The new frontier: Resurgence of Gravity and Magnetics

By Mark Wolfe

Land gravity measurement with differential GPS positioning.

Land gravity measurement with differential GPS positioning.

Advancements in earth mapping technologies, coupled with the industry's emerging need to characterize sub-surface systems, have led to a resurgence of interest in using gravity and magnetic methods in Oil and Gas exploration and development. 

Gravity and magnetic (or potential field) methods have a long history of use in the oil and gas industry dating back to the 1920s, but the petroleum industry lost interest in these techniques during the early ‘90's due to the rapid advances in seismic techniques.

Faced with the challenge of meeting growing global demand for gas supply, the industry has been steadily expanding exploration efforts into frontier and seismicly challenging areas. Both environments are ideally suited to gravity and magnetic techniques. 

"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., a Canada-based exploration software company.

Although seismic visualization 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.

Geosoft GM-SYS™ Profile model of salt structure integrating seismic reflection, FTG gravity, and magnetic data. Seismic image courtesy of Parallel Geoscience Corp.

Geosoft GM-SYS™ Profile model of salt structure integrating seismic reflection, FTG gravity, and magnetic data.  Seismic image courtesy of Parallel Geoscience Corp.

Exploration in the new frontier areas can be an order of magnitude more costly using seismic visualization 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 visualizing 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."

 

Brazilian-based 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 5,000 meters, in the Santos Basin located in the southeastern part of the country. The field, named Jupiter, may have the same dimensions as Tupi, a newly-discovered field believed to be Brazil's biggest oil-bearing area. The Jupiter well is 290 kilometres off the Rio de Janeiro state coast and 37 kilometres east of the Tupi area.

The Petrobras discovery reinforces the notion that there is decreased exploration risk in the pre-salt layer. In their announcement of the discovery, 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."

In the Santos Basin alone, the company has stakes in 13 blocks in initial exploratory phase in the pre-salt layer.
The Tupi field discovery was considered a new frontier for the industry.  The state-run oil firm reported last year that production tests confirmed the existence of a "significant volume" of 30 degrees API crude oil.  The field is located more than 7,000 metres below the ocean's surface. 

Gravity and magnetic methods are also starting to be used by junior companies engaged in frontier exploration. "There's been an explosion of smaller oil companies starting up and getting into exploration in the last several years. These companies are really flying under the radar and are having great success," Connard says.  "Many of them are starting to use gravity and magnetics in their exploration particularly in the frontier areas that have not been explored extensively in the past."

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.

"From the interpretation side, there is better software available to integrate the gravity and magnetics with the seismic and other geophysical and geological data," Connard says.  "This kind of technological integration has been key." 

Utilizing today's visualization tools, geoscientists are able to reduce risk and increase their understanding by looking at as much different data as they can, in as many different ways as they can, within compressed project time frames. 

Despite the fact that exploration companies are leaner, with fewer people and shorter project time frames, Dr. Michal Ruder, Consulting Geophysicist and Principal of U.S.-based Wintermoon Geotechnologies Inc., has seen exponential improvements in productivity and data quality as a result of new software for mapping and visualization. 

For Dr. Ruder, whose livelihood depends upon delivering accurate, up-to-date maps to clients in the oil and gas sector, recent advancements in integrated exploration tools mean smoother workflow and higher productivity in both two-dimensional and three-dimensional environments. 

Where it used to take weeks to process and interpret geoscience datasets, today it's not uncommon for geoscientists to address the salient issues of interpretations in the course of one or two days.  Increasingly, what's required for exploration is software that can handle large volumes of data and multiple data sources and data types, such as geophysical data, geochemical data, drillhole data, satellite imagery and GIS data within one single environment or transparently linked environments.

"I can remember doing batch maps, in paper copies, back in the 1980's," Dr. Ruder says. "Since then, the ability to image geoscientific datasets on a computer screen in real-time, and continual improvements in visualization software, has had an amazing impact on what we can do, as geoscientists, and how quickly we can do it."

Interpretation results are also more accurate because geoscientists have the tools to view the quality of the data in every single phase, from initial data processing and quality control through to visualization, integration, and the final interpretations.

Equipped with her laptop and mapping software, which includes ESRI's ArcGIS and Geosoft's Oasis montaj, it's not uncommon for Dr. Ruder to do on-the-spot interpretations in collaborative meetings with her major oil and gas customers.

"I can do a lot of work in my clients' offices on my laptop," she says. "My mapping software offers a lot of interactivity, and testing of their hypothesis, and I can show customers results in real time." 

Geosoft GMSYS-3D model of a salt body embedded in a 3D density volume.

Geosoft GMSYS-3D model of a salt body embedded in a 3D density volume.

Geosoft GMSYS 3D model.

Geosoft GMSYS 3D model. The three flat maps along the left side of the workspace show Observed Gravity, Calculated Gravity, and the Difference. The black line marks the location of the seismic section shown in the 3D visualization on the right. The "+" symbol in the flat maps tracks the location of the 3D cursor in the 3D visualization (the long vertical line at the end of the seismic section). The other vertical plane in the 3D visualization is the sub-surface gravity response.

There are efficiency and quality advantages to being able to dynamically pull customer data in, and immediately look at it as part of the interpretation whether it's well data, satellite imagery, or other types of data.

This type of rapid assessment and dynamic experimentation depends on the ability to interactively display, and enhance, different attributes in different ways, whether through contrast enhancement, shaded relief, angle illumination, or two-dimensional or three-dimensional displays. 

Speed in creating and recreating visualizations, or refreshment time, is also an important consideration, especially when dealing with large datasets.

The use of specialized three-dimensional modeling software for prospect modeling of salt bodies can help to further reduce risk in areas such as potential field exploration.

"It's very prudent to do three-dimensional modeling for prospect modeling of salt bodies when you're considering a very expensive well in deep water," Dr. Ruder says. 

"We've been using 3D seismic volumes, and approximations of a 3D velocity volume in our interpretations for some time.With modeling software such as Geosoft GMSYS 3D, we can convert that to depth, and ensure that it makes sense with the observed gravity and magnetic data."

Today's visualization software also supports the easy integration of different types of datasets, including geoscientific data, satellite imagery, and other GIS data into the mapping environment. Working in multiple software environments is a reality for geoscience consultants like Dr. Ruder who need to meet their needs, as well as the needs of customers with a variety of software preferences. 

While geoscientists recognize that there's a lot to gain by looking at different types of data, Dr. Ruder admits that there is still a tendency, within each discipline, to use the data that they understand the best.

"I think people don't realize how easy it is to integrate all of their datasets, whether it's seismic and non-seismic, raster and vector," she says. "GIS software and Oasis montaj provide some great tools for that."

Technology for team-based exploration

Exploration software has adapted to meet the needs of today's multidisciplinary and team-based exploration. As teams of structural geologists, petroleum technicians, seismologists and geophysicists work together to develop Oil and Gas exploration projects, maps and results need to be quickly and easily shared across different software platforms. Read more