GEOL 115: The Earth’s Energy Resources

The truly large unconventional gas accumulations are methane hydrates. Methane hydrates were laboratory curiosities until they were found to clog pipelines in cold climates and in deep water.

This phase diagram has been scaled in depth rather than pressure by assuming a hydrostatic pressure gradient (roughly 0.5 psi/ft). You can see by inspecting the diagram that methane hydrates will be stable only at pressures exceeding 250 psi and at low temperatures. As T increases the pressure required to stabilize methane hydrate also increases. The only places where methane hydrates are stable are in permafrost areas or in deep waters of the the oceans.

Methane hydrates are identified easily on seismic records. One simply looks for a BSR (bottom simulating reflector) that cuts across sedimentary strata and has a polarity opposite that of the water bottom. The BSR is the bottom of the methane hydrate zone. At the water bottom seismic waves will speed up as they pass from water to sediment. The reversed polarity of the BSR means that seismic waves must be slowing down at the BSR. The methane hydrate-cemented sediment will have a higher acoustic impedance than the uncemented sediment; thus, the base of the methane hydrate zone should have a polarity reversed from that of the water-bottom.

These systems are a potential energy bonanza: some workers have proposed that methane hydrates may contain more carbon than is present in all the fossil fuels known on the planet. They also represent a bit of a threat; destabilization of marine methane hydrates could cause tsunamis and the addition of large amounts of methane to the atmosphere could cause rapid climate change. One of the more rapid climate shifts in the geologic history occurred during what is called the Paleocene-Eocene Thermal Maximum (PETM). Temperatures during the PETM were probably considerably warmer than they are today and the event is coincident with a significant mass extinction. The wikipedia entry on the PETM is pretty good. Evidence is accumulating that destabilization of methane hydrates may have caused the rapid climate change. This is one of the reasons that scientists are concerned about climate change; we don’t fully understand the system and there is evidence that it can shift catastrophically under the right circumstances.

We’re learning about methane hydrates rapidly and are fortunate that one of the world’s experts is on faculty here at UNL.