Rare earth and trace element geochemistry of Cretaceous kerogens

Abstract

The relationship between rare earth elements (REEs) and the age of Cretaceous kerogens or petroleum source rocks is not known. In this study, kerogen samples isolated from petroleum source rocks from the Cretaceous Orange Basin, South Africa were analyzed for rare earth elements (REEs) and trace elements using inductively coupled plasma-mass spectrometry (ICP-MS). The rare earth elements were profiled in order to determine their relationship to the origin, depositional environment, and age of petroleum source rocks. Most of the rare earth elements identified are of the light REEs series (La, Ce, Pr, Nd, Sm, and Eu) while the trace elements (V, Ni, Mo, and Co) determined are those that are known to have influence on the origin and depositional environment. The results show that the kerogens are mainly of marine origin. The ratios calculated from the concentrations of the elements were able to discriminate marine organic matter from lacustrine organic matter, and also indicate that the organic matter was deposited under anoxic conditions. All the kerogen samples irrespective of the age have similar elemental distribution pattern, indicating a similar genetic origin that could be a reflection of similar organic matter input and depositional environment. The kerogen age has a significant effect on the concentration of the REEs; concentrations of the REEs increase with age. That is, the older the kerogen the greater the content of rare earth elements. Furthermore, the abundance of cerium may be a good indicator of the origin of organic matter. The study also showed that the REEs provide a unique geochemical signature of the kerogens, which can be used for fingerprinting petroleum source rocks.