The paper presents a risk management tool that assesses the impact that potential future carbon taxes will have on a company's hydrocarbon Reserves base and associated cashflows. Based on a number of case studies, this paper will present a practical application of the open-source engineering-based model called Oil Production Greenhouse Gas Emissions Estimator (OPGEE), developed by Stanford University. The paper will demonstrate the application of the OPGEE model in the assessment of a range of carbon taxes, how they may vary the economic limit of a field's Reserves, and how this may influence a company's future field development decisions. This tool becomes useful in the risk management of the portfolio planning and capital allocation process where carbon tax risk can be objectively assessed and tested. If utilised correctly, the model can help to future-proof a company's hydrocarbon assets in an increasingly carbon constrained world. Asset owners or potential asset acquirers can assess the materiality of potential carbon tax impositions on assets and can prepare and adjust portfolios accordingly on an informed basis.