Oceanic Hydrates: More Questions Than Answers

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Abstract
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, temperature and pressure interfaces. In the past, the porosity in the GHSZ was thought to be dominantly filled by hydrate, thus providing a seal to gas, at and below the base of the stability zone. However, at the Blake Ridge, ODP Leg 164 found only minor porosity (maximum of about 5%) being filled by hydrate or gas. The recent Leg 172 in the same area failed to find any hydrates at all. A much higher concentration has been indicated in the Japan National Oil Company hydrate borehole in the Nankai Trough, although this is contradicted by other reports. The Bottom Simulating Reflector (BSR) seismic reflector is caused mainly by gas bubbles at the base of the stability zone, which accordingly cannot act as a seal because the porosity is more than 95% filled by water, with the size of the pores and the gas bubbles being further factors. This is one reason why the BSR reflector does not correspond with the hydrate zones, as had been assumed. Cascadia, off Oregon, is one of the best places to investigate hydrates, as they crop out on the seafloor whereas on the Blake Ridge the first 200 m lack hydrates. Prior to 1998, the resources of hydrates were often declared to be much greater than all known fossil fuels (coal, oil and natural gas). Ginsburg (1998) disputed such claims on the grounds that the hydrates are not continuously distributed vertically or horizontally. More recently, the USGS (Course 14, AAPG 2000) has drastically reduced its past estimates to a level where it is now claimed that hydrate accumulations may only rival the known reserves of conventional gas. These dispersed hydrate deposits may be better compared with dispersed oil and gas in petroleum systems, which are very much larger than the amounts contained in commercial reservoirs. Many graphs on solubility of methane in water are computed from formulae, being rarely checked by experiments. Measurements in the laboratory seem to differ from field measurements in sediments. The solubility of methane in deep water is but poorly known, as few measurements have been taken, but it seems to be about a hundred times higher than in near surface-water. Methane released in deep water is dissolved in water, even when a large amount of methane is released. It cannot accordingly be the cause of any hazards. But little is known about the fate of the deep dissolved methane in upwelling seawater currents. Methane hydrates are less dense than water when on the seafloor down to a certain depth, which is still unknown (2650 m for CO2 hydrate). So, extrusions of hydrate tend to float upwards, disappearing into the seawater. Log measurements in sediments report hydrates being denser than water, but direct measurements are lacking, and it would seem that such sediments are also subject to buoyancy pressure. Surficial pockmarks and mud volcanoes arise from gas expelled from overpressured, underconsolidated sediments – with or without hydrates being present. Progress in understanding oceanic hydrates has not advanced much over the last twenty years because of the poor quality of measurements in soft sediments (cores, samples and logs) and because of the lack of calibration of seismic against a known oceanic hydrate system. The chance of a viable production method being developed is slim because the oceanic hydrates are dispersed and occur in erratic patches. Only national oil companies in Japan and India are actively exploring for them. Future progress may come from the deepwater exploration being undertaken by the oil industry using better tools, but oceanic hydrates seem to be similar in some respects to metallic nodules or gold in seawater-too dispersed to ever prove economic in most places. It is well said that they are a fuel for the future and likely to remain so.