Regressive expansion analysis and chemical modelling are conducted to determine the effect of concentrated pyritic zones within the Marcellus Shale Formation. Although the expansive effects of microscopic pyrite are known, concentrated pyritic zones within these carbonaceous mudstones are studied to determine their potential to expand and damage civil infrastructure. Methods of characterizing the expansion potential of pyritic mudstone are scrutinized in an effort to advance the current state-of-practice for geotechnical and geological investigations. Observational data and modelling reveal that oxidation of the concentrated pyritic zones produces microenvironments within the mudstone matrix that lead to the production of significant quantities of heave-inducing hydrous sulphates such as gypsum. These data signify a need to supplement traditional forms of sulphur testing with detailed studies of pyrite morphology, deposition and oxidation availability within fracture zones as a means of identifying swell potential. Additional laboratory testing, such as modified kinetic oxidation models, is required to further quantify the expansive effects of the concentrated pyritic zones.