Organic matter characteristics and hydrocarbon generation potential of the Middle Jurassic–Lower Cretaceous succession in the Mesopotamian Foredeep Basin, Iraq

Abstract

The Jurassic–Cretaceous source rocks and their exploratory operations in the Mesopotamian Foredeep Basin are limited, and the origins of recovered hydrocarbons have not been comprehensively investigated. Comprehensive geochemical analyses and 1-D basin models were performed on the Middle Jurassic–Lower Cretaceous succession. Additionally, geochemical analyses of eight crude oils from Upper Cretaceous reservoirs were used to evaluate the conventional petroleum resource potential and petroleum exploration and development. The total organic carbon (TOC) and Rock–Eval results reveal the Middle Jurassic–Lower Cretaceous succession to be characterized by fair to excellent source rock potential and consists mainly of Types II/III and III kerogens. Consequently, the Middle Jurassic–Lower Cretaceous succession can generate both oil and gas, with high oil generation potential. The dominance of such kerogen is confirmed by the substantial lipids derived from phytoplanktonic, bacterial, and algal organic matter, as indicated by biomarker compositions. Furthermore, biomarker parameters and isotopic compositions of oil samples provide evidence of a genetic link between the Middle Jurassic–Lower Cretaceous source rocks and crude oils. The data suggest that the non-biodegraded oils were generated from mature marine source rocks deposited under reducing conditions. One-dimensional basin models show that the oil generation from the organic matter-rich intervals within the Sargelu, Najmah/Naokelekan, and Gotina formations started during the Upper Cretaceous and continued into the Miocene (83–21 Ma) at low maturity levels (EASY%R_o > 0.50%). Oil expulsion from the Sargelu, Najmah/Naokelekan, and Gotina formations has taken place since the Miocene until now (21–0 Ma) with higher conversion ratios (%TR > 50%) and migrating through vertical pathways provided by faults and being trapped within Upper Cretaceous reservoirs.

Graphical abstract