A REVIEW OF THE GEOCHEMISTRY OF METHANE IN NATURAL-GAS HYDRATE

The largest accumulations on Earth of natural gas are in the form of g as hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with pe rmafrost. Measurements of hydrocarbon gas compositions and of carbon-i sotopic compositions of methane from natural gas hydrate samples, coll ected in subaquatic settings from around the world, suggest that metha ne guest molecules in the water clathrate structures are mainly derive d by the microbial reduction of CO2 from Sedimentary organic matter. T ypically, these hydrocarbon gases are composed of >99% methane, with c arbon-isotopic compositions (delta(13)C(PDB)) ranging from -57 to -73 parts per thousand. In only two regions, the Gulf of Mexico and the Ca spian Sea, has mainly thermogenic methane been found in gas hydrate. T here, hydrocarbon gases have methane contents ranging from 21 to 97%, with delta(13)C values ranging from -29 to -57 parts per thousand. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continenta l: gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of >99% methane, with carbon-isotopic compositions rang ing from -41 to -49 parts per thousand. These gas hydrate deposits als o contain a mixture of microbial and thermal methane, with thermal met hane likely to be dominant. Published by Elsevier Science Ltd