Re-Os Systematics of Crude Oil and Re-Os Petroleum System Geochronology

Abstract

Re and Os are present in many crude oils in measurable abundances. The Re-Os geochronometer has successfully constrained the timing of oil generation, thermochemical sulphate reduction and thermal alteration of crude oil for petroleum systems worldwide. The Os isotope composition has also been used as an oil-source correlation tool.
This thesis firstly presents two petroleum matrix-matched Re-Os measurement reference materials: the RM8505 crude oil and ~ 90 g homogeneous asphaltene powder isolated from the RM8505 crude oil. The Re-Os data are from the repeated measurements of these samples via the Carius Tube - Isotope Dilution - Negative Thermal Ionization Mass Spectrometry methodology. The normal distribution and low relative standard deviation of the abundance and isotopic data ensure them to be appropriate petroleum matrix-matched reference materials for Re-Os measurements.
A Re-Os age of 66 ± 31 Ma was defined by the Duvernay-sourced oil asphaltene fractions from Western Canada sedimentary basin. This age is in excellent agreement with the main-stage hydrocarbon generation of Duvernay Formation based on basin modelling. Further, this study supports Os isotope composition as a valid oil-source tracer and the hypothesis that the oil 187Os/188Os composition is inherited from the source unit during oil generation. This study shows limited or no influence of the Re-Os systematics of crude oil through the interaction of basinal fluids.
The progressively precipitated asphaltene fractions of six oil samples exhibit a decrease in Re and Os abundance, and diverse 187Re/188Os and 187Os/188Os patterns. This study proposes that Re and Os in crude oil are to be bound in multiple free compounds and such molecules occluded/absorbed in the asphaltene aggregate structure. No combination of the fractions of a crude oil can consistently yield geologically meaningful Re-Os age for all of the six oil samples, either the progressively precipitated asphaltene fractions or the asphaltenes and maltenes separated by n-alkanes. As such, obtaining geologically meaningful Re-Os dates from a single oil may not be viable for many oils.