Risk assessment is one of the crucial tasks to accurately account for estimating and booking technical recoverable volumes, as well as, valuating a petroleum asset. Usually these processes are affected by project maturity or non-technical issues, and usually embrace uncertainty due to lack of information and different criteria adopted by company and/or evaluators. It is believed that deterministic cash flow methods are suitable and accurate to assess a project portfolio in mature oilfields. However, even in the very well known fields with strict surveillance plans, there is still uncertainty associated to volumes of recoverable hydrocarbons, oil & gas prices, capital investment estimations, government-take share among others input variables of economic models. It is proposed a new approach for mature fields which includes a probabilistic model of reserves estimation (either applied for volumetric calculations or performance-data-based dynamic methods), an scenario matrix to account for the risk expressed in expectation curves, an estimation of the "pseudo-limiting" risk and value terms, and a decision tree to define a hierarchy of portfolio. Furthermore, it is proposed a methodology of life-cycle assessment and surveillance of reserves estimation by integrating of the "pseudo-limiting" risk and the ratio expected value to capital investment. The application of this methodology in a mature oilfield, the Peruvian Block IX, show that proposed infill drilling campaign would require a better royalty share for the company (40% additional to the current rate), while the implementation of a pilot Water flooding project would need an improve by 50% in current royalty share. Copyright 2014, Society of Petroleum Engineers.