Biodegradation and the origin of surface bitumens in the Palaeocene Kolosh Formation, Northern Iraq.

The bitumen from surface exposures of the Palaeocene Kolosh Formation in three areas (Duhok, Shaqlawa and Dokan) of northern Iraq were investigated using GC, GC-MS, carbon and hydrogen isotope ratios of aromatic, NSO and asphaltene to determine their state of alteration (biodegradation, water washing and abiotic oxidation), thermal maturity and potential source rocks. These samples have a high percentage of NSO and asphaltene and minor amounts of saturated and aromatic hydrocarbons. These bitumens have a distinguished biomarker distribution, which can be attributed to different levels of biodegradation. The samples of Group 1 are moderately biodegraded: their normal alkanes and isoprenoids have been removed, whereas their hopanes and steranes are intact or have been very slightly altered. The samples of Group 2 are heavily to very heavily biodegraded; their steranes and hopanes are degraded without the formation of 25-norhopanes. These samples are more oxidized than those of Group 1. The bitumens have different origins, and their terpane and sterane ratios indicate that they were generated by early mature marine carbonate source rocks that were likely to have originated from Jurassic–Lower Cretaceous Formations. [ABSTRACT FROM AUTHOR]