The New Frontier:
Exploring for Oil with
Gravity and Magnetics
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Innovative integration of gravity and magnetic methods help Marathon Oil Corporation maintain world leadership in oil and gas exploration
By Graham Chandler
When Pat Millegan came to Marathon Oil Corporation in 1983, he brought with him a keen interest, and experience in gravity and magnetic methods for oil and gas exploration. Now with 32 years of industry experience under his belt, there's little the company's Geoscience Consultant, Subsurface Imaging doesn't know about the techniques —the whole gamut from planning and specifying surveys, quality control of the data and its processing through to integrated interpretation; all using the latest innovations.
It's made him a valuable team asset for a company like Marathon, which has a strong record of success in exploring for oil and gas around the globe. One of the oldest oil companies in the industry, Marathon was established in 1887. Headquartered in Houston, Texas, Marathon is the fourth largest integrated oil and gas company in the US, with revenues over US $65 billion in 2007.
Marathon's exploration activities focus on adding profitable production to existing core areas – the U.S., Equatorial Guinea, Libya and the North Sea (UK and Norway) – and developing potential new core areas in Angola and Indonesia. The company has long been active in Libya's Sirte Basin, one of the most prolific oil and gas producing areas of the country; which still contains sizable undeveloped reserves. Marathon's concessions there currently produce about 345 thousand barrels of oil equivalent per day on a gross basis and encompass almost 13 million acres. Marathon holds a 16.33 percent working interest in the Waha concessions in Libya.
Behind Marathon's record of success is a focus on realizing the full potential of their upstream assets through knowledge integration and technological innovation. In the oil and gas business the key to optimizing production and resource development is quick and accurate description of reservoirs. Marathon's expertise in reservoir characterization often begins with seismic imaging, but it emphasizes integration of all geoscience, petrophysical and engineering data into fully integrated interpretations.
An important part of their integrated approach is effective use of potential fields methods. The benefits of such an approach are higher success rates in discovery, drilling and production activities and Marathon's record illustrates that.
Gravity and magnetic methods have proven to be effective in many regions of the world, particularly in sub-salt areas. Without the use of these data, prolific sub-salt traps would be much more difficult to locate and image. Using seismic alone, the high velocity salt layer absorbs and scatters the energy, distorting any picture of prospective reservoirs lying beneath the salt layer itself. Used in a tightly integrated fashion, potential fields methods can supply a better estimate of the salt shape and size, which is then used to improve the sub-salt seismic imaging.
Offshore West Africa, the Gulf of Suez, the Red Sea, and of course the Gulf of Mexico are all regions where gravity and magnetics have enhanced exploration in this way. Though generally considered to be complementary to seismic, in some cases drilling decisions have been made on the basis of gravity and/or magnetics alone particularly when seismic imaging is challenging. In one case, aeromagnetics has been credited with the discovery and delineation drilling of Ras El Ush Field in the southern Gulf of Suez in the 1990's. Today, the integrated workflow used in the Gulf of Mexico salt interpretations is an integral part of decision making for leasing and drilling.
Sub-salt imaging is a prime example of where these methods excel, but not the only one, says Millegan. "Gravity and magnetics are doing well with the current resurgence of exploration in general," he says. "Exploration is becoming more difficult, entering remote areas and areas of poor seismic imaging."
Working with the techniques for better than three decades, Millegan has seen their value and potential over the years. "Gravity and Magnetic techniques have never ceased being valuable to the exploration effort," he says. "As exploration in general increases at increasingly higher costs, gravity and magnetics' proven track record for risk reduction and integrated G&G [geological and geophysical] analysis has kept it in demand."
Integrating them with other data streams these techniques have played, and continue to play, an important part in Marathon's longstanding expertise and successes in accurate reservoir definitions and delineations. "Integrating gravity and magnetics with seismic, each measuring different, but related rock properties, offers more robust geologic interpretations," explains Millegan. "This provides management with the best information available to make leasing or drilling decisions."
Management and integration of gravity and magnetics is just part of the team effort, but it's a critical part. "My success is measured by the success of the team with which I am working," says Millegan. "Often there is a direct cause and effect, where gravity and magnetics offer something direct, such as the calculation of salt mass, or the depth and extent of a structural block, or the presence/absence of volcanics in the section." Or, sometimes the contribution is less concrete, he says, like perhaps a regional geologic/tectonic analysis that helps the team integrate various disciplines, thus helping to focus their decision making.
It's all kept Marathon at the leading edge of oil and gas exploration.
Use of gravity and magnetics with Marathon is increasingly impacting its technology needs. "We are busier than ever," says Millegan. Marathon added Dr. Neda Bundalo to their Subsurface Imaging team in 2007. "Neda has allowed us to continue to offer high quality work in a timely fashion," he says."Our workload for gravity, magnetics and EM has doubled this year. More now than ever we need our software tools to be responsive, integrated, and technically state-of-the-art. This is a huge challenge."