Effects of depositional environments on reservoir quality in Gabo Field Niger Delta

Abstract

The focus of this research is on the Effects of Depositional Environment on Reservoir Quality in Gabo Field. Data used in this research comprise Suits of Well logs, Core data and core photos for Wells 51 and 52 in the study area. The methodologies involved determination of petrophysical parameters from both core and well logs, delineation of the reservoir interval from well logs and sedimentological analysis were carried out on core photographs. Thirteen reservoir units were identified in wells 51 and 52 which had 5 reservoirs cored in each of them and correlated across the field. The lithofacies units identified in reservoirs across the study area comprise pebbly sands, coarse - very fine-grained sands, sandy mud, silty sands and sandy/muddy heteroliths. Ophiomorpha and skolithos are the major trace fossils identified and sedimentary structures such as ripple lamination, wavy lenticular and planar beds and cross bedding. The facies associations interpreted are Channel and Coastal barrier and the environment of deposition as distributary channel, upper and lower shoreface respectively. The sedimentary processes ranged from high energy regimes, reworking by waves to low energy with periodic influx of silts and muds. The average porosity and permeability for reservoirs in Well 51 is 16.7% and 1317 Md, well 52 has 28.2% and 2330Md whereas the porosity ranged from 2% - 32% and permeability 1.2 – 10600 Md in the studied sections. The reservoir quality of the sand units in Well 51 (7, 9 and 13) and Well 52 (5, 7, 9, 11 and 13) is excellent - good, this is because of the dynamics of the environments of deposition as well as the mechanisms that played out during and after deposition such as bioturbation, sorting and sedimentary structures. Whereas poor quality reservoirs were observed in lower shoreface and prodelta facies which may have been influenced by lack bioturbation, connectivity and multiplicity of burrows that may have been plugged by clay and shale. The conclusions reached in this research, is in agreement with similar works that environments of deposition have direct influence the reservoir quality in terms of porosity and permeability.

Key words: Porosity, permeability, skolithos, Ophiomorpha, lithofacies, reservoir, bioturbation, channel and coastal barrier systems.