349
OCEANIC HYDRATES:
MORE QUESTIONS THAN ANSWERS
Jean Laherrere*
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.
*e-mail: jean.laherrere@wanadoo.fr
,350 Oceanic Hydrates: More Questions than Answers
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.
HYDRATES EVERYWHERE
Hydrates are everywhere. Indeed we eat them in the form of carbohydrates as in sugar and
starch found in fruit, vegetables and cereals. The study of hydrate has a long history. H.
Davy discovered it in 1811, and M. Faraday established the chemical formula of hydrate
of chlorine in 1823. During the 1930s, several gas pipelines were put into operation in
cold climates, and it was found that methane hydrate formed in them, clogging the flow.
Methane hydrates were found in Siberia in 1964, and it was reported that they were
being produced in the Messoyakha Field from 1970 to 1978. They were also reported
in the Mackenzie delta (Bily 1974) and on the North Slope of Alaska (Collett 1983).
OCEANIC SURVEYS
The International “Deep Sea Drilling Program” (DSDP), which was financed by
several countries, started in 1968, and it organized cruises, called Legs, which
,Oceanic Hydrates: More Questions than Answers 351
investigated several Sites where closely spaced boreholes were drilled. It was
extended in 1985, and renamed the Ocean Drilling Program (ODP), stimulated in part
by an interest in hydrates. Russian research suggested that hydrates could occur at a
depth of a few hundred meters below the seabed in deep water areas. Geophysicists
simultaneously identified what was known as the Bottom Simulating Reflector (BSR)
on deepwater seismic surveys (Markl, 1970, Shipley, 1979). It was soon assumed that
the BSR marked the occurrence of hydrates, trapping free-gas below, and several
Joides sites were designed to investigate the occurrences. These sites were planned by
universities, not oilmen, although the latter were called in to advise on safety. (The
author served on the Pollution Prevention and Safety Panel in the early 1980s). A total
of 625 sites were drilled by the Glomar Challenger between 1963 and 1983 under the
auspices of the DSDP, but it was decided not to drill through the BSR to avoid the risk
of a blow-out. A further 500 sites have been drilled since by the drillship, the Joides
Resolution.
Together, these programs have investigated a large number of sites in water depths
up to 7000 m, the average being 3500 m. The ODP program concentrated in shallower
waters but took more cores and, thanks to a safer drillship, penetrated the BSR. The
details are as follows:
legs number number average cores number core/
sites holes waterdepth km cores hole m
DSDP 1 to 96 624 1092 3527 m 92 84
1968–1983
ODP 1985– 100 to 177 469 1247 2624 m 161 26287 129
1998
Figure 1. DSDP-ODP wells versus waterdepth legs 1-166
, 352 Oceanic Hydrates: More Questions than Answers
Together, as much as 250 km of cores have been recovered, with an average
recovery of about 60%. For each site, an average of 3.4 nearby boreholes were drilled
to investigate the immediate surroundings. Accordingly, it can be said that the first few
hundred meters of the seabed in oceanic areas, covering some 360 M.km2 have been
thoroughly explored. The Continental Shelves and the Slope (200-3000 m) cover
respectively 7.5 and 15 percent of the oceanic area, together comprising
some 80 M.km2
PUBLICATIONS
The cynic might say that the study of hydrates is tailor-made for academic research,
insofar it can continue for a very long time without providing conclusive results,
finding more questions than answers. It is furthermore a wide subject covering such
matters such as fuel resources, transport, environmental hazards, global warming,
turbidite formation, submarine slides and eruptions, drilling hazards, and even the
Bermuda triangle mystery. One of the reasons for the inconclusive results is that
hydrates decompose into water and methane on being brought to the surface, so it is
difficult to study them in their original state.
Literally hundreds of papers have been written, with some authors contributing in
abundance, but they are characterized by generalizations, speculations, quotations and
references, rarely supported by useful facts. In fact, many such papers are designed to
attract funding for still further research, often repeating what has already been done.
The Blake Ridge investigations are a case in point. They commenced with Leg 11
(Sites 102, 103 and 104), followed in turn by Leg 76, which allegedly found hydrate
at Sites 533, and Leg 164. The paleoclimate Leg 172 drilled more holes on the Blake
Ridge without finding any hydrates. A new 3D seismic survey is now proposed to
reinvestigate the area.
Leg 170 returned in 1997 to the Costa Rica Trench, which had been already drilled
on Leg 84 in 1982. Only one site out of five recovered hydrate in volcanic ash layers
with 0.7 to 1% methane.
Cascadia was investigated by the ODP on Leg 146 in 1992, and by Geomar and
Tecflux (Hydrate Ridge) in 1996. A further project is proposed either as Leg 198 in
2001 or by a Portable Remotely Operated Drill operated by Tecflux. Cascadia seems
the best area to search for hydrates. Since 1992, Tecflux, which is an international
collaboration led by Geomar (University of Kiel Germany), the Oregon State
University (USA) and the Monterey Bay Research Institute (MBARI, USA) has
become one of the prime researchers on the subject.
JNOC drilled (Nov. 16-Dec.1 1999) a borehole in 950 m of water finding 16 m of
hydrates in the Nankai Trough (but the results are confidential). The same area had
already been investigated by Leg 87, which observed the BSR but found no sign of
hydrates. JNOC plans to drill another borehole soon, which may suggest that the
results of the first were less than conclusive; and Leg 198 will return to Nankai in 2001
with a different objective. The research goes on; and while it never seems to deliver a
concrete answer, it often does succeed in posing a new question.
On land in permafrost, Ginsburg, the Russian expert, disputed in 1993 the claims
that the Messoyakha Field was producing natural hydrates, as quoted in US papers. A
