oa Calcareous Nannofossils from Middle to Upper Jurassic Sediments of North Kuwait Onshore

Biostratigraphic analysis of Jurassic nannofossil assemblages were performed on a total 188 core<br>samples, collected every 4 feet interval, from MU-A, MU-C and RA-A wells, North Kuwait Onshore.<br>Ninety one samples are from MU-A well represent the Middle Marrat, upper Dhruma, Sargelu, Najmah<br>and lower Jubaila formations, 61 samples from MU-C correspond to the upper Dhruma, Sargelu,<br>Najmah and lower Jubaila formations. The other 36 samples are from RA-A well represent the upper<br>Dhruma, Sargelu and Najmah formations. The rocks consists of argillaceous limestone, grainstone,<br>packstone, bituminous packstone, wackstone dolomite, anhydrite, laminated bituminous calcareous<br>mudstone and calcareous shale<br>Samples from the Middle Marrat formation are barren, whereas most of the samples from other<br>formations contain nannofossils with the total abundance fluctuates from rare to abundance allowing<br>the identification of maximum flooding surface candidates. Preservation of the nannoflora is poor to<br>moderate. The diversity of nannofossil assemblages is relatively low, dominated by the most<br>dissolution resistant species Watznaueria barnesae.<br>An index species Cyclagelosphaera margerelii is present in the samples of upper Dhruma, Sargelu and<br>lower Jubaila sediments. The first occurrence (FO) of C. margerelii was reported to occur in Late<br>Bajocian. The laminated bituminous mudstone of Najmah formation contains common to abundant<br>nannofossils but most of the specimens are poorly preserved due to most of the inner part of the<br>coccolith are covered by oil stained. Strong dissolution resistant species Watznaueria barnesae and<br>high birefringence Watznaueria manivitae however still can be identified. The W. barnesae occurs<br>abundantly whereas W. manivitae presents sporadically. Nannofossil assemblage in the Jubaila shale is<br>characterized by the association of Watznaueria barnesae, Watznaueria britannica, Watznaueria<br>communis and Watznaueria manivitae. Those fossils’s record suggesting that the interval of the upper<br>Dhruma to Najmah formations falls within Middle Jurassic Upper Bajocian to Upper Jurassic Oxfordian<br>stages and the lower Jubaila shale is Upper Jurassic Kimmeridgian stage. There is a strong possibility<br>of stratigraphic discontinuity between the Najmah and Jubaila formations and the time gap is not great<br>as that suggested by some previous workers.