SeaBed Logging (SBL) is an application of the marine controlled source electromagnetic (CSEM) method that is used to directly detect and characterize possible hydrocarbon-bearing prospects. Although the CSEM method has been used by academia for more than three decades, the application as a direct hydrocarbon indicator was first introduced about five years ago. The central idea of SBL is the guiding of electromagnetic energy in thin resistive layers within conductive sediments. Even if it has been well known for a long time that electromagnetic signals can propagate from a conductive region to another via resistive regions such as air or resistive parts of the lithosphere, the application to hydrocarbon exploration has not been developed until recently. This might be due to the uncertainty of getting any significant response from thin resistive layers such as hydrocarbon reservoirs since electromagnetic energy is highly attenuated in conductive sediments. Thus, during the early development phase of the SBL technique, a scaled laboratory experiment was performed to validate if a thin resistive layer (e.g. hydrocarbons) buried within conductive media (e.g. sediments) could be remotely detected by using electric dipoles as sources and receivers. Data from this experiment were compared to a forward modelling code for layered media, and the comparison showed good agreement between experimental and theoretical results. This suggested that thin resistive layers buried in conductive media are detectable due to the guiding of the electromagnetic field within the resistor. The successful results were vital for realizing the application of marine CSEM as a hydrocarbon exploration technique. We here present the results of the first scaled SBL experiment.

Terje Eidesmo Svein Ellingsrud Ståle E. Johansen Rune Mittet Electromagnetic Geoservices AS Trondheim, Norway An electromagnetic (EM) technique for marine hydrocarbon prospecting, first used commercially in November 2002, has gained wide industry acceptance. The technique identifies resistive reservoirs by measuring the energy received at long source–receiver offset distances (greater than three times the reservoir depth). Statoil researchers first showed the potential of the approach during the late 1990s. Electromagnetic Geoservices AS (emgs), based in Trondheim, Norway, was formed in 2002 to commercialize the technique. In the past few years, many improvements have been made to operating practices, survey equipment, and data-processing techniques. These developments have resulted in the acquisition of a considerable body of highquality data under a wide range of conditions, and the delivery of sophisticated answer products that now include depth-migrated images of resistive subsurface bodies.

Seismic imaging techniques can readily detect potential hydrocarbon (HC) traps but discriminating between the presence of water or hydrocarbons in such traps has remained a challenge. Detection of subsurface hydrocarbons by an active source electromagnetic (EM) sounding application, termed seabed logging (SBL), has recently shown very promising results, but has until now not been fully demonstrated. Here, we present SBL data from the Troll West Gas Province (TWGP), offshore Norway, providing irrefutable evidence for direct detection of a deeply buried hydrocarbon accumulation by electromagnetic sounding. A powerful horizontal electric dipole (HED) source induced up to 170 % increased subsurface returned signals above the gas accumulation. This result opens a new frontier in hydrocarbon exploration. Introduction Remote sensing techniques record variations in petrophysical parameters such as acoustic or electric properties. Seismic sounding is by far the most common of such tools and typically uses acoustic waves to map boundaries between layers with contrasting acoustic properties. Seismic data can provide detailed information about layering but is not very well suited for direct detection of pore fluid composition. Given detection of a structural geometry that may have allowed accumulation of HC within porous sedimentary rocks, the main remaining uncertainty is therefore whether the pore space is filled with saline water or HC. For this reason only 10-30% of exploration wells penetrate commercial oil or gas reserves in many areas. Electromagnetic sounding uses EM energy transmitted by an HED source to detect contrasts in subsurface resistivity. Resistivity variations in rocks are generally controlled by the interplay between highly resistive minerals (1011-1014 Ωm) and pore fluids including low resistive saline water (0.04-0.19 Ωm) and/or infinitely resistive hydrocarbons (Rider, 1996). Tight crystalline rocks such as oceanic crust typically show high resistivities (100-1000 Ωm) with variations mainly controlled by saline fluids in fracture networks. Sedimentary rocks can exhibit a wide range of resistivities (0.2-1000 Ωm) mainly controlled by variations in porosity, permeability and pore connectivity geometries in addition to pore fluid properties and temperature (Rider, 1996; Schlumberger, 1987). The high resistivity of hydrocarbon filled reservoir rocks (30-500 Ωm) compared with reservoirs filled with saline formation water (0.5-2 Ωm) makes EM sounding a potential tool for detection of subsurface HC. Although EM techniques have been used for many years, EM sounding has until recently not been applied in offshore HC exploration. A full scale EM sounding test offshore Angola in 2000 indicated that a new application of EM sounding, SBL, had a promising potential for direct detection of deeply buried hydrocarbons (Ellingsrud et al., 2002; Eidesmo et al., 2002). Until now the interpretation of SBL data has been hampered by the lack of statistically significant calibration data demonstrating that deeply buried HC accumulations were detectable by the SBL method. However, recent development of a new powerful HED source has opened the way for improved acquisition, processing and interpretation of SBL data. In this study we present SBL data across the TWGP, offshore Norway. Increased EM retur n signals over TWGP are caused by reflection and refraction of EM energy from a high resistivity HC accumulation situated ca 1100 m below the seabed. These data are in accordance with modelling results and provide the first evidence for direct detection of a deeply buried hydrocarbon accumulation by subsea EM sounding.

A system for investigating subterranean strata. An electromagnetic field and a seismic event are applied from the same location and the responses are detected using respective receivers both located at a second location spaced from the first. The responses are combined to identify the presence and/or nature of subterranean reservoir. The refracted wave components are used.

Remote resistivity sensing of buried resistive layers within conductive sediments, a concept called Sea Bed Logging (SBL), has been proven successfully by several surveys. A survey carried out offshore Angola proved to be ideal for the SBL technique due to large water depths, short distances from the sea floor to the reservoirs and high resistivity contrasts between the overburden and the hydrocarbon bearing zones. Another survey over the Ormen Lange gas field, offshore Norway, demonstrated that the technique has high potential also in more adverse areas with rough sea floor topography, rather large distance from the seafloor down to the reservoir and relatively low hydrocarbon saturation (low resistivity).

Detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry. Seismic methods have traditionally been used in this context, but the results can be ambiguous. Another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids. Modeling presented by Eidesmo et al. (2002) demonstrates that by using seabed logging (SBL), a special application of frequency domain controlled source electromagnetic (CSEM) sounding, the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified. Such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying.

In this paper we describe a technique called Sea Bed Logging (SBL), an application of marine CSEM sounding, which can be applied to detect and characterize hydrocarbon bearing reservoirs in deep water areas.

D-08 REMOTE DETECTION OF HYDROCARBON FILLED LAYERS USING MARINE CONTROLLED SOURCE ELECTROMAGNETIC SOUNDING 1 T. EIDESMO 1 S. ELLINGSRUD 1 L.M.MACGREGOR 3 S. CONSTABLE 2 M.C.SINHA 3 S. JOHANSEN 4 H. WESTERDAHL 5 and F.N.KONG 5 Introduction Measurements of sub-seafloor electrical resistivity obtained by wire line logging of wells have traditionally played a crucial role in hydrocarbon exploration and reservoir assessment and development. However there are clear advantages to developing non-invasive geophysical methods capable of providing similar information albeit at a lower vertical resolution. The vast saving in terms of avoiding the costs of drilling test wells into structures that