Linking diagenesis and reservoir quality to depositional facies in marginal to shallow marine sequence: An example from the Campano-Maastrichtian Gombe Sandstone, Northern Benue Trough, NE Nigeria

Abstract

Fluvio-deltaic strata host significant volumes of hydrocarbons in different basins across the world, and are characterized by depositional heterogeneities and varying diagenetic overprints, owing to complex interactions of different depositional processes in diverse sub-environments. An understanding of the heterogeneities at different scales is critical to the prediction of reservoir quality distribution and architecture in these strata. The Campanian-Maastrichtian Gombe Sandstone is one of the targeted reservoirs in recent hydrocarbon exploration in the Northern Benue Trough, NE Nigeria. However, to date, the diagenetic and reservoir quality evolutions of the sandstone are not well understood. In this study, a multi-technique approach (consisting of thin-section petrography, scanning electron microscopy, and X-ray diffraction analyses) was employed to link depositional facies and diagenesis to reservoir quality. Results of the study indicate that the sandstones are principally arkosic and subarkosic in composition, and were deposited in lower delta front/prodelta, upper delta front/mouth bar, distributary channel, and interdistributary tidal bar/channel depositional environments. The sandstones experienced both shallow burial and some relatively of deep burial diagenesis. Kaolinite is the most abundant eogenetic cement in the sandstones, and was particularly found to be more abundant in the interdistributary tidal bar/channel and lower delta front/prodelta depositional environments due to their significant amounts of reactive detrital feldspars and muscovite precursors, respectively. The interdistributary tidal bar/channel and the distributary channel depositional facies were highly susceptible to siderite cementation, owing to their proximity to organic-rich mudstone lithofacies. Mechanical compaction was more intense in the lower delta front/prodelta than in the interdistributary tidal bar/channel sandstones, due to the presence of high clay matrix content in the former, which enhanced ductility and close packing. The interdistributary tidal bar/channel sandstones form the best reservoir quality because of the very low amounts of detrital pore-filling matrix (average 1%; n = 24) and less compaction compared to the lower delta front/prodelta, upper delta front/mouth bar and distributary channel sandstones. Grain-coating smectitic clays were best developed in the lower delta front/prodelta, upper delta front/mouth bar and distributary channel, whereas they are poorly developed in the interdistributary tidal bar/channel sandstones. The overall low volumes of deep burial, mesogenetic cements in the sandstones (e.g., illite, dickite, and quartz overgrowths) suggest that the Gombe Sandstone spent short residence time in mesogenetic realm before being uplifted. The results of this study would provide useful input data and understanding that can be utilized for forward diagenetic modelling and burial history construction for the subsurface Gombe Sandstone reservoir in the Northern Benue Trough and similar sedimentary formations elsewhere.