A palynological investigation of the Brent Group from UK North Sea wells 211/28-H15 and 211/28-H1 (Hutton Field)

The 13 samples all produced abundant palynofloras that are dominated by characteristically Middle Jurassic spores and pollen such as Callialasporites spp., Ischyosporites variegatus, Leptolepidites spp. and Neoraistrickia gristhorpensis. Marine palynomorphs proved extremely rare, normally well below 1% of the overall association, and four samples entirely lack marine palynomorphs. Extremely low proportions of marine palynomorphs are consistent, for example, with a lagoonal setting where marine waters occasionally entered. The dinoflagellate cysts are dominated by the euryhaline genus Nannoceratopsis, so this may represent brackish water conditions. Clearly, the degree of marine influence was extremely small. The dominance of Nanoceratopsis is consistent with the Aalenian/earliest Bajocian interval. This is confirmed by the absence of the Parvocysta group, and the lack of Durotrigia filapicata and Phallocysta thomasii. This means that the succession studied ranges from the Murchisonae to Laeviuscula chronozones. In summary, the presence of the dinoflagellate cysts Nannoceratopsis deflandrei subsp. senex and Pareodinia halosa, and the pollen genus Callialasporites is indicative of the Aalenian-earliest Bajocian. Sample 3 at 16579 feet from well 211/28-H15 is within the Mid Ness Shale; it produced 13 marine palynomorphs per slide and is indicative of marginal marine conditions. A single sample of the non-marine/brackish acritarch Truncatisphaeridium clevelandense was also recovered. The occurrence of Truncatisphaeridium clevelandense in this horizon is indicative of a mid Aalenian (Murchisonae Chronozone) age, and a probable brackish water palaeoenvironent. Sample 6 at 11200 feet from well 211/28-H1 produced a relatively abundant marine palynomorph association, therefore represents open marine conditions.