Richard Morgan, 1998. "Magnetic Anomalies Associated with the North and South Morecambe Fields, U. K.", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
Download citation file:
A close line-spaced aeromagnetic data set was acquired over the East Irish Sea in 1993 by World Geoscience Corp. Ltd. Although the area was already a known hydrocarbon province containing one of the United Kingdom's largest gas accumulations (collectively, Morecambe North and Morecambe South Fields), the aeromagnetic data reveal much about the geology of the area which previously was unrecognized.
The continuation of faults known from seismic data or from surface maps of the adjacent onshore areas is represented clearly by the magnetic data, allowing the relationships between faults of different trends to be elucidated. The geometries of major faults imaged in plan-view by their magnetic expression reveal compartmentalization by secondary cross-cutting fault suites which, although not clearly resolved by 2-D seismic data, are relevant to sediment deposition and trap formation.
An unpredicted aspect of these data is the association of weak magnetic lows with known hydrocarbon accumulations. This phenomena is expressed best over the Morecambe Fields themselves, where analysis of the shape of the anomalies in relation to the structure of the fields suggests their source to be associated directly with the structure of the closures rather than as alteration plumes above them. The cause of these anomalies is considered to be secondary mineralization, believed to relate to the complex diagenetic and multiphase charging history of these fields.
The detailed comparison of the seismically defined structure of the fields with a corresponding magnetic structure would not be possible without a close line-spaced, aeromagnetic survey configuration to define clearly the margins of these intrasedimentary-sourced, nonlinear magnetic anomalies.
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.