Integrated Gravity Modeling of Salt Features in the Mississippi Salt Basin
Rhonda L. Schenk, John J. Morris, Stuart A. Hall, 1998. "Integrated Gravity Modeling of Salt Features in the Mississippi Salt Basin", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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Recent exploration in portions of the Mississippi Salt Basin indicates that some models used in the interpretation of surface and subsurface data may be inadequate to explain several salt features. Models are important in interpretation because of the depth of the objectives and the complexity of the salt ridges. Significant oil and gas reserves trapped by the salt and related faults in Upper Jurassic reservoirs were discovered from the late 1960s to the early 1980s. Early models show concordant overburden on the flanks of a simple, symmetrical salt anticline. However, with further drilling, Upper Jurassic formations and the underlying salt were not encountered where predicted by the anticlinal model, suggesting that many of the salt ridges in the basin are more complex. Well-log data indicate that the flanks of such salt features may dip at rates greater than those proposed by the symmetrical anticline model.
A study using an integrated approach to test the viability of a more complex salt-ridge model was conducted to aid in the interpretation of surface and subsurface data in the Mississippi Salt Basin. Gravity data in conjunction with seismic reflection data and well-log information form the database from which salt models were created. Seismic reflection data and well control provide a reasonable estimate of the depth of the sources of the gravity anomalies. The gravity data, on the other hand, have been used to help constrain the interpretation of the seismic data by providing a better geometric framework for the salt bodies observed on the seismic data.
Eucutta Field and Diamond Field were selected as test areas for the salt models, based on the availability of data. The two fields are located along different types of parallel salt ridges. Eucutta is located deeper in the basin, in an area of large salt features, and Diamond is located closer to the rim of the basin, where smaller salt features are more common.
Results of the study indicate that an asymmetrical pattern exists in which the flanks of the ridges may dip at different rates. Both the Eucutta and Diamond salt ridges exhibit a steeply dipping flank on the basinward side of the salt ridge and a more gently dipping landward flank. The salt ridges also appear to have a complex variation along strike in the structure of the salt. The variations of the salt features in both the strike and dip directions appear to reflect the variations in the way the overlying sediments were deposited.
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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.