Issues in the Use of Seismic Data at Agbami Field, Offshore Nigeria
Authors: Grimes David L. , Cupich Ronald M. , Angstadt David M. , Leeds Alan R. 
Published: October 2008

Seismic data at Agbami field suffer from significant water bottom multiple contamination at the main pay zone, impacting the interpretation. Initial processing of the data did not adequately address the multiples, resulting in poor data. Reprocessing the data in 1999 effectively removed the multiples, nd improved the data for mapping, but adversely affected the rimary amplitudes and left non-geologic amplitude variations in the data that precluded quantitative analysis. Recent reprocessing efforts using a proprietary Gaussian Beam method to attenuate the multiples have both removed the multiples and preserved the primary energy. This paper describes the problems related to the multiples that are present in the 1999 reprocessed data and how the recent work is impacting those issues.

Introduction

The Agbami structure is a northwest to southeast trending anticline located on the Niger Delta approximately 105 km offshore Nigeria in the Gulf of Guinea (Fig. 1). The structure spans an area of about 180 km2 at spill point and is located in approximately 1500 m of water. The discovery well, the Agbami No. 1, was drilled in 1998; subsequently, four additional wells and one sidetrack were drilled to appraise the structure. These five wells and the sidetrack all penetrated oil bearing sands with an average net pay of 107 m of oil. The structure is a world class development opportunity with up to several billion barrels of oil in place.

The pay intervals at Agbami field consist of primary reservoirs in the 17 million year (MY) sands accounting for 80% of the reserves and secondary reservoirs in the 14MY/16MY sands that hold the remaining 20%. The seismic reflections from the shallower, secondary reservoirs occur above the water bottom multiple and are of good quality, with amplitude extractions and amplitude variations with angle (AVA) interpretations that match predicted models. Stratigraphic and fluid changes can be interpreted from the data.

However, the seismic data from the deeper, main pay intervals in the 17MY interval suffer from significant multiple energy contamination. The most obvious multiples originate at the water bottom. Multiples from additional surfaces beneath the water bottom also present problems. Unaddressed, this multiple energy almost completely masks the primary reflection data from the main pay interval. Residual energy from the first and second bounces of these surfaces in the water column can be identified in the data through modeling. The migration of this energy as primaries has produced upsweeping patterns of energy that crosscut the reflections, creating a â??cat whiskersâ?? effect in the seismic image. Earlier processing of the data during the exploration and appraisal phases of the field did not adequately resolve these data problems. Recent reprocessing efforts using a Gaussian beam method for attenuation of the multiples are delivering results that are encouraging for future work to better characterize the reservoirs using the seismic data.

Discussion

1996 Initial Processing.

Initial processing of the 3D seismic data in 1996 was done in a "fast track" mode with most of the pre-stack processing done onboard the acquisition vessel.