Abstract: Background: The application of gamma ray spectroscopy on the classification for the sedimentary rocks is helpful to solve the problem of small classification and the long cycle for chemical analysis of the sedimentary rocks. Purpose: This study aims to introduce a method of determining the content of natural radionuclides K, U, Th in the well drilling debris with the low background gamma ray spectroscopy for dipartition among dark gray ash mudstone, dark grey calcareous mudstone and grey calcareous mudstone. Methods: Count rates of K, U, Th in the energy ranges of 1.37–1.56 MeV, 1.66–1.86 MeV and 2.41–2.81 MeV were selected to represent the intensity of radioactivity of K, U, Th. The fisher discriminant criterion was applied to measured count rates of various samples for classification of sedimentary rocks using the Statistical Product and Service Solutions (SPSS) software. Results: According to the count rate of K, U, Th and total count rate of the three mudstones carry out the discriminant analysis of rock lithology by using SPSS software, the accuracy rate reached 100%, 76.47% and 60%, respectively. Conclusion: Gamma ray spectrometry method is of great significance for the division of sedimentary rocks in small class. It could be used to completely identify the dark gray mudstone and the identification effect for dark grey calcareous mudstone, and grey calcareous mudstone is good, but not distinguish them completely.

Key words: Gamma ray spectrometer, Analysis of cores, Discriminant analysis