Synthetic electric sounding surveys over known oil fields

Abstract

The possibility of using various types of electrical methods to locate oil or gas fields has been proposed in recent years. In an effort to quantify the anomaly to be expected with electrical sounding methods, average geoelectrical parameters have been determined by studying electrical well logs from several oil fields characterized by different geoelectrical sections. The dc resistivity anomaly due to the presence of an oil-bearing layer at depth depends upon the sequence of resistivities above and below and the electrode array employed. The radial dipole array gives the largest anomaly values, and is followed by other arrays such as the Schlumberger and Wenner arrays. The maximum anomaly in apparent resistivity is observed when the resistivity beneath the target zone is lower than that above; the relative anomaly in apparent resistivity is almost the same as the contrast ratio of the transverse resistance of the oil-bearing layer to the overlying beds. When the radial dipole array is used, a limited areal extent of the oil-bearing layer does not cause a significant change from the anomaly value due to a layer of infinite lateral extent. In that case the least dimension is about four times the depth. Use of one buried current electrode in the vicinity of the oil-bearing layer increases the amplitude of the anomaly; the maximum anomaly appears at a separation comparable to the depth. Typical anomalies in apparent resistivity caused by these oil fields range from less than 0.1 per cent to more than 10 per cent. Such anomalies would be detectable only with an order of magnitude improvement in the capabilities of electrical sounding methods, or with considerably larger oil field targets.