Abstract Source rock shales have permeability in the nano-Darcy range and require drilling long horizontal wells and extensive hydraulic fracturing to commercially produce hydrocarbons. Recently, an electromagnetic (EM) thermal method was proposed as a primary, or secondary, stimulation method to produce light hydrocarbons from extremely tight reservoirs. This method was based on the fact that EM heating rapidly elevates the pore-water pressure in tight rocks to fracture the reservoir. Since only the in-situ pore-water is needed, this is considered to be a water-free stimulation method, which is especially desirable for regions with limited water resources. Herein this further study provides an in-depth and quantitative investigation of the effects of laboratory EM stimulation on the permeability of the source rock shales under hydrostatic confinement and its dependence on the rock water content. In order to maintain hydrostatic pressure, a special pressure cell was built using nearly microwave transparent materials. Permeability of the samples before and after EM stimulation was acquired using a novel method capable of simultaneously measuring both matrix and fracture permeability. Microwave heating was found to generate extensive fractures in the source rock samples with a moderate amount of pore-water, which resulted in an increase of more than three orders of magnitude in permeability. This study provides further comprehensive evidence suggesting that EM thermal stimulation can be an efficient production method for source rock reservoirs and can result in both a high recovery rate and large estimated ultimate recovery of hydrocarbons.