Are There Commercial Deposits of Methane Hydrates in Ocean Sediments?

Abstract

Naturally occurring gas hydrates forming in the ocean floor sediments have, ever since the 1970s, been heralded as the next century's viable energy source. Is this really so? What has the Ocean Drilling Program (ODP) found out about the nature of the Bottom Simulating Reflector (BSR) as an indicator of large volumes of gas hydrates and free natural gas? The findings so far from ODP's legs 146 and 164 have been rather discouraging with respect to the in situ amounts of both gas hydrates and free gas. In the sediments drilled on Leg 146, on the Cascadia accretionary wedge off Oregon, USA, the average amount of gas hydrates above a prominent BSR was estimated at 1 – 2 percent by sediment volume. The free gas amount below the BSR was 3–4 percent by sediment volume. On Leg 164 in the Western Atlantic Ocean (Blake Plateau) some higher amounts, averaging up to 4 – 6 percent and 5 – 7 percent by volume of sediment, respectively, were found. However, on the Hydrate Ridge, off Oregon (Cascadia, just south of Leg 146), there has recently been discovered apparently large amounts of sediment-mixed gas hydrates, which may be exploitable.Although the general percentages found so far are much too low for commercial extraction (even a high-grade copper-ore on land must average more than 6 percent by volume), there may exist some high-grade regions worth exploring. But, the question remains: Where shall we seek the high-grade gas hydrate deposits in ocean sediments? Besides the obvious deep-water high porosity conventional type of reservoirs, and judging from recent findings in the Gulf of Mexico and the Caspian Sea, perhaps the most promising locations are those where mud volcanoes occur on the ocean floor.