25th International Meeting on Organic Geochemistry IMOG 2011

P-272
Distribution of regular acyclic isoprenoidal alkanes in crude oils
of SE Hungary
Csanád Sajgó 1 , József Fekete 1 , Balázs Badics 2
1 Institute for Geochemical Research, Hungarian Academy of Sciences, Budapest, Hungary, 2 Statoil Global
Exploration, Stavanger, Norway (corresponding author:sajgo@geochem.hu)
The pristane to phytane ratio (pr/pr) is the most
important organic redox parameter to elucidate
depositional environment in organic geochemistry.
The value of the ratio increases with increasing
temperature or maturation. Nevertheless, the
temperature dependence and the reasons of the
variation are only known partly. The aim of this work
is to study the distribution of regular acyclic
isoprenoidal alkanes (iC13-iC20 range) in different oils
of the most important Hungarian oil region (e. g.
Algyő). This distribution has been only occasionally
studied [1, 2]. The interpretation of these markers is
rarely applied. The area studied (SE-Hungary)
includes two very deep subbasins of the Pannonian
Basin. These are filled with Neogene and Quaternary
marls, clays and sands up to the thickness of 7000
meters. The source rocks of the studied oils haven‘t
been proven yet, their origin is an opened question.
Over 40 oil samples have been chosen for this study
from relatively small and scattered reservoirs (depth
and temperature between 1 - 5 km, and 80 - 210°C,
respectively), trapping petroleum accumulations of
various age, origin, maturity parameters and migration
history. The oil geochemistry of the area (including
pr/ph and biomarkers) was studied [3, 4], but without
the isoprenoidal HCs in question.
The oils were sorted to 3 groups on the basis of the
distribution of 7 studied isoprenoidal HCs (in Fig 1.
typical representatives are shown). In the ―S‖ type oils
(~21) proportions of isoprenoidal HCs are similar. In
the ―D‖ types (~4) the amount of pr and ph is relatively
small, and the proportions of longer chain isoprenoids
steadily decrease. In ―I‖ oils (~16) the proportions of
given isoprenoids grow parallel with chain length. The
notations ―pr‖ and ―ph‖ denote pr or ph dominance,
respectively.
Pr vs. ph and pr/�(iC13-iC20) vs. ph/�(iC13-iC20) ratios
follow a linear trend in oils showing low (0.04-0.13)
ph/�(iC13-iC20) and high (0.12-0.28) iC13/�(iC13-iC20)
values (―low-ph‖). In these oils the pr/�(iC13-iC18) and
ph/�(iC13-iC18) ratios show a negative, parallel trend
with very close to each other, with the same slope of
tangent: 0.5. This suggests that in these oils both the
pr and the ph have produced the lighter/shorter
isoprenoid alkanes (iC13-iC18 range), in similar ratio
(cracked parallel). In the other oils (―high-ph‖)
characterized by higher (0.1-0.28) ph/�(iC13-iC20) and
low (0.05-0.12) iC13/�(iC13-iC20) values the
concentration of ph is increased (pr/ph ratio
decreased) without any trend which may refer to more
reductive depositional environment and less intensive
cracking compared to those of the other oils.
Correlation between ph and iC13-iC19 in low-ph and
between pr and iC13-iC18 in high-ph oils was
observed.
0,3
0,2
0,1
0
Üllés DK-2, "S" type
Algyő-502, "D" type
Forráskút-5, "I-pr" type
Ruzsa-6, "I-ph" type
iC13 norfar far iC16 norpr pr ph
Fig. 1 Distribution of regular acyclic isoprenoidal
alkanes in typical crude oils of the four groups
This work was funded by the Hungarian Scientific
Research Fund (OTKA) through grant K84086.
References
[1] Kissin, Y. V. 1993. Catagenesis of light acyclic
isoprenoids in petroleum. Organic Geochemistry 20.
1077-1090
[2] Illich, H. A. 1983. Pristane, Phytane, and Lower
Molecular Weight Isoprenoid Distributions in Oils
AAPG Bulletin 67. 385-393.
[3] Sajgó, Cs. 1984. Organic geochemistry of crude
oils from South-east Hungary. Organic Geochemistry
6, 569-578.
[4] Sajgó, Cs. 2000. Assessment of generation
temperatures of crude oils. Organic Geochemistry 31,
1301-1323.
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