Oceanic Hydrates: More Questions Than Answers

• Published in: Energy Exploration & Exploitation
• Main Author: Laherrere, Jean
• Format: Article in Journal/Newspaper
• Language: English
• Published: SAGE Publications 2000
• Subjects: Methane hydrate
• Online Access: http://dx.doi.org/10.1260/0144598001492175; http://journals.sagepub.com/doi/pdf/10.1260/0144598001492175

From an oil industry standpoint, methane hydrate is known as a major problem because it plugs casing and pipelines. From a media standpoint, hydrates provide an almost inexhaustible supply of articles concerning greenhouse effects, landslides, global warming and mysterious events such as the loss of aircraft in the “Bermuda Triangle”. From a scientific standpoint, they provide much scope for academic research projects. Oceanic hydrates have been recovered in some of the thousands of ODP/Joides boreholes, from which a total of over 250 km of core have been taken. Unfortunately, hydrates dissociate when brought on deck, and few samples were preserved for further analysis. Most of the oceanic hydrates are reported to be of biogenic origin, except where they overlie petroleum reservoirs, as in the Caspian Sea and Gulf of Mexico. The hydrates in the cores are found mostly as dispersed grains or thin laminae. Massive pieces of hydrate, greater than 10cm thick, have been found only at three sites. Downhole logs are unreliable indicators of hydrates due to cave-ins, and in many instances the inferred presence of hydrates depends on indirect evidence, such as seismic reflectors (BSR) or chlorinity changes in pore waters. The oil industry requires much better evidence than this before attributing reserve status to a resource, yet in the case of hydrates, enormous deposits (such as recently declared in New Caledonia) are reported on the strength of no more than uncertain seismic information. The gas hydrate stability zone (GHSZ) occurs in oceanic sediments over the first few hundred meters below the seabed. In this zone, any methane from organic material, including any seepages from below, is converted into solid hydrate, and is locked in place in the sediments. The origin of the methane is poorly understood, with even its biogenic origin being challenged. Dissolved methane or free gas may precipitate at geological discontinuities such as faults, fractures and lithological boundaries, as well as at water salinity, ...