Oil shales are organic-rich mudstones that generally have little porosity and permeability until kerogen is transformed into oil and gas. A simple mathematical model is reported for how porosity and permeability values for the Green River Formation change during retorting under confinement. Unlike when retorted unconstrained, during which numerous fractures occur due to the limited tensile strength of retorted oil shale and the permeability increases from micro or nano-Darcy levels to Darcy levels, fracture permeability is minor when constrained by lithostatic loads typical of in-situ retorting, so permeabilities increase only to the milli-Darcy level. The permeability increase is related to an increase in both porosity and pore diameter, and measured permeabilities are consistent with measurements and calculations of those properties and inter-relationships developed for naturally matured petroleum source rocks. (C) 2017 Elsevier Ltd. All rights reserved.