An integrated geochemical analysis, basin modeling, and palynofacies analysis for characterizing mixed organic-rich carbonate and shale rocks in Mesopotamian Basin, Iraq: Insights for multisource rocks evaluation

Abstract

Shales and carbonate-rich sediments are crucial in characterizing the geological history of petroleum as source rocks. This emphasizes the global significance of carbonate source rocks, and it has sparked research interests and exploration of these strata as unconventional resources. This study presents inclusive organic geochemical characteristics, palynofacies analysis, and basin modeling of the carbonate source rocks in the Ajeel oilfield, Mesopotamian Basin, Iraq. This study used traditional and advanced geochemical techniques, as well as microscopic examination and basin modeling. A total of 109 core and cutting samples from the Middle Jurassic–lowermost Early Cretaceous succession were studied via geochemical and palynological methods to evaluate their thermal maturation, depositional conditions, and hydrocarbon (HC) generation potential by combining one-dimensional (1D) modeling of the burial/thermal history and the generation/expulsion timing in the Mesopotamian Basin, Iraq. The results of total organic carbon/Rock-Eval pyrolysis revealed that the Middle Jurassic–lowermost Early Cretaceous interval predominantly comprised carbonate-rich sediments with Type-II and mixed Type-II/III kerogen. Furthermore, the results showed that these potential source rocks were of fair to excellent quality (oil window) and were deposited mainly in an anoxic marine environment. Geochemical and palynological investigation suggested that the carbonate-rich source rocks were deposited mainly under a distal suboxic–anoxic condition and contained Type-II kerogen (oil prone), except for part of the Chia Gara Formation, which displayed distal dysoxic–oxic shelf depositional conditions. The 1D basin modeling results of the burial/thermal maturation history and the generation/expulsion timing showed that these sediments reached their peak generation potential (main oil window) during the Early Miocene, and this has continued until the present day. Subsequently, the Middle Jurassic–lowermost Early Cretaceous succession represents the main source of petroleum in the Mesopotamian Basin, Iraq.This work shows how an integrated palynofacies analysis, basin modeling, and geochemical study can be used for the precise assessment of multisource rock intervals of mixed organic-rich carbonate and shale rocks. These research findings will provide guidance for better resource exploitation in northern Iraq, which has higher source-rock quality in terms of organic richness and maturity than that in southern Iraq.