Hydrocarbon field evaluation: case study of 'Tadelu' field shallow offshore Western Niger Delta, Nigeria

Field evaluation in a complex sedimentary environment is always tedious due to problems associated with imaging of the subsurface in such areas. The 'Tadelu' field in Western Niger Delta, Nigeria, consists of a series of vertically stacked reservoirs in the Agbada Formation, thus making the field a complex sedimentary environment. The main objective of this study is to perform geophysical and petrophysical interpretations of properties that are typical of a shallow marine, transitional environment, with its attendant pattern of subsurface complexities, using 3D seismic and well log data. The approach adopted in achieving this objective included 3D seismic data interpretation, petrophysical rock typing, fluid typing and contact identification and determination of reservoir properties, onwards to reserve estimation for all the hydrocarbon-bearing reservoirs in the field. Five horizons as well as major and minor faults were mapped in the field, and five structural depth maps of reservoirs A, B, B1, C and C1 were generated within the objective depth interval to define the geometry of the reservoirs. Well log data from four wells in the field were studied to characterize the porosity, shale volume and water saturation of the reservoirs. The gamma ray (GR) logs were normalized and used in the computation of shale volume, while available density logs were used in the computation of porosities. Water saturation was determined using the Waxman-Smits model to take into consideration the shaly nature of the reservoir sands. The combination of porosity, shale volume and water saturation cut-off values was used in calculating net pay and net pay averages. The result of the structural interpretation shows that Tadelu field is characterized by two major faults and synthetic faults. The synthetic faults are dipping in northeast-southwest direction, and the closure formed by their hanging walls constitutes the target area for hydrocarbon in the field. However, within the reservoirs of Tadelu field, the net sand range is from 0.17 to 0.99 ft, having an average porosity in the range of 22 to 27 %. Water saturation average ranged from a minimum of 19 to a maximum of 27 %. Porosity in the field is high on the average due to very good to excellent sand quality. These petrophysical deliverables served as an input in the estimation of reserves. The gross rock volume (GRV) was determined by establishing a cut-off at appropriate contact(s). Reserve estimation was performed for the hydrocarbon-bearing sands and a total proven recoverable oil and gas estimate put at 42.797 MMbbl and 13.256 Bscf, respectively.