FRACTURE QUANTIFICATION AND MODELING IN THE TENSLEEP SANDSTONE NEAR THE ALKALI FAULT, BIGHORN BASIN, WYOMING by Alper T. Kilic A thesis submitted to the Faculty and Board of Trustees of Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology). Golden, Colorado Date________________ Signed:______________________ Alper T. Kilic Signed:______________________ Neil F. Hurley Golden, Colorado Date__________________ __________________________ Dr. Murray Hitzman Professor and Head, Department of Geology and Geological Engineering ABSTRACT This thesis documents fractures exposed in outcrop along the Alkali fault in the Bighorn basin, Wyoming. The Alkali fault is a major east-west trending oblique-slip fault that cuts the Tensleep Sandstone, which is an important hydrocarbon producer in the area. These fractures are mainly deformation bands (gouge-filled fractures), and conjugate shear fractures that are filled with calcite. The highest fracture intensity occurs near the fault. Fracture analysis has been done by taking 20 scanline surveys at various distances, roughly perpendicular and parallel to the fault. Scanline surveys permit orientation of fracture sets and quantification of fracture spacing. Other basic fracture information was collected, such as filling and length of the fractures, and sense of offset along the fractures. The Alkali fault and locations of the scan-line surveys were determined utilizing GPS (Global Positioning System). After these scan-line surveys were interpreted, a fracture model of the study area was built. Compartment areas were calculated as a function of distance from the Alkali fault. iii Six fracture sets are detected in the study area. Sets 1, 2, 3, and 4 are controlled by the Alkali fault and spacing increases with distance from the fault. Fracture spacing increases from 0.5-11 ft (0.15-3.3 m) for Set 1, 0.5-8 ft (0.15-2.4 m) for Set 2, 0.5-8 ft (0.15-2.4 m) for Set 3, and 2-9 ft (0.6-2.7 m) for Set 4. Fracture Sets 5 and 6, which are open fractures, appear to be regional fracture sets of the Bighorn basin. The Alkali fault has a roughly east-west strike azimuth. Fracture set 1 is oriented 226/80˚, fracture set 2 is oriented 251/84˚, fracture set 3 is oriented 205/87˚, and fracture set 4 is oriented 273/88˚. None of these fracture sets are consistent with a primarily dip-slip sense of offset on the Alkali fault. Lateral sense of offset is inferred for Alkali fault movement that gave rise to the main fracture sets. Healed fractures cause compartmentalization in the study area. Compartments are bounded by healed fractures that should act as permeability barriers. Size of the compartments ranges from 1 to 20 ft2 (0.1-1.9 m2) near the fault, up to 10,000 ft2 (929 m2) at 1,000 ft (305 m) distance from the fault. iv TABLE OF CONTENTS Page ABSTRACT ......................................................................................................................iii TABLE OF CONTENTS ................................................................................................. v LIST OF FIGURES ........................................................................................................ iv LIST OF TABLES ........................................................................................................xiii ACKNOWLEDGMENTS ............................................................................................. xiv Chapter 1: INTRODUCTION .......................................................................................... 1 1.1. Research Objectives .................................................................................. 2 1.2. Previous Work ……………………………………………………………4 1.3. Research Contributions ........................................................................... 10 Chapter 2: GEOLOGICAL SETTING .......................................................................... 11 2.1. Location of Study Area ........................................................................... 11 2.2. Stratigraphy ............................................................................................. 14 2.2.1. Regional Stratigraphy .................................................................. 14 v Page 2.2.2. Local Stratigraphy ........................................................................ 17 2.2.2.1. Tensleep Sandstone ............................................................. 17 2.2.1.2. Petrography and Diagenesis................................................. 21 2.3. Structural Geology ................................................................................. 28 2.3.1. Regional Structure ........................................................................ 28 2.3.1.1. Lineaments .......................................................................... 28 2.3.1.2. Laramide Orogeny ............................................................... 32 2.3.1.3. Intermontane Basins of Wyoming ...................................... 37 2.3.1.4. Bighorn Basin ...................................................................... 38 2.3.2. Local Structure ............................................................................. 42 2.3.2.1. Tensleep Fault ..................................................................... 42 2.3.2.2. Alkali Fault ........................................................................... 48 2.4. Petroleum Geology ................................................................................ 50 Chapter 3: SCANLINES ................................................................................................ 53 3.1. Introduction ............................................................................................ 53 3.2. Data Collection Methods......................................................................... 53 3.3. Fracture Description ............................................................................... 59 3.3.1. Modes of Fracture Formation ....................................................... 59 3.3.2. Fracture Sets ................................................................................. 59 vi Page 3.3.3. Fracture Fillings .......................................................................... 74 3.4. Fracture Length ..................................................................................... 77 3.5. Discussion ............................................................................................. 85 Chapter 4: COMPARTMENTS .................................................................................... 92 4.1. Introduction ........................................................................................... 92 4.2. Fracture Spacing ..................................................................................... 93 4.3. Flow Compartment Areas .................................................................... 107 4.4. Second-Order Bounding Surfaces ...................................................... 108 4.5. Compartmentalization........................................................................... 109 4.6. Discussion ............................................................................................ 115 Chapter 5: CONCLUSIONS AND RECOMMENDATIONS .................................... 117 REFERENCES ............................................................................................................. 119 APPENDICES ............................................................................................................ 128 Abbreviations ............................................................................................. 129 APPENDIX A: Scanline Data ..................................................................... 130 APPENDIX B: Data for Fracture Spacing .................................................. 156 vii APPENDIX C: Data for Compartments Area ............................................ 158 APPENDIX D: GPS Data for Alkali Fault and Scanline Locations .......... 160 viii LIST OF FIGURES Page 1.1. Oil fields of the Bighorn basin ................................................................................. 3 2.1. Location of the study area ...................................................................................... 12 2.2. Air photo of the study area .................................................................................... 13 2.3. Stratigraphic column of the upper Paleozoic ......................................................... 15 2.4. Mineral distribution and fabric of the Tensleep Sandstone .................................... 19 2.5. Classification of the Tensleep Sandstone ............................................................... 20 2.6. Schematic summary of cement chronology ........................................................... 22 2.7. Formation of eolian marine sequences.................................................................... 22 2.8. Columnar section of Amsden Formation and Tensleep Sandstone ........................ 23 2.9. Eolian bedforms of different hierarchy ................................................................... 25 2.10. First, second and third-order bounding surfaces .................................................... 27 2.11. Tectonic map of eastern Bighorn basin .................................................................. 29 2.12. Lineaments of eastern Bighorn basin .................................................................... 30 2.13. Schematic