This research targets an in-depth understanding of the considerable disparities in petroleum properties of the oil produced from the Lower Cretaceous Zubair Formation in Central Iraq, an aspect currently lacking systematic knowledge. Utilizing newly obtained samples from various oil-bearing reservoir locations within the Zubair Formation core, we embarked on comprehensive component analysis and applied GC-MS testing for saturated and aromatic hydrocarbons. By identifying the characteristics of petroleum biomarkers, comparing the features of potential regional hydrocarbon source rocks (including hydrocarbon generation potential and biomarker composition), and considering the differences in saturated hydrocarbon content, we determined the origins of different oil properties and the controlling factors of these variations. Substantial disparities exist in the petroleum properties of oil from the Zubair Formation reservoir, with saturated hydrocarbon content ranging between 4.8% and 85%. Even within closely situated reservoirs, significant differences are observable. Distinct variances are present in the composition of saturated and aromatic hydrocarbon biomarkers in oils with differing properties. Oils with lower saturated hydrocarbon content evidently originate from carbonate-rich source rocks, predominantly marine algae organic matter, with an older sedimentary age and lower maturity. In contrast, oils with higher saturated hydrocarbon content largely derive from marine mudstone source rocks, with a more significant contribution from terrestrial plants, a more recent sedimentary age, and higher maturity. Our geochemical analysis of potential regional hydrocarbon source rocks suggests that oils with lower saturated hydrocarbon content (lower API) are products of the Jurassic Chia Gara and Sargelu formations at the immature stage of Type II kerogen. Oils with higher saturated hydrocarbon content (higher API) result from hydrocarbon mixing at the mature stage of the Zubair Formation source rock. Previous research predominantly attributes the oil from the Cretaceous reservoir to the Chia Gara and Sargelu formations. Our study contends that the contribution from the Zubair Formation source rock to the Lower Cretaceous reservoir oil is noteworthy and significantly influences the petroleum properties, particularly API gravity. This finding enriches the existing understanding within the petroleum industry.
