ASSESSMENT CATEGORIES


     The resource classification used in this study is illustrated in 
figure 4, a modified "McKelvey box." Resources can be classified along 
two axes: geologic assurance and economic feasibility. The degree of geologic 
certainty increases to the left from undiscovered resources, through inferred 
reserves (reserve growth1) to measured (proved) reserves. Degree of economic 
feasibility increases vertically upward from subeconomic to economic resources.

     Previous USGS assessments focused on undiscovered conventional
accumulations of oil and gas and additions to reserves in known fields.
This assessment is broader in scope because it considers three
categories of resources: (1) undiscovered conventional accumulations of
oil and gas, (2) future additions to reserves of known fields, and (3)
oil and gas in continuous-type accumulations (largely equivalent to
"unconventional" categories of other analysts).

1Throughout this report, the terms "reserve growth" and "field growth"
are used interchangeably.  Both terms indicate growth of reserves in
known fields.