Analysis and occurrence of C sub 26 -steranes in petroleum and source rocks

Journal Article · Mon Apr 01 04:00:00 EST 1991 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(91)90164-Z· OSTI ID:5599333

Moldowan, J M; Lee, C Y; Gallegos, E J ^[1]; Watt, D S; Jeganathan, A; Slougui, N-E ^[2]

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1. Chevron Research Company, Richmond, CA (USA)
2. Univ. of Kentucky, Lexington (USA)

The C{sub 26}-steranes previously reported in oils and source rocks have been identified as 21-, 24-, and 27-norcholestanes (1A, 1B, and 1C). Various 24-norcholesterols or stanols, possible prescursors for the 24-norcholestanes, occur widely at low levels in marine invertebrates and some algae, and 24-norcholestanes occur in marine petroleums of Tertiary through Paleozoic age. There are reports of 27-norcholesterols and stanols in recent sediments, but the precursor organisms have not been identified. The natural occurrence of the 21-norcholestane structure is unprecedented. Unlike 24- and 27-norcholestane, 21-norcholestane is in low concentration or absent in immature rocks and increases substantially relative to the other C{sub 26}-steranes in thermally mature rocks, oils, and condensates. This suggests an origin involving thermal degradation of a higher molecuar weight steroid. The ratio of 21-norcholestane to the total C{sub 26}-steranes is shown to be an effective maturity parameter in a series of Wyoming (Phosphoria source) and California (Monterey source) oils. Molecular mechanics MM2 steric energy calculations indicate a relative stability order of 21 {much gt} 27 > 24-norcholestane for the major stereoisomers. Authentic 21-, 24-, and 27-nor-5{alpha}-cholestanes and 24- and 27-nor-5{beta}-cholestanes were synthesized and subjected to catalytic isomerization over Pd/C to yield the full suite of stereoisomers for each.

• OSTI ID: 5599333
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Journal Name: Geochimica et Cosmochimica Acta; (USA) Vol. 55:4; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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