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A recent webcast highlights the value of gravity, magnetic, and electromagnetic datasets in exploration projects.
Since the 1980s 3-D seismic has been hailed as one of the most important technologies in the oil and gas industry. The ability to image buried geologic features has resulted in huge discoveries and an entire industry segment dedicated to acquiring, processing, and interpreting seismic datasets.
But seismic does not exist in a vacuum. It’s one of many measurements that can be taken to learn more about the subsurface. These additional methods were the subject of a recent Hart Energy Publishing webcast, “Using non-seismic exploration technology to better define oil and gas targets.”
Tom Austin of Austin Exploration discussed the use of gravity and magnetic data in oil and gas exploration. Magnetic surveys, he said, are often the first geophysical method used since they are relatively inexpensive and cover large areas in a short time. They provide an interpretation of basement depths, the type of basin, the thickness of the sedimentary section, and the structure of the basement and can also detect volcanic material. “This material can be misinterpreted to be basement without magnetic data,” Austin said.
Gravity surveys typically come next in the exploration lifecycle, although they can also be acquired simultaneously with magnetic surveys. Austin said these surveys are effective in salt dome areas because of the large difference between salt and sediment densities. Where no salt is present, gravity surveys map the structure of the prominent density contrast in the section.
“An example would be the Appalachian Basin, where the overlying Pennsylvanian and Mississippian clastic section is lighter density than the underlying Paleozoic section, beginning with the Devonian formation,” he said.
Next, Duncan Bate from ARKeX discussed gravity gradiometry. This technology was developed by Lockheed Martin for the US Navy and was originally deployed on boats.
Since its commercialization, the technology, known as full-tensor gravity gradiometry (FTG), has been used extensively in the mining industry.
|Three-dimensional magnetotelluric measurements can help image salt structures. (Image courtesy of KMS Technologies)|
ARKeX deploys FTG technology from the air to measure linear accelerations, which are difficult to distinguish from gravity measurements. Two accelerometers are used to provide independent measurements. The measurements are similar to the original torsion balance tools used at the beginning of the 20th century but take a fraction of the time to acquire.
Dr. Kurt Strack of KMS Technologies gave an update on the electromagnetic (EM) industry. Benefits of standard EM technology, he said, include its ability to be used where standard seismic has imaging difficulties, such as subsalt and sub-basalt. It’s also useful where information on fluid content is needed and can oftentimes be a direct hydrocarbon indicator. It costs one-third to one-tenth as much as a comparable seismic survey, can easily be used for reserve estimates, is sensitive to electrical anisotropy, and can be integrated with borehole measurements since resistivity has been measured downhole for years.
His examples showed EM being used for structural imaging, prospect identification, geothermal development, steam flood monitoring, and reevaluation of drilling targets.
Fairly new on the EM horizon is multicomponent EM, which helps measure vectors to better understand structures. Multicomponent EM allows a direct link with borehole anisotropy, he said. Ideally both magnetotellurics (natural-source EM) and controlled-source EM should be used.
KMS has developed a multicomponent marine cable called Vector EM that lies on the seabottom like an ocean-bottom seismic cable and measures the earth’s electric and magnetic fields. Ultimately the goal is to acquire EM and seismic simultaneously; Strack maintains that the workflows are very similar, making joint inversion possible.
The webcast will be available for viewing until May 12, 2010. Cost is $100. To register, click here.