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Improved P-wave imaging with 3D OBS data from the Clair field

2005; Wiley; Volume: 23; Issue: 12 Linguagem: Inglês

10.3997/1365-2397.2005023

1365-2397

Jan H. Kommedal, S. Fowler, J. McGarrity,

Reservoir Engineering and Simulation Methods

Multi-component ocean bottom seismic (OBS) data acquisition is often motivated by the ability to record converted shear waves, but such acquisition also has the potential to produce Pwave imaging superior to conventional streamer data. This paper presents such a case, where a significant improvement in S/N ratio, in large part due to improved multiple removal, has enabled us to reduce the uncertainty in the structural imaging both for the reservoir section and the overburden, and resulted in changing the geological model. Background At the Clair field, located west of the Shetland Islands, fracture characterization is crucial for planning the field development. The initial investigation into the fracturing was done by Smith and McGarrity (2001), and concluded that seismic data with full azimuth range is needed for such characterization. Consequently three 2D OBS lines were acquired in 2000 as a feasibility study in preparation for 3D OBS acquisition. The 2D programme produced good quality P- and converted S-wave data, and confirmed favourable sea floor conditions. The processed P-wave data produced imaging which seemed to be an improvement over existing streamer data, and thus improved structural imaging could be added to the list of main objectives for the 3D survey. Data The 3D OBS data was acquired by Petroleum Geo-Services (PGS) during the summer of 2002. Four multi-component cables (one hydrophone and three orthogonal, gimballed geophones at each receiver station) of 6000 m were laid 355 m apart. Over each deployment we shot lines 245 m apart and orthogonal to the cables with 2.4 km maximum offset from the cables. Both receiver and shot interval was 25 m. Twenty deployments were used to cover the survey area. Note how the cable direction is normal to the long axis of the elongated outline of field, and that the survey only covers the central part of the field. This acquisition geometry will give very high fold and uniform azimuth – offset distribution for the common depth points (CDPs). Figure 2 shows the total CDP fold for all offsets up to 10 km and including all patches. The acquisition fold footprint which appears are mainly due to fold variations from offsets of 2.5 km and greater. For the lower offsets, the fold distribution is evenly distributed down to offset ranges of 200-400 m, where the effect of cable and shot line spacing is seen. For the processing, we used offsets up to 4.5 km, which means that the average fold is about 140.