3D SEISMIC HYDROCARBON RESERVOIR CHARACTERIZATION OF AN OIL FIELD, SOUTHEAST NIGER DELTA

Abstract

Subsurface data comprising a suite of paper geophysical wireline logs and 3D seismic paper sections from an oil field offshore southeast Niger Delta supplied by Mobil Producing Nigeria Unlimited was utilized for stratigraphic and structural analysis with the aim of evaluating the hydrocarbon potential of the field. The log data was traced manually and later digitized at 1.5 m depth interval to facilitate interpretation and improvement of data quality using kingdom Suite® softwares, Petrel, CorelDraw and Excel. Lithologic interpretation and hydrocarbon reservoir identification was carried out using gamma ray, self potential and resistivity log responses. Log sequence evaluation was done by analysis of log amplitudes, and stacking patterns of facies succession. Lithostratigraphic correlation was done using the principle of similarities in log responses and rock boundaries. Chronostratigraphic correlation complemented lithostratigraphic correlation and utilized key stratigraphic surfaces, such as flooding surfaces, maximum flooding surfaces, and sequence boundaries in the subdivision of subsurface rock units into distinct depositional units. Ditch cutting information from well 06 was used for paleoenvironment of sediment deposition analysis. Synthetic seismogram constructed from velocity function at each well location derived from check shot survey data aided well to seismic tie. Petrophysical parameters of the reservoirs; thickness, volume of shale (Vsh), porosity (Ф), permeability (k), water saturation (Sw), and hydrocarbon saturation (Sh) were computed. Structural interpretation of the seismic data was carried out manually and the relevant time maps produced. Further analysis of the manually digitized seismic data was done by loading the times and depth values into an HP workstation model 2110 to generate seismic maps and 3D perspective views of the mapped horizons using kingdom Suite® software. Seismic sequence analysis was also carried out on the basis of reflection terminations and configurations to unravel stratigraphic elements controlling hydrocarbon accumulation. A fence diagram linking wells 6, 5, 4, 2, and 1 was prepared to show a 3D geometry of the subsurface stratigraphy. Ditch cutting information tied to wells were used to map major sequences. Well log correlation gave insight into vertical and lateral distribution of rocks. The general stratigraphy consists of alternation of sands and shale units. The shale units increase in thickness with depth and the sand units decrease in thickness with depth. The reservoirs are located within the Agbada Formation. The environment of deposition of the reservoirs ranges from inner, middle to outer neritic zones. Most of the reservoirs from the analysed wells are of the lowstand and highstand system tracts configuration. The transgressive system tracts serve as seal to major reservoir units. The reservoirs are located within a depth interval of 1393-2917 m subsea. Reservoir 1 cuts across wells 1, 3, 4 and 5. It has a range of thickness from 46-107m, volume of shale 0.02-0.08,, neutron porosity 11-13%, sonic porosity 13-15%, permeability 11-13 md, water saturation 25-31% , and hydrocarbon saturation 69-73%. Reservoir 2 mapped in wells 1, 3 and 4, has a thickness range of 31-76m, volume of shale 0.09-0.11, neutron porosity 10-14%, sonic porosity 13-15%, permeability 14-25 md, water saturation 24-28%, and hydrocarbon saturation 72-76%. Reservoir 3 extends across wells 1, 3 and 4. The thickness range is from 23-92m, volume of shale 0.11-0.12, neutron porosity 11-14%, sonic porosity 12-20%, permeability 18-20 md, water saturation 19-28%, and hydrocarbon saturation 72-81%. Reservoirs 4 and 5 are restricted to well 4. Reservoir 4 has a thickness of 38 m, volume of shale 0.08, neutron porosity 14%, sonic porosity 14%, permeability 14md, water saturation 23%, and hydrocarbon saturation 77%. Reservoir 5 has a thickness of 15m, volume of shale 0.04, neutron porosity 13%, sonic porosity 12%, permeability 12 md, water saturation 35%, and hydrocarbon saturation 65%. Reservoir 6 extends across wells 1, 3 and 4. The thickness range is from 15-53m, volume of shale 0.05-0.13, neutron porosity 10- 13%, sonic porosity 12-13%, permeability 13-19md, water saturation 28-35% , and hydrocarbon saturation 65-72%. A total of five faults; F1, F2, F3, F4, and F5 mapped are growth and antithetic faults. Seismic structural maps; depth, isopach, and isovelocity gave insight into rock deformation and hydrocarbon potential of the field. The internal reflection configurations of the mapped sequences is composed of parallel/subparallel, chaotic, and divergent reflection configurations suggesting a channel/slump structure. The divergent reflection pattern is associated with growth faults. The results of this research have been able to provide information concerning paleoenvironmental factors controlling sedimentary deposition processes, establish genetic relationship of seismic/log facies and general stratigraphic architecture influencing reservoir properties. Furthermore, data on porosity and permeability have enhanced reservoir characterization and hydrocarbon reservoir optimization.