Applications of Natural Gamma Ray Spectrometry Tool for Petrophysical Analysis of Hydrocarbon Reservoir in Oil & Gas Industry

Abstract

Gamma ray tool used for petrophysical analysis in Oil & Gas industry is for natural radioactivity in rocks in subsurface. Potassium (40K), uranium (236U) and thorium (232Th) are three decay series of isotopes are abundant in rocks and are detected by Natural Gamma Ray Spectrometry (NGS) tool. Potassium, thorium and uranium spectra of Gamma ray is obtained by NGS tool with NaI crystal of photo multiplier tube. NGS tool has five windows for counting the gamma rays of low energy and high energy level. Tool has a master calibration in Clamart (France). Secondary calibrators are used to check the operation of the tool and its stability. Natural Gamma Ray Spectrometry measures the contribution of gamma ray of each individual series of Thorium, Uranium and Potassium separately. Also, it measures concentration of Thorium and Uranium in ppm and potassium in percentage in any rock. Applications of NGS are many folds. This is very good shale indicator. Also, it differentiates between shale and potassium salts (Evaporites) with help of cross plots of K% versus bulk density, neutron porosity and sonic transit time. From the ratio of Th/K obtained from NGS tool against rocks, it distinguishes feldspathic sandstone, micaceous sandstone or quartzite sandstone. Feldspar contain high potassium and low thorium as a result Th/K ratio is very low against Feldspathic sandstone whereas the ratio is high in Micaceous sandstone due to high thorium in mica and low potassium. NGS tool play an important role to find the origin of carbonate either in pure chemical origin or organic by measuring the uranium. Low uranium indicates it is of chemical origin whereas rich content of uranium in carbonate indicates it is reducing environment and of organic origin. NGS also shows Th/K ratio abruptly high in case of unconformities. It is possible to find the organic content from measurement of uranium and from that its hydrocarbon potential in source rock after proper calibration with core data.