Comparative analysis of bulk and velocity-derived density data in deep wells in the Adriatic Region (Italy)

Abstract

This work is based on a systematic comparison between two datasets related to density data of geological formations crossed in 13 deep wells. The wells are located in the Adriatic area and reach depths of over 5000 meters. The main lithologies involved include sandstones, marls, clays, evaporite, and carbonate rocks. The first dataset concerns density values obtained from the analysis of sonic logs recorded along the wells, by applying the Gardner relation. The second dataset, on the other hand, refers to actually acquired density log measurements. Differences among the values in the two datasets have been calculated. The aim behind this work is to assess the reliability of rock densities estimated using the Gardner equation by comparing them to measurements obtained through density logs, despite many factors influencing the log density. This comparison of the densities obtained from various lithologies and geological formations leads us to draw, in this region, some initial considerations regarding the applicability and accuracy of Gardner formula, usually considered as standard reference, since the density log is generally available only within the reservoir. In the area we analyzed it is observed that the density values estimated from sonic velocities are underestimated for the Plio-Quaternary formations characterized by clayey-sandy lithologies by at least 0.1 g/cm3; whereas, densities of the carbonate sequences are overestimated by the same extent. Noteworthy, the density estimates deviate from the real values especially for gypsum units, overestimating by a factor of approximately 0.3 g/cm3. The results we obtained emphasize differences in density values when using the Gardner formula and suggest the need for taking into account the possible errors in the specific geological context or instead lithologies, such as those explored in this study.