This study presents a probabilistic methodology for estimating oil and gas reserves. The methodology takes into account the uncertainties of important variables present in the economic evaluation of reserves, such as oil and gas production, prices, and operational costs. In addition, this research seeks to understand the impact of oil price variations on the cost behavior in the long term. The dependence relationship between price and cost is also considered in the proposed reserves estimation methodology. A system has been designed in order to apply the methodology, allowing the verification and analysis of the results in a case study with real data from an oil field. The results show the importance of evaluating and quantifying the uncertainties in the economic assessment of the reserves, as well as the need to consider the effect of oil price changes in operational costs. This research proposes, therefore, an alternative to estimating reserves, according to a probabilistic approach, which can significantly impact the volume of oil and gas economically recoverable to be disclosed by an oil company.