B. P. Tissot D. H. Welte

Petroleum Formation and Occurrence ANew Approach to Oil and Gas Exploration

With 243 Figures

Springer-Verlag Berlin Heidelberg New York 1978

Professor BERNARD P. TISSOT Institut Francais du Petrole and Ecole Nationale Superieure du Petro Ie 4, avenue de Bois-Preau, 92502 Rueil, France

Professor DIETRICH H. WELTE Kernforschungsanlage Jtilich GmbH Institut fUr Erdal und Organische Geochemie Postfach 1913, 5170 Jtilich, FRG and Rheinisch-WesWilische Technische Hochschule Aachen

ISBN-13:978-3-642-96448-0 e-ISBN-13:978-3-642-96446-6 DOT: 1 0.1 007/978-3-642-96446-6

Library of Congress Cataloging in Publication Data. Tissot, B. Petroleum formation and occurrence. Bibliography: p. Includes index. 1. Petroleum - Geology. 2. Gas, Natural - Geology. 3. Geochemical prospecting. I. Welte. Dietrich H .. joint author. II. Title. TN870.5.T53 553'.282 78-4254.

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustra­tions, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher.

© by Springer-Verlag Berlin Heidelberg 1978. Softcover reprint of the hardcover 15t edition 1978

The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

Typesetting and printing: Beltz Offsetdruck, Hemsbach/Bergstr. Bookbinding: Konrad Triltsch, Graphischer Betrieb, Wilrzburg. 2132/3130-543210

Foreword

The publication of this book Petroleum Formation and Occurrence by Bernard Tissot and Dietrich Welte will indeed be welcomed by pertroleum geologists, petroleum geochemists, teachers and students in these fields, and all others who are interested in the origin and accumulation of hydrocarbons in nature. It is indeed a privilege for us to have the opportunity of sharing with these two eminent scientists the wealth of information they have acquired and developed during long careers devoted to concentrated scholarly study and practical investigation of the nature, origin, and occurrence of petroleum.

Professor Bernard Tissot graduated from the Ecole Nationale Superieure des Mines in 1954 and from the Ecole Nationale Superieure du Petrole in 1955. In 1955 he received a D.E.S. in geology from the University of Grenoble and then began research work on petroleum geology at the Institut Fran~ais du Petrole. He was made head of the Department of Geochemistry in 1965, and since 1970 has also been teaching organic geochemistry at the Ecole Nationale Superieure du Petrole where he became Profes­sor in 1973. Professor Tissot has had a broad and varied background of practical experience. He has been a member of exploration teams in France, New Caledonia, and Sahara. In 196G-1963 he headed a mission of the IFP to the Department of National Development of Australia. Outstanding among his achievements has been the use of the Paris basin as a laboratory in the development of an understanding of the relation between temperature and petroleum genesis. In recent years he has devoted considerable attention to North America and has published studies on the origin of the Athabasca tar sands and the Uinta Basin oil. Together with J. Espitalie he is responsible for the development of new pyrolysis techniques and instrumentation for the identification of petroleum source rocks and their stage of maturation and a mathematical model for the thermal evolution of organic matter in sediments. He is author or co-author of numerous outstanding scientific papers on origin and migration of petroleum published in both French and English in various periodicals and books.

Professor Dietrich Welte received his Ph.D. Degree in 1959 from the University of Wiirzburg in geology and chemistry. He

VI Foreword

then worked for three years as a research geochemist in the Hague for the Shell International Oil Company on the origin of oil and gas. In 1963 he returned to Wiirzburg where he established a research laboratory in organic geochemistry. In 1966 he received the President's award from the American Association of Petro­leum Geologists for his outstanding paper on Relation between Petroleum and Source Rock. During 1966 he visited various academic and industrial laboratories in the USA and in 1967 took a position as Senior Research Geochemist and toward the end of his stay was given the function of Research Coordinator for Exploration with Chevron Oil Field Research Company. While being with Chevron he was working on geological-geochemical research projects in California, the Gulf Coast, and other areas in America. In 1970-1972 he taught at the University of Gottingen and since 1972 has been Professor of Geology, Geochemistry, and Oil and Coal Deposits at the Institute of Technology at Aachen. During his scientific career Professor Welte has made numerous outstanding contributions to the knowledge of the geochemistry of petroleum, published in German and in English in various periodicals and books, in addition to numerous private reports resulting from his oil company connections. Most recently Professor Welte accepted the post of Director of the newly founded Institute for Petroleum and Organic Geochemistry at the Nuclear Research Center Jiilich, although he will continue some teaching at Aachen.

Although resident in Europe and presently working in Euro­pean institutions, Professors Tissot and Welte have both been frequent sojourners in America and have long participated actively in meetings of American geochemists and geologists. Both have worked extensively on problems of petroleum genesis in America and have published a number of papers in the Bulletin of the American Association of Petroleum Geologists. Many readers will recall the outstanding conferences on the Geology of Fluids and Organic Matter in Sediments in Banff, Alberta in May 1973 at which for several days the authors presented specific topics in this subject area and fielded questions and comments from a picked audience of petroleum geologists. Both Tissot and Welte have participated and aided greatly in work on the hydrocarbons of cores from the Joides Deep Sea Drilling Project, and are members of the Joides Panel on Organic Geochemistry.

The origin of petroleum has challenged scientists as far back as the 17th century when it was supposed by many to be associated with the mysterious "phlogiston". From being simply a matter of scientific curiosity it became, with the development of the petroleum industry, a subject of vital practical importance, knowledge of which has often meant the difference between success and failure in exploration for petroleum. Much progress

Foreword VII

has been made by the combined efforts of geologists, geochemists, and geophysicists, but the subject is still a dynamic one with many baffling unknowns and uncertainties. Perhaps even more filled with uncertainties and unknowns at the present day, and clearly just as critically important to practical oil exploration, is the manner of migration and accumulation of petroleum.

Tissot and Welte approach their subject not only as geochemists but also well-armed with geological knowledge. The book starts (Part I) with the production and accumulation of organic matter -the basic feed stock for petroleum. Then follows the very critical Part II on the transformation of this organic matter to kerogen and then to oil, or gas. Important aspects of the formation of coal and oil shales are also treated. Part III deals with the formation of oil and gas pools and the knotty problem of migration of petroleum from source to reservoir. Part IV is concerned with the composi­tion and classification of petroleum, "geochemical fossils", and the relation between character of petroleum and geological environment and subsequent alteration of oil in the reservoir. Finally, Part V covers the practical identification and evaluation of source rocks, the correlation between oils and between oils and source rocks, and an evaluation of the practical uses of geochemis­try in petroleum exploration.

The Tissot-Welte volume is a clear, practical, readable account, by two of the world's leading researchers in the field, of the formation, migration and accumulation of petroleum, primarily from the geochemical viewpoint, but adequately seasoned with geology. The text is further clarified by a wealth of tables and figures and is a mine of reference information. The topics are developed in a logical and reasonable way with full credit for other and sometimes divergent views. One of the many merits of the Tissot-Welte book is its open-minded approach to problems and its avoidance of a dogmatic attitude - the true measure of a really deep knowledge of a subject. A particularly strong point is the attention which the book gives to practical procedures and to the complex instrumentation required in modern petroleum geoche­mical investigations, as well as to the more theoretical aspects of the subject.

The book will fill a very much needed place both as an advanced classroom text, and as a guide and reference work on the subject, which should be in the hands of all practicing petroleum geologists and geochemists. These will undoubtedly emerge as better oil finders for the reading of this volume.

HOLLIS D. HEDBERG

Professor of Geology (Emeritus) Princeton University

Preface

The subject treated in this book Petroleum Formation and Occurrence is truly interdisciplinary. It has its roots in such diverse fields of science as biology, oceanography, and, most important, in various branches of chemistry and geology. Those concerned either in academic or industrial work, with research, or practical problems associated with petroleum, have long recognized this fact. A steadily increasing number of these people is looking for a comprehensive source of information on the various aspects of petroleum exploration.

For a number of years we had been giving seminars in different countries on the origin, migration and accumulation of petroleum. It was from these seminars that the idea to write a book was born. After discussing this matter, it was clear that such a book should not be written by many authors, who might each be expert in a particular field, but that it should be written just by the two of us as generalists, even if it meant an enormous amount of work. In this way we had hoped the book would be easier to read, and that the many facets of this difficult subject could be better understood.

It is our wish that people in the academic world, advanced students in geosciences and chemistry or other branches of science, would benefit from this book. The book may also help to integrate organic geochemistry more into the geosciences than has been the case up to now. Most of all, however, we hope that this book is of use for those working in petroleum exploration and fields related to it. For a long time there has been a lack of communication between geologists active in practical exploration, and researchers more involved in chemically oriented laboratory work: we hope to have bridged that gap.

Through our book we want to demonstrate that the search for petroleum can benefit greatly from the integration and application of the principles of petroleum generation and migration. For many years the decision to drill a well was largely taken on the basis of the recognition of suitable structures. Then the selection of a structure was based mainly on intuition and general experience, because very little information was available whether or not a trap would contain hydrocarbons. A systematic utilization of the new comprehensive understanding of petroleum formation and occur-

x Preface

rence, as presented in this book, can improve the success rate in predicting petroleum-filled structures and hence decrease the financial risk of drilling. To follow this concept requires that in the future petroleum geologists must acquire some knowledge of petroleum geochemistry. To this end the teaching of organic geochemistry has to be developed. We hope that this book can serve as a basic text for the petroleum geochemistry covered in such a course.

The completion of the book would have been impossible without the help of our co-workers, colleagues and friends, especially in our organizations at the Institut Fran<;ais du Petrole (IFP), Rueil-Malmaison, the Kemforschungsanlage-J iilich (KFA) and the Rheinisch-Westfiilische Technische Hochschule Aachen (RWTH). We are highly indebted to P. Albrecht, Ch. Comford, W. Dow, B. Durand, G. Eglinton, A. Hood, R. Pelet, J. Williams and M. A. Yiikler, who critically read and reviewed parts of the manuscript. We thank Mrs. R. Didelez for her indefatigable assistance during preparation and organization of the final manuscript. Last but not least we want to thank our wives for their patience with us while working on the book.

Finally special thanks go to H. D. Hedberg, who read all the text, gave very valuable advice, and encouraged us throughout the work.

April, 1978 B. P. TrssoT D. H. WELTE

Contents

Part I Production and Accumulation of Organic Matter: A Geological Perspective

Chapter 1 Production and Accumulation of Organic Matter. The Organic Carbon Cycle. . . . . . . . . . .. 3

1.1. Photosynthesis - The Basis for Mass Production of Organic Matter . . . . . . . . . . . . . . . . . . .. 3

1.2 The Organic Carbon Budget During the History of the Earth. . . . . . . . . . . . . . . . . . . . 7

1.3 The Organic Carbon Budget in the Black Sea 11 Summary and Conclusion. . . . 13

Chapter 2 Evolution of the Biosphere. 14

2.1 Phytoplankton and Bacteria. . . 14 2.2 Higher Plants . . . . . . . . . . 17 2.3 Geological History of the Biosphere. 19

Summary and Conclusion. . . . . . 20

Chapter 3 Biological Productivity of Modern Aquatic Environments . . . . . . . . . . . 21

3.1 Primary Producers of Organic Matter . . . 3.2 Factors Influencing Primary Productivity . 3.3 Present Primary Production of the Oceans

Summary and Conclusion . . . . . . . . .

Chapter 4 Chemical Composition of the Biomass: Bacteria,

21 23 28 30

Phytoplankton, Zooplankton, Higher Plants. 31

4.1 Proteins and Carbohydrates . 31 4.2 Lipids. . . . . . . . . . . . . . . . . . . . . . . 34 4.3 Lignin and Tannin. . . . . . . . . . . . . . . . . 44 4.4 Qualitative and Quantitative Occurrence of Important

Chemical Constituents in Bacteria, Phytoplankton, Zooplankton and Higher Plants. . . . . . . . . . .. 45

4.5 Natural Associations and Their Effects on Biomass Composition . . . . . . . 50 Summary and Conclusion . . . . . . . . . . . . . .. 53

XII Contents

Chapter 5 Sedimentary Processes and the Accumulation of Organic Matter . . . . . . . . . . . . . . . .. 55

5.1 Fossil and Modern Sediments Rich in Organic Matter, and Their Geological Implication . . . . . . . . . .. 55

5.2 The Role of Dissolved and Particulate Organic Matter 57 5.3 Accumulation Mechanisms for Sedimentary Organic

Matter . . . . . . . . . . 59 Summary and Conclusion . 61

References . . . . . . . . . . . 63

Part II The Fate of Organic Matter in Sedimentary Basins: Generation of Oil and Gas

Chapter 1 Diagenesis, Catagenesis and Metagenesis of Organic Matter 69

1.1 Diagenesis . . . . . . . . . . . 69 1.2 Catagenesis. . . . . . . . . . . 71 1.3 Metagenesis and Metamorphism 72

Summary and Conclusion. . . . 73

Chapter 2 Early Transformation of Organic Matter: The Diagenetic Pathway from Organisms to Geochemical Fossils and Kerogen . . . . . . 74

2.1 Significance and Main Steps of Early Transformations 74 2.2 Biochemical Degradation. 75 2.3 Polycondensation. . . . . . . . . . . . . . . . . .. 81 2.4 Insolubilization. . . . . . . . . . . . . . . . . . .. 84 2.5 Isotopic Composition of Organic Matter in Young

Sediments. . . . . . . . . . . . . 88 2.6 Result and Balance of Diagenesis . 89

Summary and Conclusion . . . . . 91

Chapter 3 Geochemical Fossils and Their Significance in Petroleum Formation . . . . . . . . . . . 92

3.1 Diagenesis Versus Catagenesis: Two Different Sources of Hydrocarbons in the Subsurface . . . 92

3.2 Hydrocarbons Inherited from Living Organisms, Directly or Through an Early Diagenesis: Geochemical Fossils ... 97

3.3 n-Alkanes......... 99 3.4 Iso- and Anteiso-Alkanes . 109 3.5 ClO-branched Alkanes . . 109

Contents XIII

3.6 AcyclicIsoprenoids.. . . . . . . . . . . . . . . .. 110 3.7 Tricyclic Diterpenoids ................ 113 3.8 Steroids and Pentacyclic Triterpenoids: Occurrence in

Recent and Ancient Sediments . . . . . . . . . . .. 114 3.9 Fate of Steroids and Triterpenoids During Diagenesis. 116 3.10 OtherPolyterpenes . . . . . . . . 119 3.11 Aromatics. . . . . . . . . . . . . . . . . . . . 119 3.12 Oxygen and Nitrogen Compounds . . . . . . . 120 3.13 Kerogen as a Possible Source of Fossil Molecules 122

Summary and Conclusion. . . . . . . . . . . 122

Chapter 4 Kerogen: Composition and Classification 123

4.1 Definition and Importance of Kerogen 123 4.2 Isolation of Kerogen . . . . . . . . . . 124 4.3 Microscopic Constituents of Kerogen . . 125 4.4 Chemical and Physical Determination of Kerogen

Structure . . . . . 131 4.5 Chemical Analysis . . . . . . 131 4.6 Physical Analysis . . . . . . . 134 4.7 General Structure of Kerogen. 138 4.8 Depositional Environment and Composition of

Kerogen: the Evolution Paths 142 4.9 Conclusion . . . . . . . . 147

Summary and Conclusion. . . 147

Chapter 5 From Kerogen to Petroleum . . . . . . . . . . . 148

5.1 Diagenesis, Catagenesis and Metagenesis of Kerogen. 148 5.2 Experimental Simulation of Kerogen Evolution. 157 5.3 Structural Evolution of Kerogen . . . . . . . . . .. 162 5.4 Formation of Hydrocarbons During Catagenesis . .. 164 5.5 Carbon Isotope Fractionation and Kerogen Evolution 177 5.6 Experimental Generation of Hydrocarbons from

Organic Material . . . . . 179 Summary and Conclusion. . . . . . . 184

Chapter 6 Formation of Petroleum in Relation to Geological Processes. Timing of Oil and Gas Generation 185

6.1 General Scheme of Petroleum Formation. . 185 6.2 Formation of Gas . . . . . . . . . . . . . . . . . 188 6.3 Genetic Potential and Transformation Ratio . . . 190 6.4 Nature of the Organic Matter. Gas Provinces Versus

Oil Provinces . . . . . . . . . . 191 6.5 Temperature, Time and Pressure . . . . . . . . . .. 194

XIV Contents

6.6 Timing of Oil and Gas Generati on. . . . . . . . . 196 6.7 Comparison Between the Time of Source Rock

Deposition and the Time of Petroleum Generation 198 Summary and Conclusion. . . . . . . . . . . . . .. 200

Chapter 7 Coal and its Relation to Oil and Gas 202

7.1 General Aspects of Coal Formation . 202 7.2 The Formation of Peat 203 7.3 Coalification Process . 207 7.4 Coal Petrography . . . 213 7.5 Petroleum Generation 217

Summary and Conclusion . 224

Chapter 8 Oil Shales: A Kerogen-Rich Sediment with Potential Economic Value. . . . . . . . . 225

8.1 Historical.................... 225 8.2 Definition of Oil Shales. Oil Shale Versus Petroleum

Source Rock . . . . . . . . . . 225 8.3 Composition of Organic Matter . 227 8.4 Conditions of Deposition . . . . 229 8.5 Oil Shale Density . . . . . . . . 230 8.6 Pyrolysis of Oil Shales. . . . . . 230 8.7 Oil Yield; Composition of Shale Oil . 231 8.8 Oil Shale Distributions and Reserves 234

Summary and Conclusion . 236

References . . . . . . . . . . . 237

Part III The Migration and Accumulation of Oil and Gas

Chapter 1 An Introduction to Migration and Accumulation of Oil and Gas . . . . 257

Summary and Conclusion. . . . . . . . . . . . . . . 259

Chapter 2 Physicochemical Aspects of Primary Migration 260

2.1 Temperature and Pressure 260 2.2 Compaction. . . . . . . . . . . . . . 264 2.3 Fluids.. . . . . . . . . . . . . . . . 271 2.4 Possible Modes of Primary Migration . 272

Summary and Conclusion. . . . . . . 283

Contents XV

Chapter 3 Geological and Geochemical Aspects of Primary Migration . . . . . . . . . . . . . . . . . . . . 285

3.1 Time and Depth of Primary Migration. . . . . . . . . 285 3.2 Changes in Composition of Source Rock Bitumen

Versus Crude Oil . . . . . . . . . . . . . . . . . . . 290 3.3 Evaluation of Geological and Geochemical Aspects of

Primary Migration . . . . . . . . . . . . . . . . . . 291 3.4 Preliminary Conclusions and Suggestions on Primary

Migration. . . . . . . . . 296 Summary and Conclusion. . . . . . . . . . . . . . . 297

Chapter 4 Secondary Migration and Accumulation. . . . . 299

4.1 The Buoyant Rise of Oil and Gas Versus Capillary Pressures . . . . . . . . . . . . . . . . . . . 300

4.2 Hydrodynamics and Secondary Migration. . . 302 4.3 Geological and Geochemical Implications of

Secondary Migration . . . . . . . . . . . 305 4.4 Termination of Secondary Migration and

Accumulation of Oil and Gas . . . 309 4.5 Distances of Secondary Migration. 312

Summary and Conclusion . . . . . 314

Chapter 5 Reservoir Rocks and Traps, the Sites of Oil and Gas Pools. 315

5.1 Reservoir Rocks . . . . . 316 5.2 Traps. . . . . . . . . . . 318

Summary and Conclusion. 323

References. . . . . . . . . . . 324

Part IV The Composition and Classification of Crude Oils and the Influence of Geological Factors

Chapter 1 Composition of Crude Oils ..... . . . . . . 333

1.1 Petroleum Versus Source Rock Bitumen . . . . . . . 333 1.2 Analytical Procedures for Crude Oil Characterization. 333 1.3 Main Groups of Compounds in Crude Oils . . 337 1.4 Principal Types of Hydrocarbons in Crude Oils 340 1.5 Sulfur Compounds . . . . . . . . . . . . . . 356

XVI Contents

1.6 Nitrogen Compounds . . . . . . . . . . . . 359 1.7 Oxygen Compounds . . . . . . . . . . . . 360 1.8 High Molecular Weight N, S, 0 Compounds 361 1.9 Organometallic Compounds . . . . . . . . 363 1.10 Covariance Analysis of Main Crude Oil Constituents 365

Summary and Conclusion . . . . . . . . . . . . .. 368

Chapter 2 Classification of Crude Oils 369

2.1 General............. 369 2.2 Historical. . . . . . . . . . . . 370 2.3 Basis of Proposed Classification of Crude Oils. 370 2.4 Classification of Crude Oils . . . . . . . . . . 371 2.5 Characteristics of the Principal Classes of Crude Oils 373 2.6 Concluding Remarks . . . 376

Summary and Conclusion. . . . . . . . . . . . .. 377

Chapter 3 Geochemical Fossils in Crude Oils and Sediments as Indicators of Depositional Environment and Geological History . . . . . . . . . . . . 378

3.1 Significance of Fossil Molecules. . . . . . . . . 378 3.2 Geochemical Fossils as Indicators of Geological

Environments. . . . . . . . . . . . . . . . . . 380 3.3 Geochemical Fossils as Indicators of Diagenesis. 385 3.4 Geochemical Fossils as Indicators of Catagenesis 387 3.5 Present and Future Development in the Use of

Geochemical Fossils ... 388 Summary and Conclusion . . . . . . . . . . . . 389

Chapter 4 Geological Control of Petroleum Type . 390

4.1 General and Geochemical Regularities of Composition . . . . . . . . . . . . . . . . . . . . . 390

4.2 Geochemical Regularities Related to the Environment of Deposition . . . . . . . . . . . . . . . . . . . . . 391

4.3 Geochemical Regularities in Relation to Thermal EvoluT!on . . . . . . . . . . . . . . . . . . . . 400

4.4 Concluding Remarks on Crude Oil Regularities. 408 Summary and Conclusion. . . . . . . ... . . . 408

Chapter 5 Petroleum Alteration

5.1 ThermalAlteration. 5.2 Deasphalting ...... .

409

410 411

Contents

5.3 Biodegradation and Water Washing. Summary and Conclusion .

References . . . . . . . . . . .

Part V A New Approach to Oil and Gas Exploration: Application of the Principles of Petroleum Generation and Migration

Chapter 1 Identification of Source Rocks .

XVII

413 419

420

429

1.1 Amount of Organic Matter . . . . 429 1.2 Type of Organic Matter . . . . . . . 431 1.3 Maturation of the Organic Matter. . 449 1.4 Conclusions on Characterization of Potential Source

Rocks. . . . . . . . . . . 464 Summary and Conclusion ............... 470

Chapter 2 Oil and Source Rock Correlation

2.1 Correlation Parameters . . . . . . . . 2.2 Oil-Oil Correlation Examples. . . . . 2.3 Oil-Source Rock Correlation Examples.

Summary and Conclusion . . . . . . . .

472

473 475 481 485

Chapter 3 Locating Petroleum Prospects: Application of Principle of Petroleum Generation and Migration 487

3.1 First Conceptual Model of Petroleum Generation in a Basin. . . . . . . . . . . . . . . . . . . . 488

3.2 The Optimization of Organic Geochemical Information from Exploration Wells . . . . 489

3.3 Determination of Most Favorable Zones of Petroleum Accumulation in a Basin: A Geochemical Approach for Locating Prospects . . 490 Summary and Conclusion . . . . . . . . . . . . . . . 499

Chapter 4 Mathematical Models: A Quantitative Approach to the Evaluation of Oil and Gas Prospects . . . . 500

4.1 Necessity of a Quantitative Approach to Petroleum Potential of Sedimentary Basins. . . . . . . . . . .. 500

4.2 Mathematical Model of Kerogen Degradation and Hydrocarbon Generation . . . . . . . . . . . . . . . 501

XVIII Contents

4.3 Genetic Potential of Source Rocks. Transformation Ratio ......................... 505

4.4 Validity of the Model . . . . . . . . . . . . . . . .. 506 4.5 Significance of the Activation Energies in Relation to

the Type of Organic Matter . . . . . . . . . . . . . . 506 4.6 Application of the Mathematical Model to Petroleum

Exploration. . . . . . . . . . . . . . . . . . 509 4.7 HeatFlowandGeothermaIGradient.. . . . 512 4.8 General Rules for Reconstitution of Ancient

Geothermal Gradient. . . . . . . . . . . . . 514 4.9 Evaluation ofthe Present Geothermal Gradient. 516 4.10 Evaluation of the Ancient Geothermal Gradient 516 4.11 Conclusion . . . . . . . . 520

SummaryandConclusion. 521

References. 522

Subject Index 529