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As petroleum exploration enters a period of searching for increasingly subtle traps, a number of exploration companies have begun to reconsider electrical techniques as a supplement to ongoing seismic programs. Many domestic companies have either established in-house electrical research groups or have utilized the services of various electrical contractors. This renewed interest in electrical techniques warrants review of the major approaches used today in exploration.
attempts the direct detection of oil by virtue of its intrinsically high resistivity. Although success has been reported for several very shallow fields, strong theoretical arguments indicate that direct detection is untenable as a standard exploration technique. A second approach utilizes magnetotelluric or similar techniques to map deep, very large-scale structures, but high cost and poor resolution confine this work primarily to areas where seismic is unobtainable. A third approach is mapping electrochemical alteration patterns in the top 1,000 m (3,000 ft) of sedimentary overburden. These patterns are attributed to clay-mineral alteration or sulfide precipitation resulting from upward seepage of light hydrocarbons and brines from the traps below. Recent advances in instrumentation and data processing have made alteration mapping a promising supplement to ongoing seismic and geologic exploration programs, especially in the search for subtle stratigraphic traps, and in distinguishing productive from nonproductive structures.
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