Oil sources and migration characteristics of Cretaceous reservoirs in the Central Iraq

Crude oil geochemical data, 3D seismic interpretation, and inclusion analyses were used to examinesource rocks and migration characteristics of the Ahdeb Oil Field in central Iraq.The results indicate that oils in the Cretaceous reservoirs in the Ahdeb Oil Field all originated from reducing and strongly reducing conditions in relatively closed marine deposits. Biomarkers showed that the Cretaceous reservoirs mainly were sourced from the Upper Jurassic Chia Gara Formation. Oil mainly migrated vertically into the Upper Cretaceous Khasib Formation through the SW-NE oriented strike-slip open faults which were controlled by the Najiad fault system. Lateral migration also took place when oils intersected high-porosity and high-permeability reservoirs (Rumaila, Mishrif and Mauddud). The Chia Gara Formation began to expel oil at the end of the Cretaceous. There were two stages of oil migration. The first one took place from the late Cretaceous to Neogene. Traps were not formed yet, and the heavy oils from low maturity source rocks migrated vertically into the Khasib Formation and laterally to the basin fringe along high-porosity and high-permeability formations. The second migration stage was since the Neogene. Due to the Zagros orogeny, source rocks entered the high maturity stage by rapid subsidence, and expelleda mass of high maturity crude oils, which accumulated after trap formation.