Electromagnetic prospect scanning moves seabed logging from risk reduction to opportunity creation

Dave Ridyard, Tor Atle Wicklund, and Bjorn Petter Lindhom of Norwegian company Electromagnetic Geoservices (emgs) describe an adaptation of conventional controlled source electromagnetic surveying aimed at marine hydrocarbon exploration in frontier areas. In the past four years emgs has conducted over 200 seabed-logging surveys. This method is now well established as an effective technique for verifying the presence of hydrocarbons in prospects initially identified by seismic analysis. Surveys can be designed to measure the resistivity of subsurface bodies in a growing number of geological settings. In the hands of a skilled interpreter, survey data can help to determine whether or not a structure contains hydrocarbons and, if it does, to delineate the reservoir. A new application of this technique, known as ‘scanning’, provides a means of identifying prospects in frontier areas working only from basin-scale knowledge. In this article, we introduce the concept of scanning and discuss some of the issues concerning survey design and application. What is scanning? The traditional approach to exploring a frontier area requires considerable effort in seismic imaging and drilling before there is any real evidence that hydrocarbons are present. In contrast, electromagnetic (EM) prospect scanning identifies prospective areas much earlier in the process. By acquiring data from a grid of seabed-logging receivers, areas containing significant resistive anomalies can be identified, and exploration efforts can then be focused on those leads. An exploration workflow that includes EM prospect scanning in this way can dramatically reduce the overall time and effort required to find a new reservoir. The difference between scanning and a conventional sea-bed-logging survey is that scanning data is acquired on a relatively sparse grid (compared with the typically 1 km receiver spacing used in traditional seabed logging). As a result, large areas can be scanned quickly. Because preliminary data analysis is also quite quick, it may be possible to identify potential reservoirs before the survey vessel leaves the area. This provides the option to undertake a more detailed infill characterization of the potential reservoir. Then, after seismic imaging, drilling can be targeted on the most prospective areas. This approach may reduce typical field development times by up to a year. The technique can also be applied in more mature areas to identify by-passed pay, which can extend the life of the field using existing production infrastructure. Another opportunity for the use of scanning is in areas where permits for seismic acquisition can be hard to obtain, such as areas of environmental sensitivity. Scanning is most effective when used to survey large areas, so collaboration between several oil companies and licensing authorities can create the greatest value.