Geochemical Analysis and Sedimentary Characteristics of the Nigerian Tar Sand; Implications on Classification, Maturity and Depositional Environment

14 subsurface samples from 3 wells (denoted as well A, B, C,) were selected for analysis of geochemical and sedimentary characteristics for the purpose of inferring on sediment classification, maturity and paleo – depositional settings of the Afowo Formation of Dahomey Basin. The study areas are located at Ayede-Ajegunle with coordinate 6.58 N, 4.62 E as Well A, Araromi-Obu/Ago Alaye with coordinate 6.59 N, 4.54 E as Well B and Ajegunle with coordinate 6.60, 4.60 E as Well C.  Grain size analysis was carried out, parameters computed for were mean, mode, standard deviation, kurtosis, and skewness. Sediments from Well A range from fine to very fine grained sizes. However, samples between depth 69m and 72m have their grain sizes ranging from medium grained to very fine grained. The grains are moderately sorted to very well sorted, platykurtic to mesokurtic and symmetrically skewed. The grains from Well B range from fine to very fine grain sizes.

The grains are mostly moderately sorted, mesokurtic to very platykurtic and symmetrical to strongly coarse skewed. The result of grain size analysis for core samples from Well C are largely fine grained. The grains are moderately sorted, platykurtic and fine skewed. The sandstones found in wells B and C should be relatively closer to the distal position of the basin and reflect single source of sediment supply (largely unimodal). This is due to the comparatively narrow range of grain size observed. Multivariate analysis shows that the environment of deposition was dominated by fluvial activities.. Major elements analysis result showed higher SiO2 with an average of 69.3%, AlO3 with an average of 12.9% and Fe2O3 with an average of 3.49%.

Relevant crossplots with the oxides were used for classification, maturity index and depositional setting inference. From the crossplot of Log (Fe2O3/K20)/Log (SiO2), 11 of the sediments are classified as Iron Sands while 4 of the samples were classified as Iron Shale. Scatter plot of SiO2/Al2O3+K2O+Na2O shows that climatic conditions at   deposition were humid to semi – humid. The chemical index of alteration (CIA) values for the sampled locations ranges from 98.97-99.90 with an average of 98.61 while the chemical index of weathering (CIW) ranges from 98.21-99.89 with an average of 99.53. Observation from the thin section showed that quartz percentage made up 75% with the absence of feldspar and lithic fragment while iron makes up for the rest 25%.

Upon this, the ferruginous sandstones can be said to belong to the class of Quartz Arenites.  It is obvious that from the above results that the sediments were of fluvial origin and its source rock was exposed to intensive weathering which depicts matured sediments.