Reservoir quality prediction of deep-water Oligocene sandstones from the west Niger Delta by integrated petrological, petrophysical and basin modelling

Petroleum exploration and production in the Niger Delta region to date has mainly focussed on the onshore, deltaic and offshore deep-water Miocene successions. Although Miocene turbidites have been the principal deep-water target to date, deeper-lying Oligocene sandstones are now being considered for exploration. This study targets an area beneath the Niger basin slope at a present-day water depth of 800-1500 m. Within this study area, the Miocene to Recent sands above a burial depth of 3600 m show very good reservoir quality with porosities as high as 35% and permeabilities in the Darcy range. The aim of this study is to predict the reservoir quality and properties of the Oligocene sandstones below 3800 m, using basin modelling to predict conditions where quartz cementation will take place and quartz cementation models to predict the amount of cementation and hence potential porosity loss. Modelling results show that the Oligocene sandstones have been exposed to conditions favourable for quartz precipitation, but that less than 14% of the original porosity will have been occluded by quartz cement. These results are in agreement with both elemental analysis from petrophysical and petrological observation of thin-sections. Although the deeper-lying Oligocene sandstones are likely to have reduced reservoir quality caused by the presence of quartz overgrowth cementation, it appears likely that the volume of cement is relatively low and that the Oligocene succession should be considered a viable play.