Experts may disagree on when world oil production will peak, but there is general agreement that marginal fields will contribute a greater percentage of world supply in the future. As fields mature and operations transition into the later stages of their production cycle, decreasing revenue streams, higher operating costs, and fewer upside opportunities lead to declining profitability Eventually, all properties are abandoned when cost exceeds the revenue of production In this two-part paper on marginal production in the Gulf of Mexico, the number of marginal structures in the gulf and then expected contribution to future production is forecast using established economic models. In Part 1. a historical perspective on producing assets is provided and the inventory of committed assets is modeled and categorized. We operationalize the definition of marginal production based on a structure's economic limit. The model framework to identify marginal assets in the Gulf of Mexico is discussed and a step-by-step description of the methodology is provided. In Part 2, the results of the model are described (C) 2010 Elsevier Ltd. All rights reserved