Acquisition and Processing of Shallow Water 3-D Seismic Surveys Over Producing Fields in the Northwest Niger Delta

Abstract

ABSTRACT Most of the fields operated by Gulf Nigeria on behalf of Nigerian National Petroleum Corp. (NNPC) and Gulf Joint Venture, lie in the shallow waters of the northwest Niger Delta. The techniques of radio-telemetric transmission of seismic data, shallow-draft seismic source vessels, and receiver swath survey methods have been used to gather data accurately and efficiently. Processing steps such as 3-D residual statics, dip move-out (DMO), and steep dip migration have been used to produce highly resolved images of the subsurface. The fields surveyed are mostly in mature stages of development and the 3-D data acquired in these surveys are better defined than the 2-D lines used for original mapping. Because of the new methods used to acquire and process the 3-D data, seismic interpreters have identified untested fault blocks, stratigraphic plays, and deeper pool potential. The fields can now be more completely and efficiently developed. INTRODUCTION Between 1984 and 1987, western Geophysical Company acquired and processed three 3-D seismic surveys for Gulf Oil Company in the following producing oil fields offshore northwest Niger Delta : Malu-Parabe, Okan, and Mina-Meren-Ewan (Fig. 1). The first 3-D survey was conducted between August 1984 and March 1985 (processing inclusive) in Malu-Parabe fields using air gun sources and streamed cable. About 4266 mi (6,865 kID) of 4800% seismic data were acquired [equivalent to approximately 116 sq mi (300 sq kID)] in Malu-Parabe fields located in water depths varying between 20 and 100 ft (6.1 to 30.5 m). To date, a total of 61 wells have been spudded in these two adjoining fields which have been producing oil since 1971. The second 3-D survey was carried out between December 1985 and January 1987 (processing inclusive) in Okan field located in 10 to 60 ft (3.1 to 18.3 m) water depth, using a radio telemetric acquisition system with airgun sources. Okan is a giant oilfield, and is the oldest field in Gulf's concession offshore Niger Delta. To date, 76 wells have been spudded in this field and it has been producing oil since 1965. The 3-D survey also encompassed parts of Mefa and Delta South fields which adjoin Okan field to the southwest and northeast respectively. The size of this survey was 96 sq mi (250 sq km) with a nominal fold of coverage of 4400%. The third survey, which is the largest, embraced three field areas called Mina-Meren- Ewan. The water depth in Mina-Meren-Ewan area varies between 10 ft (3.1 m) and 80 ft (24.4 m). A similar but slightly modified radio telemetric shooting technique as was used in the Okan 3-D survey was applied to the Mina-Meren- Ewan area. About 154 sq mi (400 sq km) of 3-D data were successfully acquired in this area between November 1986 and May 1987 with 6000% multiplicity.