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The relative amounts of C₂₇, C₂₈ and C₂₉ steranes in oils are controlled by the types of photosynthetic organisms that contributed to the organic matter. A dominance of C₂₇ steranes is almost always associated with marine organisms. Most nonmarine organic matter has a dominance of the C₂₉ sterane precursors, but C₂₉ steranes can dominate in marine systems as well. The abundance of C₂₈ steranes in marine systems may depend primarily on geologic age<ref name=ch08r16>Grantham, P. J., and L. L. Wakefield, 1988, Variations in the sterane carbon number distributions of marine [[source rock]] derived crude oils through geological time: Organic Geochemistry, vol. 12, p. 61–73, DOI: 10.1016/0146-6380(88)90115-5.</ref> but this idea is controversial. In nonmarine systems, there is no proposed relationship between C₂₈ sterane abundance and age. C₃₀ steranes (''n''-propylcholestanes) are usually less abundant than the other regular steranes and occur only in samples deposited where marine organisms lived.<ref name=ch08r32>Moldowan, J. M., W. K. Seifert, and E. J. Gallegos, 1985, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0069/0008/1250/1255.htm Relationship between petroleum composition and depositional environment of petroleum source rocks]: AAPG Bulletin, vol. 69, p. 1255–1268.</ref>

The relative amounts of C₂₇, C₂₈ and C₂₉ steranes in oils are controlled by the types of photosynthetic organisms that contributed to the organic matter. A dominance of C₂₇ steranes is almost always associated with marine organisms. Most nonmarine organic matter has a dominance of the C₂₉ sterane precursors, but C₂₉ steranes can dominate in marine systems as well. The abundance of C₂₈ steranes in marine systems may depend primarily on geologic age<ref name=ch08r16>Grantham, P. J., and L. L. Wakefield, 1988, Variations in the sterane carbon number distributions of marine [[source rock]] derived [[crude oil]]s through geological time: Organic Geochemistry, vol. 12, p. 61–73, DOI: 10.1016/0146-6380(88)90115-5.</ref> but this idea is controversial. In nonmarine systems, there is no proposed relationship between C₂₈ sterane abundance and age. C₃₀ steranes (''n''-propylcholestanes) are usually less abundant than the other regular steranes and occur only in samples deposited where marine organisms lived.<ref name=ch08r32>Moldowan, J. M., W. K. Seifert, and E. J. Gallegos, 1985, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0069/0008/1250/1255.htm Relationship between petroleum composition and depositional environment of petroleum source rocks]: AAPG Bulletin, vol. 69, p. 1255–1268.</ref>

[[:file:oiloil-and-oilsource-rock-correlations_fig8-23.png|Figure 1]] shows m/z 217 mass fragmentograms from two oils showing quite different distributions of regular steranes. The top example is dominated by C₂₉ steranes with only moderate amounts of C₂₇ and C₂₈. The bottom sample, in contrast, shows similar amounts of all three homologs plus moderate amounts of the C₃₀ steranes (four unlabeled peaks to the far right).

[[:file:oiloil-and-oilsource-rock-correlations_fig8-23.png|Figure 1]] shows m/z 217 mass fragmentograms from two oils showing quite different distributions of regular steranes. The top example is dominated by C₂₉ steranes with only moderate amounts of C₂₇ and C₂₈. The bottom sample, in contrast, shows similar amounts of all three homologs plus moderate amounts of the C₃₀ steranes (four unlabeled peaks to the far right).