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Taxon-specific Biomakers As Indicators of Geologic Age And Depositional Environment of Petroleum

 

Barbanti, Silvana M.1, J. Michael Moldowan2 (1) Petrobras, Rio de Janeiro, Brazil (2) Stanford University, CA

 

The well-documented evolution and radiation of taxa suggest that their specific biomarkers may follow similar occurrence and abundance patterns. Several such biomarkers have been tested against occurrence in source rocks to determine their correlation with geologic age and depositional environment. This work has been followed up by analyzing a suite of 100 oil samples from diverse global locations, derived from a wide variety of source rock types with ages ranging from Precambrian to Miocene. Analytical methods included gas chromatography-mass spectrometry and metastable reaction monitoring of saturate branched plus cyclic fractions and aromatic fractions.

Most biomarkers follow the geologic record of their precursor taxa, but there are exceptions, which could indicate a pre-history of the taxon or its progenitors. (1) Triaromatic 23,24-dimethylcholesteroids largely mimics the dinoflagellate cyst record, (2) Oleanane Index, traces angiosperm evolution, (3) bicadinane, which, although found in certain angiosperms probably extends to the Permian, and (4) the C25 highly branched isoprenoid, which follows the radiation of certain diatoms.

 

The interplay of various parameters related to depositional environment was also tested. Gammacerane Index, a proxy for water chemo-stratification, and C35 Homohopane Index, a proxy for marine anoxia, strongly correlate for marine, but not for lacustrine oil sets. The Tetracyclic Polyprenoid Ratio (TPP), may reflect certain green algae that thrive in non-marine conditions. High values are recorded for most lacustrine oil samples, but marine oil samples show a broad range suggesting great variability among the represented marine source rock depositional environments.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California