Atlas of Natural and Induced Fractures in Core

An invaluable reference that helps geologists recognize and differentiate the many types of natural fractures, induced fractures and artefacts found in cores

Atlas of Natural and Induced Fractures in Core offers a reference for the interpretation of natural and induced fractures in cores. The natural and induced fracture data contained in cores provides a wealth of information once they are recognized and properly interpreted. Written by two experts in the field, this resource provides a much-needed tool to help with the accurate interpretation of these cores. 

The authorsinclude the information needed to identify different fracture types as well as the criteria for distinguishing between the types of fractures. The atlas shows how to recognize non-fracture artefacts in a core since many of them provide other types of useful information. In addition, the text’s illustrated structures combined with their basic interpretations are designed to be primary building blocks of a complete fracture assessment and analysis. The authors show how to recognize and correctly interpret these building blocks to ensure that subsequent analyses, interpretations, and modeling efforts regarding fracture-controlled reservoir permeability are valid.

Presented in full color throughout, this comprehensive reference is written for geologists charged with interpreting fracture-controlled permeability systems in reservoirs as well as for students or other scientists who need to develop the skills to accurately interpret the natural and induced fractures in cores.

John C. Lorenz and Scott P. Cooper are Senior Geologists with FractureStudies LLC in Edgewood, New Mexico.

Foreword xi

Preface xiii

Acknowledgments xv

Introduction 1

Part 1 Natural Fractures 9

Section A Extension Fractures 11

A1 High‐Angle Extension Fractures 13

A1a Introduction 13

A1b Fractography of High‐Angle Extension Fractures 14

A1c Extension Fracture Dimensions 21

A1d Extension Fracture Variations and Lithologic Influences 33

A1e High‐Angle Extension Fracture Intersections 43

A1f High‐ Angle Extension Fractures in Deviated Core 47

A2 Inclined Extension Fractures 51

A2a Inclined Extension Fractures in Horizontally Bedded Strata 51

A2b Inclined Extension Fractures in Inclined Strata 53

A2c Vertical Extension Fractures in Inclined Strata 54

A3 Horizontal Extension Fractures 55

A3a Beef‐Filled Fractures 55

A3b Other Calcite‐Mineralized Horizontal Extension Fractures 57

A3c NOT Horizontal Extension Fractures 57

Section B Shear Fractures 59

B1 Introduction 61

B1a Nomenclature 61

B1b Anderson’s Shear Fracture/Fault Classification 62

B2 Shear Fracture Dimensions 65

B3 Shear Fracture Fractography 67

B3a Slickensides, Slickenlines, and Accretionary Steps 67

B3b En Echelon Segments 69

B3c Steps 71

B3d Pinch and Swell 72

B3e Sheared and Glassy Surfaces 75

B3f Slickencrysts 75

B3g Other Evidence for Shear 76

B4 High‐Angle Shear Fractures 79

B4a Introduction 79

B4b High‐Angle Strike‐Slip Shear Fractures 79

B4c Non‐ Ideal High‐Angle Shear Fractures 82

B5 Intermediate‐Angle Shear Fractures 85

B6 Low‐Angle Shear Fractures 89

B7 Bed‐Parallel Shear Fractures 93

B8 Deformation Bands 97

B9 Faults 101

Section C Other Types of Natural Fractures 105

C1 Introduction 107

C2 Microfractures 109

C3 Ptygmatically Folded Fractures 111

C4 Fissures 117

C5 Veins 119

C6 Expulsion Structures 121

C7 Syn‐Sedimentary Fractures 125

C8 Compound/Reactivated Fractures 133

C9 Shattered Rock 137

C10 Karst Breccias 139

C11 Pocket‐Size Geomechanical Systems 143

C12 Stylolites 147

Section D Mineralization 151

D1 Mineralization 153

D1a Introduction 153

D1b Calcite Mineralization 154

D1c Other Types of Mineralization 160

D1d Oil and Bitumen 163

D1e False Mineralization 167

Part 2 Induced Fractures 171

2A Introduction 173

2B Petal and Saddle Fractures 175

2C Centerline Fractures 185

2D Disc Fractures 197

2E Scribe‐Knife Fractures 209

2F Torque and Helical Twist Fractures 213

2G Core‐Compression Fractures 219

2H Percussion‐Induced Fractures 221

2I Bending Fractures with Barbs 225

2J Irregular Crack Networks 229

2K Induced Fractures with Curved Strikes 233

2L Waterflood‐Related Fractures 237

2M Cored Hydraulic Fractures 241

Part 3 Artifacts 245

3A Introduction 247

3B Core Tops and Core Bases 249

3C1 Core‐Catcher Drag 253

3C2 Core Orientation Scribe Grooves 257

3C3 Irregular Core Diameters 261

3C4 Pinion Holes 263

3D1 Spinoffs 265

3D2 Twice‐Turned Core 269

3E Saw Scars 271

3F1 Core Plucking 277

3F2 Scratches 279

3F3 Drill‐Mud Erosion 281

3F4 Core‐Parting Enigmas 283

3F5 Polished Fracture Surfaces in Horizontal Cores 285

3F6 Tip Polish 287

3F7 Slab‐Plane Consistency 291

3F8 Illusions 295

3F9 Coring‐Related Rock Alteration on Core Surfaces 299

Index 301

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