Trace metal and Mo isotope systematics in petroleum fluids

The presence of trace metals in oils has long been of interest for both their detrimental and beneficial effects during hydrocarbon exploration. For example, elevated levels of trace metals can cause problems during downstream refining processes due to catalyst poisoning. In contrast, the diagnostic potential of trace metals incorporated in fluids and host rocks has been successfully applied in various fingerprinting studies. The majority of work to date has focussed on the two most abundant metals found in oils, vanadium (V) and nickel (Ni), which have been used as tools in correlation and fingerprinting, and provide information on redox conditions during source rock deposition. Here we focus on the molybdenum (Mo) isotope system, well known to display bi-polar redox chemistry [1], and thus providing a potentially valuable new tool to exploit in petroleum systems’ studies. We have developed techniques for the determination of Mo isotopes in hydrocarbon (HC) fluids. Mo is extracted from its complex organic fluid matrix, through high pressure ashing, which renders the complete decomposition of the HC fluid, allowing the Mo to be purified via conventional ion-exchange methods [2]. Preliminary data from replicate samples suggest a precision of better than 0.1‰ in #98/95Mo. We combine the Mo isotope data with more conventional elemental fingerprinting, as determined through isotope dilution HR-ICPMS analysis with a new multi-element spike. This method allows greater accuracy and precision than has been previously possible, both being better than 10%. Samples display variably positive #98/95Mo ratios, ranging from +0.4‰ to +1.7‰, within the range of sediments deposited under variably reducing conditions [3]. We will further explore the relationship between oil Mo isotope composition and its source rock, as well as biodegradation and maturation processes.