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Oil men, geophysicists, and geologists have been searching for years to find a direct method for locating oil and gas deposits. Strangely enough, an old method, gravity, comes very close to being such a method in certain areas such as the salt-dome province of South Louisiana. Most of the oil and gas fields of South Louisiana, both onshore and offshore, are associated with salt domes or salt masses. The gravity method is almost infallible in locating salt domes or salt masses.
The usual geophysical exploration procedure, a reconnaissance gravity survey followed by detailed seismic surveys of the gravity anomalies found, was not practical or feasible in the beginning of offshore exploration. Gravity exploration costs approximately ten times as much offshore as onshore and seismic exploration costs one third as much offshore as onshore. Except for a few instances, most companies completely dismissed gravity data in connection with their offshore prospecting. Improved underwater gravity meters and electronic surveying now make offshore gravity surveys practical and feasible but still rather costly. However, since the advent of joint participation by companies in extensive gravity programs, gravity data are now available at a nominal cost.
The next problem was to convince geophysicists, geologists, and management that they should obtain and use gravity data together with seismic data to evaluate offshore prospects. Many exploration people had been led to believe that the seismic method had rendered the gravity method obsolete and unnecessary.
There are numerous examples of failures on offshore seismic prospects which were selected and drilled without confirmation by gravity anomalies. In some areas seismic data are not at all conclusive, due to misleading multiple reflections, poor energy return and stray reflections, the sources of which can not be definitely determined. Since most oil fields in South Louisiana are
associated with gravity anomalies, it makes sense to be sure there is a gravity anomaly to confirm the existence of a seismic structure.
There are still many structures, or prospects, to be leased and drilled offshore. The proper use of gravity data will greatly increase the ratio of successes to failures.
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