Spectral gamma-ray logging of the Grès d'Annot, SE France: An outcrop analogue to geophysical facies mapping and well-log correlation of sand-rich turbidite reservoirs

Abstract

Spectral gamma-ray (GRS) logging is a powerful tool in cyclo- and sequence stratigraphy of carbonate depositional systems. In siliciclastic systems with low chemical maturity, the multi-component character of sediment complicates the interpretation of such logs. This work focuses on understand better the relationships between GRS logs and siliciclastic facies stacking patterns, using GRS measurements combined with lithology, modal composition and geochemistry. This study is focused on outcrops in the St. Antonin–Annot–Grand Coyer–Chalufy areas of the Oligocene Grès d´Annot Formation (SE France).The studied siliciclastics reveal moderately high total radioactivity and mean concentrations of K: 3.8 %, U: 4.5 ppm and Th: 13 ppm. U and Th are predominantly contained within heavy minerals and so tend to be concentrated in mudstones and heterolithic sand-mud facies whereas sandstone and conglomerate facies have slightly higher levels of K due to higher contents of K-feldspars and micas. The abundance of minerals containing radioactive elements and the relatively low compositional contrast between different facies are considered the main reasons why the standard gamma-ray measurements provide only a limited proxy for lithology in this case.Two complete regressive-to-transgressive cycles were identified based on combined Th and Th/K logs and facies stacking patterns. The cycle boundaries are marked by shifts towards coarse-grained facies, abrupt increase of U and Th concentrations and Th/K ratio within the proximal deposits at the St. Antonin section. In the distal Chalufy section, the response of the gamma-ray proxies to facies shifts is reversed. These boundaries, interpreted as basal surfaces of forced regression, can be correlated with prominent erosional surfaces at the Annot section. GRS logging is a sensitive method capable of indicating changes of genetic depositional units in sand-rich turbidite systems. However, detailed facies analysis, mineralogy and geochemistry producing the radioactive signal are necessary to correctly interpret such logs.