Reservoir sandstone grain-size distributions: Implications for sequence stratigraphic and reservoir depositional modelling in Otovwe field, onshore Niger Delta Basin, Nigeria

Abstract

The grain-size distribution and the complexity of the sand-body suggests a high degree of heterogeneity of the depositional environments of clastic reservoirs. To understand the facies and depositional environments distribution of the oil-bearing sandstone reservoir in the Otovwe field, onshore Niger Delta Basin, an integrated sedimentological approach was employed. This involves the integration of well log signatures and log-probability plots of grain-size analyses of ditch-cuttings obtained from 3 wells that showed variability in the rates of oil production. The result suggests that the sediments were deposited in a fluvial environment that contains point bars of a lowstand systems tract; delta front environment that contains mouth bars and distributary channel deposits of a highstand and lowstand systems tracts, respectively; and the nearshore environment with tidal flat sediments of a transgressive systems tract. In some cases, stratigraphic correlation shows variations in facies succession at the same stratigraphic level in individual wells. A systematic variation in grain-size distribution parameters of the various depositional facies, however, appears similar in individual wells. These suggest that the location of the wells within the field experienced different depositional processes, fluvial controlled, and rates of reworking by marine-influenced. Grain-size distribution pattern shows very fine, and fine sands constitute the grain framework of the reservoir sandstone and a minor amount of silt and medium sands with an insignificant amount of sediment size ranging between coarser sand and fine gravel with clay. These suggest that possibly the very fine-grained and fine-grained sands constitute thick beds, silty-sand and shaly-sand as thin beds. Conversely, the clay and gravelly-sand occur as dispersed sediments constituted laminae, and lenticular beds, respectively. The mixture of silt and gravel possibly constituted the sediments that gave rise to the poorly sorted status of the reservoir. Variations in depositional environment and facies, together with grain-size distribution across the reservoir, possibly constituted heterogeneous elements that instituted the variability in the oil production rates of these wells. This study demonstrates the importance of “textural-electrofacies” analytical method in sedimentological studies in a situation where other subsurface data are not readily available for interpretation in anticipation of hydrocarbon reservoir sandstone management and field development in siliciclastic settings. • Very fine and fine sands constitute the framework of the reservoir sandstone. • Dispersed coarser and mud institute a poorly sorted sediment status of the reservoir. • Sediments were transported in a bottom suspension and rolling prior to deposition. • Reservoir facies contains three paleoenvironments: fluvial, delta front and nearshore. • The reservoir experienced more than six sedimentation cycles during development.