Seismic imaging techniques can readily detect potential hydrocarbon (HC) traps but discriminating between the presence of water or hydrocarbons in such traps is a challenge. Detection of subsurface hydrocarbons by a controlled source electromagnetic surveying application, termed seabed logging (SBL), has recently showed very promising results, but has until now not been fully demonstrated. Here we present SBL data from the Troll West Gas Province offshore Norway, providing irrefutable evidence for direct detection of a deeply buried hydrocarbon accumulation by electromagnetic sounding. This survey also demonstrates that the SBL can be applied in much shallower water depth than previously thought. This result opens a new frontier in hydrocarbon exploration.
Electromagnetic seabed logging (SBL) is an advanced electromagnetic method developed by Statoil, and com-mercialized through ElectroMagnetic GeoServices (emgs), for remote and direct identification of high-resistivity hydrocarbon saturated reservoirs layers in large and medium water depth areas.
Offshore hydrocarbon exploration is both challenging and expensive. Acquisition and processing of 2D and 3D seismic data occupy a significant portion of any exploration budget. It is, however, the drilling of wells that represents the main cost. It is an unfortunate fact that the risk of an exploration well failing to discover hydrocarbons is high. Any means to increase the discovery rate will be of great interest and benefit for the oil and gas industry.
Remote sensing techniques record variations in petrophysical parameters such as acoustic, elastic, electric or gravimetric properties. Seismic exploration is by far the most common tool and uses acoustic and elastic waves to map boundaries between buried layers with contrasting P- and S-wave velocities. Seismic data can provide high-resolution information about geological strata but have limited capabilities to characterize the properties within the reservoirs. Given a mapped structural geometry that may allow for accumulation of hydrocarbons within porous sediments, the uncertainty is whether the reservoir pore space contains oil, gas, brine or some combination of these. Seismic data are not particularly good at determine the fluid content of a reservoir rock and in especially estimating the water saturation. However, hydrocarbon-filled reservoirs are often better characterized by the electrical resistivity rather than seismic parameters. For instance, P-wave velocity is affected relatively less than electrical resistivity as a function of varying brine saturation (Figure 1).
Figure 1: Representative P-wave velocity (red line) and resistivity (blue line) as a function of brine saturation:
oil case and
gas case.
The P-wave velocity is calculated using Gassmann's equation assuming 30% porosity and 10% fraction of shale in reservoir rock with "loose" sandstone. The effective bulk modulus is calculated using Voigt's model for solid and Wood's model for fluid. The resistivity is calculated using Archie's law with resistivity of clean brine equal to 2.0 ?m. (Available in full paper)
(Available in full paper)
Electromagnetic (EM) methods are sensitive to changes in electrical resistivity. For this reason resistivity is one of the most common borehole logging measurements used in the oil and gas industry. Resistivity values from well logs of hydrocarbon saturated rocks are several orders of magnitude greater than the same rock saturated with brine.
