Understanding the Resistivities Observed in Geothermal Systems

Abstract

The low resistivity geophysical anomalies observed over most geothermal systems have been very useful exploration targets. As better and deeper imaging of the resistivity structure of geothermal systems has become possible with the use of methods such as MT surveying, it has been shown that the lowest resistivity is usually in a zone above the reservoir and that the resistivity of the actual reservoir can be much higher.

To help understand this distribution of resistivity, we have reviewed the factors affecting resistivity in geothermal systems. Conductive clay products of hydrothermal alteration are the most common cause of low resistivity in a zone above the reservoir. Higher temperature alteration products are less conductive so that in systems of low to moderate salinity the reservoir has higher resistivity.

Associations between temperature and clay alteration assemblages have been well established and used as tools for predicting reservoir temperature during drilling. Inferred correlations between alteration type and resistivity can extend this further to enable better prediction of reservoir temperature distribution from surface geophysical measurements. This has the potential to greatly improve reservoir definition and the success rates for exploration drilling.