Page 45 - Introduction to Petroleum Engineering

OIL AND GAS RESOURCES 29
2.3 OIL AND GAS RESOURCES

Oil and gas resources may be characterized as conventional and unconventional
resources. Snyder and Seale (2011) defined conventional oil and gas resources as
formations that can be produced at economic flow rates or that produce economic
volumes of oil and gas without stimulation treatments or special recovery processes
and technologies. Unconventional oil and gas resources refer to formations that
cannot be produced at economic flow rates or do not produce economic volumes of
oil and gas without stimulation treatments or special recovery processes and technol-
ogies. Figure 2.7 presents a classification of oil and gas resources that is consistent
with these definitions. Following Fanchi and Fanchi (2016), more information about
several unconventional oil and gas resources near the base of the resource triangle in
Figure 2.7 is provided in the following text.
Large oil and gas fields can be characterized as giant or supergiant fields. A giant
oil field contains from 500 million barrels to 5 billion barrels of recoverable oil. Oil
fields with more than five billion barrels of recoverable oil are supergiant oil fields.
3
A giant gas field contains from 3 to 30 trillion ft of recoverable gas. Gas fields with
3
more than 30 trillion ft of recoverable gas are supergiant gas fields.

2.3.1 Coal Gas
Gas recovered from coalbeds is known as coal gas (Jenkins et al., 2007). The gas
can be present as liberated gas in the fracture system or as a monomolecular layer
on the internal surface of the coal matrix. The composition of coal gas is predomi-
nately methane but can also include constituents such as ethane, carbon dioxide,
nitrogen, and hydrogen (Mavor et al., 1999). Gas content in coal can range from
approximately 20 standard cubic feet (SCF) gas per ton of coal in the Powder River

Conventional: High quality
Smaller volume
Easy to develop Medium quality

Low perm oil Tight gas sands
Unconventional:
Larger volume
Dif cult to develop
Gas shales Heavy oil Coal gas

Gas hydrates Oil shales

FIGURE 2.7 Resource triangle. (Source: Adapted from Snyder and Seale (2011) and
Holditch, 2007.)