An Integrated Approach to Reservoir Quality Prediction of Untapped Oligocene Reservoir Offshore Western Niger Delta

Abstract

Abstract A belief that has been widely accepted amongst petroleum engineers and geoscientists in the oil industry is that early hydrocarbon emplacement in sandstone reservoirs is capable of inhibiting quartz cementation, thereby leading to porosity preservation. Results from this study of the timing and degree of cementation of a deep Oligocene potential clastic reservoir are used to predict: (1) the onset and volume of quartz cementation in the potential reservoir, and (2) the effect of early hydrocarbon emplacement on reservoir quality. In this study, burial and thermal history of untapped deeper Oligocene plays that lie below the highly productive clastic Miocene plays of the western deep Offshore Niger Delta are investigated using basin modelling enhanced by quartz cementation modelling. Of particular interest is the timing of charge to the reservoirs and the timing of diagenesis. Quartz cementation was modelled using the Walderhaug quartz cementation algorithm as a precipitation rate limiting reaction controlled by the rock fabric and temperature. Model calibration was primarily achived using physical properties measured from well A1- the only well that penetrated the top of the Oligocene sequence. Simulation results indicate that less than 0.5% of pore space of the shallow proven hydrocarbon-bearing Miocene sands have been cemented, having been exposed only to low temperatures unfavourable for quartz precipitation. This is in agreement with measured porosity values exceedin 30% and permeability in the Darcy range. The deeper Oligocene succession has been buried to depths where temperatures are simulated to exceed 70° C favouring quartz cementation. The cementation model of the Oligocene sediments indicates that less than 14% of the total porosity has been occluded by quartz cement. However, the charge model simulates peak hydrocarbon saturation being reached when less than 7% of the pore space, in all four simulated accumulations, was quartz cemented. Hence the rate of quartz cementation is likely to have reduced from the time of peak oil charge onwards, thereby preserving a significant fraction of the porosity in the potential Oligocene reservoirs. The integration of all techniques adopted in this study that results in a holistic outcome is believed to represent a novel approach in reservoir quality prediction. If the Oligocene is proven then it would represent a significant new play that is likely to extend beyond the study area across the western offshore Niger Delta.