Variable alteration in heavy crude oils of west-central Saskatchewan, Canada

Heavy crude oils occurring in the Mississippian Bakken and Lower Cretaceous Mannville reservoirs in west-central Saskatchewan display variable geochemical composition in their light (C sub(5)-C sub(8)) and medium (C sub(12)-C sub(30)) molecular weight fractions. All oils show a depletion of normal a...

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Bibliographic Details
Published in:Organic geochemistry 2004-04, Vol.35 (4), p.469-491
Main Authors: OBERMAJER, Mark, OSADETZ, Kirk G, FOWLER, Martin G, MAOWEN LI, SNOWDON, Lloyd R
Format: Article
Language:English
Subjects:
Earth sciences
Earth, ocean, space
Exact sciences and technology
Geochemistry
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Summary:Heavy crude oils occurring in the Mississippian Bakken and Lower Cretaceous Mannville reservoirs in west-central Saskatchewan display variable geochemical composition in their light (C sub(5)-C sub(8)) and medium (C sub(12)-C sub(30)) molecular weight fractions. All oils show a depletion of normal alkanes, and to a lesser extent acyclic isoprenoids, which appears to increase progressively southward. As biodegradation seems to be the primary control of the n-alkane distributions, it appears that oils produced from deeper reservoirs in the south are more biodegraded than the oils in shallower Mannville reservoirs in the north. However, several other compositional characteristics, including gasoline range and aromatic fractions, are inconsistent with the trend in biodegradation defined by the relative n-alkane abundance. Oils from the shallower northern reservoirs are enriched in cycloalkanes, show lower relative concentrations of isoheptane, lower paraffin indices, much higher K1 ratios and lower relative concentrations of lower molecular weight naphthalenes. Moreover, some oils show a persistent presence of lower homologs of n-alkanes (C sub(5)-C sub(8)) and light aromatics such as benzene and toluene when higher (>C sub(15)) homologs are not present. Such compositional characteristics may indicate that some of the reservoirs containing biodegraded oil were subjected to a subsequent secondary charge of lighter, unbiodegraded crude or to different post-generative alteration mechanisms. This later charge of light hydrocarbons suggests the timing of most biodegradation to be pre-Eocene. Destructive microbial action resulting in preferential removal of n-alkanes, isoprenoids, alkylbiphenyls and methylated naphthalenes had little effect on higher molecular weight fractions. Therefore, despite their apparently very low gravities (average API of 14 degree ), most of these oils suffered only light to moderate biodegradation. Although generally similar biomarker signatures indicate an overall genetic affinity and the same source rock for these oils, minor but systematic variations in the biomarker compositions likely reflect regional source organic facies variation or indicate a progressive change in oil maturity.
ISSN:0146-6380
1873-5290
DOI:10.1016/j.orggeochem.2004.01.010