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Petroleum System Analysis in the Central Alborz Basin, Iran -Geochemical
Characteristics of Oil Seepages and 
Source
Rocks-
By
Kunihiro Tsuchida1, Yoshihiro Tsuji1, Rasoul Sorkhabi1, Akihiko Okui1, Osamu Himeno1, Mohammad Reza Kamali2, Mahmoud Memariani2, Mohammad Moein Pour2
(1) Japan National Oil Corporation, Technology Research Center, Chiba, Japan (2) National Iranian Oil Company, Research Institute for Petroleum Industry, Tehran, Iran
Geochemical analyses on oil seepages in the Central Alborz Basin reveal that
the seeps belong to two distinct oil families, and thus two different types of
source rocks possibly exist in this region. These two types of seep oils are
correlated to the Jurassic coal and the carbonate rocks of unknown age.
Biomarker analyses by GC/MS and GC/MS/MS show that the first family is
characterized by high concentration of C29 steranes and lack of C30 marine
steranes. High diasteranes/steranes ratio also suggests that clastic input was
significant. Hence it can be correlated to terrestrial source rocks. Lack of
oleanane proves that the source rock was not younger than the Cretaceous. As the
biomarkers extracted from the Jurassic coals and shales, which are thought to
have been a part of deltaic sediments, are very similar to those of the seep and
the hydrogen index of these rocks are plotted between 100 and 250, it is
interpreted that the Jurassic rock is the source of the oil seepage.
The second family is characterized by high concentration of C27 and C30
steranes, high C29/C30 hopanes ratio and irregular distribution of homohopanes.
These characteristics point to a marine carbonate source rock
, which may have
been of Cretaceous or younger age as evidenced from the high concentration of
24-norcholestanes and 24-nordiacholestanes, and from the occurrence of
dinosteranes.