It is well established that geology, geomechanics, and completion play an important role in gas productivity from shale formations. In particular, overpressured shale — along with higher fracture intensity — generally correlates with higher production rates. Conventional pore-pressure prediction techniques do not work well for black shale formations, primarily because a basin has often experienced multiple episodes of tectonic activity and subsequent overpressure generation; secondly, because the recorded petrophysical logs for pore-pressure estimation are significantly affected by the presence of organic content and gas. Overpressure mechanisms and distribution are studied for the Middle Devonian Marcellus Shale Formation of the Appalachian Basin in Pennsylvania to understand their relationship with well productivity. A sonic velocity-density crossplot does not reveal a clear picture of overpressure generation, primarily due to complex geologic history and distinct compositional variation between Marcellus Shale and overburden shale. 1D pore pressure was estimated from sonic velocity using traditional methods with pressure reference trend for the Marcellus Shale interval in the drilled wells and calibrated to the available in situ pressure measurements. 3D pore pressure was then estimated with upscaled 1D pore-pressure profiles and 3D seismic velocity data. Results suggest that the area having higher overpressure of about 160 psi produces about 55% more gas than the lower overpressure area. Further, understanding the spatial variation of overpressure in the study area may facilitate better field development planning.