New developments in conventional hydrocarbon exploration with electromagnetic methods

Over recent decades, electromagnetic methods have beenviewed with considerable suspicion by many in the hydro-carbon exploration industry. While highly effective inmineral and environmental geophysics, electro m a g n e t i cmethods have played a minor role in hydrocarbon explo-ration. Since electromagnetic (EM) methods use signals thatdiffuse in the Earth, they cannot provide the same verticalresolution as modern seismic exploration. However, in thelast decade, electromagnetic methods in general, and magne-totellurics (MT) specifically, have become more widely usedin hydrocarbon exploration. This change is clearly not due toany change in the underlying physics but is due to:(a)The significant improvements that have taken place inmagnetotelluric data collection, processing and interpre-tation.(b)The application of MT in settings where other explorationmethods (seismic, gravity, magnetic) encounter problems,are cost prohibitive, or yield ambiguous results.(c)The realization that MT data can provide complementaryinformation to that derived from seismic exploration. Forexample, the diffusive signal propagation used in MT canbe an advantage in a region of intense fracturing. Whileseismic signals will be scattered, the MT signals diffuseand give a reliable estimate of bulk properties such asporosity. Studies of the shallow structure of the SanAndreas Fault illustrate the ability of MT to image rockvolume properties (Unsworth et al., 2004).Just as in seismic exploration, electromagnetic geophysics can contribute to effective hydrocarbon explorationin twodistinct ways. Most often, EM methods are used to imagestructures that could host potential reservoirs and/or sourcerocks. In certain cases, they may also give evidence for directindication of the presence of hydrocarbons.In this article, the focus is on the magnetotelluric method asbeing representative of the developments in EM techniquesin general. Areview of the MT method is presented, recentdevelopments are highlighted, and typical applications arediscussed. Active source EM methods have seen a similaradvance in technology and they have been applied to explo-ration for shallow gas and oil sand, or in a deep water setting.Table 1 summarizes the most common EM methods used in oil and gas exploration and outlines typical depths of investigation.