This log records the radioactivity of a formation. Shales (or clay-minerals) commonly have a relatively high gamma radioactive response, and consequently gamma ray logs are taken as good measures for grain size (and subsequently inferred depositional energy). Thus coarse-grain sand, which contains little mud, will have low gamma ray value, while a fine mud will have a high gamma ray value. The values range of gamma ray is measured in API (American Petroleum Institute) units and range from very few units (in anhydrite) to over 200 API units in shales.
Gamma ray logs are one of the most commonly used logs for sequence stratigraphic.
A good source of further information on well logs, one that this web site has used, is Malcolm Rider, 1996, "The geological interpretation of well logs", 2nd edition, Gulf Publishing Company, Houston p. 280 ISBN 0-88415-354-1.
