Migration and Accumulation of Hydrocarbons in Cretaceous and Tertiary Reservoirs of Iraq: Implications of a 2D/3D Basin Modeling Study
Janet K. Pitman1, Douglas Steinshouer2, Michael Lewan3
(1) U.S. Geological Survey, Lakewood, CO (2) GeoLukas, Denver, CO (3) U. S. Geological Survey, Lakewood, CO
Multilayer, 2D/3D petroleum system modeling was undertaken to evaluate the origin and extent of hydrocarbon (HC) generation, expulsion, and secondary migration in the Upper Jurassic Sargelu/Naokelekan source rock system of Iraq. Structural restorations of source rock surfaces, regional isopach and facies maps, and thermal maturity data were used as input to the model. Petroleum generation and potential migration pathways on the top of a (Lower Cretaceous) composite reservoir unit immediately overlying the principle source rock unit were modeled for three geologic time periods: 25, 8, and 0 Mybp. Petroleum accumulations predicted by the model were compared with the location of known fields that produce from the Cretaceous reservoir interval. Modeling results show that in the early Miocene, at about 25 Mybp, the first major stage of HC generation and expulsion was initiated in two depocenters. The petroleum that migrated laterally from these areas began to fill most fields in northern Iraq and the largest fields in southern Iraq. By 8 Mybp, in the late Miocene, the area of main stage HC generation extended along the Zagros Mountain Front due to extensive foreland basin deposition associated with the Zagros Orogeny, and folding and faulting caused larger petroleum drainage basins to be segmented into smaller subbasins. Related southwest tilting set up a northeast-southwest HC flow pattern across the region. Modelling results demonstrate that Cretaceous reservoired oil in northern Iraq remigrated vertically along faults and fractures ultimately charging younger Tertiary reservoirs. At present day, most subbasins containing hydrocarbon accumulations are underlain by active source rock, and most fields lie on or close to migration pathways and have been subject to filling and spilling. It follows that prospects closest to modeled pathways are highly likely to be charged and thus have low exploration risk.