An extension of the novel fully coupled thermo-hydro-mechanical Open-System Geomechanics (OSG) model is developed for the analysis of coupled flow and oil-shale geomechanics. The model is cast within the framework of Biot's elasticity and classical thermoelasticity. A new term in the equations due to pyrolysis is included in the present model to capture the removal of mass from the reactive solid phase due to the kerogen conversion process modeled through a simplified chemical reaction model. The proposed novel formulation approach and its numerical implementation differs from traditional methods and offers a step improvement in the geomechanical modeling of thermally-reactive porous media, such as, oil shales. The numerical implementation of the OSG model, a first in the literature, is developed within the framework of a proprietary coupled thermal-reactive flow and geomechanics simulator, which was extensively validated in a previous publication. In this paper, we compare the thermal-reactive OSG model against experimental measurements. Numerical results from the validation test shows that the model captures the fundamental physical behavior of oil-shale geomechanics realistically and correctly. Parametric analyses of the OSG model indicate that the chemical conversion term is the critical term that dictates the magnitude of the compaction in the solid equation and a further investigation illustrates the importance of the mobility term in the pore pressure buildup. It is also noticed that the initiation and rate of compaction of the oil-shale sample are governed by the chemical activation energy and reaction-rate constant.