In a previous article the authors presented a mathematical model for the description of the rate of decomposition of kerogen and carbonate minerals in a large block of oil shale. Emphasis was placed on the effect of an internal temperature gradient as opposed to the uniform temperature model, which neglects it. This more comprehensive model involves the solution of partial differential equations in terms of the radial position and time. In an overall retorting model of an oil shale bed with pieces of a wide size distribution, such as the one developed by Braun and Chin of LLL a direct incorporation of this comprehensive model greatly increases the difficulty of computation despite the availability of a large computing system at LLL. In this communication we discuss an approximate solution that obviates the necessity for numerically solving the partial differential equation.