Micropalaeontology and palynology of the Lower and Middle Cretaceous Thamama and Wasia groups, DK-C well, Dukhan oil field, Western Qatar. / Micropaléontologie et palynologie dans le Crétacé inférieur et moyen des groupes de Thamama et de Wasia, puits DK-C, champ pétrolifère de Dukhan, Qatar occidental

Abstract

Well preserved, diverse sporomorphs, phytoplankton and foraminifers have been found in cores from the DK-C well, Dukhan Oil Field, Western Qatar, Arabian Gulf. Most samples studied contained dinocysts, acritarchs, foraminiferal test linings, spores, pollen grains and benthic and planktic forams. From the entire palynomorph assemblage mainly dinoflagellate cysts and miospores were studied to give a stratigraphie outline for the upper Lower and Middle Cretaceous Thamama and Wasia groups. The age of the Shuaiba Formation as indicated by dinocysts is possibly early Aptian. Deposition took place in a marine environment which could possibly reflect the extreme distal locus of deposition. A tentative late middle Albian age could be assigned to unit A of the Nahr Umr Formation on basis of palynology. However unit В is dated as late Albian by palynomorphs and early Albian by foraminifera. In view of the extremely low faunal recovery and unavailability of samples from unit С of the Nahr Umr Formation, no age assignment could be postulated. The clastic facies of the Nahr Umr Formation showed high recovery of fern spores which allows us to assume marshy environment with fluviatile deposits that phased out Albian sedimentation. The dating of the Mauddud shelf carbonate Formation as late Albian, based on foraminifera, is not supported palynologically. Unit A of the Ahmadi Formation did not yield any diagnostic fauna that did allow an age assignment. The presence of elater-bearing palynomorphs, diagnostic miospores and dinocysts in this interval confined the age of this unit to the late Albian-early Cenomanian. This aspect of the Albian-Cenomanian flora could possibly indicate the prevalence of marine conditions in its upper part and the elaters are suited to a presumed arid or semi-arid climate. Units В, С and D are of early and middle Cenomanian, based on the presence of Hensonina lenticularis and Asterohedbergella asterospinosa zones. The Mishrif shallow water limestone Formation which closes out the Middle Cretaceous with alternations of limestone and shale is attributed to the late Cenomanian Dictyoconella minima Zone reported from adjacent wells in the same region.