Geochemical and petrographic characterization of Late Jurassic–Early Cretaceous Chia Gara Formation in Northern Iraq: Palaeoenvironment and oil-generation potential

The marine Late Jurassic to Early Cretaceous Chia Gara Formation is well exposed in northern Iraq. It is built of organic-rich limestones and calcareous shales. The organic-rich sediments have been investigated to determine the type and origin of the organic matter as well as their petroleum-generation potential. n microscopy shows that these sediments are characterized by large amounts of predominantly amorphous organic matter with a Total Organic Carbon content of 7.42%. The large amounts of organic matter are mainly due to good preservation under anoxic conditions, as evidenced by numerous pyritized fragments and biomarkers that are diagnostic for the depositional environment. The investigated biomarkers are characterized by a dominance of low to medium molecular weight compounds, a low Pr/Ph ratio (<1.0), a composition of C27–C29 regular steranes, and the presence of tricyclic terpanes, indicating a strong decay of marine organic matter preserved under reducing conditions. A small amount of terrigenous organic matter is, according to the n-alkane distribution, also present. ia Gara sediments thus have a high oil- but a low gas-generation potential due to the high content of hydrogen-rich Type II and mixed Type II–III kerogens with a minor contribution of Type III kerogen.