The oil industry strives to create an international standard for classification and estimation of resources since the 1930s. The goal is to provide investors with information obtained under the same assumptions, as to facilitate the comparison between petroleum companies. In 2007 the four major international organizations, Society of Petroleum Engineers (SPE), American Association of Petroleum Geologists (AAPG), World Petroleum Council (WPC) and Society of Petroleum Evaluation Engineers (SPEE), jointly released a single set of guidelines for classification and evaluation of oil and gas resources, the Petroleum Resources Management System (PRMS, 2007).

Several methodologies for estimating reserves can be employed within the PRMS’s (2007) premises, which can be classified as deterministic or probabilistic. Unconventional resources emerge as a new frontier for the oil industry, thus implying high uncertainty levels in both technical and economic assessments. The main purpose of this paper is to explore this issue and to propose a correlation-based probabilistic methodology for aggregating oil and gas reserves of conventional and unconventional resources. The methodology is in accordance with the guidelines of the PRMS (2007) and with the new rules of the Securities Exchange Commission 2009 (SEC). The correlation assessment evaluates technical and operational features, and the probabilistic aggregation is performed by Monte Carlo Simulation (MCS).

Besides the introductory section, this paper comprises four other sections. A literature review presents definitions of conventional and unconventional resources, and an examination of classification, estimation and aggregation of reserves, important for better understanding the following sections. The third section describes the proposed correlation-based probabilistic methodology. Afterward, a case study presents an application of the methodology. Finally, the last section synthesizes the main conclusions.

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