25th International Meeting on Organic Geochemistry IMOG 2011

P-291
Geochemical parameters for unravelling mixtures. Examples
from the South Atlantic continental margins and the giant northcentral
West Siberian gas fields
John Moldowan 1,2 , David Zinniker 1 , Zhaoqian Liu 1 , Alla Rovenskaya-Nemchenko 3 ,
Jeremy Dahl 1 , Tatyana Nemchenko 4
1 Geological & Environmental Science, Stanford University, Stanford, United States of America, 2 Biomarker
Technology, Sebastopol, United States of America, 3 The Foundation for East-West Cooperation, Moscow,
Russian Federation, 4 Vernadsky Institute of Geochemistry and Analytical Chemistry (GEOKHI), Moscow,
Russian Federation (corresponding author:moldowan@yahoo.com)
The recognition, correlation and quantification of oil
mixtures remain challenging in many petroleum
system studies. Most prolifically productive basins or
regions have multiple source rocks generating at wide
ranges of maturity. Mixtures present themselves in
many forms, including any proportions of black oil,
light oil and condensate, which can be single-sourced
or co-sourced. Knowledge of all active sources and
the maturities of their generated hydrocarbons is the
critical starting point to develop correct basin models.
Selection of the best parameters to de-convolute oilsource
mixtures can best be determined from singly
sourced end-member oil and source rock samples.
The ideal parameters show very large differences in
magnitude between the candidate sources and are
independent of maturity difference. In parallel,
maturity differences can be taken into account and
also provide useful information about the mixtures.
In the absence of data from end-member samples
such parameters can be construed by inference
based on information about source rock depositional
environment and burial depth or history (maturity)
where different source types often have widely
contrasting characteristics.
In the south atlantic margins, we are faced with oil
mixtures that may include oil generated in the
lacustrine rift sections of Barremian-Aptian age,
transitional to marine post-rift sources of late Aptian
into the late Cretaceous and Tertiary marine-deltaic
oil from the latest Cretaceous through the Tertiary.
Parameter value ranges depend on the specific basin
in question and analysis of some end-member
samples from the area of interest can more precisely
define those values. However, we find that purely
lacustrine sourced oil samples from Angola show C29hopane
C-isotope ratios of < -35 ‰, near absence of
C30 (desmethyl) steranes (C30/C29 ≤ 0.01%), and
concentration ratio differences from other sources for
diamondoid pseudohomologues (e.g., adamantanes /
diamantanes).
Post-rift marine sources generally show C29-hopane
C-isotope ratios > -24‰, C30/C29 sterane ratios ~10%.
Younger marine-deltaic oil often shows geochemical
parameter values intermediate between the marine
and lacustrine end-members. TPP ratios and certain
plant biomarkers (diterpanes, oleanane, bicadinane
and their C-isotope ratios) differentiate those sources.
The source(s) of gas in the giant gas fields (e.g.,
Urengoi, Yamburg) of north central Western Siberia
has been a subject of studies that have reached
different conclusions. The possibilities include
thermogenic origins of various source ages and facies
versus biogenic. Our recent studies show that at
least some of the gas is thermogenic in origin. This is
indicated by elevated diamondoid concentrations in
light oil samples obtained from those fields. Further,
the C-isotope ratios of diamondoids correlate them to
end-member Early – Middle Jurassic Tyumensourced
oil (Figure 1). This correlation strongly infers
a Tyumen source for the gas, which is probably,
therefore, the major source. Some samples in the
basin also show mixtures of Late Jurassic Bashenov
oil with Tyumen condensate that can be unravelled by
compound specific isotope analysis of biomarkers
(CSIA-B) and diamondoids (CSIA-D).
� 13 C PDB (‰)
Urengoi & Yamburg
cracked -19 oil samples
-20
-21
-22
-23
-24
-25
-26
-27
-28
-29
-30
-31
-32
-33
-34
-35
-36
All cracked oil & Tyumen
end-members
(heavy isotope group)
Only non-cracked oil and
Bashenov end-members
(lighter isotopes group)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Diamondoids (adamantane & alkyladamantanes)
Figure 1. CSIA-D distinguishes Bashenov and Tyumen oil. In every
case the deep source correlates with the Tyumen
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