The structure of hydrate bearing fine grained marine sediments

Abstract

Recent advances in pressure coring techniques, such as the HYACINTH and IODP PCS pressure cores deployed during Expedition 1 of the India National Gas Hydrate Program using the JOIDES Resolution have enabled the recovery of fine grained sediments with intact gas hydrates contained within the sediments. This has provided the opportunity to study the morphology of gas hydrates within fine grained sediments which until now has been hindered due to the long transit times during core recovery leading to the dissociation of the gas hydrates. Once recovered from the seafloor, rapid depressurization and subsequent freezing of the cores in liquid nitrogen has enabled the near complete fine fracture filling nature of the gas hydrates to be largely preserved. High resolution X-ray CT (computer tomography), which has a pixel resolution of approx. 0.07mm, has been used to provide detailed images showing the 3-dimensional distribution of hydrates within the recovered fine grained sediments. Results have shown that in fine grained sediments gas hydrates grow along fine fracture faults within the sediment. Although the fractures were predominantly sub-vertical and continuous through the cores, stranded fractures were also observed suggesting that hydrate formation is episodic. However, within the cores open voids were observed which were not evident in low resolution CT images taken before the depressurization step suggesting that during depressurization either finely disseminated gas hydrate was dissociated or that gas exsolving from solution created these voids in the sample prior to freezing in liquid nitrogen. These detailed observations of gas hydrate in fine grained sediments will help us understand the differing morphology of gas hydrates in sediments. They also show that sample disturbance is still a major concern and further techniques are required to restrict these effects so that meaningful laboratory tests can be undertaken on recovered samples.

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