Marine controlled-source electromagnetic surveying has emerged as a new tool for remotely detecting reservoired hydrocarbons offshore. The technology was pioneered by university and government researchers over the past 25 years, and recently has benefited from development by contractors and the oil industry, including ExxonMobil. When integrated carefully with other geoscience information, primarily seismic, marine CSEM shows promise for adding considerable value in Upstream applications. Remote reservoir resistivity detection and imaging results from a recent ExxonMobil survey demonstrate this technology.
Offshore CSEM research began in the 1920's with studies of seafloor power cables, and saw the first commercial minerals survey offshore Cornwall, England a decade later. The research that led to today's commercial methods began in earnest much later, primarily at the British Geological Survey and UCSD Scripps Institution of Oceanography in the 1970's. Many other university and government groups started research on the technology about that time, and many continue to the present. Exxon began investigating marine CSEM in early 1981 using theory and computer modeling, and scoped the feasibility of field tests. Results looked promising for very deep water applications. However, the lack of suitable acquisition equipment, limited deep water opportunities, and the emphasis on the (then) new 3D marine seismic technology resulted in deferring further marine CSEM research.
The technology was re-examined in late 1996 in a workshop that focussed on potential breakthrough methods for the Upstream, championed by senior management. Recognition of advances in acquisition and computing capabilities, together with growing opportunities in very deep water, led to the identification of a breakthrough project in CSEM methods constrained by seismic to deliver the sensitivity and resolution required for Upstream applications. The Remote Reservoir Resistivity Mapping ("R3M") project was created, and by mid-1998 research was underway. From its inception, R3M focussed on full 3D methods for survey design, acquisition, data processing, imaging, and interpretation.
Technology leveraging has been essential for the project's success. Leveraging included modeling and imaging (inversion) algorithms from universities and national laboratories, seafloor receivers from Scripps, deeptow sources from the Southampton Oceanography Center, and specialized navigation and positioning systems from commercial contractors. For each of these technology elements, significant upgrades were made in-house to improve their performance for hydrocarbon applications. As the project progressed, the technology elements evolved at varying rates, creating both technical opportunities and project management challenges. Several contractors offering marine CSEM services emerged during this period.
Following equipment trials in 2001, three successful proof-of-principle surveys were acquired offshore West Africa. Thirty-four additional surveys were acquired through the end of 2004 in a variety of basins and geologic settings offshore West Africa, South America, and North America in order to evaluate the range of applicability of this technology. Technology developments have produced a factor of nearly 50 improvement in the processed data signal-to-noise floor since surveys began, opening new application opportunities.
