| Popis: |
Stable carbon isotope signatures of individual hydrocarbon components provide important information on the source rock facies of oils as well as their asphaltene fractions, despite maturity variations and secondary processes such as phase fractionation and biodegradation. Our study is based upon petroleum fluids and source rock samples from offshore Mid-Norway, where the main source rock is the marine shales of the Upper Jurassic Spekk Formation. An alternative source is provided by coals and coaly shales of the Lower Jurassic Are Formation. In terms of maturity the source rock samples are distributed throughout the main phase of oil generation. In this study compound-specific carbon isotope analysis has been performed on thermal extracts and kerogen pyrolysates of the source rocks, as well as on the oils and the pyrolysis products of their asphaltenes. We demonstrate that trends of absolute isotope data and specific isotopic fingerprints after normalizing the data are useful for petroleum-source rock correlations. The carbon isotope ratios of the n-alkanes separate the Are from the Spekk Formation samples in both thermal extracts and kerogen pyrolysates, becoming less negative with increasing terrestrial input. The Spekk samples also tend to become isotopically heavier with increasing maturity, while the Are samples do not show any systematic difference with increasing maturity. The carbon isotope ratios of the n-alkanes also discriminate between the oils as well as the asphaltenes from the fluids. The individual components generated by pyrolysis of kerogens are isotopically heavier than those from the thermal extracts. This is expected from models of kinetic isotope effects which predict that the isotopically lightest products are generally released at an earlier stage in the maturation process. A similar isotopic difference is also observed between the pyrolysis products of asphaltenes and the original oils derived from the same source rock type. However, in a sample where the oil and its asphaltene fraction originate from two different source rocks, the opposite relationship between the carbon isotope composition of the oil and its asphaltene fraction is observed. Bulk isotope data of kerogen concentrates, pyrolysates and the residue after pyrolysis of the source rock samples as well as bulk isotope data of whole oils are also presented. We show that besides separating Are from Spekk, the bulk carbon isotope values of the Spekk samples are considerably heavier than the n-alkanes from the thermal extracts, whereas those of the Are samples are almost equal. |
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