Gamma-ray spectrometry as a tool for stratigraphic correlations in the carbonate-dominated, organic-rich, pelagic Albian sediments in Central Italy

Portable gamma-ray spectrometry provides a valuable method for detecting li thological variations and establishing correlations in monotonous sedimenta ry sequences. It has been applied to the pelagic, organic-rich and carbonat e-dominated "Marne a Fucoidi" Formation in the Marche-Umbria Basin. The rel ationship between the natural radioactivity parameters (potassium, thorium and uranium) and the carbonate and organic contents has been analysed in a reference section. The potassium and thorium signals appear as very good in dicators of the clay content, whereas the uranium signal is not as straight forward. The latter is influenced by the content in organic matter, the thi ckness of black shales and the type of rocks alternating with black shales (marls or carbonates). In marl-dominated rocks, the uranium curve is a good indicator of the organic-rich zones, whereas, in carbonate-dominated sedim ents, the signal is more subtle and zones with a higher organic content do not always appear as good log markers. The total gamma-ray signal is strong ly influenced by the potassium and amplifies the effects of the clay conten t with respect to the organic matter. The validity of field gamma-ray investigations as a lithological and intra- basin correlation tool is confirmed by the excellent match between the tota l gamma-ray curve of the reference section, that from another field section and geophysical logs from a petroleum well. The precise correlation of fie ld gamma-ray data with subsurface logs permits results from field studies ( e.g., biostratigraphic boundaries) to be directly plotted onto the well dat a.