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