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Earth Explorer is an online source of news, expertise and applied knowledge for resource explorers and earth scientists. Sponsored by Geosoft.

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New Technique Reduces Risk for Offshore
Oil Explorers

California-based eField Exploration LLP goes to the Norwegian Sea and completes first offshore survey using its new-generation airborne Electro-Magnetotelluric technology

By Graham Chandler

Electro-Magnetotelluric (EMT) doesn't easily roll off the tongue and it's not exactly a household word but that doesn't detract from the technique's exciting possibilities when it comes to looking for oil and gas from the air.

After proving their EMT technology with five successful airborne surveys over shale gas properties in Texas, culminating in mapping the complex Rhombochasm field at a depth of 10,200 feet flying over the Palo Duro Basin of Cottle County, eField Exploration LLP of Yorba Linda, California, decided to broaden the concept to offshore surveys. "The minute you say offshore, that opens up the whole world," says Ed Johnson, president.

Their first overseas stop was Norway. Contracted by a Norwegian JV, eField undertook an airborne survey in the Nordland area of the Norwegian Sea with the objective of rapidly assessing the potential of multiple blocks for the Norwegian Petroleum Directorate's 20th Licensing Round in 2008.

"Rapid" was the operative word here. Flying 100 miles offshore over water up to 1000 feet deep, the data acquisition was completed in less than a week with the company's specially-designed sensor package. Johnson says conventional methods such as marine seismic would have taken months at several times the cost. "Our offshore Airborne System can cover up to 500 line miles per day at a fraction of the cost of a marine survey," he says.

Several airborne methods utilize measurement of electric and magnetic fields for resource exploration but the key difference with eField's EMT is the use of the natural electrical flows though the earth called telluric currents. These are induced by natural electric phenomena such as energy from solar flares and lightning strikes that penetrate deeply into the earth. As they travel beneath the surface, telluric currents become distorted by different types of rock and subterranean structures. By recording and processing these signals, the nature of these underground features and compositions can be deduced.

It's a passive system; using natural EM fields as the energy source rather than transmitting a high-energy EM signal. Measurements include electrical resistivity and conductivity, interfacial polarization, dielectric properties and magnetization. Normalized components of E and H are measured to obtain Natural Field Apparent Resistivity, or NFAR, and conductivities down to 30,000 feet and more.

Because of their depth, detection and interpretation of their behaviour can be used to look much further below the surface than traditional methods—up to seven kilometres or more. They become useful in the search for oil and gas because these deposits are natural insulators with high resistivity compared to water. Such differentiation allows modeling and identifying anomalies which can be attributable to the structures that potentially could host hydrocarbons.

Oil and gas reservoirs are generally a combination of brackish water and dissolved hydrocarbons. When telluric currents flow through the boundaries between hydrocarbons and water, they develop strong interfacial double layer electric charge effects. The accumulation of these charges results in an anomaly seen from the air by the eField Airborne EMT System. It's a phenomenon known as Natural Field Induced Polarization, or NFIP.

While the use of ground electric and magnetotelluric surveys have long been important oil and gas exploration tools, no one had applied the science in an airborne system focusing on these natural electric fields before. "We were the first for oil and gas, in 2002," says Johnson.

Conceptually, detection is relatively simple. Using a typical survey aircraft such as a Piper Navajo, Cessna 320 or KingAir, the patented system detects telluric flows by way of electric field sensors mounted in wingtip pods. The sensors include three orthogonal dipoles oriented along the X, Y and Z axes, and are attached to angular motion sensors that compensate for the angular motion of the airplane in strong fields. A total field magnetometer mounted in an eight-foot long tail stinger provides phase and amplitude references for the electric field sensors that enable them to provide accurate flow information about the telluric currents.

Not only location, but the source and composition of the hydrocarbon reservoirs are determined using the Airborne Offshore System. It maps the electric charge effects associated with dissolved hydrocarbons—microseeps—as they migrate to the ocean's surface, exploiting the fact that the hydrocarbons transform to aerosols at the surface. In moving from a highly conductive environment (seawater) to a highly resistive environment (air) they generate two distinct and measurable anomalies.

To further map these hydrocarbon seepages, eField sources information from the ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite sensor from a strategic partner. It's a useful and proven data source as scientists have demonstrated good correlation between spectral anomalies observed on ASTER data and known production.

Utilizing all these sources the partner has developed a general seepage model. Johnson explains that the imagery is processed using their Sea Enhancement Algorithm, then enlarged and printed for seep detection analysis and interpretation. "They employ a rigorous ranking system that assigns a value ranging from 1 to 10 to convey the analyst's level of confidence that a detected sea surface anomaly actually represents and oil slick caused by natural seepage, rather than some other source," he says. The ranking considers image data, typical anomaly characteristics, geologic and cultural setting, illumination parameters, and several other factors that may affect seep detection and discrimination.

The data reside on the company's custom database which interfaces with Geosoft's Oasis montaj software. That allows the integration of several other earth science datasets and images, including 2D and 3D seismic, satellite imagery and surface geochemistry. "The Geosoft platform has been very easy to adopt and apply to our incoming data stream needs," says Johnson. "Flying the first passive EM system for oil and gas means that the existing minerals based architecture didn't suit our sensor outputs. But we were easily able to build back-end databases to accept incoming sensor information (14), create automatic QC routines, and then build front-end tools for data analysis." He says the tools can almost run in real time and permit analysts to spot potential data for detailed interpretation.

"With the total package," Johnson says, "eField's technology enables airborne exploration of up to 100 square miles a day, detecting the presence of hydrocarbons to depths of well over 20,000 feet."

"The projected savings in cost and time, as well as the elimination of environmental damage caused by conventional surveying, will potentially enable the oil and gas industry to enter a new phase of exploration that has thus far been cost prohibitive," he says.

Moreover, there's the element of ease of access. "Early adopters are using airborne EMT where seismic is difficult or where access is a problem," says Johnson. "Or where additional data will help to high-grade many prospects (de-risking)." It doesn't replace seismic, however. "At this stage in development we are an adjunct to seismic or can plan where seismic would be more beneficial," he explains. "The known presence of hydrocarbons permits us to refine our interpretation using analogues."

It's a promising start as the company refines its offshore method. "It is developmental," says Johnson. "We are cataloguing responses in different settings so we are still discovering the full potential. Offshore Norway proved that there is a whole new world of potential available."

Many clients come to eField after having difficulty using traditional methods. "You know you are brand new from a technology standpoint when you tend to get used where other things don't work," says Johnson. "People will try new technology. Where seismic effectively doesn't work in these [Texas] shale formations for example, they've been trying other methods." And it's not only oil and gas, he adds. "It's the same in northern Saskatchewan where there's a lot of overburden—we're being looked at to fly for uranium because the existing systems don't go deep enough," he says, "and yet the passive systems like magnetics and gravity don't do the job. So there are applications where we can enter the market maybe because nothing else works." The only drawback to keeping busy in learning new applications he says is "it means extra time to figure out what we're seeing."

For eField's future, "data modeling is the next step as well as quantifying the results," says Johnson. "Integrating the results with existing oil and gas industry IT platforms is our immediate next priority."