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The present study focuses on the Chia Gara Formation that is part of the oil-prone sedimentary formations of Northern Iraq, being under extensive scientific concern in recent years. The Chia Gara Formation is considered one of the major source rocks of the prolific Kirkuk oil field of Northern Iraq, consisting of alternating layers of bituminous limestones and calcareous shales that were deposited during Late Jurassic – Early Cretaceous. The studied sequence from the Miran-2 well (depth 2185–2315 m) comprises mostly argillaceous limestones with calcite and quartz being the main minerals with variable amounts of clay minerals, K-feldspars, pyrite, gypsum, and anhydrite. TOC wt% ranges between 0.73 and 7.24 wt%, pointing to a significant variation regarding organic matter content. The average random vitrinite reflectance (VR o) is 1.00% and the average value of solid bitumen random reflectance (BR o) is 0.73%. Solid bitumens and bituminite (the latter corresponding to kerogen type II) are the predominant organic particles, particularly in the bottom to middle part of the Formation, whereas alginite occurs subordinately. Additionally, the common occurrence of vitrinite fragments indicates a significant terrestrial input (kerogen type III). Organic petrography proved to be the most important tool to provide with clear-cut results regarding the organic matter characteristics. Chia Gara Formation was deposited in a shallow marine palaeoenvironment probably shelf-slope, under reducing conditions. In particular in Miran-2 the studied strata are within the oil window, and the obtained results provide a better understanding of the organic matter type and maturation of this highly prolific and productive Formation. • Chia Gara Formation in NE Iraq is one of the most interesting "natural laboratory" for maturation studies. • Bituminite and solid bitumens dominate the organic matter of Chia Gara Formation. • Chia Gara Formation represents a shallow marine palaeoenvironment, with intense terrigenous siliciclastic influx. [ABSTRACT FROM AUTHOR]