FROZEN HEAT | Volume 1

Box 1.2 Identifying gas hydrate in specimens of natural sediment

Gas hydrates are now considered one of the largest storehouses of potentially mobile organic carbon on the planet. However, their very existence on Earth was not confirmed until the first samples were observed during scientific drilling programs in the early 1980s (see TEXT BOX 1.1). One reason gas hydrates eluded detection for so long is that the unique combination of high- pressure/low-temperature conditions required for their stability is restricted to some of the more remote places on Earth, including in and beneath permafrost in Arctic regions and within the marine sediments of continental margins. Like water ice, when a gas hydrate is removed from the environment in which it is stable, it melts into a liquid water phase. Gas hydrate also releases its trapped methane gas in the process. Since gas hydrates achieve this phase change rather quickly, much of the gas hydrate present in specimens collected at or below the sea floor in conventional marine studies will have disappeared (dissociated) by the time the specimens arrive on deck for inspection. Only the largest solid masses persist long enough to be physically observed. Initially, scientists developed special means to infer the presence of gas hydrates from the impact their dissociation has on the chemistry of the surrounding sediment: that is, the stronger the shift of pore-water salinity to fresher values as compared to the local background condition, the greater the gas-hydrate volume that had recently been present. In addition, infrared scanners are used to detect cold spots in recovered cores. These spots indicate where gas hydrates have been and where their melting has cooled the surrounding sediment. The ability to conduct direct measurements in situ using geophysical well-logging tools has advanced significantly (Tsuji et al. 2009), and currently much can be determined with great confidence using such tools, particularly when gas- hydrate concentrations are high. Predicting gas-hydrate occurrence using remote sensing (such as seismic or electromagnetic surveys conducted from the surface) is possible, and this ability becomes more accurate with each detailed field study.

To fully assess gas-hydrate-bearing sediments, scientists have devised pressure-coring technologies that allow samples to be collected and retrieved without ever exiting gas-hydrate stability conditions. This technology continues to advance, with increasingly complex measurements being made on acquired samples. X-ray images taken of such samples have demonstrated the wide variety of forms gas hydrates can take in the subsurface, ranging from broadly disseminated pore- filling grains to complex arrays of delicate tabular veins and fracture-filling forms (see Fig. TB-1.2) (Holland et al. 2008; Rees et al. 2011). Such measurements and images provide critical ground-truth data to confirm the impact of gas-hydrate occurrence on the physical properties of the sediment.

B A

C

Figure TB-1.2: X-Ray-computed tomography images for gas- hydrate-bearing clays from the Krishna-Godavari Basin offshore eastern India. Gas hydrates are shown in white, clay is shown in grey, and blue represents ice. (A) Gas hydrates are generally observed as near-vertical veins in this 90-centimetre-long core. The diameter is 5.7 centimetres (Holland et al. 2008). (B) In this micro-computed tomography scan (Rees et al. 2011), a 23-centimetre-long sample, also 5.7 centimetres in diameter, illustrates how the large gas hydrate veins observed in the full- core scan are themselvesmade up of small, interconnected veins. Ice has formed in this specimen during sample transfer and handling, and it is not representative of the in situ environment, which is well above the freezing temperature of water. (C) A natural-light image of gas-hydrate-bearing clay from the region.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 25

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