Appraisal of the Middle Cretaceous Sediments in Ubiaja-1 well Through the Use of Palynology in Anambra Basin, Southeastern Nigeria

Abstract

Revelation of the presence of older sediments continues to evolve in the Anambra Basin through the biostratigraphy study of Ubiaja-1 well located in the Anambra Basin, Nigeria. The study is predicated with a view to determining the age of the oldest sediments, evidence of rework sediment and chronostratigraphy of the studied section. The method of analysis includes digestion, sieving, oxidation, floatation of residues and final mounting of them on glass slides for petrographic observation using binocular microscope. The lithostratigraphy of the formations varies from dark grey fissile shale and shaley siltstone at the base through heterolith facies of sand and shale, dark grey fissile shale at the middle section to sandstone and intercalated sand and shale of various proportions at the upper section of the interval. Three palynological zones were erected: Triorites africaensis assemblage zone 1 (2510-3130ft) characterized by co-occurrence of Triorite africaensis and Lycopodiumsporites sp dated Turonian age; belonging to Eze-Aku Formation; Droseridites senonicus assemblage zone 2 varying from interval 2052-2510ft, characterized by the co-occurrence of Droseridites senonicus, Trifossapollenites sp and the dominance of angiosperm pollen, dated Coniacian age and belongs to Awgu Formation. This is overlain by the Milfordia spp. acme zone 3, ranging from interval 1440 to 2050ft, marked by the appearance of Milfordia jardinei, Milfordia sp and paucity of palynomorphs. The interval is dated Campanian age and belongs stratigraphically to Nkporo Shale. The results show that sediments older than Santonian age, devoid of rework sediment and which ranges from Turonian to Campanian age are present in Anambra Basin. The paleoenvironment of deposition is mainly deltaic to marginal marine defined by the occurrence of Andalusiella sp, Senegalinium sp andCriproperidinium sp.