Case Studies - Oil & Gas Exploration

GEOTEM/Aeromagnetic Definition of Hydrocarbon Alteration Plumes in Nevada

Introduction

An airborne electromagnetic and magnetic survey was flown over a number of oilfields in Nevada to map alteration plumes. In some of the areas, there is an anomalous magnetic response and an excellent correlation between an enhanced conductivity anomaly and the oilfields. The approach indicates that alteration plumes may be detected and defined with greater confidence if a multi-sensor system is employed rather than only a single sensor one such as magnetics.

In recent years, there has been an increased interest in identifying oil deposits by detecting changes in conductivity due to possible alteration plumes in the rocks above an oilfield. This enables techniques with penetration depths of a few hundred metres to detect anomalies associated with deeply buried oilfields, but requires that the alteration is near to surface and has resulted in anomalous conductivities being mapped.

In the case of magnetic methods, anomalous magnetic susceptibilities above an oil reservoir have been documented. The conditions required to obtain anomalous susceptibilities indicate that not all oilfields will show a magnetic alteration. The effect of man-made culture (wells, pipes) can also be great.

Tests of the radiometric method have indicated that the potassium and uranium count rates, when normalised by the thorium count rate, are sensitive to hydrocarbon microseepage alteration.

Electrical and IP measurements have also been tried with the strongest anomalies occurring where the near-surface rocks are porous and iron-rich. Again cultural responses from fences, pipes, well casings can be significant. Electrical methods are sensitive to conductivity, as are electromagnetic (EM) methods.

The advantage of airborne techniques is that large areas can be covered, increasing the possibility of traversing an oilfield with an alteration halo.

Fugro Airborne Surveys, has performed a number of airborne transient electromagnetic (GEOTEM) surveys over oilfields with known alteration haloes. The surveys were flown to test the hypothesis that airborne EM methods can detect alteration in cases when the conductivity of the altered material varies from the background.

The Airborne EM System

The airborne GEOTEM system is installed in a CASA-212 STOL aircraft and incorporates a large vertical-axis three-turn transmitter coil generating a dipole moment of approximately 450,000 Am². The sensor is a horizontal-axis ferrite core which is housed in a towed bird attached to a fully digital receiver (Fig. 1). The surveys were designed with flight lines oriented parallel to the larger dimension of the oilfield at 500 metre intervals and with tie lines planned at 3000 metre intervals. The areas were sufficiently large to sample the region surrounding the oilfield. 

Figure 1: Magnetometer sensor (L) and GEOTEM sensor.

Field Example- Railroad Valley, Nevada