Role of diagenetic alterations on porosity evolution in the cretaceous (Albian-Aptian) Bima Sandstone, a case study from the Northern Benue Trough, NE Nigeria

The Cretaceous Bima Sandstone is a potential hydrocarbon reservoir that is currently being explored in the Northern Benue Trough, NE Nigeria. The sedimentological characteristics and tectonic evolution of the formation have been extensively studied by previous researchers. However, the impact of diagenetic alterations on porosity evolution of the sandstone is poorly understood. This study employed a multi-technique approach, including thin-section petrography, SEM, XRD, and cathodoluminescence analyses to investigate the role of framework composition, primary depositional characteristics, and diagenetic alterations on porosity evolution of the sandstones. The results of the study indicate that the Bima Sandstone has a present-day intergranular porosity ranging from 0 to 25.9%, averaging 5.8%. Diagenetic alterations, including mechanical and chemical compaction and quartz cementation, have significantly modified the porosity of the sandstones more than the primary depositional features (grain size and sorting). Porosity reduction was mainly due to mechanical compaction (av. 32.1%) than cementation (av. 7.4%). Quartz overgrowths (up to 15%) are the most abundant cement in the sandstones, formed primarily due to lack of grain-coating clays. The main eogenetic alterations related to shallow burial include dissolution and kaolinitization of feldspars and, to some extent, mechanical infiltration of smectitic clays, which formed largely discontinuous clay coatings and pore-filling clays. Mesogenetic alterations related to deep-burial include the formation of quartz overgrowths, transformation of kaolinite and smectite into illite, and pervasive albitization of both plagioclase and K-feldspar, which is exclusively associated with the deepest, Lower Bima Sandstone Member, and has significantly reduced the secondary porosity in the sandstones. Telogenetic alterations related to uplift, mainly include dissolution and kaolinitization of unstable feldspars, which has significantly increased the present-day, secondary porosity of the sandstones to up to 15%. Pore-filling clays (chlorite, illite, and kaolinite) have had a small impact on the intergranular porosity of the sandstones, mainly because of their low volume. The study provides insights into how diagenesis can significantly affect the reservoir porosity of the sandstones and could also assist in reducing the risk of exploration of the fluvial reservoir in the basin and elsewhere.