The USGS FORSPAN model is designed for the assessment of continuous accumulations of
crude oil, natural gas, and natural gas liquids (collectively called petroleum). Continuous (also
called ?unconventional?) accumulations have large spatial dimensions and lack well defined
down-dip petroleum/water contacts. Oil and natural gas therefore are not localized by buoyancy
in water in these accumulations. Continuous accumulations include ?tight gas reservoirs,?
coalbed gas, oil and gas in shale, oil and gas in chalk, and shallow biogenic gas.
The FORSPAN model treats a continuous accumulation as a collection of petroleumcontaining
cells for assessment purposes. Each cell is capable of producing oil or gas, but the
cells may vary significantly from one another in their production (and thus economic)
characteristics. The potential additions to reserves from continuous petroleum resources are
calculated by statistically combining probability distributions of the estimated number of
untested cells having the potential for additions to reserves with the estimated volume of oil and
natural gas that each of the untested cells may potentially produce (total recovery). One such
statistical method for combination of number of cells with total recovery, used by the USGS, is