In 1997, ElectroMagnetic GeoServices AS (EMGS) founders pioneered a technology called SeaBed Logging (SBL) to detect and characterize hydrocarbon layers prior to drilling through the use of electromagnetic energy. The company made the first discovery of commercial quantities of hydrocarbons solely on the basis of the technology in 2001. To date, EMGS has successfully completed more than 70 surveys for more than 10 oil companies.

SBL has been demonstrated effective through calibration surveys over a known oil field offshore Angola and the Ormen Lange gas field offshore Norway. These surveys were acquired in deepwater areas (greater than 3,050 ft or 1,000 m) to minimize the background return signal from the air-water interface.

SBL will increasingly become a critical tool in the exploration and development process as it both lowers capital costs and increases corporate reserves by offering an innovative solution that detects the presence of hydrocarbons before drilling. Of the 70 commercial surveys performed to date, 18 have been drilled, and SBL has correctly predicted the existence or absence of hydrocarbons in every one of these instances.

To highlight new use of the technology, this article presents the results from a SBL calibration survey acquired in shallow water (1,080 ft or 330 m) over the giant Troll gas field offshore Norway. The Troll SBL survey successfully detected the Troll West Gas Province (TWGP), demonstrating that SBL can be applied in much shallower water depths than previously thought. Shallowwater SBL opens a new frontier in cost-efficient exploration.

Method and equipment

The SBL technique utilizes a horizontal electrical dipole (HED) emitting a low-frequency electromagnetic (EM) signal into the underlying seabed and downwards into the underlying sediments. EM energy is rapidly attenuated in the conductive seafloor sediments because the pore space is normally water-filled. In highly resistive layers such as hydrocarbon-filled sandstones, the energy is guided along the layers and attenuates less. Energy is constantly refracted back to the seafloor and is detected by seafloor EM receivers.

Each EM receiver is dropped from the vessel and freely sinks to the seabed. At the seabed the receivers are held in position by concrete anchors. After the recording period, an acoustic signal from the vessel triggers a release mechanism, causing the receivers to release from their anchors and float back to the sea surface, where they are retrieved (Figure 1).

The HED antenna consists of two electrodes separated approximately 750 ft (230 m) from each other with electrical contact to the seawater. The electrodes are positioned on a streamer section providing neutral buoyancy at depth. The streamer is towed behind an instrumented tow fish. Each electrode is connected electrically to a signal source located on the tow fish. The output signal to the electrodes is monitored at the tail of the tow fish. The source transmits a continuous periodic signal with any curve shape and frequency ranging from 0.05 Hz to 10 Hz. The peak-to-peak current varies from zero to several hundreds Ampere. Target altitude for the source is around 100 ft to 165 ft (30 m to 50 m). The depth of the tow fish and the HED are controlled by the length of the umbilical running from the survey vessel. The umbilical cable also provides power and signal transmission between the vessel and the tow fish (Figure 2).

The Troll gas field

The Troll field is the largest gas discovery on the Norwegian shelf, located in the northeastern part of the North Sea. The field is roughly separated in three parts, the oil province in the west and the western (TWGP) and eastern gas provinces. From TWGP there is also oil production from a thin oil leg. Operators of the field are Statoil (gas production) and Norsk Hydro (oil production). Water depth in the area varies between 985 ft and 1,120 ft (300 m and 360 m). Gas-filled reservoir intervals have resistivities around from 50 Ohm up to several hundred Ohm, while water-bearing sands and overburden generally show resistivities in the 0.5-2 Ohm range.

Data acquisition and quality

The survey was designed with one 2-D line across TWGP. A total of 24 EM instruments were deployed, and the line was towed from SW to NE with a sine wave signal of 0.25 Hz. The SBL data were acquired as time series and then processed by a windowed Fourier series analysis. The data quality was high with reliable information to an offset higher than 5 miles (8 km). This is due both to a high power source and high-quality receivers. Data were processed and displayed as magnitude vs. offset (MVO) at the transmitted frequency.

Results and conclusions

Figure 3 shows median normalized magnitudes between 3.7 miles and 4.3 miles (6 km and 7 km) offset for all receivers along the TWGP towline. The onset of increased MVO matches closely with the boundaries of TWGP with maximum increase in MVO occurring at the apex of the hydrocarbon accumulation.

SBL over the TWGP reveals systematically higher MVO for EM receivers above TWGP compared to EM receivers outside TWGP. The observed EM anomaly correlates with the inferred boundaries for TWGP, showing up to a 200% increase in magnitudes over central parts of the hydrocarbon accumulation.

The Troll survey opens a new and much needed frontier in application of shallowwater SBL. According to Wood Mackenzie, the oil and gas industry spends more than US $3 billion annually drilling unsuccessful exploratory wells in offshore basins and more than $10 billion developing offshore hydrocarbon fields. The ability for SBL to lower costs by offering an innovative solution that detects the presence of hydrocarbons before drilling is clear. In addition, SBL can locate reservoirs that are currently undetected by other geological or geophysical techniques.

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