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Fluid Chemistry; Metal-Organic Interactions |
Department of Chemistry, Washington State University, Pullman, WA 99164-4630, USA
Crude oils and bitumens contain trace elements whose abundances range from less than a part per billion (ppb) to more than several thousand parts per million (ppm). Nickel and vanadium are usually the most abundant metals with values ranging up to 200 ppm Ni and 2000 ppm V. Abundances of other trace elements are generally less than 100 ppm. Nickel and vanadium occur in oils and source-rock bitumens as metalloporphyrins and non-porphyrin species. The nickel and vanadium porphyrins are predominantly DPEP or etio complexes of Ni2+ or VO2+ although smaller amounts of benzo and other complex porphyrins are found. The metalloporphyrins are formed during early diagenesis of source rocks and the relative abundances of nickel and vanadium are related to the depositional environment. The origin of the non-porphyrin nickel and vanadium complexes and the complexes of other trace elements in crude oils may be primary or secondary. Primary processes include release of metal complexes from kerogen during categenesis and formation of metal-organic complexes by mineral-kerogen reactions during catagenesis. Secondary processes such as interaction of oils with mineral matter or formation waters during migration, maturation or biodegradation may also affect the concentrations of some elements in crude oils, e.g. Hg.
