Other
Scientific paper
Apr 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.1065m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 4, pp.1065-1081
Other
1
Scientific paper
The C 26 -steranes previously reported in oils and source rocks (MOLDOWAN et al., 1985) have been identified as 21-, 24-, and 27-norcholestanes (1A, 1B, and 1C). Various 24-norcholesterols or stanols, possible precursors 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 26 -steranes in thermally mature rocks, oils, and condensates. This suggests an origin involving thermal degradation of a higher molecular weight steroid. The ratio of 21-norcholestane to the total C 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 >> 27 > 24-norcholestane for the major stereoisomers. Authentic 21-, 24-, and 27-nor-5 -cholestanes and 24- and 27-nor-5 -cholestanes were synthesized and subjected to catalytic isomerization over Pd/C to yield the full suite of stereoisomers for each. In immature rocks the 5 ,14 ,17 (H),20R isomers predominate. Mixtures of 24- and 27-norcholestane in oils and mature rocks show a familiar elution pattern of four major peaks presumed to be 14 ,17 (H),20S, 14 ,17 (H),20R, 14 ,17 (H),20S, and 14 ,17 (H),20R in order of elution, as well as putative 20S and 20R rearranged steranes [13 ,17 (H)-diasteranes]. However, the 21-norcholestanes are represented by a single peak in mature sediments under normal GC conditions, which is shown to consist of a major 14 ,17 (H) peak and a minor 14 ,17 (H) peak using an extended injection-temperature-hold-time gas Chromatographic technique.
Gallegos Emilio J.
Jeganathan Alwarsamy
Lee Cathy Y.
Moldowan Michael J.
Slougui Nacer-Eddine
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