As conventional gas (CG) resources are depleted, unconventional gas (UG) resources (gas from tight sands, coalbeds, and shale) are becoming increasingly important to US and world energy supply. The volume of UG resources is generally unknown in most basins outside North America. However, in many mature North American basins, UG resources have been produced for decades, and resources and reserves are well characterized. The objective of this work was to determine the quantitative relations between known conventional and unconventional hydrocarbon resources in mature North American basins, with the ultimate goal of using these relations to estimate UG resources in frontier basins outside North America.
We used assessments by the US Geological Survey (USGS), Potential Gas Committee (PGC), Energy Information Administration (EIA), National Petroleum Council (NPC), and Gas Technology Institute (GTI) to evaluate relations among hydrocarbon-resource types in the Appalachian, Black Warrior, Greater Green River, Illinois, San Juan, Uinta-Piceance, and Wind River basins. We chose these seven basins for initial analysis of relations between CG and UG resources because they are mature basins for both conventional and unconventional oil and gas production. To conduct this analysis, we wrote a computer program that we call Petroleum Resources Investigation Summary and Evaluation (PRISE). Input data for PRISE, obtained from published data sources and resource assessments, were values of technically recoverable resources, consisting of the following resource categories: (1) cumulative production, (2) proved reserves, (3) probable and possible reserves and contingent resources (PPC), and (4) prospective resources. We then analyzed these data in each of the seven basins to assess the relationship between conventional- and unconventional-resource volumes.
For the seven basins studied, we found that approximately 10-20% of the total recoverable hydrocarbon resources are conventional oil (CO) and CG, whereas 80-90% of the recoverable hydrocarbons are UG resources. We suggest that the results of this study may be used to estimate recoverable resources from UG reservoirs in frontier basins worldwide, where CO and CG resources are known but UG resources have not been evaluated.