PART II: This is the second in a series on oceanic methane hydrates. Part 1 dealt with the current state of our knowledge.
Methane is continuously produced at the surface of the earth from the fermentation of organic material by termites, bovine digestion, rice paddies, and marshes. It also comes from hydrocarbon source rocks and escapes from imperfectly sealed source rocks and reservoirs.
Gas generation from source rocks may be due to bacterial action, providing biogenic gas. Its isotopic signature may distinguish this gas type from thermogenic gas, (formed by the heating of the source rock on burial).
It is widely accepted that most oceanic methane is of biogenic origin (Monasterksy, 1998). Occurrences of hydrates in the form of a few small, dispersed gains or lamina are likely biogenic. Some massive hydrate deposits are associated with seabed disturbances, suggesting faults and fractures, and are probably formed from the escape of gas from depth. Kvenvolden (1993) has described the environment of hydrate formation in many different areas. He found methane with a thermogenic signature only in three places: the Gulf of Mexico, the Caspian Sea and the Mid-Atlantic Trench.
It is important to recognize that the methane in hydrates is held in the molecular structure as a solid, as soon as it is generated in the zone of hydrate stability. It cannot therefore migrate and accumulate in large deposits. It also follows that true biogenic hydrates are represented only by the minor occurrences of dispersed grains and laminae as found in many boreholes. The so-called free gas, which is commonly observed below, is the same biogenic gas. It is not free gas, but only bubbles in water. The bubbles have either not converted into hydrates or have decomposed from hydrates. In some cases, the source may be deep-seated thermogenic gas.
Resource estimates
Estimates of the size of hydrate deposits range greatly, highlighting the extreme uncertainty that surrounds the subject (Prensky, 1995). These estimates are extremely unreliable, most failing to indicate the area, thickness and concentration of the alleged deposits.
The estimates of Kvenvolden and MacDonald accompany this article (Kvenvolden, 1998). Of note was that MacDonald was obliged by recent data to reduce the hydrate saturation in the pore space from 10% to 1% and, to obtain the same order, he multiplied the area by 2.5, demonstrating again the wild speculation attached to the subject.
It would be much more realistic to greatly reduce the thickness estimate because there is no evidence from the thousands of boreholes throughout the world's oceans of anything approaching a 500-meter column of methane hydrates. Furthermore, it is important to distinguish net from gross thickness, given the real occurrence of dispersed hydrate grains.
The claim by the USGS that "the worldwide amounts of carbon bound in gas hydrates is conservatively estimated to total twice the amount of carbon to be found in all known fossil fuels on Earth" is preposterous.
Kvenvolden estimates the distribution of organic carbon, excluding dispersed carbon in rocks and sediments (which equals nearly 1,000 times the total amount), as follows in gigatons (Gt): gas hydrates - 10,000; fossil fuels - 5,000; soil - 1,400; dissolved organic matter in water - 980; land biota - 830; peat - 830; detrital organic matter - 60; atmosphere - 4; marine biota - 3.
The comparison is greatly flawed. The hydrocarbon in oil and gas fields is estimated to be only about 1% of the hydrocarbons generated. Since the reported hydrate volumes cover disseminated deposits, it is necessary to multiply the oil deposits by at least 100 to obtain a comparable disseminated value.
A recent estimate of the hydrocarbon endowment (Perrodon, Laherrere, Campbell, 1998) gives proper ranges for the ultimate recoverable resource, showing them as a small part of the resource.
The most optimistic estimate of ultimate oil is 10,000 Gb (1,500 Gt) compared with the above mean of 2,750 Gb for oil. The maximum hydrocarbon amount, including gas is about 3,000 Gt. This with a further 2,000 Gt from coal gives a total for fossil fuels of about 5,000 Gt.
But fuel reserves have to be compared to hydrate recoverable resources and not to carbon disseminated in the sediments. Combaz (1991) estimated that the total fossil organic matter in its scattered state weighs up to 1016 tons (10,000,000 Gt) of organic carbon, when the fossil fuels concentrated stock reaches about 1014 tons (100,000 Gt) of carbon. The oceans produce annually 23 Gt/yr of organic matter compared to 0.7 Gt/yr for land. The 10,000 Gt estimate by Kvenvolden for hydrate is only one-tenth of Combaz's estimate of the fossil fuels.
Volumetric flaws
In assessing a new oil prospect, the oil industry evaluates the necessary parameters, including source rock, maturation, migration, reservoir, trap, and seal. But in the case of hydrates, the source, reservoir and seal are the same, comprising the 500 meters of unconsolidated sediment beneath the seabed. Since hydrate is a solid, there is no possibility of migration.
Hydrate is only 1% of the porosity, the rest is 99% water. So, hydrates do not correspond to a seal. Free gas stays because it is the equivalent of hydrate in the zone where hydrate is unstable. Leg 164 in the Blake Ridge found that the percentage of methane is about the same above and below the seismic reflector (BSR).
The estimates of hydrate volume assume that most of the available organic material is converted into hydrate. The percentage of total organic carbon (TOC) is around 1%, when the percentage of hydrate is around 1% of porosity, or 0.5% of the total volume.
Time factor
We have also to take into account the geological time factor. It is difficult to believe that hydrates contained in the first 600 meters of oceanic sediments covering a period of less than 10 million years could hold two times more carbon than the fossil fuels from 6,000 meters of sediments covering a period of more than 500 million years.
Holbrook (1996) estimated in the Blake Ridge study that 30 Gt of hydrate (23 Gt of carbon) occurred in deposits laid down over six million years, 5 kt/year. This compares with the estimates from other methane sources (Neue, 1993).
The methane hydrate accumulated in the Blake Ridge at 5 kt/yr represents about half the present methane coming from all the oceans. It is noteworthy that the amount of biogenic marsh gas (wetlands) or bovine methane (enteric fermentation) is about ten times that in oceanic methane hydrates.
US estimates
In 1995, the U.S. Geological Survey (USGS) completed its most detailed assessment of U.S. hydrate resources, with the in-place gas resource estimated at 112,000 tcf to 676,000 tcf, with a mean value of 320,000 tcf. Subsequent refinements of the data in 1997 have suggested that the mean should be adjusted slightly downward, to around 200,000 tcf. In fact, it seems that 200,000 tcf is closer to the most likely value (probability around 65%) than the mean-value (probability around 40%), which is still 300,000 tcf.
The comparison with the US proved conventional gas reserves of 1,400 tcf is unrealistic. Some authors claim that about only 1% recovery is enough to provide huge reserves, but it has to be a significant recovery or nothing.
Non-conventional sources
There are other sources of non-conventional gas better known and more accessible than hydrates. For example, Bonham (1982) estimated that there was as much as 50,000 tcf in geo-pressured brines in the Gulf Coast. This is much larger than the 1,300 tcf attributed to the Blake Ridge hydrates by the USGS, and is obviously a much more reliable resource.
There is more in Russia. Zor'kin & Stadnik (1975) estimated the amount of gas dissolved in water at around 35,000 tcf for West Siberia and as much for Caspian. That said, there remain technical difficulties in producing gas from these sources, including the disposal of the brines, but they must be small compared with those of oceanic hydrates. Geopressured brines have been tested in the Gulf Coast and found uneconomic and full of problems.
Production prospects
Thus far, there is no commercial hydrate production and none is planned. The reports of production in the Messoyakha Field have been challenged. To produce hydrates, it would be necessary to first find a concentrated deposit and then apply thermal or depressuring energy or solvents to release the gas. Many ideas have been voiced.
Iseux (1991) proposed hydraulic fracturing, and the injection of hot solvent. Islam, 1994, proposed electromagnetic heating of hydrates in permafrost. The problem with that process is the low flow rate. The only valid interest seems to be in ideas for transporting the methane from remote gasfields in the form of hydrates, being more condensed than LNG.
There is no commercial interest in the oceanic hydrates. Chevron, testifying to a US Senate Committee in 1998, correctly stated that hydrates occur in low concentrations and have no commercial potential. Gazprom likewise dismissed submarine hydrates (Krason, 1999), on the strength of substantial Russian research. Oil companies are involved in hydrate projects only in Japan and India, which are countries with limited indigenous oil and gas who face the growing cost of imports.
Conclusions
Methane hydrates are well known to the oil industry as a material for clogging pipelines and casings. They are also present in permafrost areas and in the oceans. Oceanic hydrates are mainly biogenic and different from thermogenic hydrates.
Claims for widespread hydrate occurrence in thick oceanic deposits are unfounded. The thickest interval recovered from a total of 250,000 meters of core from 2,300 ODP/DSDP boreholes was one meter. Mostly, they occur as dispersed grains and laminae. Indirect evidence from BSR, seismic direct hydrocarbon indicators, logs, and free gas samples is unreliable and highly speculative.
Being a solid, methane in oceanic hydrates cannot migrate and accumulate in deposits sufficiently large to be commercially exploited. The published estimates of the size of the resource are highly unreliable and give flawed comparisons with conventional fossil fuels. There are other non-conventional sources of gas which are better known and more accessible than hydrates, yet remain uneconomic. The prospects for the commercial production of oceanic hydrates in the foreseeable future are negligible. The academic research dedicated to hydrates has produced more questions than answers at this stage.
