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