Subsurface hydrocarbons detected by electromagnetic sounding

Abstract

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.