Evaluation of porosity and permeability of sandstones within the Oti Group of the Volta Basin using petrophysical and petrographic techniques

Abstract

This study investigates the reservoir quality of sandstones in the Oti Group of the Volta Basin of Ghana. Geological field mapping, petrographic, petrophysical, mineralogical, and geochemical techniques are used to investigate the reservoir parameters of the sandstones by evaluating the fluid holding and transmission capabilities of the rocks. Results from the comprehensive study identified two sandstone formations of interest; viz. the Bimbila Sandstone and Yabraso Sandstone. Both sandstones were found to be quartz sandstones (sub-arkose and quartz arenites). The Bimbila Sandstones proved to have better porosity and permeability as compared to the Yabraso Sandstones. The Yabraso Sandstone showed porosity between 7-22 % with an average porosity of 13 % (helium gas) and permeability of 63.41 mD, which may be linked to intense cementation and intermediate compaction as well as grain size, shape and arrangement. The Bimbila Sandstones showed better porosity and permeability with a porosity range of 6-24 %, an average porosity of 14 % (helium gas) and 131.80 mD permeability. This is seen to be due to lower compaction supported by framework-stable quartz resulting in a well-connected pore system with high permeability. Further mineralogical data show that the clay minerals present are in minor concentrations. Also, the position of the Yabraso and Bimbila Sandstones in the project area as plotted on the geological map show that there is a close proximity relationship between these sandstones and the limestones; hence forming a conducive system such that if hydrocarbons are produced by the possible source rocks (limestones), they can be housed by the sandstones.