For the past three decades, discussion of naturally-occurring gas hydrates has been framed by a series of assessments that indicate enormous global volumes of methane present within gas hydrate accumulations. At present, these estimates continue to range over several orders of magnitude, creating great uncertainty in assessing those two gas hydrate issues that relate most directly to resource volumes – gas hydrate’s potential as an energy resource and its possible role in ongoing climate change. However, a series of recent field expeditions have provided new insights into the nature of gas hydrate occurrence; perhaps most notably, the understanding that gas hydrates occur in a wide variety of geologic settings and modes of occurrence. These fundamental differences - which include gas hydrate concentration, host lithology, distribution within the sediment matrix, burial depth, water depth, and many others - can now be incorporated into evaluations of gas hydrate energy resource and environmental issues. With regard to energy supply potential, field data combined with advanced numerical simulation have identified gas-hydrate-bearing sands as the most feasible initial targets for energy recovery. The first assessments of potential technically-recoverable resources are now occurring, enabling a preliminary estimate of ultimate global recoverable volumes on the order of ~3 × 10¹⁴ m³ (10¹⁶ ft³; ∼150 GtC). Other occurrences, such as gas hydrate-filled fractures in clay-dominated reservoirs, may also become potential energy production targets in the future; but as yet, no production concept has been demonstrated. With regard to the climate implications of gas hydrate, an analogous partitioning of global resources to determine that portion most prone to dissociation during specific future warming scenarios is needed. At present, it appears that these two portions of total gas hydrate resources (those that are the most likely targets for gas extraction and those that are the most likely to respond in a meaningful way to climate change) will be largely exclusive, as those deposits that are the most amenable to production (the more deeply buried and localized accumulations) are also those that are the most poorly coupled to oceanic and atmospheric conditions.

Technological advances have opened up natural gas resources that were previously unobtainable, including deep-water areas (depths >305 m) and unconventional resources, such as coal-bed methane, and gas in shale, that do not readily release their gas to wells. The next resource poised to be delivered is gas hydrates, which form from methane and water at low temperatures and moderate pressures. Gas hydrates occur in permafrost (1), but most of this vast resource occurs in marine sediments on the outer continental shelves (2). Physical barriers posed by Arctic and deep-water settings, as well as a lack of proven extraction methods, have made them an unexploited resource. However, a series of international field programs in the last 5 years, in conjunction with experimental studies and numerical simulations, show that it should be possible to extract the most favorable gas hydrates—those enclosed in sandy sediments that lay at the apex of the gas hydrate resource pyramid (see the figure)—with existing technologies.

Summary: Gas hydrates (GHs) are a vast energy resource with global distribution in the permafrost and in the oceans. Even if conservative estimates are considered and only a small fraction is recoverable, the sheer size of the resource is so large that it demands evaluation as a potential energy source. In this review paper, we discuss the distribution of natural GH accumulations, the status of the primary international research and development (R&D) programs, and the remaining science and technological challenges facing the commercialization of production. After a brief examination of GH accumulations that are well characterized and appear to be models for future development and gas production, we analyze the role of numerical simulation in the assessment of the hydrate-production potential, identify the data needs for reliable predictions, evaluate the status of knowledge with regard to these needs, discuss knowledge gaps and their impact, and reach the conclusion that the numerical-simulation capabilities are quite advanced and that the related gaps either are not significant or are being addressed. We review the current body of literature relevant to potential productivity from different types of GH deposits and determine that there are consistent indications of a large production potential at high rates across long periods from a wide variety of hydrate deposits. Finally, we identify (a) features, conditions, geology and techniques that are desirable in potential production targets; (b) methods to maximize production; and (c) some of the conditions and characteristics that render certain GH deposits undesirable for production.

Summary: The current paper complements the Moridis et al. (2009) review of the status of the effort toward commercial gas production from hydrates. We aim to describe the concept of the gas-hydrate (GH) petroleum system; to discuss advances, requirements, and suggested practices in GH prospecting and GH deposit characterization; and to review the associated technical, economic, and environmental challenges and uncertainties, which include the following: accurate assessment of producible fractions of the GH resource; development of methods for identifying suitable production targets; sampling of hydrate-bearing sediments (HBS) and sample analysis; analysis and interpretation of geophysical surveys of GH reservoirs; well-testing methods; interpretation of well-testing results; geomechanical and reservoir/well stability concerns; well design, operation, and installation; field operations and extending production beyond sand-dominated GH reservoirs; monitoring production and geomechanical stability; laboratory investigations; fundamental knowledge of hydrate behavior; the economics of commercial gas production from hydrates; and associated environmental concerns.