The C26-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 C26-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 C26-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-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 steroisomers for each. In immature rocks the 5-alpha,14-alpha,17-alpha-(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-alpha-17-alpha-(H),20S, 14-beta,17-beta-(H),20R, 14-beta,17-beta-(H),20S, and 14-alpha,17-alpha-(H),20R in order of elution, as well as putative 20S and 20R rearranged steranes [13-beta,17-alpha-(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-beta,17-beta-(H) peak and a minor 14-alpha,17-alpha-(H) peak using an extended injection-temperature-hold-time gas chromatographic technique.
