The St. George Basin, Alaska, COST #1 Well: An Example of the Need for Integrated Interpretation
David A. Chapin, SubbaRao V. Yalamanchili, Paul H. Daggett, 1998. "The St. George Basin, Alaska, COST #1 Well: An Example of the Need for Integrated Interpretation", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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The 1976 St. George Basin COST #1 well, Bering Sea, Alaska, produced unexpected results. Twenty oil companies participated in the drilling of this expensive stratigraphic test, and the well was expected to encounter a thick nonigneous sedimentary section in the deepest portion of the basin. Instead, the well drilled through more than 1000 m (3300 ft) of Tertiary basalt and other volcanic extrusives before it was abandoned. Gravity and magnetic data were available before the well site was chosen, but the information apparently did not impact the site selection. A recent reexamination of the data available in 1975 reveals that significant gravity and magnetic anomalies occur over the well site. These data point to an interpretation that the St. George COST #1 well is located on a collapse feature associated with an igneous calderalike structure. The experience gained from this well demonstrates the need to integrate all available types of data prior to making exploration decisions.
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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.