Albian-Cenomanian Palynology, Paleoecology and Organic Thermal Maturity of Well DK-B in the Dukhan Oil Field of Western Qatar

Abstract

ABSTRACT A palynological investigation of the Nahr Umr, Mauddud and Ahmadi formations of the middle Cretaceous Wasia Group in Well DK-B in the Dukhan oil field of Qatar, yielded 30 species of dinoflagellate cysts, 18 of pteridophytic spores, 14 of gymnosperm pollen, and 16 of angiosperm pollen. Based on the investigation, the age of the Nahr Umr Formation is middle to late Albian. The basal part of the Formation was deposited in a marine prodelta or shallow shelf environment, whereas sedimentation of the upper part took place in normal marine conditions of an inner to middle shelf at depths of 10 to 80 meters. The carbonates of the Mauddud Formation are of late Albian (Vraconian) to early Cenomanian age and were deposited in an inner-middle shelf environment (20–100 meters). The shales and limestones of the Ahmadi Formation are of early to middle Cenomanian age and accumulated in open-marine conditions within an outer-shelf environment (100–200 meters). Two regressive pulses or lowstand system tracts can be detected in the lower and middle parts of the Ahmadi Formation that are consistent with published short-term global eustatic curves. Rocks of the Wasia Group studied in Well DK-B are in general enriched in kerogen type II (oil-prone material) except for the lower part of the Nahr Umr Formation that can be attributed to kerogen type III (gas-prone material). The thermal alteration index range of 2, 2+ to 3− suggests that the sediments of the Ahmadi Formation are immature whereas those of the Mauddud and Nahr Umr formations are slightly mature. The Albian-Cenomanian palyno-assemblage of western Qatar is referable to the African-South American Phytogeoprovince as shown by the presence of Crybelosporites pannuceus, ephedroid pollen, elater-bearing pollen, Afropollis, Stellatopollis, Reyrea and Cretacaeiporites species. Arid to semi-arid (tropical to subtropical) climatic conditions prevailed in the African-South American province at this time. Dinoflagellate cysts suggest a Tethyan connection.