Methodology and Interpretation for Rare Earth Elements (REE) in Various Hydrocarbons

Abstract

The use of Rare Earth Elements (REE) in the petroleum industry is still in the developing stages. Various capabilities of different universities and commercial labs have produced a handful of different approaches to the digestion processes of oil sample preparation for extracting very low concentrations of REE (measured in ppm, ppb, or even ppt). However, no standard for oil digestion has been established as it has been for digestion of sedimentary rock samples (U.S. Geological Survey; Meier and Slowik, 2002).

Different kerogen types may cause some samples not to produce REE results after same or similar digestion processes. Also, high maturity oils may not exhibit heavy rare earth elements (HREE) due to the smaller size of HREE versus medium rare earth elements (MREE) or light rare earth elements (LREE), or due to complexing of the HREE. Number of published data was evaluated for the digestion process; including the mud volcanoes in China, Niger Delta oils, Devonian and Mississippian oils from Oklahoma and Pennsylvanian age Tyler oils from the Williston Basin, North Dakota. The study revealed that only higher temperature heating (at least 200 ^oC) and stronger concentrations of acids, such as nitric acid (HNO₃) and sulfuric acid (H₂SO₄) with peroxide (H₂O₂) provide results greater than zero (measurable by the Inductively Coupled Plasma-Mass Spectrometer; ICP-MS).

There is a distinct difference in results between chondrite and North American Shale Composite (NASC) normalization method of REE analysis. The chondrite REE normalization is appropriate for igneous petrography and the mining industry where different metals are involved, but for thee petroleum analyses it is better to use the NASC REE normalization, as it better represents the environment and the source rock in which the oil is found.

The ternary diagram with separated light (LREE), medium (MREE) and heavy (HREE) rare earth elements was used for the first time to determine the paleoenvironments in which the oils were formed.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018