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.