ALTHOUGH methane of bacterial origin is ubiquitous in marine and feshwater sediments, economic accumulations of bacterial gases occur mainly at depths of several kilometres in Tertiary basins that had high sedimentation rates(1’2). Here we present an integration of geochemical and isotopic data from gas and water extracted from the Upper Devonian Antrim shale, along the northern margin of the Michigan basin, which demonstrates that significant volumes of bacterial gas have been generated in organic-rich shales at depths of less than 600 metres. The Antrim shale is mainly a self-sourced reservoir, in contrast to conventional gas deposits that have migrated from a source to a reservoir, and has become one of the most actively exploited gas reservoirs(3) in the United States. The gas-forming processes operating at shallow depths in the Antrim shale are noe unique(4) and an understanding of these processes should lead to the identification acid development of other economic, non-conventional gas deposits around the world.