Positron annihilation spectroscopy of sandstone and carbonate rocks

Abstract

The techniques of positron annihilation spectroscopy have been applied to the study of well-characterized reservoir rock samples. In this article we focus primarily on sandstone and carbonate samples. We report (a) measurements of the Doppler broadening (DB) parameters and mean lifetime values (〈τ〉) of dry samples; (b) measurement of the DB parameters as a function of temperature from room temperature to 200 °C; and (c) observation of the outgassing of heated samples simultaneously with the DB measurements. We interpret the measurements in terms of positron annihilation with both valence and core electrons and discuss how the DB parameters depend on the major chemical constituents (lithology) and to a lesser degree on the structure of the samples. The major result of this work is that the positron parameters such DB parameters (S, SW, and W) and mean lifetime values are shown to be reproducible and reliable bulk properties of well-characterized rock samples and can be used to categorize rocks of geophysical interest in the same way as the more familiar bulk properties such as density, porosity, resistivity, etc. These results provide the first set of data for and a physical description of some of the processes involved in the interaction of positrons with rocks. This research provides a basis for further laboratory studies and is necessary for the eventual development of a new nuclear well-logging tool based on Doppler broadening of the positron annihilation gamma ray.