the gas is being converted to liquified natural gas for shipping, and the helium in it is more highly concentrated (Francis, 1998). Algerian helium principally serves the European market.
Evaluating U.S. helium reserves and resources is the responsibility of BLM. BLM has constructed a 19,000-sample database of helium concentrations, with much of the measuring having been done at its own laboratory in Amarillo. BLM also uses data from a variety of sources for its analyses, including Potential Gas Committee reports (see, for example, Colorado School of Mines, 1995) and data from private producers of helium-rich natural gas.
BLM categorizes helium reserves using a United States Geological Survey classification system that considers both physical uncertainty and economic viability. Physical uncertainty is conveyed by dividing resources into those that are "identified" and those that are "undiscovered." Identified resources are estimated from specific geological evidence, while undiscovered resources are postulated to occur in unexplored areas. Identified resources are further divided into ''measured," "indicated," and "inferred" resources. Measured resources are based on production tests and other measurements made during well drilling. Indicated and inferred resources are based on progressively less certain geological data. The combination of measured and indicated resources are referred to as "demonstrated."
Economic considerations are conveyed through a division of the resource into "reserves," "marginal reserves," and "subeconomic reserves." Reserves refer to resources that can be economically extracted. Marginal reserves border on being economically producible. Subeconomic reserves are clearly not economically producible.
The total identified U.S. helium resource base is estimated by BLM to have been approximately 589 billion scf (16 billion scm) as of December 31, 1996, of which 217 billion scf (6 billion scm) is classified as measured reserves. Of this total, 35 billion scf (1 billion scm) is in storage in the Bush Dome reservoir, 4 billion scf (110 million scm) of which is privately owned. The BLM category measured reserves comes closest to the definition of "proved reserves" used by the petroleum industry to signify actual anticipated recoverable volumes of a resource (Society of Petroleum Engineers/World Petroleum Congress, 1997). However, measured reserves are larger than proved reserves because BLM's measured resources numbers include both "nondepleting" reserves (i.e., known but not developed) as well as those from gas that is being produced but from which helium is not being extracted.
The terminology used by BLM makes it difficult to understand how much helium is potentially available. The classification scheme used by the natural gas industry is clearer, and all new helium resources are coming from that industry.
BLM estimates nondepleting measured reserves of helium to be around 53 billion scf (1.5 billion scm) of helium, the bulk of which lie in deposits in the Riley Ridge area (Gage and Driskill, 1998). The Riley Ridge nondepleting reserves are not likely to be produced in the foreseeable future because of poor gas quality. In addition, it is expected that only 60 to 65 percent of helium-rich natural gas is being processed for helium from the Hugoton-Panhandle complex (Gage and Driskill, 1998). Although this number is estimated to approach 75 percent (Gage and Driskill, 1998), a significant portion of these reserves will still be lost when helium-containing gas is ultimately burned as fuel. Accounting for these factors to attempt to arrive at a more realistic proved reserves estimate results in the data presented in Table 4.2.