupled plasma-mass spectrometry. Preliminary data indicates that average REE concentrations in Phosphoria-sourced oils in the Bighorn Basin are ~2.5 ppb, consistent with REE concentrations reported in the few studies available in the public domain, 2 to 3 orders-of-magnitude lower than chondrite, and 2 orders-of-magnitude higher than average REE concentrations reported for basinal brines sampled across Wyoming. In contrast, published values for REEs in Phosphoria Formation rocks range from 160-3,000 ppm. Though the concentrations between the rocks, oil, and water vary widely, differences in concentrations and partitioning will shed light on the way in which REEs behave in low temperature basins with high volumes of fluid migration.