REE Geochemistry of Organic Fractions in Mudstone Source Rock and Oil Sand From Aer Sag, Erlian Basin: Implications for Oil-Source Correlation

Abstract

Whether rare earth elements (REE) can be used for oil-source correlation is not yet known. In order to investigate the reliability of REE as a tool for oil-source correlation, we employed traditional biomarkers to identify the source rock of oil sands, isolated organic fractions in mudstones and oil sands, including soluble organic fraction (extract) and/or insoluble organic fraction (kerogen), and analyzed the isolated fractions as well as corresponding whole rock for their REE compositions by ICP-MS. Based on the study of REE composition in the isolated organic fractions of the Lower Crataceous source rocks and oil sands from Aer Sag in Erlian Basin, the following conclusions can be drawn. (a) REE distribution characteristics of mudstone organic fractions are significantly different from those of corresponding whole rocks. The sum of rare earth elements (∑REE) in kerogens is higher than that in whole rocks. ∑REE value in extracts is much lower than that in the corresponding whole rocks and kerogens, indicating that only small amounts of REE migrated from kerogens to the soluble hydrocarbons during primary migration. ∑REE value in extracts is associated with not only maturity but also the relative content of asphaltenes, suggesting that REE in crude oils mainly occurs in the asphaltenes. (b) Compared with mudstone whole rocks, the corresponding kerogens are enriched in the LREE relative to the HREE. This distribution patterns are also observed in the mudstone extracts, showing that REE in the mudstone extracts should be directly originated from kerogen. (c) During the oil generation of kerogen, middle rare earth elements (Sm-Ho), especially Eu, migrated from keogen to the soluble hydrocarbons more easily than the rest REEs. Eu3+ can be reduced to Eu2+ under the acidic and reducing environments formed by oil generation, and it then shifts into soluble organic matters (extracts), resulting in the higher concentrations of Eu relative to the neighboring REEs (Sm and Gd) and the pronounced positive Eu anomalies. (d) ∑REE and REE patterns are very useful oil fingerprinting parameters for oil-source correlation, and REE pattern is proved to be unaffected by petroleum migration. Moreover, REE can be used as a novel complementary approach for the oil-source correlation in complicated petroleum systems.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015