ABSTRACT: Impact of 3-D Seismic Data on the Nigerian National Petroleum Corporation/Chevron Nigeria Limited Joint Venture Development Drilling Program

The Nigerian National Petroleum Corporation/Chevron Nigeria Limited joint venture has been acquiring three-dimensional (3-D) seismic data over its concessions since 1984. To date, 1700 sq km have been recorded and processed at a cumulative cost of US $39 million. During 1991-1992, 20 development wells were drilled based directly on new 3-D seismic interpretations. These wells have added 148 million bbl of oil in new recoverable reserves, and to date have added 37,000 bbl/day to the joint venture's production. In addition, the 3-D interpretations have resulted in a sizable inventory of wells for future development drilling.

The new 3-D interpretations provided more accurate pictures of fault patterns, fluid contacts, channel trends, stratigraphic continuity, and velocity/amplitude anomalies. In addition, the 3-D data were invaluable in designing low risk, directional well trajectories to tap relatively thin oil legs under large gas caps. Wells often were programmed to hit several objectives at their respective gas/oil contacts, resulting in maximized net oil sand pays and reducing the risk of gas production. In order to do this, directional "sharpshooting," accurate depth conversion of the seismic time maps, was critical. By using the 3-D seismic, checkshot, and sonic data to develop a variable velocity space, well-top prognoses within 50 ft at depths of 6,000-10,000 ft were possible, and were key to the success of the program.

As the joint venture acreage becomes more mature, development wells will be drilled for smaller numbers of stacked objectives,

and sometimes for single sands. Highly accurate 3-D interpretations and depth conversions will become even more critical in order to tap thinner pay zones in a cost-effective manner.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.