Organic geochemical investigation and 1D basin modeling of the Chia Gara source rock (Tithonian-Berriasian) interval in the Zagros Basin, Iraq: implications for oil generation potential

The generation potential of the Zagros Basin in Iraq is a critical aspect of exploration and development strategies in the province. A specific maturation mechanism and the timing of petroleum expulsion from the probable source rock interval remain unresolved. The gaps in this study are addressed through investigations of organic-rich intervals with accompanying oil expulsion processes. In this regard, forty-seven core and ditch-cutting samples were geochemically analyzed to evaluate organic matter enrichment, maturation, hydrocarbon potential, and time for source-rock-derived hydrocarbons and expulsion. The geochemical results revealed fair to very-good potential as evidenced by elevated organic carbon (TOC) content which reached up to 3.95 wt% and genetic potential (GP) of up to 8.52 mg HC/g rock. Hydrogen index (HI) values in the range of 171 to 462, along with Rock-Eval T_max values above 430 °C, confirmed that the evaluated interval has Type II and mixed II/III kerogens. The results of basin models demonstrated that the conversion of kerogen into oil began during the upper Cretaceous, precisely between 77 and 67 Ma, while the expulsion started during the middle Miocene (22 Ma) till the present day (0 Ma). Time plots of this basin showed that the modeled vitrinite reflectance (Easy%VRo) values during the oil expulsion phase are in the range of 0.74–0.83, and the transformation ratios (%TR) are higher than 55%. These findings establish the Chia Gara Formation as the main organic-rich source rocks in this region, highlighting its significant oil generation/expulsion capabilities, which may serve as a source for future hydrocarbon exploration in the Iraqi Zagros Basin.