As conventional gas resources are depleted, unconventional gas (UG) resources (gas from tight sands, coal beds, and shale) are becoming increasingly important to U.S. and world energy supply. The volume of UG resources is generally unknown in most basins outside North America. However, in 25 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 evaluate recoverable UG resources and determine the quantitative relations between known conventional and unconventional hydrocarbon resources in mature North American basins, with the goal of using these relations to estimate unconventional hydrocarbon resources in basin outside North America, which we call frontier basins.
We used data from the U.S. Geological Survey, Potential Gas Committee, Energy Information Administration, National Petroleum Council, and Gas Technology Institute 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 preliminary analysis of relations between conventional and unconventional gas 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 PRISE (Petroleum Resources Investigation Summary and Evaluation). Input data for PRISE, obtained from the published data sources, were the values of technically recoverable resources, which were the sum of the following resource categories: cumulative production, proved reserves, growth, and undiscovered recoverable resources. We then compared technically recoverable conventional and unconventional resources for each basin to evaluate relationship between conventional and unconventional resource volumes in these seven basins.
For the seven basins studied, we found that 10% of the total recoverable hydrocarbon resources are conventional oil and gas, whereas 90% of the recoverable hydrocarbons are unconventional resources. We propose that the results of this study may be used to estimate recoverable resources from unconventional gas reservoirs in target (frontier) basins, where conventional oil and gas resources are known but unconventional resources have not been evaluated.
World energy supply must continue to grow to satisfy increasing energy demands. In North America, unconventional resources already play a major role in fulfilling national energy needs, but internationally, unconventional resources are not major contributors to energy supply. International conventional resources are widely explored, but with growing demand, international unconventional resources must also be explored and developed. We define unconventional hydrocarbon resources as those oil and gas accumulations that, owing to their special reservoir rock properties (i.e., low matrix permeability, presence of natural fractures), charge (adsorbed gas in self-sourced reservoirs, methane clathrates), and/or fluids characteristics (high viscosity), are economically exploitable only by using advanced technologies, massive stimulation treatments, and/or special recovery processes (Singh 2006, Singh 2007). Unconventional resources include tight gas sands, coalbed methane, shale gas, gas hydrates, and heavy oil (Singh 2006, Singh 2007).
The database used for PRISE was compiled for a broader study in which software called BASIN (Basin Analog System Investigation) was developed to determine whether one basin or petroleum system is analogous to another (Singh 2006). In the following sections, we first describe BASIN, and then we and explain how we used PRISE with publicly available data to determine the distribution of natural gas resources in sedimentary basins.