GEOLOGY, RESERVOIR DIAGENESIS AND QUALITY OF ALBIAN - CENOMANIAN KEANA /AWE POTENTIAL RESERVOIR SANDSTONE IN MIDDLE BENUE TROUGH, NIGERIA

Abstract

The Keana/Awe formations constitute important reservoirs across the Middle Benue Trough, North-central, Nigeria. Ongoing 3D seismic data acquisition in search of hydrocarbon deposits in the Middle Benue Trough has necessitated the need to enhance geological and geophysical data to minimize exploration risk.  In this study, surface outcrops were mapped and described to determine depositional environment and further subjected to petrographic description, petrographic image analysis (PIA) and scanning electron microscopy (SEM) to evaluate reservoir quality of the sandstone formation. Geological field mapping reveals light gray to white, fine to coarse-grained, sub angular to rounded, moderately to poorly sorted sandstones which are calcareous at some sites and bioturbated with vertical and horizontal worm (Rhizocorallium, Diplocaterion, and Skolithos) burrows at other locations. It exhibits structures such as graded bedding; fining and coarsening upward pattern at Old Awe/Tsoho-Gari, Trough cross-bedding at Tunga, Ribi and Tsoho-Gari. At Gidan Barenyi-Tsoho Guaria road and Tsoho Gari, an observed high-low angle and directional tabular cross-bedding, Ripple marks at Abuni and sinistral sense of fault movement of 10 cm at Tsoho-Gari, joints around Old Awe/Tsoho Gari, Gidan Osande - Gara road and fold generally trending NW- SE with fold plunge direction of 145°and plunge amount of 064° as observed at Tsoho Gari. All these are inferred to influence of tides in a shallow marine which supports fluvio-deltaic depositional environments. Results from integration of petrography, petrographic image analysis (PIA), and SEM analysis highlights reservoir diagenesis processes of a mechanical grain compaction and cementation; calcite and siderite are main authigenic minerals resulting in reduction and obliteration of intergranular pore spaces ranking the Keana/Awe reservoir sandstone a moderate reservoir quality grade formation with porosity of ɸ = 10.91% and calcite cement of 33% composition.