Biomarker assessment of oil biodegradation, water washing, and source rock characteristics of oil seeps from the Foothill Zone along the Tigris River, Northern Iraq

Highlights

• Hammam Al-Alil and Fat'ha seeps are more affected by alteration processes than Qaiyarah seeps.

• The oil seeps are moderately biodegraded: normal alkanes and isoprenoids are almost completely removed, while steranes and hopanes are intact.

• Biomarker analysis of seep samples indicates a common origin and carbonate source rocks.

• The most likely source rock candidates of these seeps are Jurassic strata including the Sargelu, Naoklekan, and Chia Gara Formations.

Abstract

Oil seep samples from the Foothill Zone of northern Iraq were geochemically characterized to determine the level of biodegradation and water washing of the seeps and to determine the depositional environment (including sedimentary facies, oxicity, and organic matter type) of correlative source rocks. Eight samples from three seeps sites (Hammam Al-Alil, Qaiyarah, and Fat'ha) were investigated. Detailed saturated and aromatic biomarkers were analyzed using GC-FID and GC-MS, in addition to measurement of carbon isotopes and the sulphur content. Qaiyarah samples range from non-biodegraded to moderately biodegraded; whereas Hammam Al-Alil and Fat'ha samples are moderately biodegraded. The Hammam Al-Alil and Fat'ha samples are also affected by water washing. Biomarker, carbon isotope, and sulphur data suggest that these oil seeps contain oil expelled during the early oil window from carbonate source rocks of Jurassic age. Correlative source rocks were deposited in an anoxic marine environment. The most likely source rock candidates include the Sargelu, Naoklekan, and Chia Gara formations.