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A Geochemical Approach to Defining the Active Petroleum Systems of Iraq

Abstract

Geochemical analyses of 250 production oils together with over 600 core, cuttings, outcrop and seep samples provides a comprehensive, regional picture of the petroleum systems that are active across Iraq and the bordering lands within the Arabian Plate. Sample selection exemplified the geochemical diversity of petroleum systems and the intimate relationships between stratigraphic placement of source and reservoir units. For the rocks we evaluated source potential using TOC and Rock-Eval measurements before and after solvent extraction to distinguish active and potential source rocks from those with staining by migrated bitumen. Effective source rocks with high TOC and hydrogen indices were identified in marls of the Triassic Kurra Chine Fm. and in carbonate sediments of the Upper Jurassic Sargelu, Najma and Naokelekan fms. and the Lower Cretaceous Ratwi and Sarmord fms. Moderate source potential was identified in the Middle Cretaceous Nahr Umr Fms and minor source potential was identified in an evaporitic facies of the Gotnia Fm. Neither of these latter rock units, however, appeared to contribute significantly to produced or seep oils in the study area. Oils were classified on the basis of geochemical parameters diagnostic for source facies and lithology, thermal maturity and geological age. Included in our analyses were several novel geochemical parameters that reflect anoxic and euxinic conditions extending into surface waters. Based on a statistical analysis of geochemical characters, we found that oils of the Kurdistan region of northern Iraq were predominantly related to sources in the Sargelu and Naokelekan fms. Oils sourced from Cretaceous sediments appeared to be confined to fields in the northern Zagros fold belt. Notably, oils sourced from Triassic sediments were also confined to the far north of the study area in Iraq and we identified no Paleozoic petroleum in Iraq although oils from rocks of this age occur nearby in Syria and Turkey. The geographical distribution of the regional salt seal exerts a strong control on the stratigraphic placement of oils derived from Jurassic source rocks.