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Deimos, discovered in 2003, consists of numerous high quality oil reservoirs which increased the depth and subsalt extension of the Mars Field. It was followed up one year later by the up-dip dry-hole exploration well, Boreas. The subsurface results from Boreas were sufficiently disappointing to discourage further exploration in the subsalt portion of the Mars Basin. Both Deimos and Boreas were drilled using our best seismic of the time, PrestackDepth Migrated narrow azimuth (NAZ) streamer data. However this data was insufficient in imaging the structural and stratigraphic complexities apparent in this subsalt environment.
Following the success of the Deimos Phase 1 Development, Shell acquired its first Ocean Bottom Seismic – Full Azimuth (OBS) survey in 2007 to plan Phase 2 of Development. The improvement in the seismic image over Deimos and the surrounding subsalt area was dramatic. The OBS along with an anisotropic corrected velocity model seismic data set resulted in a step change in our geologic and stratigraphic understanding of the subsalt portion of the Mars Basin. Two exploration opportunities, West Boreas and South Deimos were quickly identified. The new data also indicated that the Boreas dry-hole missed West Boreas by < 200 ft, and drilled through a previously un-imaged fault which cut out most of a high quality reservoir sand.
The OBS seismic image facilitated a high quality interpretation of the subsalt portion of the Mars Basin that not only enabled the rapid maturation of the exploration opportunities, but also allowed for the sanctioning of the Mars B subsea development with only one penetration into each of the newly discovered fields. The seismic-to-well ties had an excellent match and instilled high confidence in our seismic interpretation and geologic modeling which enabled Shell to progress the Mars B subsea development at a rapid pace.
One of the largest risks to any subsalt development is the uncertainty of the target reservoir's quality, extent and structure. To date, two of the six proposed subsea development wells have been drilled, and the well results have confirmed our confidence in the quality of the OBS seismic. These subsea wells will be tied back to Mars B's newly installed Olympus TLP and deliver its first oil in 2014.