TY - JOUR
T1 - Analysis and occurrence of C26-steranes in petroleum and source rocks
AU - Moldowan, J. Michael
AU - Lee, Cathy Y.
AU - Watt, David S.
AU - Jeganathan, Alwarsamy
AU - Slougui, Nacer Eddine
AU - Gallegos, Emilio J.
PY - 1991/4
Y1 - 1991/4
N2 - 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-5α-cholestanes and 24- and 27-nor-5β-cholestanes were synthesized and subjected to catalytic isomerization over Pd/C to yield the full suite of stereoisomers for each. In immature rocks the 5α,14α,17α(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 14α,17α(H),20S, 14β,17β(H),20R, 14β,17β(H),20S, and 14α,17α(H),20R in order of elution, as well as putative 20S and 20R rearranged steranes [13β,17α(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 14β,17α(H) peak and a minor 14α,17α(H) peak using an extended injection-temperature-hold-time gas Chromatographic technique.
AB - 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-5α-cholestanes and 24- and 27-nor-5β-cholestanes were synthesized and subjected to catalytic isomerization over Pd/C to yield the full suite of stereoisomers for each. In immature rocks the 5α,14α,17α(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 14α,17α(H),20S, 14β,17β(H),20R, 14β,17β(H),20S, and 14α,17α(H),20R in order of elution, as well as putative 20S and 20R rearranged steranes [13β,17α(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 14β,17α(H) peak and a minor 14α,17α(H) peak using an extended injection-temperature-hold-time gas Chromatographic technique.
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U2 - 10.1016/0016-7037(91)90164-Z
DO - 10.1016/0016-7037(91)90164-Z
M3 - Article
AN - SCOPUS:0026053231
SN - 0016-7037
VL - 55
SP - 1065
EP - 1081
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 4
ER -