Gas hydrates mapped using marine CSEM methods

Methane hydrate is an ice/gas mixture stable at low temperatures and high pressures. Such conditions are found worldwide on continental shelves in water depths greater than approximately 300 m (1,000 ft), and wherever there is a source of methane, either from microbial activity or from leakage of deeper hydrocarbon reservoirs, hydrate is likely to form. Hydrate is stable in seafloor sediment until the geothermal gradient makes temperatures too high, at which point a potent seismic reflector called the bottom-simulating reflector (BSR) is often observed, associated with the small amount of free gas that can accumulate at the bottom of the hydrate stability zone.

This illustration shows the gas hydrate stability zone and seismic BSR. (Images courtesy of Steven Constable)

Hydrate is hugely important: it represents a large part of the global carbon inventory and probably contains more methane than conventional gas reserves. As such, it is considered by some to represent a commercial source of energy, and tests have been carried out to produce gas by depressurization. It can be a hazard to drilling and infrastructure if warm production fluids or exothermic cementation reactions destabilize hydrate in the sediment, and it could be responsible for submarine landslides during periods of sea level change or global warming. Rapid release of methane, a potent greenhouse gas, also could have played a role in past climate change.

Yet scientists know little about the global volume and distribution of hydrate, and, in particular, commercially viable deposits have proved elusive. One reason for this is that simple seismic reflection methods tend to respond to the edges of structure and are poor at identifying hydrate in the section through bulk properties. The BSR, once thought to be a good indicator of hydrate occurrence, has proved otherwise because it is simply evidence of trace gas at the edge of the stability field.

Hydrate resistivity

Gas hydrate has long been known to be electrically resistive, determined both from well logs and from qualitative laboratory measurements. Recently, laboratory work was extended to quantify the electrical resistivity of pure methane hydrate and showed it to be 20,000 ohm at 0°C (32°F) and about 40% less resistive at 10°C (50°F). The addition of sediment and pore fluid makes resistivity relationships much more complicated, and a great deal of laboratory work remains to be done.

In a marine CSEM hydrate survey, deployed OBEM receivers are augmented by receivers towed at fixed offset behind an EM transmitter.

In spite of the complications, gas hydrate will present a viable target to electrical prospecting methods, and researchers are using marine controlled-source electromagnetic (CSEM) surveys in the search for submarine hydrate.

Over the last decade, marine CSEM methods have been adopted by the oil and gas industry as a deepwater exploration tool. Ocean-bottom electromagnetic (OBEM) recorders are deployed on the seafloor, and a powerful electromagnetic (EM) transmitter is towed through this array in close proximity (50 m to 100 m or 165 ft to 330 ft) to the seabed. In this way, data are collected to transmitter-receiver offsets of many kilometers, providing data sensitive to depths of several kilometers into the crust.

Results from Mississippi Canyon 118 in the GoM are presented as apparent resistivity pseudosections. On the left are data from OBEM instruments using source-receiver offset as a proxy for depth. On the right are results from a receiver towed at a fixed offset of 400 m (1,300 ft), using a frequency-dependent skin depth as a proxy. In this figure, the blue regions are resistive and probably represent hydrate in the seafloor section.

One of the principle applications of this relatively new technology is discriminating between seismic targets that are caused by a small amount of gas in the pore volume ("fizz gas") and viable hydrocarbon reservoirs, which are more resistive. This traditional CSEM method has successfully been used to study hydrate, notably offshore Oregon at a location called Hydrate Ridge, but a sparse array of seafloor receivers is not ideal for studying structure in the upper few hundred meters of sediment. To address this problem, a three-axis electric field receiver was developed that can be towed at a fixed offset of 300 m to 1,000 m (985 ft to 3,300 ft) from the transmitter ("Vulcan"). Unlike similar systems that are dragged in contact with the seafloor, by "flying" the receivers at a similar height to the transmitter, the system can operate in areas with installed infrastructure such as wellheads, pipelines, etc. This complicates the problem of locating the positions of the instruments, but the towed receivers record depth, pitch, roll, and heading as well as electric field amplitude and phase.

For traditional CSEM surveys with deployed OBEM receivers, the various transmitter-receiver offsets provide depth sensitivity in the data. Although an array of four Vulcans at offsets between 400 m and 1,000 m (1,300 ft and 3,000 ft) was successfully deployed, transmitter waveforms with broad frequency content can be used to provide sensitivity at different depths. High-frequency EM energy gets absorbed more rapidly in conductive seafloor sediments, while low-frequency energy can propagate more deeply, and the characteristic penetration distance, or skin depth, can be used as a depth discriminator.

In results collected over Mississippi Canyon Block 118 in the Gulf of Mexico (GoM), hydrate and active methane vents have been observed on the seabed, but it is not known if hydrate occurs buried in the sediment. Both a conventional array of OBEM recorders and towed receivers at 400-m offset were deployed behind the transmitter. Both the OBEM data and towed receiver data show a region of increased electrical resistivity in the vicinity of the methane vents. However, the towed receiver data exhibited higher resolution and better ties between crossing lines. Also, about half the survey time was taken deploying, navigating, and recovering the seafloor receivers, and so the ability to collect data without the use of OBEM instruments significantly reduces the cost of such shallow surveys (of course, larger offset, deep-target surveys will still rely on OBEM instruments, which have a significantly lower noise floor at lower frequencies).

It is hoped that in the future a broad application of CSEM technology will increase the industry's understanding of the amount and distribution of submarine gas hydrate and will provide the offshore exploration industry with a useful geophysical tool for hazard mitigation.