COMMODITIES ASSESSED


     The commodities considered in this study were crude oil, natural 
gas, and natural gas liquids that can be expected to be produced from 
the subsurface through a well. Most heavy oil deposits were assessed as
conventional resources. Specifically excluded from consideration were
gas dissolved in geopressured brines and resources in tar deposits and
oil shales. Gas in clathrate structures (gas hydrates) were not assessed 
as technically recoverable resources; however, a chapter concerning these 
in-place volumes of gas is included in the supporting CD-ROM (Gautier and 
others, 1995). Specifically included in this assessment were technically 
recoverable gas from low-permeability "tight" sandstone reservoirs, gas and 
oil from fractured shale reservoirs, and coal-bed gas. The systematic 
inclusion of unconventional resources marks a significant departure from 
previous USGS assessments.

     Crude oil, as considered in this assessment, is a natural liquid
consisting mainly of a mixture of complex hydrocarbon molecules.
Natural gas is a mixture of hydrocarbon gases, mainly methane, and
certain non-hydrocarbon gases such as carbon dioxide, hydrogen sulfide,
nitrogen, and helium. This analysis assessed hydrocarbon gases, although 
minor amounts of non-hydrocarbon gases may be included.  Natural gas liquids 
(NGL) are the heavier homologs of methane, which are in the gas-phase under 
reservoir pressure and temperature conditions. NGL includes those portions of 
the reservoir gas that are liquefied at the surface in various field facilities
and in gas-processing plants. NGL commonly includes propane, ethane, butane,
pentane, natural gasoline, and condensate.