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Reservoir Characterization of Sand Units Penetrated by Four Wells in the Nigerian Sector of the Chad Basin and its Implication for Hydrocarbon Generation and Accumulation

Ofoma AE, Chiaghanam OI, Amobi JO and Okogbue CO

The reservoir properties of over six hundred sand units penetrated by four exploratory oil wells (Zy-01, Kt-01, Ks-01 & Kd-01) in the Nigerian sector of the Chad Basin Northeastern Nigeria was carried out using the PRIZM GeoGraphix software. Results of the evaluation show that eighty-seven sand units have good reservoir properties for all the wells. Their sand thicknesses are between -1 m and -160 m while averages of effective (φE), density (φD) and sonic porosities (φS) are between 10 & 41%, 10 & 54% and 10 & 89% respectively. These porosity results are fair to very good. Average shale volume content (Vshl) is between 23% & 67%, these values are beyond the acceptable limit in reservoirs. The averages of permeability (k) and fluid saturation (SwA & Sh) are between 15md & 7477.8md, 11% & 71% and 29% & 89% respectively. The permeability values are therefore moderate to excellent. Bulk volume water (BVW) is between 0.028 & 0.188, suggesting very fine to silty grained sands. A comparison of the values of apparent water resistivity (RwA) with water resistivity (Rw), suggest imprints of hydrocarbon in these sands. Also a further comparison of water saturation of flushed zone (SxO) with the Archie’s water saturation (SwA), moveable hydrocarbon index (MHI) and moveable oil saturation (MOS) revealed possible hydrocarbon mobility in the flushed zone by invading drilling fluid. Calculated temperatures and pressures of the wells show increasing trend with depth. Geopressures were encountered at some shale beds within the wells. The sand units of the wells with the good reservoir properties correlate fairly well along Ks-01, Kt-01 & Zy-01 wells. This study has shown that some of the sand units in the study wells have favourable reservoir properties that will enable the generation, accumulation and preservation of hydrocarbons, especially within Ks-01 and Zy-01 wells. However, the very high values obtained for Vshl requires that further estimation of SwA be carried out using other methods and models for a better understanding of the hydrocarbon potential of the Basin. Furthermore, drilling to deeper depths is strongly recommended due to the very favourable reservoir properties observed at deeper depths for Ks-01 and Zy-01 wells respectively.