The branching of fractures in shale formations surrounding a wellbore can be enhanced so that more rock surface is exposed in the formation and more hydrocarbon resources can be recovered using smaller quantities of fracturing fluids. The branching of the fractures can be enhanced by establishing a substantially static sub-threshold fluid pressure in a well in a geological formation, and generating a pressure pulse in the well such that the pressure pulse combined with the substantially static sub-threshold fluid pressure forms a plurality of fractures in the geological formation. An arc jet insertable into a passage in the geological formation and having an electric arc channel can be used to generate the pressure pulse.