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Rare Earth Elements as an Indicator of Organic Maturation: A Case Study From the Woodford Shale, Oklahoma

Abstract

Exploitation of unconventional shale reserves provides an opportunity to understand the transformation of organic matter to petroleum, without introducing additional variations due to migration or post-migration processes. Ten samples of the organic matter fraction of the Woodford shale from north central Oklahoma were analyzed for their Rare Earth Element (REE) distributions and compared to the REE distributions in oils from the same formation. The REE concentrations in the organic matter of the Woodford Shale samples analyzed range from 300 to 800 ppm. This is much higher than REE concentration levels in an average shale (170 ppm to 185 ppm), and orders of magnitude higher than concentration levels in modern day plants (which occur in ppb levels). As expected, total REE concentrations in Woodford oils are at the ppb level. There is also a distinct difference in the REE patterns between the organic matter, which are heavy REE enriched, and the crude oils, which are light REE enriched. We suggest that the liquid hydrocarbons inherited their light REE enrichment characteristics at least in part from their original organic source and that the organic source material in turn became relatively heavy REE enriched. The oil-source bed REE fractionation relationship and the specific REE distribution patterns, such as Eu and Ce anomalies in them, may be investigated to establish REE geochemistry as a reliable indicator, or a supporting evidence, of the degree of organic maturation, oil to oil correlation, and oil to source bed correlation. In conjunction with K/Rb ratios (ranging from 100 to 500 in the oils), this research brings further evidence about the paths of interactions between the organic materials, the mineral matrix, and the water during the hydrocarbon generation.