| Searching for Oil from Space
An interpreted RADARSAT-1 SAR image illustrates a variety of water conditions off the coast of Nigeria. SAR is sensitive to changes in surface texture and gives different return signal information for different surfaces. Because oil dampens capillary waves, oil-covered surfaces are smoother than surrounding water and reflect the radar signal away from the satellite. Oil seeps are characterized by distinct areas of darkness on a radar image. By examining the shape and size of the dark regions on an image, in conjunction with ancillary data, an image interpreter can discriminate between pollution, phytoplankton, wind shadow, grease, ice or oil seep.
Oil and gas exploration is inherently risky, and offshore oil exploration is an especially huge gamble. According to a joint study by energy consultants Wood MacKenzie and Fugro Robertson, two-thirds of all oil-and-gas reserves discovered globally in 2002-2003 were in water 1,200 feet or deeper. In the Gulf of Mexico, Unocal's Trident well took 66 days to drill at a cost of $34 million, and the company's St. Malo well had to be drilled nearly 36,000 feet deep—almost seven miles—before striking oil. Seeking every possible competitive edge, oil companies are turning to radar satellite imagery to improve deepwater exploration success. The SAR Advantage
Combining synthetic aperture radar (SAR) imagery with other data such as satellite-derived gravity and bathymetry enables companies to clearly detect and map natural oil seeps on the sea surface and correlate them to sub-surface structures. It's a method that has proved effective in Angola, Nigeria, Brazil and the Gulf of Mexico.
So well documented is the use of SAR imagery as a long-range reconnaissance guide to indicate seep activity that most oil companies now routinely study data from Canada's RADARSAT-1 satellite, as well as the European Space Agency's Envisat and ERS satellites, to help detect seepage.
That reliance isn't likely to diminish in the current environment of increasing demand and pressures to meet unabating demand with tight supply conditions and a substantial number of aging global fields in the Middle East. Adding to this deepwater race are the prospects of devising solid bid strategies for new frontiers coming online in offshore Mexico, West Africa, Venezuela, Vietnam, Libya and India. The high price of oil also makes smaller and more obscure fields more economically attractive.
"SAR imagery is a key part of the new venture exploration," says Alan Williams, oil and gas manager at Nigel Press Associat es Limited (NPA) in Edenbridge, England, a company specializing in satellite applications for more than 25 years. "Mapping subtle, yet clear, examples of natural hydrocarbon seepage to within a few hundred meters using radar imagery presents an important confidence builder to petroleum companies in deciding to commit exploration expenditure in an area where doubts previously existed about the presence of a working petroleum system. That data helps them to decide which license rounds they want to bid on based on perceived acreage value." Radar's Edge
NPA has been providing oil seep detection studies to oil and gas companies such as ExxonMobil, Shell, Chevron, Total and BP since 1994 through its Offshore Basin Screening (OBS) technique. The method combines seepage detection using RADARSAT-1, ENVISAT and ERS imagery with high-resolution satellite gravity to investigate basin structure, and to map regional seep distribution patterns on the sea surface and oil migration pathways at the seabed. Williams says NPA has screened and mapped 75 percent of the world's basins to ultra deep water depths up to 10,000 feet and maintains more than 6,000 interpreted radar scenes in its Global Offshore Seeps Database (GOSD).
Petroleum companies can receive an archived GOSD digital dataset of seep-related information in 24 hours that comprises the locations and classifications of the seeps. Each has been categorized and rated by characteristics such as size, shape, slick orientation and repeatability, and interpreted with collateral data like water depth, wind speed and geographical location. Each slick is also hyperlinked to a high-resolution image in the database, enabling users to view the slick in a wider context of the whole scene. Clients can also receive a more detailed map of the basin complete with bathymetry, gravity and regional geology.
NPA has used the OBS technique worldwide to provide oil companies with natural seepage evaluations in areas such as Vietnam, Venezuela, Brazil, the Gulf of Mexico, Norway and Angola. Angola, in particular, has yielded positive results for petroleum players, as deepwater studies conducted by NPA in 1998 correctly predicted where eventual discoveries would be. "There have been a recent string of major discoveries along deepwater offshore Angola where we had previously mapped an extensive zone of intense and repeated oil seepage based on our analysis using primarily RADARSAT-1 data," says Williams. "The companies that bought those studies before they were licensed have now been very successful."
Capable of being applied in any offshore environment, Williams says the OBS provides competitive intelligence at a competitive price. "For many regions, there isn't any way petroleum companies can obtain widespread geological information with traditional methods. Using SAR imagery, we can now screen large areas for less than $0.50 per square kilometer because the satellites provide wide and continuous coverage."
The low-cost technique will offer an even better package with the launch of Canada's RADARSAT-2 satellite. Adds Williams, "RADARSAT-2's dual polarization modes could help better define oil slick composition. Also, the high 3-meter-resolution swath will be ideal for finding smaller seeps that are associated with leakage from light oil and condensate traps that are invisible to conventional SARs."
RADARSAT International (www.rsi.ca), now doing business as MDA, Geospatial Services International, will be responsible for RADARSAT-2's operations and data distribution rights after launch in early 2006. Global Seeps
Helping oil companies see the most important element in the exploration process—the geological structure of a basin—is the premise behind the Global Seeps (GS) Database that Infoterra has been offering to petroleum clients for 10 years. Infoterra ( www.infoterra-global.com) is a geoinformation products and services provider based in Leicester, England, and is a wholly owned subsidiary of the European space company EADS Astrium.
According to Paul Russell, Infoterra's general manager of remote sensing applications, in recent months the company has provided oil seep detection studies to clients for offshore Africa, Australasia, South America and the Arctic. The Arctic region — an exploration hotbed at present — has yielded particularly promising results, as the GS identified previously undetected oil seeps in the Barents Sea and offshore Siberia using RADARSAT-1 Wide Beam imagery (30-meter-resolution and nominal coverage of 150 kilometers x 150 kilometers) and ERS imagery.
Under the GS program, Infoterra has screened offshore basins worldwide for oil seeps with more than 10,000 ERS and RADARSAT-1 scenes. Coupling those with an array of ancillary information, Infoterra has created off-the-shelf oil seep detection products for the oil industry. In a matter of hours clients can receive full-resolution subset imagery of each identified seep from the archive. The imagery is complete with seep points, lines, individual scene outlines, and ship and rig locations, as well as full scenes at reduced resolution to obtain an impression of the seeps in an overview. In addition, all seeps found will be mapped in a regional context according to bathymetry, as well as field and well data.
"Clients are interested in exploring different areas at different times, so quickly providing off-the-shelf data gives them the opportunity to explore where they want to when they want to," says Russell.. Turning to the SEA
The turnaround time to receive a study may be quick, but the time between assessing that study and ultimately drilling sedimentary rock is not.
"It may be five years before an oil company drills an exploratory well and another year before it makes a discovery," explains Roger Mitchell, vice president of business development for Earth Satellite Corp. (EarthSat) in Rockville, Md., a company specializing in satellite marine oil seep detection applications. "Our data are used by oil companies as part of a comprehensive oil exploration program. And it can be a long time before they're ready to drill somewhere. But almost any company exploring offshore is at least going to consider, if not use, a seep program. It's involved in virtually all offshore exploration."
EarthSat (www.earthsat.com), now doing business as MDA, Geospatial Services U.S., developed an oil seep detection technique called the Seep Enhancement Algorithm (SEA) 14 years ago. Designed to rapidly assess hydrocarbon seepage potential, SEA integrates either radar or optical imagery with gravity, bathymetry, offshore structure locations and other information to correlate natural oil seeps with their hydrocarbon sources and to map those seepage points.
Clients receive classified slick distribution maps overlaid on corresponding gravity and bathymetry models to produce detailed maps of the basin in the area of interest. This visual representation includes the basin's geological structure, the number of natural seeps present and the location of seepage points.
According to Mitchell, the SEA uses an adaptive processing algorithm that identifies seeps in varying water and temperature environments such as the cold, choppy waters of the North Sea or the warm, often polluted Mediterranean Sea. Because seeps are intermittent in nature, EarthSat typically acquires three data passes at a minimum to establish seep locations. One of the earliest exhaustive SEA studies EarthSat completed was a survey for the entire U.S. economic zone of the Gulf of Mexico, an area covering roughly 900,000 square kilometers, using several RADARSAT and ERS images. It is perhaps the most compelling evidence of just how well radar imagery can detect natural seeps. According to Mitchell, more than 450 seep anomalies were located in this region that has long been a natural hydrocarbon haven, and is still one of the most exciting exploration and development areas in the world today. Indications of seeps were identified in 15 protraction areas, eight of which had no previously recorded evidence of hydrocarbon seepage.
"We believe surveys such as these provide unique insights into the petroleum systems of offshore basins and at a very low cost," says Mitchell. "We surveyed this area, which is eight times larger than the Green Canyon region, for about 8 cents US per square kilometer." Although a radar image can't definitively tell oil companies where to drill, it does provide them with a starting point with which to plan more data sampling, and subsequently helps to lower the risk of investigating active migration seepage. And minimizing risk is what it's all about as petroleum players continue on their pioneering quests to realize the tantalizing promise offered by the world's offshore virgin basins. |
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