Abstract

An attempt has been made to estimate the formation factor and porosity of the Quaternary Alluvial aquifers in parts of Bayelsa State, in Southern Nigeria. Formation factor (F) is a very prominent parameter in most Geophysical investigations; it is fundamentally an indispensable factor in the evaluation of the petrophysical characteristics of repositories. The electrical formation factor (or formation resistivity factor), Fa, was evaluated from modelled resistivity data. In this study, the formation factor was estimated following Archie’s original formulation and the corrected formation factor Fc, was estimated following the Waxman-Smits model. Formation factor obtained ranges from 1.41 – 47.5 with an average of 12.49. The spatial distribution of formation factor in the study area as here presented, shows that formation factor is of highest values in the west central parts and lower in the central parts of the Study area. We attribute the relatively low Formation factor values, in some areas to low pore water resistivity, which ranges from 6 - 179 Ωm, with an average of 69.2 Ωm; as well as high bulk resistivity values ranging from 82 - 1894 Ωm, with an average of 441 Ωm. The estimated porosity in the study area ranges from 12 to 57%, at VES stations of 7, 34 and 21. The spatial distribution of the estimated porosity is shown in Fig. 8. The Porosity values obtained from the inversion of VES modelled parameters (Table 1) show that the porosity (φ) values range from 12% to 57%. The spatial distribution map of the estimated porosity is presented in Fig. 8, the Fig. shows that porosity values vary spatially from low values around the west central and south western parts to relatively high values around the north eastern parts of the study area. The calculated porosity values of an aquifer using average resistivity values are considered average porosity values, this is worthy of note because the resistivity values obtained from VES analysis are themselves average values, constructed from all of the smaller scaled heterogeneities within each layer.