Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks

Details Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks

Aug 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48.1651m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 48, Issue 8, pp.1651-1661

Computer Science

1

Scientific paper

Two C 28 H 48 -pentacyclic triterpanes were isolated from Monterey shale. X-ray crystallography of a crystal containing both compounds proved their structures as 17 ,18 ,21 (H)-28,30-bisnorhopane and 17 ,18 ,21 (H)-28,30-bisnorhopane. Several differences are found between 28,30-bisnorhopanes and the regular hopanes. Unlike the regular hopane epimers, for practical purposes the three epimeric 28,30-bisnorhopanes [17 ,21 (H)-, 17 ,21 (H)-, and 17 ,21 (H)-]cannot be distinguished by their mass spectra. Special conditions are needed to separate them by gas chromatography. The diagenetically first-formed epimer is thought to be 17 ,21 (H)- because it predominates in immature shales. The order of thermodynamic stability is 17 ,2l (H) < > 17 ,21 (H) > 17 ,21 (H), and all three epimers are present in petroleum. 25,28,30-Trisnorhopanes can be analyzed in similar fashion and are found to have similar thermodynamic characteristics. The percent of the ring D/E cis epimer of 28,30-bisnorhopane and/or 25,28,30-trisnorhopane is a useful maturation parameter similar to the 20S/20R sterane ratio. Evidence indicates 25-demethylation of 28,30-bisnorhopane to 25,28,30-trisnorhopane during advanced stages of biodegradation. Hence, percent ring D / E cis 25,28,30-trisnorhopane has an application to maturation assessment in heavily biodegraded oils.

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