Estimates of the Methane Resource in Hydrates

Over the past 20 years, a consensus has been reached in the hydrate R&D community that 1) the global volume of methane contained in hydrate is huge—far exceeding the volumes of methane present in other forms, and 2) although significant volumes occur in polar settings, the vast bulk of this methane occurs in deep marine environments. However, estimates of the global volume of methane contained in hydrate still varies widely, due largely to a lack of real field data. Providing more accurate estimates is critical to constraining models of methane hydrate's role in natural processes as well as determining hydrate's energy potential.

Historically, estimates of the total volume of methane in natural gas hydrate have ranged widely, from roughly 100,000 trillion cubic feet (Tcf) to as much as 270,000,000 Tcf. In recent years, as more information is gained and real data slowly replace best guesses, estimates have tended to fall in a narrower range—from 100,000 to 1,000,000 Tcf.

Estimated U.S. Methane Resource

Domestically, the current best estimate of the methane-in-place resource is derived from an appraisal conducted by the U. S. Geological Survey in 1995. That study allocated 320,000 Tcf of methane to various hydrate plays along each coast and in the permafrost regions of Alaska. The USGS methodology included assessing both the probability of hydrate presence in each region and the likely ranges of values for the various geologic parameters that control hydrate volumes. Subsequent to that work, analysis of core samples taken on the Blake Ridge indicated that a modest downward reduction in the assumed values for hydrate saturation was needed, resulting in a revised, as-yet unofficial, estimate of 200,000 Tcf.

USGS 1995 play boundaries map. Courtesy of USGS. D

All current estimates of methane content in hydrates are based primarily on data indicating the location and potential thickness of the Gas Hydrate Stability Zone (GHSZ). Critical data, such as concentrations within the GHSZ, must be extrapolated from detailed analyses that have thus far only been accomplished at a few localities. As more sites are studied in detail, new data may lead to further significant refinement of these numbers. For example, recent data from Japan's Nankai Trough indicate concentrations much larger than anyone had expected. Conversely, data from the Blake Ridge in the North Atlantic indicate lower-than-expected concentrations.