Organic geochemistry of hydrocarbon seeps associated with sulfurous spring water, western Iraq: Biodegradation, source rock and sedimentary environment

Abstract

Oil seep samples from Abu-Jir Fault Zone (AJFZ) were geochemically characterized to determine the level of biodegradation and the depositional environment of correlative source rocks. For this purpose, saturated and aromatic biomarkers and stable carbon isotopes of thirteen samples were analyzed and measured. Saturated hydrocarbons of the AJFZ are low; whereas polar fractions are relatively high. Normal alkanes and isoprenoids of these oil seeps are totally removed; steranes are still intact in some samples, and slightly to heavily degraded in other samples; whereas hopanes are intact These criteria indicate that these samples are moderately to heavily biodegraded. low C24/C23, high C22/C21 tricyclic terpanes and C29/C30 hopane ratios suggest that these oil seeps were generated from carbonate source rocks. In addition, the high C35/C34 hopanes and homohopane index values suggest deposition under marine anoxic conditions. Low values of diasteranes/regular steranes, Ts/Tm, and moretane/hopane for most samples suggest early-mature source rocks. Sterane distributions and carbon isotopic composition of the AJFZ oil seep samples suggest they are not genetically related with the oils of the southern oilfields (Majnoon, Nasiriyah, West Qurna, North Rumaila, Luhais, Abu Gharab, Faka, Buzergan), nor consistent with the main source rocks (Sargelu and Naokelekan), but they are closely related to the East Baghdad oilfield and Chia Gara extracts. This study suggests the oil seeps were generated from Chia Gara Formation (upper Jurassic-lower Cretaceous).