Identifying the source of natural gamma-rays in shallow-marine siliciclastic strata and their significance for shale evaluation: A case study of the CO2 storage aquifer at the Nagaoka site, Japan

Abstract Gamma-ray (GR) logging measures naturally occurring gamma radiation in strata, and is a widely used method for reservoir characterization. The objectives of this study are to identify the source of total GR, and to discuss criteria for reasonable shale evaluation by the combined use of sediment core and GR logging analysis using a case study of the CO 2 storage aquifer at the Nagaoka site in Japan. The aquifer is comprised of shallow-marine siliciclastic deltaic deposits characterized by prodelta to delta front facies. The profile of total GR output is consistent with that of mud content, suggesting that the main source of GR can be attributed to minerals in the mud fraction. Prior to comparison of the radioactive elements measured by spectral gamma-ray (SGR) logging and elements from core analysis with total GR, elements were transformed to centered log-ratios to remove constant-sum constraints. Evaluation of the total GR profile with radioactive elements from SGR logging indicates that the distribution of thorium (Th) exhibits a high contrast between prodelta and delta front deposits. Th content has a significant positive correlation with total GR, implying that Th-bearing minerals mostly appear in fine-grained fraction. Conversely, the distribution of potassium (K) has a significant negative correlation with total GR, implying that K-bearing minerals appear in coarse-grained fraction. Based on geochemical analysis of the sediment core, minerals composed of Mg, Ti, Fe, Zr, and Th are likely to be radioactive carriers in the fine-grained fraction, and those containing Na, Al, Si, K, Rb, and Sr are considered to be the radioactive carriers in the coarse-grained fraction. The former can be attributed to detrital zircon grains and clay minerals, including smectite and chlorite, which contain or adsorb the radioactive Th, and the latter can be attributed to K-feldspar. The dispersion diagram between total GR and radioactive elements at observation wells emphasizes the differences in the endmembers of total GR between the two depositional sequences. The estimated shale volume is concordant with measured values if we select the endmembers at each depositional sequence. This means that the endmembers should be selected independently for each depositional sequence to enhance the reliability of shale evaluation. For improving shale evaluation, the combined use of GR logging and detailed core analysis in terms of sedimentology and geochemistry appears to be effective for reliably assessing shale volume in the target strata.