The Antrim Shale of Upper Devonian age produces along a belt across the northern part of the Michigan Basin. Although the Antrim Shale has produced gas since the 1940s, the play was not active until the late 1980s. Unlike other shale gas plays such as the Barnett Shale, the natural gas from the Antrim appears to be biogenic gas generated by the action of bacteria on the organic-rich rock.[42]