Geothermal application of spectral gamma ray logging in the South Kansas Subsurface, USA

Abstract

Spectral gamma ray (SGR) is a logging tool that allows computation of the radioactive heat produced through radioactive decay of uranium, thorium and potassium within rocks. Radioactive heat-producing rocks are common targets for geothermal exploration. Gamma rays have longer half-length life in comparison to other radiations generated during radioactive decay. This characteristic allows the gamma rays to permeate far enough through the media to be measured by a logging tool. Radioactive heat production in this study was computed using spectral gamma ray and density logs. High-temperature geothermal reservoirs could provide heat that could be used to produce electricity from steam turbines. To provide domestic hot water, space heating, or process hot water, both low- and high-temperature geothermal systems could be used. This article is a case study illustrating subsurface radioactive heat production (A) calculated using well log data recorded in 48 wells located at 16 counties in the South Kansas State, USA. The results show that radioactive heat production (A) varies from approximately 0.1 to more than 10 μW/m3, whereas the average arithmetic mean is 1.15 μW/m3 and the average standard deviation is 0.57 μW/m3, which varies with lithology. The high measurements of radioactive heat production (A) in the studied rocks are predominantly related to the respective increase in uranium content in both shale and carbonate formations that could be future geothermal targets.