Figure 1.1), allowing researchers to refine their geologic models and improve their interpretations of geophysical data. Whereas some early global estimates of methane occurring as methane hydrate were as high as 1018 m3 (~35 million trillion cubic feet [TCF] methane at standard pressure and temperature [STP] conditions), estimates by Milkov (2004) decreased the range to 1-5 × 1015 m3 (~35,000-177,000 TCF). But later estimates by Klauda and Sandler (2005) are much larger (1.2 × 1017 m3 or 4,200,000 TCF), demonstrating that even recent estimates range over several orders of magnitude. However, even the lowest global resource estimates are 2 to 10 times greater than global estimates of the conventional natural gas endowment of 4.4 × 1014 m3 (~16,000 TCF) of reserves and technically recoverable undiscovered resources (Ahlbrandt, 2002; TEA, 2006). Recalling that the United States in 2008 consumed 6.5 × 1011 m3 (23 TCF; see Chapter 1) of natural gas, the global estimates of volumes of methane in methane hydrate are significant.
Although the global methane hydrate resource inventories illustrate the importance of methane hydrate as a component of the global carbon cycle, their utility to address the energy potential of methane hydrate is limited. The majority of the enormous global methane hydrate inventory occurs as dispersed concentrations over large areas and therefore recovery of the methane, for the most part, is unfavorable technically and economically. Conversely, areas with concentrated methane hydrate deposits that may be the appropriate candidates for economic development are more limited in size. Boswell and Collett (2006) reviewed the challenge of appraising the energy potential of the large but uncertain global inventories of methane hydrate and introduced the resource pyramid concept which qualitatively appraises the distribution of the global methane hydrate resource and evaluates which type of deposit holds the greatest economic potential for development (Figure 2.1). They conclude that the deposits that are most concentrated and hold greatest potential for exploitation occur in sandstone reservoirs in the Arctic and deepwater marine environments.