Gravity’s Role in a Modern Exploration Program
In this age of 3-D seismic surveys, seismic inversion, depth migration, analysis of amplitude variation with offset (AVO), and personal workstations, can gravity data contribute to a modern exploration program? The answer is definitely yes!
The following is an attempt to define the role of gravity in oil and gas exploration, especially in the Gulf Coast, explaining its strengths and limitations.
Much has been written about integrated exploration programs that incorporate all geologic and geophysical data available, but in practice, most prospects presented to management or to prospective investors consist only of subsurface geologic information and seismic data. Many prospect generators do not realize the existence of, or take the time and effort to use, gravity data already in their files or readily available for purchase. Most prospects can be enhanced and better defined by including information derived from a gravity survey.
Most geologic features in the sedimentary section associated with the accumulation of oil and gas are related directly to horizontal density changes of magnitudes large enough to be mapped by an accurate gravity survey. A partial list of such features includes anticlines, synclines, reefs, faults, and horizontal changes in the thickness of salt beds, which, of course, include salt domes, pillows, and ridges.
The map resulting from a gravity survey is a Bouguer map. A Bouguer gravity map consists of gravity values (the vertical component of the Earth's gravity) which have been corrected for latitude, elevation, and terrain. In the Gulf Coast, these maps are reduced to a sea-level datum. The Bouguer map is the response of all the horizontal changes in density over the mapped area, from the surface to the center of the Earth. To derive the maximum information from these data, they must be “processed” and interpreted.