Electromagnetic seabed logging (SBL) has the potential to eliminate a large chunk of exploration uncertainty. Early success is impressive, but commercial uptake has lagged.

The response of hydrocarbon-bearing strata to an electromagnetic field has been known for some time, and is the principle behind modern electric logging tools. In fact, the Schlumberger brothers developed the electric log in the early part of the 20th century following experiments on their estate in Normandy, where they laid out an array of electrodes on the ground to map the presence of conductive strata thought to contain iron ore. They proved that an electrical field reacts to the conductivity of its environment.

However, logging tools require a borehole, so in essence, they can only confirm and help to quantify the presence of hydrocarbon after an exploratory well is drilled. A new technique called SeaBed Logging, developed by Statoil and offered by ElectroMagnetic GeoServices, AS (EMGS) of Trondheim, Norway, shows great promise in detecting and confirming the presence of commercial quantities of hydrocarbon before drilling.

How it works

Briefly, the system involves two elements. An array of autonomous electromagnetic receivers is laid in a pattern on the seabed from a work vessel. The survey area has been predetermined from surface seismic, and overlies structures identified as potential reservoirs. Then, a survey vessel tows an electromagnetic streamer sonde containing a horizontal electric dipole (HED) transmitter in a criss-cross pattern over the sensor array. The sonde, which is held in neutral buoyancy a fixed distance above the seabed, emits a strong omni-directional electromagnetic signal. If resistive hydrocarbon-bearing strata are located beneath the seabed, the signal wave is guided along it. A portion of the electromagnetic signal leaks back to the seabed receiver array, altering the natural pattern of current flow in the overburden layer. The seabed receivers store the data in non-volatile memory, which is downloaded to the processor when the unit is retrieved.

A detailed discussion of the physics of the SBL measurement can be found in EAGE Z-99 (Florence, Italy 27-30 May 2002) and in recent articles appearing in The Leading Edge (Oct. 2002) and First Break (Mar. 2002) magazines.

The key attribute that makes this technique valuable is the use of the dipole antenna. Whereas a single in-line antenna cannot generate guided waves, the HED antenna has different sensitivities to thin resistive layers. Through modeling and interpretation, in conjunction with surface seismic surveys, a clear picture of the areal extent of the hydrocarbon-bearing portion of the structure emerges.

The principle was tested offshore Angola in November, 2000, in 4,329 ft (1,320 m) of water over a known hydrocarbon reservoir featuring two overlying channel sands and a highly-resistive salt diapir. An array of 26 seabed receivers was laid in a pattern that included the salt structure to ascertain whether the survey could discriminate salt from hydrocarbon resistivity. 17 tow lines were made totaling 196 miles (314 km).

Results were impressive. Azimuthal measurements from a single receiver located roughly in the center of the survey area show clearly how measured data compare with two models - one with oil and one without. The oil water contact can be seen in relationship with the structure as shown in the seismic section.

According to Per Brandeshaug, chief engineer of the Norwegian Petroleum Directorate (NPD), "In 2001, seven dry holes were drilled on the Norwegian shelf at a cost of about 700 million NOK [about US $100 million]. With seabed logging, some, if not all, of this cost might have been saved." One can imagine that an explorationist, armed with a good seismic picture of the subsea structure and confirmation of the presence and areal extent of hydrocarbon would be encouraged to drill with reasonable certainty of hitting commercial pay.

Since the Angola test, several commercial surveys have been run offshore Norway. One, at Ormen Lange gas field, attempted to calibrate the SBL technique in a known gas reservoir. Data were processed and displayed as magnitude-versus-offset (MVO) at a predefined frequency or frequencies. Higher MVO was observed over the gas-filled zones than water-filled ones.

Since June, 2003, EMGS has run continuously back to back surveys for customers in the North Sea, Norwegian Sea and Barents Sea. SBL data have been collected over more than 40 prospects.

The devil's in the details

While the acquisition of electromagnetic data in the subsea environment is fairly straightforward, interpretation can be complex, and requires a synergistic approach. Several obvious questions come to mind. We put these to Terje Eidesmo, President of EMGS, and co-inventor of the technique:

Q. How does the measurement discriminate between a single oil-bearing reservoir and two or more formations that overlie each other?


A. We have seen from the Angola example that it is possible to discriminate between two overlaying hydrocarbon-bearing reservoirs by using different frequencies.


Q. What challenges must be overcome to survey in very deep water [>6,562 ft (>2,000 m)]?


A. The equipment is rated to 11,483 ft (3,500 m) which will cover most of the present known reservoirs. We have done surveys in the range from 393 ft (120 m) water depth down to 9,843 ft (3,000 m) plus.


Q. What is the practical limit of formation depth below the seabed that can be probed?


A. This depends on the complexity of the overburden, the reservoir resistivity, reservoir thickness and reservoir size. In each case we model the prospect beforehand, either with 1D, 2D or 3D, to evaluate if there is a reasonable chance to image the reservoir with SBL.


Q. What is the minimum practical hydrocarbon layer thickness that can be detected?


A. Again this depends of the complexity of the overburden, the resistivity of the reservoir and depth of burial. We typically say that it is possible to detect a 50 ohm reservoir, 65 ft (20 m) thick, 6,562 ft (2,000 m) below the mud line.


Q. How can the measurement discriminate between oil and gas?


A. As the method is today, it is not possible to discriminate between oil and gas. But when interpreted in conjunction with seismic it is possible to discriminate oil and gas.


Q. How does the measurement respond in low-contrast pay zones?


A. SBL typically responds to the resistivity contrast between overburden and reservoir. Typically, low-contrast formations are indicative of high connate water saturations, either due to small grain sizes or the presence of dispersed clay. But through a combination of velocity modeling and local knowledge, it may be possible to identify these zones on the seismic section, and configure the SBL tool to image a complementary frequency spectrum that will see the oil.


Q. How long does interpretation take?


A. We typically process, interpret and report the job within 2 months after data have been brought on shore.


Q. Where are you looking to grow the company?


A. We have just been granted our US Patents, and are looking to offer our services worldwide.


Experience to date indicates that SBL can be a valuable aid in reducing exploration uncertainty. As more surveys are run and interpretation techniques are refined, the system's potential to save drilling dollars should grow. A significant reduction in dry holes is clearly possible. The key is to remain conservative so that no commercially viable hydrocarbon-bearing prospects are passed up.




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