Integrated geochemical characterization and geological modeling of organic matter-rich limestones and oils from Ajeel Oilfield in Mesopotamian Basin, Northern Iraq

Abstract

The current study focuses on the organic geochemical and basin modeling of organic matter-rich limestones and oil samples from the Ajeel Oilfield. The objective was to characterize the organic matter and its hydrocarbon-generating potential, and its relevance to future hydrocarbon exploration in the northwestern Mesopotamian Basin. The organic biomarker results of the extracted bitumen samples indicated that the Sargelu, Naokelekan, and Chia Gara limestones received abundant planktonic-bacterial organic matter input, with a limited terrestrial contribution. Lipid-rich organic matter that originates from phytoplankton and bacteria with low amounts of terrestrial organic matter suggest a high abundance of Type-II or mixed-II/III kerogen as the original organic matter at time of sedimentation.Bulk property and geochemical analyses of the recovered oil samples, with mediumAPI gravities between 31 and 33°, and relatively high sulfur concentrations of up to 3.95 wt%, suggest that examined oil samples were derived mainly from Type II-S kerogen at moderate maturity stages. Biomarker ratios and parameters, as well as carbon isotopes, further indicated that these oils were derived from marine carbonate-rich source rocks deposited under reducing environmental conditions. These biomarker fingerprints and carbon isotopes show a genetic link between the oil examined samples and the extracted rocks of the Sargelu, Naokelekan, and Chia Gara source rocks and represent a mix of these carbonate-rich source rocks. Therefore, these source rocks contained kerogen Type II-S with high sulfur, and generated commercial oil at relatively low thermal maturity of early to moderate stages, as demonstrated from maturity parameters such as Tmax and PI as well as several biomarker ratios.Burial/maturity models with kerogen Type–II–S kinetic parameter revealed that the Sargelu, Naokelekan, and Chia Gara intervals began to generate oil at slightly lower thermal maturity (maturity levels = 0.55–0.79 EASY%Ro). As a result, about 10%–50% of the organic-matter was converted to oil during the Late Paleocene. The basin models also suggested that the organic-rich units including Sargelu and Naokelekan source rocks reached higher maturity levels, with TR of 71%–79% in the Early Miocene (20 Ma) and continued to the present. Trapping large quantities of oil lead to the high pressures and expulsion of these oils. The high conversion ratios further suggested large oil volumes accumulated in the northwestern Mesopotamian Basin were mainly originated from the Middle-Late Jurassic Sargelu and Naokelekan source rocks.