The volcanic complex of Phlegrean Fields, located northwest of Naples (Italy), has been the site of deep geothermal exploration in the 1980s. Several wells were drilled by an Agip-Enel joint venture, with downhole continuous physical properties acquired in each well. The main purpose of this study is to map and describe the spatial variations of the volcanic deposits, in terms of their nature, thickness and in situ physical properties. We apply fuzzy k means classification of the well-logging data available in the wells cored in the San Vito area. Fuzzy set theory provides an approach that quantitatively assigns individuals to physically continuous classes. The optimal number of classes is found by minimization of three mathematical functions, thus reducing subjectivity. The resulting classifications are found (1) to reflect the main lithologies in the San Vito plain, (2) to provide more detail on the geological stratigraphy, allowing a more precise volcanic history, and (3) to differentiate between transition zones and interbedded well-defined deposits. In addition to discriminating between lithologies that have different physical properties, this study gives information on the degree of homogeneity of each lithologic unit, and the range of variation for the measured properties. Finally, using the physical classification of the deposits, we are able to propose a detailed two-dimensional compressional acoustic velocity structure of the studied area.