SOURCE:

Faculty: Physical Sciences
Department: Applied Geophysics

CONTRIBUTORS:

Chinwuko A. I.
Anakwuba E. K.
Onwuemesi A.G.

ABSTRACT:

Co-blending of seismic attributes was used in the interpretation of channel geometries and reservoir characterization in Rence Field of Niger Delta, Nigeria. The study aims at defining the geometries of hydrocarbon reservoirs with particular emphasis on channels in shallow marine (offshore) Niger Delta. The co-blending application enhances both the ease of detection and continuity of channels, leaving the channel environs unchanged.
The methodology involved the use of three dimensional seismic reflection volume, checkshot (velocity) and well log data in order to generate all necessary interpretation and input for geological modelling from seismic and well data interpretations. Eight single seismic attributes such as average energy, root-mean-square amplitude, instantaneous frequency, reflection acoustic impedance, iso-frequency, dominant frequency, signal envelope and coherence attributes were used for interpretation in the study. Four sets of co-blending were carried out. The execution of these steps was done with the aid of Schlumberger Petrel. The result of the seismic facies analysis reveals that the Rence Field can be distinguished into two seismic facies, namely, layered complexes and chaotic complexes. The channel elements deduced from facies analysis are meander loop cut-off deposits, lateral accretionary channel deposits, and shore deposits. The result of well to seismic ties reveals high and low amplitude reflection events for sand and shale units respectively. Seismic structural interpretation of Rence Field reveals four major regional faults and twelve minor faults. Seven of the faults are antithetic and the rest are synthetic faults. One mega-channel feature that trends east - west was identified in the attribute maps generated. It is characterized by sinuosity of 1.3, with a meandering length of 22500 m, and a distance of 17500 m. The average depth of the channel is about 170 m with amplitude of 1670 m, and wavelength as high as 7640 m. A depositional model generated from the attribute maps indicates a sub-aqueous channel environment of deposition. The attribute map also shows that there was shifting in location of barrier bars within the area. The shifting could be attributed to growth faults mechanism. At the flanks of the sinusoidal channel there are prominent sand point bars sequences. The petrophysical analysis of the well data shows 90% Net-to-Gross, 28% porosity, 27% volume of shale, and 24% water saturation. These values indicate that the reservoir is of pay quality. The oil reservoir has been estimated to be 474.4 million stock tank barrels of oil initially in place. The oil/water contact was delineated at a depth of 1294m (4180ft) with the oil bearing sands having low acoustic impedance values ranging from 4600 to 5450 g/cm3.cm/s with highest porosity value of 0.36. The water sand has higher acoustic
impedance values ranging from 5500 to 6150 g/cm3.m/s with the lowest porosity value of 0.12. The validity of this work is in the application of the seismic time slice at 1703ms from the attribute map generated which shows that, all the existing wells in the study area are targeting the mega-channel for oil exploitation. Based on the petrophysical analysis, results, and identification of channel deposits, the study area proves highly promising for hydrocarbon accumulation and definition of the geometries of channel in study area