GEO ExPro

Marine CSEM – what’s the future like?

Offshore Electromagnetic surveying, CSEM, was commercialized in 2002 as a new exploration technology for finding hydrocarbons. Following some seven years of trial and failure, we have asked one of the early users of the technology to elaborate on his views of the value and future of this innovative tool.
This article appeared in Vol. 6, No. 2 - 2009

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Len Srnka has a PhD from University of Newcastle upon Tyne, UK (1974). From 1979 he has been employed with ExxonMobil; Exxon Production Research Co. 1979-1993, Exxon Exploration Co. 1993-1998, and from 1998 to present ExxonMobil Upstream Research Co., where he is now Chief Research Geoscientist. In 2007 Len Srnka received the Society of Exploration Geophysicists Virgil Kaufmann Gold Medal. In your view, what is the main purpose of acquiring electromagnetic data when exploring for oil and gas?  

Marine CSEM provides information on rock and fluid properties independent from the seismic method. When interpreted in conjunction with seismic, this information can reduce exploration risk particularly when the seismic interpretation is uncertain.

There are still, in contrast to seismic, certain limitations for the use of EM in offshore sedimentary basins. I am curious to know more about this.

Current marine CSEM data have less depth of penetration than reflection seismic, although this depth now exceeds 3000 meters sub-seafloor in several places. A second challenge is spatial resolution, which is considerably less than seismic, especially in the vertical direction due to the much longer wavelengths of CSEM.  

What kind of geological conditions are the most favourable for EM surveying?  

Increased resistivity in a potential reservoir formation is most easily detected when the resistivity contrast of the reservoir with its surroundings is large, and the thickness of the reservoir is also large. High contrasts occur most readily in moderate-to-high porosity clastic reservoirs that also have high hydrocarbon saturation. In addition, a reservoir is most readily detected when at least one of its lateral dimensions is at least comparable to its depth of burial below the seafloor.  
  We have heard that some surveys are successful, others are not. Which risk factors do you look upon as the most important when assessing the possibility for surveying a prospect or an area?  

It is vital to remember that marine CSEM measures subsurface electrical properties, and not the presence of hydrocarbons directly. So it is necessary to map all of the subsurface formations in three dimensions that exhibit enhanced resistivity, and to interpret those resistivities in terms of rock and fluid types. Examples of non-reservoir resistive formations include evaporites such as salt and anhydrite, volcanic sills, low-porosity marls, sandstones whose pores are tightly cemented with calcite and other minerals, and shallow features such as hydrates that can confuse interpretation of responses from deeper geology. Depending upon the area, the presence of geologic facies such as these must be considered when scoping the utility of a CSEM survey for hydrocarbon identification, in addition to the depth and resolution restrictions.

ExxonMobil has done many surveys in several basins. Are you pleased with the results you have achieved?
 
We have now acquired 53 surveys over a range of discovered reservoirs, for calibration purposes, in field development areas, and over exploration prospects. We are very pleased with the results from these surveys, all of which increased our learning such as the necessity to acquire wide-azimuth CSEM data. Our pre-drill prediction technical success rate for CSEM surveys is very high. One measure of success is our plan to continue using this technology in a number of basins, in both previously visited and in new areas.

While the seismic technology has been developed for decades by numerous individuals and companies, EM is a juvenile in comparison. Is it too early to assess its value in the future?
 
It is true that in comparison to 3D marine reflection seismic technology, also invented by ExxonMobil, marine CSEM is in its relative infancy. However, given our extensive survey experience, it is now possible for us to calculate the probable value-of-information (VOI) for a given survey in order to assess whether it makes business sense to proceed. We have published this approach in the technical literature.  

Do you believe that EM, one day, will become a commodity in exploration along with seismic?  

It is not our practice to speculate, but in the appropriate geologic settings - and also for development and production, at least offshore, our technology is promising.

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