Peter Howard Wertheim
OGJ Correspondent
RIO DE JANEIRO, July 18 -- Petroleum Geo-Services ASA (PGS) completed a four-component (4C) 3D seismic survey of a 45 sq km area of giant Roncador field in as much as 1,860 m of water off Brazil.
The firm said the water depth was triple that of previous 3D surveys and the first 4C survey conducted in ultradeep water. It shot the survey for Petroleo Brasileiro SA (Petrobras). Processing will be finished in September.
The three-boat survey used ocean-bottom cables 6 km long deployed from the Bergen Surveyor seismic vessel and steered into position with a real-time acoustic navigation system. PGS said 4C sensors landed within 0.8% of the water depths of their planned positions.
Roncador target sands occur at 3,000-3,700 m and are difficult to map and interpret with conventional compressional, or longitudinally vibrating, seismic waves.
A 4C survey makes use of laterally vibrating (shear) energy introduced in the subsurface when compressional energy propagating downward through water reaches the sea floor.
PGS hopes to use information from the converted waves to plot reservoirs transparent to compressional waves in Roncador and nearby Albacora field. It also hopes to validate several oil-water contacts.
