Hydrocarbon Habitat of Mesozoic-Cenozoic Sequences in Iraq

Alsharhan, A.S., Middle East Geological Enterprises, Al Ain, United Arab Emirates

Iraq ranked one of the top leading producing countries in the world with its proved reserves of 113 BB of oil and 110 TCF of gas from about 84 fields.

Major reservoirs are in the Cretaceous and Tertiary carbonates and sandstones and Early Paleozoic sandstones. These reservoirs are sealed either by shales or evaporites and trapped in structural traps. Trap-forming mechanisms are due to reactivation of deep-seat-ed Precambrian faults and diapiric growth, synorogenic faulting and folding and restricted movements.

Cretaceous and Tertiary reservoirs in the Mesopotamian Basin and fold belt are domi­nantly oil-prone, while free gas encountered locally in the Tertiary reservoirs along the east­ern margin of the fold belt and in Paleozoic reservoirs of western desert.

The Jurassic Sargelu and Naokelekan formations are the main source rock potential. The shales in the Sargelu has TOC ranges from 2% to 6% with higher values of up to 20%; while in the Naokelekan TOC ranges from 3-9% and locally reaches at 15%. The present HI values ranges from less than 100 to more than 600 mg HC/g TOC and maturites ranges from 0.5-1.9% Ro.

Oil generation connected in the Late Cretaceous and reached completion in the Late Paleogene. Timing of oil generation and expulsion postdated the formation of major traps. In the southern fold belt oil generated and expelled during the Late Cretaceous presumably accumulated in the Cretaceous and older traps, whereas the later generated oil charged Tertiary traps. While in the northern fold belt oil generation commenced during Paleogene-Neogene folding and faulting and trap formation.