Wells drilled in the earth in the search for oil are logged with tools lowered into the hole to measure the natural and induced radioactivity of the rock formations. Studies of the nuclear properties of common earth elements should improve interpretation of existing radioactive logs and provide the basis for new, improved methods of logging.

This paper describes laboratory experiments on the interactions of neutrons and gamma rays with common earth elements, sedimentary rocks, and the fluids found in these rocks and in the bore hole. The results show that a number of common earth elements can be identified by the gamma rays produced when rocks are bombarded with high-energy neutrons. The density of formations, of borehole fluids in producing wells, and of cement behind steel pipe can be measured by the use of a collimated source of gamma rays and a scintillation-crystal detector.

These results, supported by preliminary field tests, show that new methods of radioactive logging of both old and new wells are useful for defining zones of oil, gas, and water saturation; for identifying various lithologic types; for shutting off water zones in the reworking of old wells and for recovering steel pipe when wells are abandoned.