Geophysical Well Logging

Abstract

INTRODUCTION Uses of Well Logging Well logging involves measuring the physical properties of surrounding rocks with a sensor located in a borehole. The record of the measurements as a function of depth is called a well log . Geophysical well logging has become a standard operation in petroleum exploration. Identification of geological formations and formation fluids, correlation between holes, and evaluation of the productive capabilities of reservoir formations are usually the principal objectives. Except for the natural γ-ray log, which is used routinely in uranium exploration, well logging is still not used extensively in the search for metallic minerals. Some argue that the complete recovery of cores in diamond drilling eliminates the need for logging holes because the information is laid out in the core box. It is unfortunate that this attitude still prevails because well logging is cheap compared to drilling and would be valuable in correlation and identification of mineral-associated anomalies, particularly when core is lost or difficult to identify. Geophysical well-logging methods include mechanical methods, passive and a number of active electrical methods (including self-potential, resistivity, induction, induced polarization), several nuclear methods (natural γ-ray detection and observations from induced nuclear reactions), acoustic logging, and measurement of magnetic and thermal properties. The emphasis in what follows will be on logging for petroleum (Pickett, 1970) because this is the major application.