Texaco Inc.

Richard A Deans

Alfred Brown

Joseph H Frantz

A potassium salt which is soluble either in water or in an oil-based drilling mud is forced via filtrate invasion into permeable subsurface formation zones adjacent a well borehole via an over-balanced drilling mud or completion system. Well logging is then performed in an interval of interest in the formations using natural gamma ray spectrometry. Gamma radiation at an energy of 1.46 MeV from the naturally occurring radioactive potassium isotope .sup.40 K is detected. Gamma radiation levels of the isotope .sup.40 K above normal background readings indicate the presence of permeable formations.

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Stamford, CT, United States of America

Richard A Deans