Geological assessment of Usani reservoirs from petrophysical, Sedimentary facies and Seismic studies, Niger Delta Basin, Nigeria

Abstract

The Usani wells, Niger Delta basin, Nigeria have problems of unexpected hydrocarbon decline in the field. This study aimed to evaluate the causes of the sharp decline of hydrocarbon resources in the wells. Geophysical, seismic, and core well logs from the research region were used in the investigation. To assess the hydrocarbon saturation (Sh), the petrophysical properties of the reservoirs, core data, seismic data, and well logs were integrated. The petrophysical data was used to create reservoir models for the sequence that the wells in the Usani oil field penetrated. Eight reservoirs were mapped, with depths ranging from 2886 to 3533 meters and thicknesses spanning from 12 to 407 meters. Facies analysis indicate that the reservoirs comprise four main lithofacies: bioturbated sandstone, heterolithic sandstone facies, parallel laminated sandstone, structure less sandstone reservoirs. Hydrocarbon saturation (Sh) ranges from 45.50% to 78.50 percent, water saturation (Sw) from 21.65% to 54.50%, and permeability (K) from 245.70 md to 454.7 md, based on petrophysical data. Porosity (Ø) in the reservoirs varies from 14% to 28%. The productive reservoirs A, B, C, D1, D2, D4, and D5 were found to contain
hydrocarbons based on the seismic attribute map. In relatively poor resistivity reservoirs D1 and D2 at depths of 2884 and 2940 meters, and 3114 and 3126 meters, respectively were discovered to contain significant amount of hydrocarbon. Our estimates indicate that while the Sw values in the wells are below the essential water saturation values, the main reservoirs A, B, and C will mostly produce oil and gas without water. The study has shown that reservoirs of the Usani Field have good Ø and K. The unexpected decline in performance was due to shale effect. Predicting reservoir performance and addressing the issues of unanticipated hydrocarbon depletion in the field were accomplished by integrating seismic, well, and core data. The core sedimentary features have revealed deposition in fluvio-deltaic environments which produce good reservoirs for oil and gas. The result of the petrophysical, seismic attributes, depositional sedimentary facies and geological porosity reservoir models have provided valuable basis for ongoing field development studies, reservoir simulation modelling and monitoring well performance.