--> Abstract: Characterization of South American Oils to Determine Source-rock Age and Provenance Using New High-Resolution Biomarker Methods, by J. M. Moldowan, J. Dahl, P. W. Brooks, and M. R. Mello; #90933 (1998).

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Abstract: Characterization of South American Oils to Determine Source-rock Age and Provenance Using New High-Resolution Biomarker Methods

Moldowan, J. M. and Dahl, J. - Stanford University; Brooks, P. W and Mello, M. R. - Petrobrás/Cenpes

Oil samples generated from all the major known hydrocarbon systems in South America were analyzed for their contents of taxon-specific biomarkers in order to form a calibration set that can be applied to unknown samples. Taxon-specific biomarkers, compounds related to specific taxa of organisms can be useful to determine the age and provenance of an oil. The occurrence of such compounds is related to the evolution and radiation of the taxa responsible for these biomarkers. Examples of such biomarkers include oleanane for angiosperms (flowering plants, range Cretaceous to present), 24-norcholestanes for diatoms (diatom tests known Jurassic to present), dinosteroids for dinoflagellates (dinoflagellate cysts known Triassic to present). In addition to the presence or absence of these compounds, relative concentrations are important. Thus, oleanane is often found higher relative concentrations in Tertiary oils than in Cretaceous oils. Abundances of 24-norcholestanes relative to 27- norcholestanes are elevated in many Cretaceous oils and are even higher in some Tertiary oils. Dinosteroids are seen in all Triassic to Tertiary marine oils but are not detected in most Paleozoic oils. The results of the South American oils are compared to a world-wide calibration set based on more than 350 source rocks and oils.

Identification of source rocks and therefore the petroleum systems for oils can be difficult when source rocks are not available for comparison. Furthermore, sources of different ages can have very similar depositional environments resulting in biomarker fingerprint similarities in some unrelated oils. For the most part these fingerprints are composed of biomarkers derived from widespread taxa. They are effected mostly by depositional environment and are not particularly effected by the ages represented. Examples stem from the input of dinoflagellates, that appear in the Triassic and continue to diversify and radiate throughout the Mesozoic. Their input can be recognized by abundant dinosteroids in oils. Diatoms have been linked to 24-norcholestanes and 24-nordiacholestanes and high abundances of these steroids relative to others can be tied to diatom evolution and radiation in the Jurassic through Tertiary, with the highest ratios observed in the Tertiary. Oleanane parallels the evolution and radiation of flowering plants and is detected beginning in the Cretaceous and through the Tertiary with higher relative amounts observed in many Tertiary oils.

This study focuses on distinguishing oils by the ages of their source rocks. An example is shown from the Llanos basin of Colombia in which there are at least 2 sources (Fig. 1). One is Upper Cretaceous marine to marine deltaic/siliciclastic, and the other is a Tertiary age deltaic to marine deltaic/siliciclastic. When these sources contribute together to an oil it may be difficult to know which source(s) is represented and to what extent. The ratios 24-/(24 + 27)-norcholestanes and 24-/(24 + 27)-nordiacholestanes describe the input of certain algae (diatoms) that showed continued radiation during these time periods. These C26-steranes are the ideal compliment to the widely used oleanane index because the C26-steranes derive from aquatic algae that are relatively independent of events that occur on land. They depend upon conditions in the aqueous environment. In contrast, oleanane input may depend upon paleo-floral ecology on land and the circulation of higher plant debris into the marine basin, not to mention conditions of water salinity and oxicity during deposition.

Age-related and new environmentally specific biomarkers can be used to determine the petroleum systems in various parts of Latin America. Distinguishing Upper Cretaceous and Tertiary petroleum systems across northern South America appears to be especially suitable for the application of biomarkers related to flowering plants (oleanane) and diatoms (24-nordiacholestanes). Paleozoic marine oils can be distinguished from Mesozoic marine oils in the Sub-Andean basins by the absence or low abundance of dinosteroids related to dinoflagellates.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil