Solid bitumen in shales from the Middle to Upper Jurassic Banik section of northernmost Iraq: Implication for reservoir characterization

Iraq is the sixth largest oil reserve in the world, with current proven reserves of 115 billion barrels of oil (BBO) and 110 trillion cubic feet (TCF) of gas. The study area is situated within the Kurdistan Region of Iraq which is estimated to contain about 39% of Iraq’s total barrels of oil reserves. Petrographic, organic, and inorganic geochemical analysis of the solid bitumen and host shales from the Middle and Late Jurassic-age Sargelu and Naokelekan Formations of the Banik section, northernmost Iraq, were undertaken with the aim of understanding their derivation and preservation, as well as examining the carbon and oxygen isotopes, and paleoredox proxies under which the solid bitumen and host sediments were deposited. Petrographic analysis of both formations revealed the presence of solid bitumen high reflectance (first phase) and solid bitumen low reflectance (second phase). The equivalent vitrinite reflectance indicates that the solid bitumen of the Sargelu and Naokelekan Formations probably accumulated within the shale reservoirs following oil migration from source rocks located within the same formations. Mineralogical study (XRD and SEM - EDX) revealed that the shales hosting the solid bitumen also contain clay minerals (illite, rectorite, chlorite, montmorillonite, and kaolinite) as well as carbonate minerals, quartz, alkali feldspar, and pyrite. Carbon and oxygen isotope data along with paleoredox indicators suggest that both the solid bitumen sources and host shales in the Sargelu and Naokelekan Formations formed within a shallow-marine setting, most probably under anoxic conditions where water circulation was restricted.