Oil and gas fields in Iraq are distributed in geographic locations with giant oil fields toward the east and giant gas field toward the west of Iraq, except Diyala region that has separated gas and oil fields in the northeast. Tertiary Petroleum systems such as oil and gas fields from pays of the Middle Miocene Jeribe Euphrates Formation in the Diyala region, . Jeribe Formation (Lower Miocene) in Northern and Northeastern from Iraq is composed principally of dolomitic limestone with typical porosity in the range of 10–24% and mean permeability of 30 Md. The formation serves as a reservoir for oil and gas at east Baghdad field, gas at Mansuriya, Khashim Al-Ahmar, Pulkhana fields, and oil at Injana, Gillabat, Qumar, and Jambur. A regional seal is provided by the anhydrites of Lower Fars (Fat'ha) Formation. Iraqi oil accumulations could be grouped into varieties of total or part of formational petroleum systems.