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.
Figures & Tables
The idea for this book came from a perceived lack of recent, instructive examples of exploration-oriented interpretations of gravity and magnetic data. The Society of Exploration Geophysicists two volumes, Geophysical Case Histories, are probably closest in philosophy to this book. Published in 1948 and 1956, many of the examples in the Case Histories are relatively dated and specific to particular areas. We hope this new book provides an update that includes lessons about gravity and magnetic exploration that can be applied to many parts of the world. The Utility of Regional Gravity and Magnetic Anomaly Maps (SEG, 1985, W.J. Hinze, editor) contains some excellent papers dealing with tectonics that have clear bearing on hydrocarbon exploration, but no paper shows the relationships among hydrocarbon accumulations, exploration, gravity, and magnetics. Geophysical texts focusing on gravity and magnetics, including L.L. Nettleton's classics, include only a few (albeit often excellent) case histories, and many are dated.
Thus, this book's target audience is geologists and geophysicists in operations offices, actively involved in exploration at any level from basin analysis to prospect generation. Although most of the papers deal with hydrocarbon exploration, several papers relate to gravity and magnetic data in mining and environmental applications. A final section is included on new developments, the state of the art.
The book is not intended for gravity and magnetics specialists (although we hope they will find it interesting), or for geophysicists interested in theory, acquisition, and processing, unless those aspects are important to the geologic exploration problem and to the decisionmaking process.
We believe that the philosophical approach to interpretation is almost as important as some aspects of a technical interpretation itself. This book reveals the diversity of philosophies that gravity and magnetic interpreters embrace, as well as the common threads to which all interpreters aspire.
This book is not a textbook, although we have tried hard to highlight the exploration lessons inherent in each technical paper. Additional instructional aspects of the book are the glossary of gravity and magnetic terms, provided by Integrated Geophysics Corporation (with assistance from Richard Hansen of Pearson, DeRidder & Johnson) and an annotated bibliography, which has pointers to the rich literature of gravity and magnetics. Other short "lessons" can be found in stand-alone illustrations or short features throughout the book.
We thank Ray Thomasson for continual encouragement, suggestions, and prodding. Reviewers, whose efforts are appreciated greatly, include Dale Bird, Bill Pearson, Mark Odegard, and several anonymous reviewers. We appreciate the help of the AAPG, especially Ken Wolgemuth, in this, the first effort at serious book publication by the coeditors.