“While research on methane hydrates is still in the early stages, these research efforts… could potentially yield significant new supplies of natural gas and further expand US energy supplies.”
On August 31, 2012 US Secretary of Energy, Dr. Steven Chu, announced an investment of $5.6 million in research on methane hydrates.
From Part I (GEO ExPro Vol. 9, No. 3) of our series on gas, the key learning is that four ‘magic ingredients’ must be present for gas hydrates to exist. They form when there is a sufficient supply of water and gas, predominantly methane (99%), at relatively low temperatures and high pressures, with temperature and pressure in the so-called Gas Hydrate Stability Zone (GHSZ), (see box, p34). Favourable hydrate formation conditions exist off the coasts on the continental margins and below the permafrost.
In marine settings, temperature is controlled by the ocean bottom water temperature and the geothermal gradient at any given location, while pressure is controlled by sea level. In aquatic sediment where water depths exceed about 300m and bottom water temperatures approach 0°C, gas hydrate is found at the seafloor to sediment depths of about 1,100m. The general temperature range is from 2 to 20°C.
In a permafrost setting, however, temperature gradients are considerably lower than in the ocean. The ambient temperature and the thickness of the permafrost layer therefore are of significant importance for the stability of gas hydrate. In polar continental regions, methane hydrate can occur at depths ranging from 150 to 2,000m, with a general temperature range from -10 to 20°C.
Bottom Simulating Reflector (BSR)
The geothermal gradient is important. At a certain depth in ocean sediment the geothermal gradient makes the sediment too warm to support the solid gas hydrates, so any methane produced below this depth will be trapped as a layer of free gas in the pore space beneath the solid gas hydrate layer. Often, but not always, the interface between the gas hydrate and the free gas is an anomalous seismic reflector called a Bottom Simulating Reflector (BSR), as this reflector necessarily is roughly parallel to the seafloor morphology along isotherms. BSRs therefore need not follow the trend of stratigraphic horizons, but may intersect them.
In seismic sections, BSRs are usually characterized by large amplitudes but exhibit reversed polarity compared with the sea-bottom reflection.
The BSR indicates the lower boundary of gas hydrate stability. Consequently, gas hydrate is often assumed to exist above the BSR; otherwise, the free gas below the BSR would have migrated upwards. But, while a BSR does illustrate the volume of sediment inside the stability zone it does not provide information on the actual hydrate saturation in-place. BSRs can be observed even when very little hydrate is present, and BSRs need not always be observed in hydrate-bearing sediments.
Exploration for Gas Hydrates
To date, around 100 sites have been identified as containing gas hydrate deposits. Samples have been taken at approximately 20 different sites, while at another 80 sites the existence of gas hydrate has been suggested by seismic evidence, in the form of BSRs.
Exploration for gas hydrates is not much different from exploration for conventional hydrocarbons: important factors to recognize are source, migration, reservoir, and seal.
If there is not sufficient gas supply, there will be no gas hydrates. Two distinct processes produce hydrocarbon gas: biogenic and thermogenic degradation of organic matter. Biogenic gas is formed at shallow depths and low temperatures, up to 75–80°C, by anaerobic bacterial decomposition of sedimentary organic matter. It is very dry and consists almost entirely of methane. In contrast, thermogenic gas is formed at deeper depths, much deeper than the GHSZ, in the temperature range 50–200°C by thermal cracking of sedimentary organic matter into hydrocarbon liquids and gas. This type of gas, which is common in conventional gas reservoirs, can be dry, or can contain significant concentrations of ‘wet gas’ components (ethane, propane, butanes) and condensate.
Fluid migration from the source through faults, folds, and fractures into the GHSZ plays a critical role in the formation of a gas hydrate accumulation. Rapid gas transport is required to concentrate gas in permeable reservoir sediments where gas hydrate crystallizes. Water transport is usually thought to be less important because water is virtually omnipresent in sediments, although it may be a limiting factor for gas hydrate crystallization in some areas. Sand-rich reservoir environments are better than clay-dominated systems. As far as seals are concerned, gas hydrates themselves are the seals.
The possibility of production from hydrates is highly dependent on the particular reservoir characteristics. Many of the known marine deposits are probably unfeasible for hydrate production. The candidates that are currently being explored are high concentration accumulations in coarse-grained sand environments with high porosity and permeability.
Global GHSZ Thicknesses
Burwicz et al (2011) have calculated GHSZ thicknesses based on the global bathymetry, salinity, bottom water temperature, and heat flow (as a proxy to geothermal gradients as they are not globally available).
GHSZ thicknesses can be considered a proxy for potential hydrate deposits distribution but not necessarily for the real volume of hydrate-bearing sediments. The formation of hydrates is mainly controlled by methane supply either through the direct degradation of organic matter within the GHSZ or through an upward flux of deeper biogenic and thermogenic methane. Global estimates of methane fluxes from deep sediments are poorly constrained.
Acknowledgement: We would like to thank the Directorate General of Oil and Natural Gas at the Ministry of Energy and Mineral Resources of Republic of Indonesia for permission to show the seismic example.