Figure 8.17: Seismic example of Bottom-Simulating Reflector (BSR), from Indonesia. The water depth is 1,800–2,000m. Observe that the BSR has opposite polarity to that of the seabed. The average thermal gradient, about 35°C, permits the gas hydrate to exist down to around 300m sediment depth – below that depth the temperature is too high and methane can exist only as free gas.


amplitudes but exhibit reversed polarity compared with the sea-bottom reflection. Te BSR indicates the lower boundary of gas hydrate


stability. Consequently, gas hydrate is often assumed to exist above the BSR; otherwise, the free gas below the BSR would have migrated upwards. But, while a BSR does illustrate the volume of sediment inside the stability zone, it does not provide information on the actual hydrate saturation in-place. BSRs can be observed even when very little hydrate is present, and BSRs need not always be observed in hydrate-bearing sediments.


8.3.2 Exploration for Gas Hydrates


To date, around 100 sites have been identified as containing gas hydrate deposits. Samples have been taken at approximately 20 different sites, while at another 80 sites the existence of gas hydrate has been suggested by seismic evidence, in the form of BSRs. Exploration for gas hydrates is


not very different to explora tion for conventional hydro carbons: important factors to recognise are source, migration, reservoir, and seal. If there is not sufficient gas


supply, there will be no gas hydrates. Two distinct processes produce hydrocarbon gas: biogenic and thermogenic degradation of organic matter. Biogenic gas is formed at shallow depths and low temperatures, up to 75–80°C, by anaerobic bacterial decomposition of sedimentary organic matter. It is very dry and consists almost entirely of methane. In contrast, thermogenic gas is formed at deeper depths, much deeper than the GHSZ in the temperature range


271


50–200°C, by thermal cracking of sedimentary organic matter into hydrocarbon liquids and gas. Tis type of gas, which is common in conventional gas reservoirs, can be dry, or can contain significant concentrations of ‘wet gas’ components (ethane, propane, butanes) and condensate. Fluid migration from the source through faults, folds and


fractures into the GHSZ plays a critical role in the formation of a gas hydrate accumulation. Rapid gas transport is required to concentrate gas in permeable reservoir sediments where gas hydrate crystallises. Water transport is usually thought to be less important because water is virtually omnipresent in sediments, although it may be a limiting factor for gas hydrate crystallisation in some areas. Sand-rich reservoir environments are better than clay-dominated systems. As far as seals are concerned, gas hydrates themselves are the seals. Te possibility of production from hydrates is highly


Figure 8.18: Distribution of known methane hydrate accumulations. The yellow dots show where actual samples of gas hydrate have been recovered, whereas the red dots show where gas hydrate occurrences have been inferred based on BSRs and well logs. It is evident that gas hydrates are found along most continental shelf and slope regions and in many permafrost areas. Hydrates have also been found in inland seas (e.g., Black Sea and Caspian Sea) and in fresh water lakes (Lake Baikal). (Courtesy of Council of Canadian Academies (2008), based on data from Kvenvolden and Rogers, 2005.)


Statoil Indonesia


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