GC-C-IRMS : a new technique to characterize and correlate oils and source rocks
Date
1994
Authors
Murphy, Dena Elizabeth
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
In this study, individual gasoline range hydrocarbons in Canadian crude oils are analyzed by a new isotope technique called gas chromatography-combustion-isotope ratio mass spectrometry using an alumina substrate, a helium purge, and a liquid nitrogen trapping mechanism for volatile sample application. The oils chosen for this analysis originate from a variety of source materials and are differentially affected by alteration processes. The choice of sample set enables testing of the new isotope technique to determine whether or not compound-specific, gasoline isotope ratios can be used as an independent parameter to characterize and correlate oils and source rocks. In addition, the effects of alteration on the isotopic composition of individual compounds are investigated.
The main conclusions of this study are as follows :
(1) The isotope ratios of Cs to C 10 hydrocarbons can be used to characterize and correlate related oils. The isotope variation between different oils is about 10 %0 while the experimental error only ranges from ± 0.3 ‰ to 0.5 ‰.
(2) The Saskatchewan, Arctic, and East Coast oils separate into the three different groups consistent with previously known sources for the oils. The Alberta oils, from a variety of source environments, correlate with other, similar-sourced oils from other areas.
(3) Thermal maturation does not appear to result in a significant isotope effect since the high maturity Turner Valley oil correlates with low maturity Arctic oils and no significant isotopic differences were observed between Arctic condensates (higher maturity) and oils (lower maturity).
(4) The isotope ratios of the volatile hydrocarbons are resistant to secondary alteration processes such as biodegradation. The change in the isotope ratios (or isotope effect) due to alteration processes is generally less than 1 ‰ and often within the analytical uncertainty.
Previous molecular and isotopic geochemical techniques for correlating oils and sources can not be used independent of each other. The molecular compositions of oils change due to processes such as maturation, biodegradation, and water-washing. This makes the recognition of chemical similarities between related oils difficult. Traditional bulk oil and major oil fraction isotope analyses often yield non-specific information (averaged ratios of many compounds) which hinders the achievement of reliable correlations. The success of the individual gasoline range isotope ratios in correlating Canadian oils (which are differentially biodegraded) establishes this new isotope technique as a promising geochemical tool for correlation problems.