The 3D/4C OBS Survey at Valhall
Authors: Thomsen L. , Thomsen O.I. , Rosland B.O. 
Published: October 2008

Abstract
A 3D/4C Ocean Bottom Seismic dataset was acquired at the Valhall field, Norwegian Block 8, North Sea in January 1998. (Rosland, et al, 1999). This dataset was acquired after prior 2D/4C converted-wave data showed coherent images in the crestal portion of the field (Thomsen, et al 1997), despite the presence of a "gas cloud". The 2D survey both demonstrated the viability of the OBS technique (not well proven in those days), and showed significant small-scale structure (e.g. faults). However, since the fluids flow in 3 dimensions, this 2D survey was viewed primarily as an enabler for the subsequent 3D survey which we describe here, and in companion papers (Brzostowski, et al, 1999, Rosland, et al, 1999).
The 3D multicomponent data volumes demonstrate significant potential for improved reservoir definition across the field - a key element for proper reservoir management and for optimal well positioning. Seismic data play a definitive role in establishing this model and, in the flank areas of the field, the seismic data quality is considered to be of excellent quality. However, the crestal area is rather poorly defined by conventional streamer data, due to gas residing in the overburden, with consequent P-wave attenuation and distortion. With the acquisition of the new 3D converted-wave volumes, integrated with the usable portions of the P-wave volumes, it is now possible to approach seismic interpretation in a vector manner (Mueller, et al, 1999). With this emerging methodology, details regarding the internal structure and properties of the reservoir both on the flank and at the crest are available for the first time.

The Valhall Field: Setting, Geology, Drilling
The Valhall field, located in block 2/8 in the southern part of the Norwegian North Sea has been on production since 1982. The current estimate of original oil in place is 2.5 billion barrels; this number has been steadily rising over the years, as a result of improved reservoir definition, improved drainage by horizontal wells, and more efficient completion techniques (Farmer, et al, 1997). Daily production is today on the order of 100.000 barrels. The field is producing from the Tor and lower Hod formations -both upper Cretaceous chalks. The Tor formation is by volume the most important. Thickness ranges from 0 to 100 m in the area, with average thickness around 25 m. The porosity is locally higher than 50%. The primary reservoir in the Hod formation (Hod 4), is normally around 30 m thick, with porosity around 35% and above.
The conventional marine seismic streamer and ocean bottom P-wave data quality varies across the field (Fig. 1a, c). Shallow gas distorts and attenuates the compressional seismic waves such that a reasonable image is difficult to obtain. In the flank areas of the field the seismic quality is considered to be very good.
Although the seismic quality is far from ideal, seismic data continues to be the driver for ongoing field development, by identifying possible undrained compartments and by providing the basis for detailed well planning. The flank areas of the field are developed using horizontal wells.