Molecular geochemistry and biomarker analysis of the Garau Formation, western Iran

The Garau Formation is one of the important oil source rocks in Zagros basin, Western Iran, which several geological studies have been performed about it. However, current article attempts to study biomarkers related to organic matter sources, thermal maturity and geological age in the extracted bitumen samples from geological sections of the Garau Formation. Biomarkers were analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) techniques. As well as, stable carbon isotope composition analysis has been used to investigate the weathering processes and genetic association of extracted bitumen with source rock samples. The study of sterane biomarkers (C28/C29) indicates the Garau Formation has been formed during phytoplankton assemblages bloom of the Cretaceous period, and includes predominantly algal matter along with bacterial matter, and some reworked higher plant inputs. Sedimentation eventually occurred in a marine carbonate basin. Due to predominance of reduced (anoxic) conditions in the sedimentary basin, the bitumen composition is rich in nitrogen-sulfur-oxygen (NSO compounds) and asphaltenes. Based on bitumen fractions, aromatic-asphaltic oil is expected to be generated. These findings indicate the occurrence of alteration processes on the organic matter of the Garau Formation. As a result of biodegradation, a type of solid and insoluble bitumen, called biobitoman, has been formed. Stable carbon isotope analysis shows that biodegradation and water-washing have lost some more saturates and lower molecular weight aromatics in the surface bitumen samples of the Garau Formation. The thermal maturity of the organic matter is almost high and the source rock has entered to the oil window.