One of the most critical stages after a field discovery is to define the Field Development Plan (FDP) for the new field; usually with limited information, managers must define the marketability of a prospect by taking as input the Hydrocarbon In Place (HIP), Hydrocarbons Rate of Production (HRP), Number of Wells (NW) and Drainage Location (DL), Plateau, Well Completion, Surfaces Facilities (SF) including pipelines and separation/compression systems; all these elements have to be economically evaluated and finally a decision must be taken based on most probable production scenario and economic optimization. In the last five years the importance of hydrocarbons gas has increased due to the increase in oil prices, the limited number of new big oil fields discoveries, the environmental contamination due to oil combustion and the development of new technologies to use gas as a competitive energy source. In this paper we implement a methodology to build an FDP for a gas field, Reggane, located in the South-West of Algeria. Reggane concession is a structural trend including Azrafil, Reggane, Sali, Tiouliline and Tif-Taf fields. Some of these fields have been tested in Devonian, Ordovician and Carboniferous formations with mixed results. Here we will focus on Devonian formation, Azrafil and Reggane fields, which has been tested in 5 wells and shown remarkable results. The methodology uses an Integrated Asset Management (IAM) approach, linking geological with reservoir information to model hydrocarbon flow going from the reservoir to wells, wellbore, through the pipeline network and the compressor system up to the point of delivery based on defined HIP, HRP and plateau; uncertainties associated with these parameters were identified and ranked to be included in the FDP. As a conclusion, the FDP shows that around 30 wells must be drilled during the life cycle with a plateau of about 8.5 MM3/D of production gas for more than 10 years. Introduction During the life cycle of a Hydrocarbon Asset, when going from the Exploration Phase to the Development Phase, there is a critical period where important decisions for going or not ahead must be taken: The Appraisal phase. This paper is related to the difficulties usually found at that stage. The available set of initial data and worked out figures, does not normally correspond from a qualitative and quantitative point of view, with our wishes of a very solid background with no uncertainties that will allow us to take solid decisions and so acquiring strong commitments for the future development. This situation becomes worse in those cases where commitment for development, meaning investment, should be taken prior to what is commonly named Basic Engineer or FEED period.