be intrinsic permeability. Sediments of high intrinsic permeability may have the capability to host hydrate at high saturations (50–90% of pore space).


Mud and fine silt occurences: Te category for muds and fine silt lies below marine sands in the gas hydrate resource pyramid. Fractured muds are less permeable, usually smaller-grained sediments that may host gas hydrates in fracture-related permeability. Drilling on the Indian and Korean margins and in the Gulf of Mexico has found gas hydrate filling pervasive fractures within low permeability sediments (e.g., silts and clays). Such sediments may not have a high average saturation of gas hydrate, maybe around 20%, but targeted production from gas hydrates within the fractures could theoretically yield significant gas. At the base of the resource pyramid lie gas hydrates in low


Figure 8.26: Hydrate Energy International (HEI) recently released estimates of the gas hydrate resource potential, utilising a petroleum systems approach.


permeability, undeformed fine-grained muds. Such sediments host most of the global gas in place in methane hydrates and are unlikely to become a target for commercial production of gas from methane hydrates. Te saturation typically is only 5%. Sea-floor mound deposits are small size and ephemeral.


Tey are environmentally sensitive due to associated unique biological communities and thus unattractive as a resource target.


8.4.2 Potential Worldwide


In conventional petroleum systems analysis, the geological components and processes necessary to generate and store hydrocarbons are well established: source, migration, reservoir, seal, and timing. To apply this petroleum system model to a methane hydrate resource system, one needs also to incorporate the parameters that determine methane hydrate stability conditions: formation temperature and pressure, pore water salinity, water availability, gas source, gas transport, gas concentration, and the time over which the system evolves. Recently, Hydrate Energy International (HEI), as part


of the Global Energy Assessment being conducted by the International Institute for Applied Systems Analysis (IIASA), released the results of a new evaluation of the gas hydrate resource potential, utilising a petroleum systems approach (Figure 8.26). Teir median assessment is around 43,000 Tcf.


8.4.3 Gas Hydrate Accumulations


Gas hydrates occur in a wide variety of geologic settings and modes of occurrence. Tese include gas hydrate concentration, host lithology, distribution within the sediment matrix, burial depth, water depth, and many others. Te major controlling factor on where gas hydrate forms is lithology and availability of methane. Figure 8.27 from Boswell (2011) gives a schematic depiction


of the components of various methane hydrate systems. Examples A and B represent massive forms in hydrate-bearing marine clays. Example C shows a hydrate-bearing marine sand.


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Examples D and E represent sea-floor mounds (outcrops) and hydrate-bearing clays (finely dispersed). Tree dominant types of gas hydrate accumulations can


be defined and distinguished based on the mode of fluid migration and gas hydrate concentration within the GHSZ (Milkov and Sassen, 2002). Te end-members are structural and stratigraphic accumulations, but combination accumulations controlled both by structures and stratigraphy may occur.


Structural accumulations: Structural gas hydrate accumulations occur in advective high fluid flux settings, where highly permeable fractured conduits like fault systems, mud volcanoes and other geological structures facilitate rapid fluid transport from depth into the GHSZ. Te gas hydrate concentration in the sediments is relatively high. Gas hydrate deposits associated with active faults and craters of deepwater mud volcanoes usually present high gas hydrate concentrations, with 30–50% of the pore space filled by hydrates. Te shallow seafloor consists typically of non-consolidated


silts and clays. Various types of gas hydrates may occur: layers of hydrates of thicknesses from millimetres to tens of centimetres, massive hydrate deposits, or hydrate outcrops (mounds) on the seafloor. Bottom-simulating reflectors (BSRs) are not common in


structural accumulations as they do not typically seal much gas below the gas hydrate layer. If present, they are patchy and displaced and they do not parallel the seafloor.


Stratigraphic accumulations: Stratigraphic gas hydrate accumulations generally occur in advective low fluid flux settings within passive margins in relatively coarse-grained sediments, from biogenic methane gas generated in situ, or gas which is slowly supplied from deeper in the subsurface. In stratigraphic accumulations, gas hydrate tends to


be highly dispersed through the GHSZ, and low hydrate concentrations are commonly measured with 1–12% of the pore space filled by hydrates. Te low hydrate concentration can be explained by the low permeability and porosity in clay-rich sediments, which hinder the mobility of both water and gas,


Johnson, 2011


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