Although conventional streamer seismic data serves exploration purposes well in many cases, the quality may not be sufficient to support an adequate model for reservoir development, in particular below complex overburden. As discussed in GEO ExPro No 1-2/2008, this experience led to the development of innovative ways of acquiring seismic data. Here, we address the OBS technique, and briefly show a few of the successful results.
Ocean bottom seismic surveys
In the early 1990's, Eivind Berg at Statoil led the development of the SUMIC (SUbsea seisMIC) system, whereby both shear and pressure waves were recorded by sensors implanted in the seabed. The 1993 pilot survey over the Tommeliten structure in the North Sea demonstrated that the SUMIC technique can successfully image subsurface structures through and below gas chimneys by the use of shear waves. For this achievement, Berg and his colleagues received the prestigious Kauffman Gold Medal from the Society of Exploration Geophysicists.
In the OBS survey every receiver station is a four-component (4C) sensing system: a three-component geophone and a hydrophone. While the sensing system is stationary on the seabed, a source vessel towing a marine source array shoots on a predetermined dense grid on the sea surface. When the shooting is complete, the sensing system is retrieved and redeployed in a nearby location and the shooting campaign continues.
The uses of 4C-OBS data can be divided into three broad categories: firstly, lithology and fluid prediction by the combined analysis of pressure and shear waves; secondly, time-lapse (4D) seismic monitoring; and thirdly, imaging in geologically complex areas.
Seismic from all directions
The benefit of OBS for detailed structural imaging by the exploitation of pressure reflections was not fully realized by everyone in the seismic community before 2005. In a summary from the 2005 EAGE/SEG research workshop on multi-component seismic it was stated: "Surprisingly, the driver behind the multi-component business was not shear waves but better pressure wave data" (Lynn and Spitz 2006).
Some major oil companies, however, were fully aware of this application. In 1999 an extensive research program led by StatoilHydro's R&D group concluded that detailed structural seismic imaging of complex geology required the acquisition of high-fold seismic data from all directions around a reservoir. This result was obtained by careful planning of the world's first dedicated 3D imaging OBS cable survey over the Statfjord field offshore Norway, followed by thorough and consistent evaluation of image quality versus acquisition geometry (Thompson et al., 2002).
Elucidating the Statfjord field, Norway
Discovered in 1979 in approximately 150 m water depth, Statfjord is one of the oldest producing fields on the Norwegian continental shelf, and the largest oil discovery in the North Sea. The reservoir units are sandstones located in the Brent group and in the Cook and Statfjord formations. Structurally the field is dominated by a single rotated fault block dipping towards the west, with a more structurally complex area on the East Flank characterized by small rotated fault blocks and slump features.
Previously, imaging from conventional 3D streamer acquisition had been difficult in this field, due to gas in the overburden and multiples in the lower reservoir zones. Therefore, an OBS cable pilot survey was shot in late 1997 with the main objective of improving the seismic imaging of the structurally complex East Flank.
Once the 3D OBS survey was processed, it was possible to see that the definition of the Base Cretaceous unconformity and the Base Slope of Failure had improved over a large portion of the survey area. More accurate definition of faults and improved resolution of small scale structural elements were also achieved. The new interpretation resulted in more confident mapping of intact rotated fault blocks with a better understanding of the areal extension and the internal stratigraphic dip within the East Flank area (Osmundsen et al., 2002, Force Meeting, Stavanger).
After the success of the 1997 pilot survey, a 120 km2 3D OBS survey was commissioned in 2002, which showed a consistent uplift in image quality compared to the existing conventional 3D marine seismic. To date, 3D OBS has been actively used for planning at least eight successful wells.
Imaging Cantarell's daughter
The Cantarell complex in Mexico, discovered in 1979 in 40 m water depth, is the third largest producing oil field in the world. The field has five blocks bounded by faults, Akal being the most important. Geologically, it is one of the most interesting oil fields in the world, because the reservoirs are formed from carbonate breccia of Upper Cretaceous age - the rubble from the enourmous Chicxulub meteorite that created the Chicxulub Crater. In addition, the region has a complicated tectonic history including major compressional and extensional episodes.
As Cantarell's production is now dwindling, Pemex is dedicating billions of dollars to finding more oil. Cantarell is believed to hide a huge secret: a daughter reservoir immediately underneath the Sihil field, which is underlying the giant Akal field. But imaging difficulties are numerous - in addition to being a fractured carbonate reservoir with salt and an overthrust structure, Cantarell is populated with dozens of platforms, with lots of vessel activity, so that traditional seismic is difficult to acquire due to the risk of tangling streamers.
Therefore, to better image the Akal field and map Cantarell's daughter, Pemex turned to ocean bottom seismic nodal acquisition. SeaBed Geophysical was awarded the contract in 2003/2004. More than 1,500 node sensor units were deployed into seven swaths in a 400 m by 400 m grid, covering a total of 230 km2, at the time the largest survey of this type ever conducted.
Compared with the existing OBS cable 1996 data, Seabed's OBS node 2004 data demonstrated higher resolution, excellent reflector continuity, and improved structural definition of both top Akal and Sihil levels below. The new data mapped structural compartments at the targeted Sihil level.
Deepwater Gulf of Mexico survey
BP has also made a strong R&D effort towards imaging challenges in the subsalt area of their developments.
Discovered a decade ago, the reservoir structure of BP's Atlantis field, the third-largest in the Gulf of Mexico, has posed particular imaging problems which called for major innovations. Regular streamer seismic had not allowed its northern flank, where the crest of the field is partially obscured below a thick sheet of salt, to be mapped. as a result, poor seismic imaging has hindered development.
To meet the Atlantis challenge, and obtain higher-quality seismic images, in 2005 BP contracted Fairfield Industries to deploy 900 autonomous recording units in a grid-pattern on the seafloor. The acquisition program encompassed 240 km2 in water depths between 1,300 and 2,200 m. Completed in 2006, the Atlantis project is the deepest OBS survey acquired by the industry to date.
The OBS node survey produced significant image improvement of the Atlantis field over the existing towed streamer seismic. The improved images are impacting current extra-salt field development and laying the foundation for development expansion into the sub-salt areas which make up two-thirds of the structure, previously viewed as high risk because of the very poor imaging from towed streamer data (Howie et al 2008).