The new frontier

Mark Wolfe investigates the resurgence of gravity and magnetics.

“Faced with the challenge of meeting growing global demand for gas supply, the industry has been steadily expanding exploration efforts into frontier and seismically challenging areas”
-Mark Wolfe

Advancements in earth mapping technologies, coupled with the industry's emerging need to characterise sub-surface systems, have led to a resurgence of interest in using gravity and magnetic methods in oil and gas exploration and development. Faced with the challenge of meeting growing global demand for gas supply, the industry has been steadily expanding exploration efforts into frontier and seismically challenging areas. Both environments are ideally suited to gravity and magnetic techniques (also known as potential field).

"Potential fields are typically used in frontier areas to do reconnaissance exploration, to look for new basins, and to investigate large areas that might be prospective for doing more detailed and expensive seismic work," says Gerry Connard, Petroleum Industry Market Manager with Geosoft Inc.

Although seismic visualisation technology still remains the powerhouse of oil and gas exploration, the industry now has the tools and the knowledge to integrate the best of this technology with gravity and magnetic methods. By combining technologies, the industry has added a robust new dimension that is enabling explorers to look more quickly and efficiently into the uncharted frontier, while reducing the risks associated with technically challenging exploration.

Exploration in the new frontier areas can be an order of magnitude more costly using seismic visualisation when compared to the less expensive potential field methods. Economics aside, Connard says that gravity and magnetic methods are commonly used in areas where seismic work is difficult or impossible. He notes as an example instances where you are trying to gather images beneath basalt-covered areas that have high-velocity rocks near the surface or exploring sub-salt plays. “When visualising salt structures, seismic technology is very effective for imaging top of salt but has difficulty imaging below the salt because of the high velocity of salt,” Connard says. “Gravity and magnetics have been used extensively in sub-salt exploration to integrate with the seismic data and image the base of the salt, or to assist in the processing of the seismic data.” Petrobras has experienced major success using gravity and magnetic methods with the discovery of a major natural gas and condensate field in the pre-salt layer, at a depth of 5000 meters, in the Santos Basin located in the south-eastern part of Brazil.

The Petrobras discovery reinforces the notion that there is decreased exploration risk in the pre-salt layer. In their discovery announcement, Guilherme Estrella, Petrobras Exploration and Production Director, is quoted as saying, “All of the pre-salt blocks achieved exploratory success, something that confirms the region's high prospectivity”.

"There's also been a recent explosion of smaller oil companies getting into exploration,” Connard says. “Many of them are starting to use gravity and magnetics in their exploration of the frontier areas with great success.”

Advances in software that provide the ability to effectively display, rapidly assess, and dynamically experiment with multiple datasets have helped to reduce risk and increase prospecting capabilities in exploration. These technological innovations have helped to make the use of gravity and magnetic methods in the oil and gas industry more effective.

The use of specialised three-dimensional modelling software for prospect modelling of salt bodies can help to further reduce risk in areas such as potential field exploration. “We've been using 3D seismic volumes and approximations of a 3D velocity volume in our interpretations for some time.” says geophysicist Dr Michal Ruder of Wintermoon Geotechnologies. “With modelling software such as Geosoft GM-SYS, we can convert that to depth, and ensure that it makes sense with the observed gravity and magnetic data."

Today's visualisation software also supports the easy integration of different types of datasets, including geosciences data, satellite imagery, and other GIS data into the mapping environment. And while geoscientists recognise that there's a lot to gain by looking at different types of data, Dr Ruder admits. "I think people don't realise how easy it is to integrate all of their datasets, whether it's seismic and non-seismic, raster and vector.”

Mark Wolfe is a Canada-based freelance writer, editor, researcher and photographer who specialises in business and industry, oil and gas, construction, health care, education, and technology. His articles and photographs have appeared in over 18 publications across North America over the past 12 years.