Geochemical characterisation of Tithonian–Berriasian Chia Gara organic-rich rocks in northern Iraq with an emphasis on organic matter enrichment and the relationship to the bioproductivity and anoxia conditions

•Chia Gara organic-rich rocks represent excellent oil source rocks in Kurdistan.•Organic matter is dominated by aquatic organisms, with a minor terrigenous contribution.•Salinity stratification and reducing conditions occurred during deposition of Chia Gara rocks.•Organic matter enrichment in the Chia Gara Formation is due to increased preservation and productivity.

Tithonian–Berriasian Chia Gara organic-rich rocks, located in Kurdistan (northern Iraq), were analysed based on inorganic and organic geochemistry to define the origin, type of organic matter, and the influencing factors of organic matter (OM) enrichment, including the OM input and preservation, and their relationship to the paleoenvironment conditions.The δ13Corg values of the Chia Gara rocks range from −29.99‰ to −26.93‰, with average values of approximately −28.8‰, indicating that the organic matter has a predominantly marine origin. Enhanced biological productivity within the photic zone of the water column during deposition of the Chia Gara Fm. is indicated by consistently high organic carbon content in most samples (average 3 wt.%), as well as by bulk geochemical and biomarker characteristics. Major contributions by aquatic algae and microorganisms with a minor terrigenous organic matter contribution are indicated by the n-alkane distribution patterns and the composition of the hopanoids, steroids, and tricyclic terpenoids. Strongly reducing bottom water is indicated by low pristane/phytane ratios, homohopane distribution patterns, and the redox-sensitive trace elements geochemistry. Salinity stratification with alkaline bottom waters is indicated by high Sr/Ba ratios and the presence of gammacerane in the analysed Chia Gara samples. These data indicate that OM enrichment in the Tithonian–Berriasian Chia Gara rocks results from the combination of enhanced biological productivity and salinity stratification with anoxic bottom water conditions, which favoured OM preservation.