A process is disclosed for stimulating the activity of microbial consortia in a subterranean formation to convert hydrocarbons to methane, which can be produced. An n-type clad layer, a first guide layer, an active layer, a second guide layer, a p-type clad layer, a first semiconductor layer, and a second semiconductor layer of ZnSe are successively grown on an n-type substrate. Said inner gear ring is composed of a number of teeth and a pair of tapered discs which are coaxially with said gear ring and the teeth are inter-connected to form a curled rack.