Compaction-induced porosity loss in the Cretaceous Bima Formation around Lakwaime and Dogon Dutsi, Yola Subbasin, Northern Benue Trough, NE Nigeria

Abstract

The Bima Sandstone is an alluvial fan to braided river sedimentation that represents purely a siliciclastic depositional system. It is the oldest formation and the main reservoir unit in the entire Benue Trough. Porosity loss is an essential component of petroleum reservoirs that needs to be properly investigated for the purpose of hydrocarbon exploitation. Compaction and cementation processes are partly responsible for porosity loss in many known reservoirs around the world. Outcropping units of the formation around Lakwaime and Dogon Dutsi in Yola Sub-basin have been subjected to an integrated approach involving field observations, petrographic analysis and porosity measurements in order to understand the contribution of each of the processes. Modal composition of the samples shows that they are mostly lithic arkose to feldspathic litharenite, with quartz as the most common detrital mineral grain. Grain fracturing has been attributed to mechanical compaction, whereas grain contacts like sutured and concavo-convex are products of chemical compaction. The average values of compactional-porosity loss (COPL), cementational-porosity loss (CEPL), and compaction index (ICOMPACT) in the studied samples stand at 19.51%, 4.18% and 0.82 respectively. The study indicates that the role of compaction in porosity loss is greater than that of cementation as shown by the COPL-CEPL diagram and, as such considered to be generally responsible for the porosity loss in Bima Sandstone.