Exploration and development of a trolley field face some uncertainties linked to yields and necks throughout the lifecycle of the projet. By way of example, it is often uncertain what is needed, investments, industrial necks, production and price. At the same time, it is difficult to see cash flows for the simplest projects. Different policies are available to help them to assess uncertainties, and to reduce the risk of investment opportunities upstream. The simulation has been recognised as a tool for risk analysis and decision-making in the evaluation of upstream investment projects linked to the objectives of the projects 60. Today, all major companies have incorporated the simulation into their project studies. In this article, we try to explain the statements of assessment and risk analysis of upstream investment projects based on cash flows and to show how Monte Carlo’s simulation can be useful in this context.