This study presents the results of the reprocessing and geological reinterpretation of a trio of regional seismic reflection lines acquired by the Ministère des Ressources naturelles du Québec in 1978. The three profiles (M-2001, M-2002 and M-2003), which represent 280 km of data, cross the Chambly-Fortierville syncline and provide images of the St. Lawrence Platform and the Appalachian foreland thrust belt of southern Quebec. Profile M-2001, the longest profile, extends across the full width of the southern Quebec Appalachians. Post-stack seismic data processing has improved the quality of the original seismic sections by enhancing coherent reflection events and attenuating reflection noise. Time migration further improved the seismic data. The seismic interpretations provide new information on subsurface geology, including the recognition of complex structural patterns in platform and foreland units, the presence of a triangle zone at the structural front, and the geometry of thrust slices of platform units and Appalachian thrust nappes. The M-2001 line also provides images of the internal Humber Zone, including back-thrust faults on the flanks of Notre Dame Mountains Anticlinorium and mega thrust wedges, possibly involving Grenvillian basement, within the core of the anticlinorium. In southern Quebec, six conventional petroleum plays, plus shale gas potential, are recognized in the St. Lawrence Platform and Appalachian Foreland. From these, the Saint-Flavien field (161 million m3 of past natural gas production) and other potential targets are imaged by the three seismic profiles studied. The reprocessing and reinterpretation of these seismic reflection profiles provide new insights into the structure, geological evolution and petroleum potential of the St. Lawrence Platform and the Appalachian foreland thrust belt of southern Quebec.