Geochemical Implication of some Chemical Fossils as Indicators of Petroleum Source Rocks

Abstract

Chemical fossils (biomarkers) are molecules in crude oils, source rocks and sediments whose carbon structures or skeletons can be traced back to living organisms. Three of such diagnostic fossils have been typically reviewed with respect to maturation changes, precursor-product relationship, utility for source rock/crude oil, crude oil/crude oil correlations and role in basin development. Fingerprints (composition and distribution) of hopanes, oleananes and steranes have been highlighted. Defunctionalization and breakage of double bonds which occur during diagenesis as well as stereochemical changes which occur during catagenesis have also been highlighted. Napthenic hydrocarbons, such as steranes (C27 - C29) and triterpanes (C29 - C32), are useful in geochemical correlations because of their unique compound distributions and the fact that they are not seriously affected by migration, thermal alteration and biodegradation. Although individual biomarkers are in concentrations of only 10 to 200 ppm in crude oils, they can be accurately measured in spite of their unusual complexity and variety which find use in source rock correlation and maturation studies. Correlations using chemical fossil technology can be applied in real cases by recognizing the source chromatogram (GC) or fragmentogram of the hydrocarbon molecules in the oils to know whether they have the same biomarkers or similar geohistory of origin and migration. Therefore, genetically related oils are differentiated from unrelated oils on the assumption that the same source material and environment of deposition produce the same oil. Thus, a chemical fossil compound in a particular source rock would be expected to appear in the oils it generated. @JASE