DETERMINATION OF TRACE-ELEMENTS IN PETROLEUM-EXPLORATION SAMPLES BY INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY AND INSTRUMENTAL NEUTRON-ACTIVATION ANALYSIS

A suite of geochemical oil samples from a wide geographic area and a v ariety of depositional environments was analysed by INAA and ICP-MS. G ood correlation between the two techniques was obtained for V, Ni and Co (r = 0.95-0.90). The correlation decreased in the order: Zn (r = 0. 87), Fe, Sb, Se, U, Ti, La, As, Ba, Mn and W (r = 0.18). The contribut ion of trace elements to oils from produced waters, drilling fluids, b iodegradation and migration has been suggested by some of the samples for which there was disagreement. Laboratory experiments confirmed tha t barite contributes Mn, Fe, Ga and Pb to oil samples that have not be en filtered. On the ICP-MS Ba-130(2+) interferes with Cu-65(+) whereas Cu-63(+) suffers from interference by (VC+)-V-51-C-12. If formation w ater and produced water is not removed from the oil, the water contrib ution of elements e.g., As and Pr is superimposed onto the organically bound contents in the oils. In such instances, it appears that these elements are not useful in fingerprinting and classifying oils. Howeve r, when the formation water contribution is removed, elements e.g., As are shown to be geochemically significant. It has thus been demonstra ted that it is important to water wash and filter oil samples before a nalysing for geochemical exploration purposes. This work has also show n that metals such as Zn, Cd and Pb are picked up by oils migrating th rough ore bodies of these metals. Prior to examining the contributions of water, barite and migration, oils could only be classified using t heir V, Ni and Co contents. The new understanding acquired from this w ork, has made it possible to identify and eliminate samples with sever e contamination. Application of multivariate statistics to the V, Ni, Co, Mo, As, Sb, Fe, Mn, Zn and Bi data for the Exxon oils enabled appr opriate oils to be correlated with each other.