Expanding Range of Geophysical Exploration Methods

For many years, the chances of exploration success in potential oil and gas regions have been improved through the use of dierent geophysical exploration methods, which can be used to detect or infer the presence and position of hydrocarbons in the subsoil. By using physical methods, exploration geophysics technologies are able to measure the dierences between various types of rocks, in order to discern between those that contain hydrocarbons and those that do not.

Seismic reflection techniques have long been the standard method of geophysical exploration in the oil and gas industry. Using a seismic source of energy, operators send seismic waves into the selected geological region, and based on the data they receive back are able to estimate the properties of the subsoil subjected to the seismic waves. However, gravity and magnetic techniques are increasingly being adopted by the oil and gas industry, as they can be used to determine the geometry and depth of covered geological structures. Magnetotellurics and electromagnetic techniques can provide results by detecting resistivity changes in the subsoil, particularly useful when trying to detect hydrocarbons through a salt layer.

Dave Ridyard, President of EMGS Americas, a Controlled Source Electromagnetic (CSEM) surveying company, is keen to stress that his technology is not a replacement for traditional seismic survey. “You have to have some seismic first. 90 percent of our customers will do 3D seismic first and, following the completion of a seismic survey, use CSEM to reduce risk in the well,” Ridyard says. “Many companies would spend a couple of million dollars to increase the chances of success when drilling their US$100 million well.”

During the exploration stage, CSEM is typically used after the various stages of seismic survey, including 2D seismic, infill seismic, exploration 3D and even time-lapse 4D seismic. However, some companies, including Statoil, have started using CSEM at dierent stages in the exploration process. In April 2011, Statoil announced that it had discovered reserves of 250 million bbl in Norway, using only existing 2D seismic data collected by the Norwegian Petroleum Directorate (NPD) and EMGS multi-client 3D EM data. The total exploration project cost less than US$1 million, but Ridyard describes such projects as a “radical concept” at the moment.

Despite such developments, Ridyard does not believe that CSEM has the potential to damage the seismic industry: “When E&P companies want to do their appraisal programme, they will now come in and do a very detailed 3D survey. What’s more, they’ll do it in full knowledge that they have got a discovery. In the end they would actually spend more on getting higher quality seismic because they have no risk to worry about.”

OVERVIEW OF GEOPHYSICAL EXPLORATION METHODS

1. Seismic methods, including seismic refraction, seismic tomography and reflection seismology

2. Geodesy and gravity techniques, including gravity gradiometry

3. Magnetic techniques, including aeromagnetic surveys

4. Electrical techniques, including induced polarization and electrical resistivity tomography

5. Electromagnetic methods, including magnetotellurics, ground penetrating radar and transient electromagnetics

6. Borehole geophysics, or well logging

7. Remote sensing techniques, such as hyperspectral