Delineating a Sub-Salt field in the Central North Sea Using High Density OBC - A Case Study from the Culzean Development

The Culzean discovery, located in block 22/25a in the UK Central North Sea, was discovered by Maersk Oil and partners in 2008. The High Pressure, High Temperature discovery contains hydrocarbons in reservoir sand of Triassic age, within a rotated fault block at a depth of approximately 18000 feet. At the time of discovery, the reservoir outline was bounded to the south by a seismic shadow zone caused by the presence of the Merganser salt diapir. Full assessment of this area was not possible using the conventional Narrow Azimuth (NAZ) streamer data that was available. In order to improve seismic data quality around the salt body, a High Density Ocean Bottom Cable (HDOBC) survey was acquired during 2010 and 2011. The survey objective was to improve image quality by utilising Full Azimuth (FAZ) illumination of the subsurface in the vicinity of the salt diapir. The HDOBC survey proved highly successful in imaging the previously obscured area, and has provided key insights into the understanding of the geology and structural setting of the discovery.