- GeoRef, Copyright 2005, American Geological Institute. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
A great deal of interest has been expressed in Western countries during recent years on possible applications of surface electrical methods in oil and gas exploration. It has been reported that these methods are widely used in the Soviet Union, but to date few technical details have been available. In late 1979, the author visited the Soviet Union under the auspices of the Australia-USSR Agreement on Scientific Cooperation, with the purpose of studying recent developments in electrical and electromagnetic (EM) methods. This paper presents a summary of the use and applications of those methods in oil and gas exploration, and includes a number of case histories.The methods can be broadly classified as follows: sounding for structural mapping, sounding to measure the geoelectric properties of the oil-bearing horizon, and indirect methods which detect the presence of a geochemical plume or halo above the oil deposit. The magnetotelluric (MT) method and transient sounding in the near zone (ZSBZ) are widely used for deep sounding, especially in areas where seismic methods are ineffective. Modern ZSBZ equipment can measure over 5 1/2 decades of time, from 0.1 msec to 44.8 sec. The transmitter consists of a large (1000 m) square loop carrying a 40 A rectangular waveform, and the receiver is a multiturn coil located at the center. Depths of exploration depend upon geologic conditions, but they are typically of the order of 3 to 4 km.Statistical data, based on logging, of the geoelectric properties of a large number of oil fields have also been collected. There is generally an increase in resistivity in both the oil-bearing horizon and overlying layers. The resistivity of near-surface layers appears to be partially dependent upon climatic conditions.Experimental induced-polarization (IP) surveys have been carried out over oil and gas deposits for about ten years. Both time-domain and frequency-domain systems are used, and sometimes an orthogonal electric array is employed to reduce inductive coupling. Either low frequencies (less than 1 Hz) or late times (greater than several seconds) are measured. IP anomalies are thought to be caused by epigenetic pyritization located in a plume over oil fields in a geochemically active environment. The anomalies are either of the form of a wide anomaly centered over the top of the deposit, or the shape of a halo, surrounding the deposit. There is evidence that the behavior of the IP decay at very late times (several minutes) can be used to distinguish IP anomalies inside and outside the boundaries of a hydrocarbon deposit.