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Rare Earth Elements (REE) as a Proxy for Oil Migration: Tyler Formation, North Dakota


The Rare Earth Elements (REE) and other trace elements (TE) have proven to be an excellent indicator of depositional environments. REE low mobility during diagenesis makes their unique REE signatures within certain fractions of the sediment an ideal tool for tracing oil migration. The Tyler Formation is part of the Williston Basin has been described as a transgression sequence with elements of marginal-marine, near shore to terrestrial types of depositional environment. The very thin discontinuous sand bodies of the Tyler Formation with small reservoirs displayed in any number of orientations are corresponding to high risk reservoirs. The Tyler Formation contains all the four kerogen types, and is a significant hydrocarbon source and reservoir. It is relatively thin (140 feet) and geographically constrained within North Dakota. This particular setting provides a unique opportunity to test the REE and TE as a proxy for oil migration. This study examined REE and TE in oil from active oil wells and cores from those wells, when available, or from nearby wells. The samples were analyzed for REE composition using Inductively-Coupled Plasma – Mass Spectrometry (ICP-MS). This research has enabled a comparison of REE signatures between oils and also between oils and cores for paleoenvironmental interpretation, correlation of the wells and migration pathways. Sequential extraction of the sediments was conducted to identify the REE signature in particular fractions. These REE signatures were then compared using Discriminant analyses to identify the similar environments for finer resolution. The geochemical data has been mapped and analyzed using Arc GIS software. The results proved that REE are useful tool in studying paleoenvironments and of the oil migration in the Tyler Formation. The REE analysis was also instrumental in identifying the sandstone and the limestone units as the source of the Tyler oil.