Source Rock Evaluation of the Chia Gara Formation in Central Iraq: Results from Rock-Eval Pyrolysis and Biomarkers

Chia Gara Formation is considered an important hydrocarbon source rock in northern Iraq, as it has high organic carbon content due to deposition under reducing conditions. This study analyzes rock samples from Chia Gara Formation in Well A, located in central Iraq, to evaluate the organic richness, type of organic matter, thermal maturity level, and conditions of the sedimentary environment of Chia Gara Formation in central Iraq. The evaluation utilized total organic carbon content, Rock-Eval pyrolysis, and biomarker ratios. Chia Gara Formation is classified as a source rock that ranges from poor to excellent quality, with organic carbon content ranging from 0.2% to 17.6%, averaging 5.64% in the examined well. The highest organic carbon content values are found in the lowermost part of the formation and decrease upward. The kerogen type present is mixed type II/III and II, which exhibit favorable properties for oil generation. A low oxygen index and low pristane to phytane ratios of 0.79 to 1.09, combined with a high hydrogen index (HI) averaging 394 mg HC/g organic carbon content, indicate that this formation was deposited under anoxic conditions. Ratios of regular steranes (C27, C28, C29), terpanes (C35/C34 hopane), and aromatic hydrocarbons suggest a dominance of marine organic matter in the depositional environment. Tmax values exceeding 430 °C reflect the early mature-to-peak oil window for these samples. Additionally, the ratios ββ/(ββ+αα) and SS/(SS+RR) of the C29 steranes, along with moretane/hopane ratios, Production Index, and methydibenzothiophene ratios, indicate mature organic matter, highlighting the formation's capacity for oil generation.