Project portfolio management is a comprehensive framework for decision making and selecting the portfolio of projects to achieve the goals of the organization by considering resource constraints. The importance of this issue in Iran's oil and gas industry is even more remarkable than ever due to its unique position in the country's economy, capital-intensive and capital budget constraints that have been intensified in recent years. Identifying and defining different scenarios for each oil and gas field, determining the parameters of the mathematical model, the required data to calculate the parameters of the model and the process and methods of identifying this data, indicate the distinction and necessity of this research. This study is an applied research in terms of objective, using mathematical modeling approach, has provided a pattern to determine the optimal portfolio of capital plans of oil and gas companies. The research method is case study which has studied one of the most important oil and gas producing companies in the country and the only offshore company. In this study, a framework for selecting the optimal portfolio of capital projects is determined and after gathering required data, the zero-one integer linear mathematical programming model with the objective function of maximizing the net present value from fields (as the strategic goal of company) by considering investment constraints was designed and solved by GAMS software. Finally, according to the defined constraint, the best investment mode for each field was identified and the optimal portfolio was defined.