Development options used for conventional oilfields have sometimes erroneously been extended to marginal oilfields and this affects the decision process. Objective of study is to provide a perspective on portfolio diversification and justification using key economic parameters such as Net Present Value (NPV), Internal Rate of Return (IRR), Productivity Index (PI), Profit-To-Investment Ratio (PIR) and Payback Period (PBP) in order to encourage investment and resource development on marginal oilfields in Nigeria and other world marginal oilfields. Economic analysis was carried out using both deterministic and Probabilistic approach incorporated by using Monte Carlo Simulation through the use of Crystal Ball Software. Key uncertainties such as fluctuation in oil price, inflation rate and tax rate were represented and their impacts on economic viability defined. The study based on economic evaluation recommended Nigeria National Assembly to pass into law the Petroleum Industry Bill (PIB) fiscal regime and now passed into law in May 2017. This monograph will be beneficial for academic study for universities and higher education students including marginal oilfields operators.