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IDOWU, AYORIND O., Nigerian National Petroleum Corporation, Lagos, Nigeria

ABSTRACT: Areal 3-D Seismic Technique for Reservoir Delineation: Case History from Offshore Niger Delta

In the 1950s, early exploration period in the Niger Delta witnessed the use of 2-D (two-dimensional) seismic reflection method which adequate for imaging large subsurface geologic features including growth faulting and roll-over anticlines. This technique involves the Common-Depth-Point method (CDP) which acquires a plane of seismic information in distance along the surface and in time into the geological section, and is used to improve the signal-to-noise (S/N) ratio, to remove multiples and consequently give a representation of the subsurface particularly if the data are collected up- or downdip.

By mid-1980s, the obvious geological structures have, in general, been discovered and it became necessary to adopt a more sophisticated technique such as the 3-D (three-dimensional) seismic method to delineate more subtle reservoirs and resolve complex fault patterns in order to aid exploration as well as facilitate efficient field development. The case history discussed in this paper involves the use of areal 3-D seismic method for delineating the reservoir characterization of the O-field located in a shallow water area of the western Niger Delta.

The areal 3-D seismic technique is superior to the earlier CDP method in that a cube of seismic data can be collected in two dimensions in space and one in time by a variety of techniques including the swath seismic shooting pattern adopted for gathering the 3-D data for the O-field's reservoir which involves the shooting into lines of detectors which are at some angle to the line of sources. The objective is to adequately sample the subsurface so that changes in various parameters such as the amplitude, phase or power in the seismic signal or velocity of propagation can be mapped areally and interpreted as an indication of changes in the physical properties of the rock matrix.

The 3-D data for the field was acquired, processed through migration and it was possible to display what is essentially a cube of seismic data as a series of layers cut horizontally and vertically through the reservoir complex. This is done on a television screen for video presentation and reproduction, while the addition of an interactive computer system enables a three-dimensional interpretation of the seismic data to be made. The net effect is that an efficient and cost-effective reservoir development was accomplished by proper timing and appropriate location of production wells for the drainage of this field.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.