Devonian oil shales have produced gas since the mid-1800s and include some of the longest life reservoirs in existence. They are geographically widespread and of low geological risk. Devonian shales produce gas from the Appalachian Basin, the Michigan Basin, the Fort Worth Basin, the Illinois Basin, and the Anadarko Basin. Reservoirs developed in Devonian shales have a num-ber of features in common. Fracture porosity is associat-ed with high permeability and dominates reservoir flow capacity.
Fracture porosity is the focus of most explo-ration efforts.
Natural high-angle fractures have been observed in nearly all cores. There is a short term production kick, followed by a long, slow decline. In contrast, matrix porosity in Devonian shales is asso-ciated with very low permeability. Porosity is present only as micropores associated with microfractures. Decline curves typically include an initial steep region as fractures produce and a long low decline as the matrix desorbs gas.