RESUMEN
Sedimentological and palynostratigraphical studies were performed on 189 ditch-cutting samples composited at 50 ft intervals from four well sections located in KW field, onshore western Niger Delta Basin, Nigeria, using sieving and visual microscopic observations, hydrochloric and hydrofluoric acids digestion methods to determine their lithological character and palynologic content of the sedimentary successions. The basis of these data is to interpret the lithofacies and biostratigraphy, establish the age of the strata, establish the palynological zones and infer the environments of deposition. Five lithofacies delineated based on the sedimentological study were very coarse sand, coarse to fine sand, shaly sand, muddy sand, and sandy to silty shale. The age of the sedimentary successions ranges from Early Miocene to Pliocene based on the bio-assemblages of these key species. The Early Miocene age is characterized by the occurrence of Magnastriatites howardi, Psilatricolporites triangularis, Praedapollis africanus, Pachydermites diederixi, Spirosyncoilpites spiralis, Praedapollis flexibility, Racemonocolpites hians, Verrutricolporites rotundiporus, Verrutricolporites microporus, and Psilatricolporites divisus. The Middle Miocene age is characterized by the FAD of Crassoretitriletes vanraadshooveni, the occurrence of Belskipollis elegans, in association with Striatricolporites catatumbus, Verrutricolporites rotundiporus, Verrutricolporites microporus, Crassoretitriletes vanraadshooveni, Racemonocolpites hians, Retibrevitricolpites obodoensis, and Retibrevitricolpites protrudens. The Late Miocene age is characterized by the occurrence of Multiareolites formosus, Cyperaceaepollis sp., Stereiosporites sp., Peregrinipollis nigericus, Gemmamonoporites sp. (Cleistopholis patens), Laevigatosporites sp., Verrutricolporites sp., Nymphaeapollis clarus, and Verrutricolporites usmensis. The Pliocene age is characterized by the basal occurrence of Retistephenocolpites gracilis in association with Nymphaeapollis clarus, Cyperaceaepollis sp., Laevigatosporites sp., Verrutricolporites sp., and Crassoretitriletes vanraadshooveni. The boundary between the Early Miocene and Middle Miocene from the studied oil well sections is marked by the first appearance datum (FAD) and numerical maximum occurrence of Striatricolporites catatumbus and Belskipollis elegans and FAD of Crassoretitrilettes vanraadshooveni. The boundary between the Middle and late Miocene is defined by the LAD of Verrutricolporites rotundiporus, the quantitative top and numerical maximum occurrence of Racemonocolpites hians, and the numerical maximum occurrence of Zonocostites ramonae. The boundary between the late Miocene and the Early Pliocene is marked by the first appearance datum (FAD) and the numerical maximum occurrence of Retistephenocolpites gracilis, the numerical maximum occurrence of Echiperiporites icacinoides, and the highest abundance of Monoporites annulatus. Four palynozones PAZ-1, PAZ-2, PAZ-3, and PAZ-4, and ten sub-zones were erected. Finally, Fluvial-coastal to shallow marine environments of deposition were inferred for the sedimentary successions using sedimentological study, nature of organic matter, and association of key environmentally-sensitive biomarker species.
RESUMEN
A sedimentological investigation was carried out in Agu-Awka and Umunya localities to interpret the facies, depositional environment, sequence stratigraphy, reservoir potential, and architecture of the outcropping Eocene Nanka Formation of the Ameki Group in southeast Nigeria. Petrographic analysis reveals that the sediments are composed of predominantly subangular to subrounded recycled quartz grains with a minor amount of rock fragments. It indicates that the sediments are texturally submature and mineralogically mature. Lithofacies analysis indicates that the formation is composed of fine to coarse-grained, trough and planar cross-bedded sandstone with clay-drape units, interpreted to be a subtidal bar facies, and a minor mudstone interval with sand to muddy heterolith interpreted to be a tidal mudflat facies. This suggests that the formation is of mixed environments, such as a tidally influenced high-energy fluvial and low-energy marsh to lagoonal settings. The lower sand unit of the subtidal bar facies (fine to coarse-grained cross-bedded sandstone with clay-drape) represents a highstand systems tract. The sharp base sandy heterolith lithofacies that forms the upper unit of the subtidal facies, and the overlying tidal mudflat facies that comprises a bedded mudstone and overlying wave rippled sandy heterolith intervals represent a transgressive systems tract. The reservoir quality of the sandstone facies is predicted to be moderate to good based on textural statistical analysis. The various relationships of the sedimentological parameters of the outcrops revealed by this study may assist and contribute to the prediction and understanding of the reservoir stratigraphic heterogeneity and properties of the subsurface depositional facies of the Nanka Formation.