Online Journal for E&P Geoscientists

Tai, Po C.^*1; Grabowski, George J.¹; Schafer, Kirk W.¹; Liu, Chengjie ¹; Wilson, Augustus O.²
(1) ExxonMobil Exploration Company, Houston, TX. (2) Consultant, London, United Kingdom.

The Cenozoic strata of the Mesopotamian Basin contain important reservoirs and seals, and they recorded the basin-filling history of the Zagros Foreland Basin. The Neogene clastic interval has been considered as foredeep and wedge-top deposits. The tectono-stratigraphic setting of the Paleogene interval has received less attention. We have revised the Paleocene-Early Miocene stratigraphy in the central Mesopotamian Basin to provide constraints on the early evolution of the Zagros Foreland Basin.

Our regional analysis shows that during Paleogene time, two margins developed along the Mesopotamian Basin and prograded toward each other as the basin center narrowed and was filled with deep-water calcareous muds and evaporites. In NE Iraq, the Paleocene-Early Eocene interval consists of a terrestrial to deep-water clastic system with minor shallow-marine carbonates. The clastics were mainly sourced from an obducted ophiolite located to the NE. This interval is succeeded by Middle-Late Eocene aggradational to progradational platform carbonates with minor clastics that are mainly slope deposits. The Late Eocene carbonates are unconformably overlain by Early Miocene (Burdigalian) shallow-marine and terrestrial sediments. The missing to highly-condensed Oligocene-basal Miocene interval in NE Iraq is interpreted as the expression of a flexural forebulge. The corresponding time-equivalent backbulge depozone, in central Iraq, consists of highly progradational to downstepping platform carbonates. In S Iraq and SW Iran, the Oligocene to earliest Miocene interval is characterized by a mixed system. The clastics are fluvial-deltaic to shallow-marine sands that were derived from the exposed Arabian Shield.

The interpretation of Oligocene forebulge (NE Iraq) and backbulge depozones (central Iraq) implies the onset of Arabia-Eurasia collision and the establishment of an initial orogenic wedge by Late Eocene. Our study suggests that an early flexural basin first developed in NE Iraq from Late Cretaceous to Eocene by ophiolite loading. At the onset of collision (Late Eocene) a distal foreland basin system was established in central and S Iraq and mostly filled by basin-central evaporites and progradational to aggradational carbonates from Oligocene to Early Miocene. An abrupt increase in subsidence of the inferred backbulge depozone at ~19 Ma indicated the arrival of the Zagros Foredeep, which then was quickly filled by mixed carbonates, evaporites, and clastics.