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Cretaceous Petroleum Systems of Southern Iraq

Alsharhan, Abdulrahman S.1
1 Middle East Geological Establishment, Al Ain, United Arab Emirates.

Three major events controlled the deposition of the Cretaceous sediments in Southern Iraq. These are the culmination of the Neo-Tethys rifting, the advent of the Arabian Plate to the tropical and subtropical climatic zones and the contemporaneous growth of giant structures due to salt diapirism. The sediments are divided among seven megasequences, each contains source, reservoir and cap rocks. The reservoirs characterized by deltaic sandstones and shallow-water carbonates. The sandstone reservoirs has 1-30% porosity and 0.1-5 D permeability. The carbonate reservoirs also contains an excellent petrophysical properties (porosity exceeds 30% and permeability 1D) with karstification and fracturing.

The Jurassic Sargelu Formation is the main source rock. The shales have TOC ranges from 2-6% with higher values up to 20%, and HI values ranges from 100-600 mgHC/g. Maturities ranges from 1.2-1.9% Ro, and the oil generation connected in the Late Cretaceous and reached completion in the Late Paleogene.

Structural traps are large, broad, N-S striking, basement-cored anticlines. Trap forming mechanisms are due to reactivation of deep-seated Precambrian faults and diapiric growth, synorogenic faulting in Mesozoic and restricted movement in the Cenozoic. Cretaceous reservoirs hold major hydrocarbons (76%) and are dominantly oil-prone.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009