Variation of geochemical data and sedimentary characteristics in the Upper Cretaceous Ilam Formation, a case study from southwest Iran

The Upper Cretaceous Ilam Formation in southwest Iran was studied in order to provide a better understanding of the palaeoenvironments that lead to the deposition of heterogeneous carbonate rocks. This formation is an important hydrocarbon reservoir of the Bangestan Group in the Zagros Basin and the Abadan Plain. The Ilam Formation is comprised mainly of carbonate rocks and was studied from three subsurface sections (100, 74, and 100 m thick). Petrographic investigation reveals that the Ilam Formation consists of 12 microfacies and a shale petrofacies that were deposited in lagoonal, shoal, and open marine facies belts of a carbonate ramp. Two third–order depositional sequences are recognised. Comparison of Ni/Co reveals that the limestones of the Ilam Formation formed under oxic to anoxic conditions. The main diagenetic features of the Ilam Formation include compaction, dissolution, dolomitization and cementation that occurred in marine, meteoric and burial diagenetic environments. We document for the first time the relationship between facies and geochemistry in the carbonate rocks of the Ilam Formation. These findings provide a more comprehensive understanding of the palaeoenvironments that lead to the deposition of heterogeneous carbonate rocks and their relationship with geochemistry and reservoir characteristics.