Depositional facies analysis of coastal to shallow marine deposits in the onshore Niger Delta Basin: Accessing the influence of sedimentology and depositional environments on reservoir quality

Abstract

Sedimentology and depositional environment of E2000-Sand in the southern part of the Central Swamp depobelt have been studied using core and wireline log data. Nine wells were used for the study, one of which has some 200ft of core in the southern part of the field. Key wells were correlated based on normalized gamma-ray and other logs. Using two main panels, one along the depositional dip across syn-sedimentary intra-field faults and another along strike, lateral continuity, reservoir development, and shoreline proximity were evaluated. The cores were described to identify lithology, sedimentary structures, depositional processes, and genetic units. The results from the electro-facies analysis, wireline log correlation, core description, and core permeameter measurements were integrated to interpret depositional environments. The E2000-Sand normalized gamma-ray log profile showed three broad sections made up of (from bottom to top) a coarsening upward funnel-shaped basal section overlain by an overall cylindrical-shaped gamma-ray log signature capped by a short coarsening upward funnel-shaped interval. The overall gamma-ray log profile is consistent with a deltaic progradational setting typical of a shoreface sequence inundated by channel activities. Seven genetic units were identified in the cored interval comprising Marine Shale, Offshore Transition Heteroliths, Lower Shoreface, Upper Shoreface, Lagoonal Shale/ Heterolithics, Tidally Influenced Channel/Crevasse Splay, and Distributary Channel. Petrophysical analysis of these units showed a direct correlation between lithofacies type and grain size with flow properties deteriorating with decreasing grain size. Using such attributes as permeability, porosity, and grain size, four genetic units in the sand namely Lower Shoreface, Upper Shoreface, Tidal Channel, and Distributary Channel were interpreted as reservoir units. The best reservoir flow properties were preserved in the Distributary Channels with a porosity range of 20-29%, permeability in the range of 3,300-9,900mD and average grain size ranging from 177-500μ, while the Lower Shoreface corresponded to the worst quality reservoir units with porosity ranging from 17-26%, permeability varying from 0.01-180mD, and average grain size varying from 62-125μ. Three of the genetic units including Offshore Transition Heteroliths, Lagoonal Shales/Heterolithics, and Marine Shale were interpreted as non-reservoir units with porosity and permeability ranging from 4-17%, and 0.03-36mD respectively, while average grain size was below resolution. The E2000-Sand is interpreted as deposited in a coastal shoreface/delta mouth shallow marine setting. Reservoir quality in the sand is strongly faciesdependent with sedimentology and depositional environments controlling the reservoir properties of the sand bodies.