Borehole-image log interpretation and 3D facies modeling in the Mesaverde group, Greater Natural Buttes field, Uinta Basin, Utah

BOREHOLE-IMAGE LOG INTERPRETATION AND 3D FACIES MODELING IN THE

MESAVERDE GROUP, GREATER NATURAL BUTTES FIELD,

UINTA BASIN, UTAH

by

Mirna I. Slim

A thesis submitted to the faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology).

Golden, Colorado

Date ___________

Signed: _____________________

Mirna I. Slim

Approved: ___________________

Dr. Neil F. Hurley Thesis Advisor

Approved: ___________________

Ir. Max Peeters Thesis Advisor Golden, Colorado

Date ___________

_______________________

Dr. John Humphrey

Acting Department Head,

Department of Geology and

Geological Engineering

ABSTRACT

Four borehole image logs from wells in the Greater Natural Buttes field, Uinta basin, Utah constitute the data set of this study. The main objective of the research was to study structural and sedimentary features on the image logs to determine: (1) the orientations of the horizontal principal stresses, (2) the fluvial and shoreface facies proportions, and (3) model the subsurface for productivity prediction.

Fractures were picked on the image logs and classified into various fracture sets.

The orientation of open natural fractures and drilling-induced fractures helps determine the orientation of the present-day maximum horizontal stress (S

). Hydraulic fractures occur parallel to this stress orientation. The mean S

and S

obtained from the 4 wells trend 103 and 17 degrees from north, respectively.

Sedimentary features, dip-magnitude, dip-azimuth patterns, and gamma-ray log shapes were studied to determine the environment of deposition of the various sandstone packages. The vertical distribution and proportions of the various facies in well NBU-222 and their dimensions, based on outcrop analogs, provided the input for a 3D facies model.

Pseudo-wells were inserted in the model to check the N/G (net-to-gross ratio) and calculate an average number of sand bodies intersected by a well in the modeled area.

The average net-to-gross (N/G) percent in the Mesaverde Group and the Castlegate Sandstone was found to be 55% sandstone. For all wells combined, channel, crevasse-splay, and shoreface sandstones formed 33%, 5%, and 10% of the vertical sections, respectively. Coals, lagoonal, and washover sandstones existed in minor amounts. Also, the facies proportions showed that the Upper Mesaverde Group formed in a fluvial environment, the Lower Mesaverde Group contained deposits from fluvial, shoreface, and transition environments, and the Castlegate Sandstone was deposited in a shallow-marine environment. Finally, the facies model showed that, in a depositional system similar to the Mesaverde Group, half of the sand bodies present may be penetrated by more than one well if the wells are drilled at a 1.35-ac (0.005 km

) spacing.

A relationship between the numbers of wells in a drilling scenario to the percentage of

sand bodies intersected was obtained and helped determine the best scenario to adopt in a

similar fluvial environment. Between 6 and 50% of sand bodies can be intersected when well spacing varies between 40-ac (0.17 km

) and 5-ac (0.02 km

), respectively.

We assumed that all the sand facies have the same sand quality. No porosity or

permeability data were included in the facies model to quantitatively check the hydraulic

connectivity of the sand bodies. Similarly, no production data were used to check the

sand volume predicted by the model or the subsurface sand distribution. Therefore, future

work should integrate outcrop and core descriptions and production data.

TABLE OF CONTENT

ABSTRACT ... iii

LIST OF FIGURES ... xi

LIST OF TABLES ... xix

ACKNOWLEDGMENT ...xx

CHAPTER 1: INTRODUCTION...1

1.1. Research Objectives...1

1.2. Studied Wells and Data...3

1.3. Previous Work ...3

1.4. Methodology ...5

1.5. Research Contributions...7

CHAPTER 2: REGIONAL GEOLOGY ...8

2.1. North America and Global Tectonics ...8

2.2. The Rocky Mountain Region Tectonics ...10

2.2.1. The Ancestral Rocky Mountains ...10

2.2.2. The Sevier Orogeny ...10

2.3.3. The Laramide Orogeny ...12

2.2.4. The Wasatch Range ...13

2.3. The Cretaceous Geology of Utah...13

2.4. The Uinta Basin ...16

2.5. Regional Stratigraphy ...18

2.5.1. The Mesaverde Group in the Studied Wells ...24

2.5.2. Mesaverde Group Environment of Deposition ...25

2.6. The Mesaverde Total Petroleum System ...29

2.6.1. Source Rocks ...29

2.6.2. Maturation...31

2.6.3. Migration...32

2.6.4. Reservoir Rocks ...32

2.6.5. Traps and Seals ...32

2.7. Summary of the Greater Natural Buttes Field Characteristics...32

CHAPTER 3: BOREHOLE IMAGE LOGS...34

3.1. Formation MicroImager...34

3.2. The Physics of FMI Measurements ...44

3.3. FMI Image Processing ...45

3.3.1. Data Load...45

3.3.2. GPIT Survey ...45

3.3.3. ICS Super Caliper Recalibrator ...47

3.3.4. BorEID...47

3.3.4.1. EMEX Correction ... 47

3.3.4.2. Data Equalization... 48

3.3.4.3. Depth and Speed Corrections... 48

3.3.5. BorNor ...50

3.3.6. BorScale...50

3.3.6.1. Conductivity Matching ... 50

3.3.6.2. Depth Shift ... 53

3.3.7. BorDip...53

3.3.8. BorView...57

3.3.9. Data Save ...59

3.3.10. Dip to ASCII ...62

3.4. Log Quality ...62

3.4.1. Gas Entry ...62

3.4.2. Tool Pull...62

3.5. Fractures and Structural Analysis ...65

3.5.1. Open Fractures ...65

3.5.1.1. Open Natural Fractures ... 67

3.5.1.2. Drilling-Induced Fractures... 67

3.5.1.3. Borehole Breakouts... 69

3.5.1.4. Borehole Elongations... 72

3.5.2. Healed Fractures ...75

3.5.3. Fracture Aperture ...75

3.5.4. Fracture Porosity...78

3.5.5. Fracture Total Trace Length ...79

3.6. Bedding planes and Sedimentary Analysis...79

3.6.1. Lithology Determination...79

3.6.2. Environments of Deposition ...80

3.6.3. GR Signature of Sandstone Facies...80

3.6.4. Open-Hole Logs...84

3.6.5. Coal ...84

3.6.6. FMI Signatures of Sandstone Lithologies...84

3.6.6.1. Dip Pattern ... 87

3.6.6.2. Dip Magnitude ... 87

3.6.6.3. Scour Surfaces ... 87

3.6.6.4. Dip Azimuth Vector Plots... 87

3.6.7. Paleocurrent Directions from Image Logs ...89

3.6.8. Structural Dip Removal ...90

3.6.9. Cumulative Dip Plot ...92

3.7. Log Presentation ...95

3.7.1. Track One...95

3.7.2. Track Two ...95

3.7.3. Track Three...97

3.7.4. Track Four...97

3.7.5. Track Five ...98

3.7.6. Track Six...98

3.7.7. Track Seven ...100

3.7.8. Track Eight...102

CHAPTER 4: BOREHOLE IMAGES: FRACTURE INTERPRETATION ...103

4.1. Well One: Natural Buttes Unit 222...103

4.1.1. Open Natural Fractures ...104

4.1.2. Drilling-Induced Fractures...106

4.1.3. Borehole Breakouts...106

4.1.4. Healed (Resistive) Fractures ...109

4.2. Well Two: Bonanza 4-6 ...109

4.2.1. Open Natural and Partially Healed Fractures ...109

4.2.2. Drilling-Induced Fractures...111

4.2.3. Borehole Breakouts...113

4.2.4. Healed (Resistive) Fractures ...113

4.3. Well Three: Pawwinnee 3-181...113

4.3.1. Open Natural and Partially Healed Fractures ...115

4.3.2. Drilling-Induced Fractures...118

4.3.3. Borehole Breakouts...118

4.3.4. Healed (Resistive) Fractures ...123

4.4. Well Four: Kennedy Wash Federal Unit 16-1 ...123

4.4.1. Open Natural and Partially Healed Fractures ...123

4.4.2. Drilling-Induced Fractures...125

4.4.3. Borehole Breakouts...125

4.5. Summary ...127

4.6. Discussion ...127

CHAPTER 5: BOREHOLE IMAGES - FACIES INTERPRETATION ...134

5.1. Well One: Natural Buttes Unit 222...135

5.1.1. Structural Dip Removal ...135

5.1.2. Cumulative Dip Plot ...138

5.1.3. Dip Azimuth Vector Plot ...140

5.1.4. Facies Examples from Well NBU-222 ...143

5.1.5. Facies Proportions...154

5.2. Well Two: Bonanza 4-6 ...154

5.2.1. Structural Dip Removal ...154

5.2.2. Cumulative Dip Plot ...157

5.2.3. Dip Azimuth Vector Plot ...160

5.2.4. Facies Examples from Well Bonanza 4-6...160

5.2.5. Facies Proportions...166

5.3. Well Three: Pawwinnee 3-181...174

5.3.1. Structural Dip Removal ...174

5.3.2. Cumulative Dip Plot ...177

5.3.3. Dip Azimuth Vector Plot ...177

5.3.4. Facies Examples from Well Pawwinnee 3-181 ...182

5.3.5. Facies Proportions...193

5.4. Well Four: Kennedy Wash Federal Unit 16-1 ...193

5.4.1. Structural Dip Removal ...198

5.4.2. Cumulative Dip Plots...198

5.4.3. Dip Azimuth Vector Plot ...202

5.4.4. Facies Examples from Well KWFU 16-1 ...202

5.4.5. Facies Proportions...209

5.5. Discussion ...209

CHAPTER 6: 3D FACIES MODEL ...218

6.1. Model Grid Design ...220

6.2. NBU-222 Facies Proportions...226

6.3. Petrel Input: Sand Element Dimensions and Shapes ...226

6.4. Petrel Model Results ...232

6.5. Discussion ...240

CHAPTER 7: CONCLUSIONS AND RECOMMENDATIONS...243

7.1. Conclusions...243

7.2. Recommendations...244

REFERENCES...246 APPENDIX A: SUMMARY OF FACIES INTERPRETATION...CD APPENDIX B: MAPVIEWS OF WELL-SPACING SCENARIOS AND

CONNECTIVITY ANALYSIS...CD

APPENDIX C: WELL DATA ...CD

LIST OF FIGURES

Figure 1 - 1: The Greater Natural Buttes field in the Uinta basin...2

Figure 1 - 2: Map view showing the four studied wells ...4

Figure 2 - 1: Physiographic provinces of the Uinta-Piceance basin region...11

Figure 2 - 2: The main structural elements that resulted from the Laramide Orogeny in Utah...14

Figure 2 - 3: Late Cretaceous Paleogeography of the western United States ...15

Figure 2 - 4: Outcrops of Upper Cretaceous rocks and the main uplifts in Utah and surrounding states...17

Figure 2 - 5: Generalized stratigraphic column of Uinta-Piceance province...19

Figure 2 - 6: Divisions and nomenclatures of the different sandstones of the Mesaverde Group...20

Figure 2 - 7: Fluvial environment from borehole image logs ...26

Figure 2 - 8: A model of nearshore sand deposits ...26

Figure 2 - 9: Depositional model showing the sedimentary structures in (A) longitudinal bar and (B) transverse bar in a braided channel ...27

Figure 2 - 10: Depositional model showing the sedimentary structures in (A) a mid- or downstream point bar and (B) upstream end of a point bar...28

Figure 2 - 11: Sedimentary features of a prograding shoreface succession with (A) high energy and (B) low energy ...30

Figure 3 - 1: Different generations of borehole image tools...35

Figure 3 - 2: Schlumberger‘s FMI tool ...36

Figure 3 - 3: A pad and a flap mounted on each of the caliper arms ...37

Figure 3 - 4: The coverage of various borehole image tools ...39

Figure 3 - 5: Microresistivity curves recorded by each pad...43

Figure 3 - 6: The sequential image processing chain in Geoframe...46

Figure 3 - 7: Depth correction to align the microresistivity curves ...49

Figure 3 - 8: Static and dynamic normalization...51

Figure 3 - 9: Static vs. dynamic image ...52

Figure 3 - 10: A schematic diagram showing the dip and azimuth of a bedding plane...54

Figure 3 - 11: Cross-correlations between the FMI pads...56

Figure 3 - 12: Dipping bedding plane traced with a sine wave ...58

Figure 3 - 13: Structural cross section built using the monocline model...60

Figure 3 - 14: Structural cross section built using the similar-fold model...61

Figure 3 - 15: Gas entry in well Bonanza 4-6...63

Figure 3 - 16: Tool pull in well Pawwinnee 3-181 ...64

Figure 3 - 17: Fracture morphologic types ...66

Figure 3 - 18: Dynamic image from well Bonanza 4-6 showing an open fracture and cross stratifications ...68

Figure 3 - 19: Drilling-induced fractures ...70

Figure 3 - 20: Borehole breakouts and drilling-induced fractures from well Bonanza 4-6 ...71

Figure 3 - 21: Asymmetric elongation in well section (A) with and (B) without borehole breakouts ...73

Figure 3 - 22: Fracture classifications and their orientation with respect to the maximum and minimum stress directions ...74

Figure 3 - 23: Healed fractures with the halo effects...76

Figure 3 - 24: FMI response to conductive fractures...77

Figure 3 - 25: GR curve signatures in different fluvial facies ...81

Figure 3 - 26: Tadpole patterns and GR signatures for continental shelf ...82

Figure 3 - 27: Tadpole analysis and GR signatures in continental and shelf-

delta...83

Figure 3 - 28: The GR curve signature of the braided-channel facies of Figure 2-9 ...85

Figure 3 - 29: The GR curve signature of the meandering system bars ...86

Figure 3 - 30: Different dip patterns ...88

Figure 3 - 31: A dip-azimuth vector plot of (A) all shale beds in well NBU- 222 and (B) sections of opposed accretions...88

Figure 3 - 32: A Schmidt plot prepared using Geoframe for shale beds in well NBU-222...91

Figure 3 - 33: Cumulative dip plot of the shale beds in well NBU-222 ...93

Figure 3 - 34: Slumps and sedimentary deformation in well Pawwinnee 3-181 ...94

Figure 3 - 35: The first five tracks of the FMI PDS log presentation...96

Figure 3 - 36: The sixth track of the FMI log presentation...99

Figure 3 - 37: Tracks seven and eight of the FMI log presentation...101

Figure 4 - 1: The open natural fracture set in well NBU-222...105

Figure 4 - 2: The drilling-induced fracture set in well NBU-222 ...107

Figure 4 - 3: The borehole breakout set in well NBU-222 ...108

Figure 4 - 4: The open natural and partially healed fracture sets in well Bonanza 4-6 ...110

Figure 4 - 5: The drilling-induced fracture and borehole breakout sets in well Bonanza 4-6 ...112

Figure 4 - 6: The healed fracture set in well Bonanza 4-6...114

Figure 4 - 7: The open natural and partially healed fracture sets in the shallower interval of well Pawwinnee 3-181...116

Figure 4 - 8: The open natural fracture set in the deeper interval in well

Pawwinnee 3-181...117

Figure 4 - 9: The drilling-induced fracture and borehole breakout sets in the

shallower interval in well Pawwinnee 3-181 ...119

Figure 4 - 10: The drilling-induced fracture and borehole breakout sets in the deeper interval in well Pawwinnee 3-181...120

Figure 4 - 11: The healed fracture set in the shallower interval of well Pawwinnee 3-181...121

Figure 4 - 12: The healed fracture set in the deeper interval of well Pawwinnee 3-181 ...122

Figure 4 - 13: The open natural fracture set in well KWFU 16-1...124

Figure 4 - 14: The drilling-induced fracture and borehole breakout sets in well KWFU 16-1 ...126

Figure 4 - 15: (A) Vertical extension fractures in the MWX well, and strike directions of (B) open natural fractures and (C) drilling-induced fractures from the 23 wells in the Piceance basin...130

Figure 4 - 16: Section of the geologic map of the Vernal area in Utah ...131

Figure 5 - 1: Stereonet plot of the “Sedimentary_Dip” set in well NBU-222 ...136

Figure 5 - 2: Stereonet plot of the “Bed_Boundary” dip set in well NBU-222 ...137

Figure 5 - 3: Cumulative dip plot of the bedding planes in well NBU-222...139

Figure 5 - 4: Cross section showing the shale bedding planes in well NBU-222...141

Figure 5 - 5: Dip azimuth vector plot of all shale beds in well NBU-222 ...142

Figure 5 - 6: Channel-sand fill in well NBU-222 ...144

Figure 5 - 7: DAVP of the sand package seen in Figure 5-6 ...145

Figure 5 - 8: Point bar from well NBU-222...147

Figure 5 - 9: Section from well NBU-222 of a sand package from the Lower Mesaverde ...148

Figure 5 - 10: DAVP of the sand layers seen in Figure 5-9...149

Figure 5 - 11: Channel-fill deposits from well NBU-222...150

Figure 5 - 12: The DAVP of amalgamated sand of Figure 5-11 ...151

Figure 5 - 13: Two sand packages from well NBU-222...152

Figure 5 - 14: The DAVP of both sand layers of Figure 5-13. ...153

Figure 5 - 15: Stereonet plot of the various bedding plane dip sets in well BON 4-6...156

Figure 5 - 16: Cumulative dip plot of the bedding planes in well BON 4-6...158

Figure 5 - 17: Cross section showing the shale bedding planes in well BON 4- 6...159

Figure 5 - 18: DAVP of all shale beds in well BON 4-6 ...161

Figure 5 - 19: Fining-upward meandering channel fill in well BON 4-6 ...162

Figure 5 - 20: The DAVP of the sand in Figure 5-19 ...163

Figure 5 - 21: Washover sand in well BON 4-6 ...164

Figure 5 - 22: The DAVP of the lagoon washover sand of Figure 5-21...165

Figure 5 - 23: Washover fan/lagoonal shoreline sandstone from well BON 4-6...167

Figure 5 - 24: DAVP of the washover fan seen in Figure 5-23 ...168

Figure 5 - 25: Washover fan sand in well BON 4-6 ...169

Figure 5 - 26: The DAVP of the washover fan sand seen in Figure 5-25...170

Figure 5 - 27: A thick amalgamated shoreface sand section in the Lower Sego Formation in well BON 4-6 ...171

Figure 5 - 28: Vector plot of a lower shoreface sand in well BON 4-6 ...172

Figure 5 - 29: Stereonet plot of the various bedding plane sets of the shallower interval of well Pawwinnee 3-181 ...175

Figure 5 - 30: Stereonet plot of the various bedding plane sets in the deeper interval of well Pawwinnee 3-181 ...176

Figure 5 - 31: Cumulative dip plot of the bedding planes in well Pawwinnee 3-

181...178

Figure 5 - 32: Cross section showing the shale bedding planes of the upper

interval of well Pawwinnee 3-181 ...179

Figure 5 - 33: Cross section showing the shale bedding planes of the lower interval of well Pawwinnee 3-181 ...180

Figure 5 - 34: The DAVP of the bedding planes picked in shale sections in the (A) shallower interval and (B) deeper interval of well Pawwinnee 3- 181...181

Figure 5 - 35: Fluvial deposits in the Upper Mesaverde in well Pawwinnee 3- 181...183

Figure 5 - 36: DAVP of the fluvial sand in Figure 5-35...184

Figure 5 - 37: Amalgamated fluvial channel deposits in well Pawwinnee 3- 181...185

Figure 5 - 38: DAVP of the sand section seen in Figure 5-37...186

Figure 5 - 39: Washover fan from well Pawwinnee 3-181...187

Figure 5 - 40: DAVP of the washover fan sand of Figure 5-39...188

Figure 5 - 41: Shoreface sand seen in the Lower Sego Formation in well Pawwinnee 3-181...189

Figure 5 - 42: DAVP of the shoreface sand seen in Figure 5-41...190

Figure 5 - 43: Upper shoreface sand from the Castlegate Formation in well Pawwinnee 3-181...191

Figure 5 - 44: DAVP shows episodes of progradation and retrogradation...192

Figure 5 - 45: Planar cross stratification in a silty sand package from the Mancos Shale in well Pawwinnee 3-181 ...194

Figure 5 - 46: DAVP of the silty sand in the Mancos Shale seen in Figure 5-45...195

Figure 5 - 47: Turbidite deposits from the Mancos Shale in well Pawwinnee 3- 181...196

Figure 5 - 48: Stereonet plot of the various bedding plane sets in well KWFU 16-1 ...199

Figure 5 - 49: Cumulative dip plot of the bedding planes in well KWFU 16-1 ...200

Figure 5 - 50: Cross section showing the shale bedding planes in well KWFU

16-1 ...201

Figure 5 - 51: DAVP showing an inflection point at 10,231ft...203

Figure 5 - 52: DAVP of the shale bedding planes in well KWFU 16-1 ...204

Figure 5 - 53: Fining-upward fluvial sand package from the Upper Mesaverde Group in well KWFU 16-1 ...205

Figure 5 - 54: DAVP of the fluvial sand in Figure 5-53...206

Figure 5 - 55: Fining-upward fluvial sand from the Upper Mesaverde Group in well KWFU 16-1...207

Figure 5 - 56: DAVP of the fluvial sand in Figure 5-55...208

Figure 5 - 57: Braided stream sand deposits from the Upper Mesaverde Group in well KWFU 16-1 ...210

Figure 5 - 58: DAVP of the braided stream channel fill of Figure 5-57...211

Figure 5 - 59: Shoreface sand section from the Castlegate Formation in well KWFU 16-1 ...212

Figure 5 - 60: DAVP of the shoreface sand in Figure 5-59 ...213

Figure 5 - 61: Facies proportions in all 4 wells ...216

Figure 5 - 62: Facies proportions calculated from all 4 wells in (A) MVU, (B) MVL, and (C) the CGATE Formation...217

Figure 6 - 1: A map view to show the locations of the four wells in Petrel ...219

Figure 6 - 2: Well NBU-222 facies log...221

Figure 6 - 3: Surfaces created to connect the formation tops ...222

Figure 6 - 4: Polygon that designates the area modeled around well NBU-222...223

Figure 6 - 5: (A) The horizons relating formation tops and separating (B) the modeled zones...224

Figure 6 - 6: Example of the input data in Petrel ...227

Figure 6 - 7: Elliptical shape used to model sand bodies in Petrel ...230

Figure 6 - 8: Petrel 3D facies model ...233 Figure 6 - 9: A map view showing the 34 wells distributed at 5-ac (0.02 km

)

spacing around well NBU-222...234 Figure 6 - 10: Number of sand bodies intersected in different infill-drilling

scenarios...238 Figure 6 - 11: Relationships of the number of wells in a drilling scenario to

(A) the number and (B) percentage of the sand bodies intersected ...239

Figure 6 - 12: Facies proportions in MVU and MVL of well NBU-222 ...242

LIST OF TABLES

Table 1 - 1: Four studied wells and their locations in the Uinta basin...5

Table 2 - 1: Greater Natural Buttes Field summary...33

Table 3 - 1: Generations of borehole image tools...38

Table 3 - 2: FMI Specifications ...41

Table 3 - 3: FMI Applications ...42

Table 3 - 4: Cumulative dip calculations spreadsheet ...92

Table 3 - 5: Dip sets, their color codes, and tadpole symbols used on the FMI log presentation...100

Table 4 - 1: Structural analysis summary ...128

Table 5 - 1:Formation tops in the studied wells...135

Table 5 - 2: Facies proportions in well NBU-222...155

Table 5 - 3: Facies proportions in well BON 4-6...173

Table 5 - 4: Facies proportions in well Pawwinnee 3-181...197

Table 5 - 5: Facies proportions in well KWFU 16-1 ...214

Table 6 - 1: The coordinates, KB elevations, and TD of the four studied wells...225

Table 6 - 2: Sand element orientations and dimensions used for the modeled zones ...228

Table 6 - 3: Elements/facies proportions in 5-ac (0.02 km

) spaced wells around well NBU-222...235

Table 6 - 4: Number of the sand bodies penetrated by 34 wells distributed in a 5-ac (0.02 km

) grid spacing around well NBU-222 ...236

Table 6 - 5: Summary of the well-spacing scenarios, the wells they contain,

and the number of sand bodies intersected in each...239

ACKNOWLEDGMENT

Many people have contributed to make my research and 2 years at the Colorado School of Mines an interesting experience. I acknowledge the Kerr McGee Corporation and the Center of Petrophysics for funding my research and study.

I would like to thank my advisor, Dr. Neil F. Hurley, for his guidance, support, revision, and comments on my work. I feel lucky to have known a person like you that can teach a lot, even outside a classroom. It was a privilege to work with you and be your student.

Thanks go to my co-advisor, Professor Max Peeters, for his support and help during my last semester. Also, special thanks go to Dr. Donna Anderson for her time and insights to better understand fluvial sandstones and their architecture. Thanks go to Dr.

Piret Plink-Bjorklund for her comments and revisions. I extend my special thanks to Jerry Cuzella at Kerr McGee Corporation, who helped provide the logs and the required data to build the Petrel model.

I feel grateful to Mrs. Charlie Rourke for many things. Charlie, your smile and kindness made it possible to go through tough days. Even during the days when I did not need to bother you to help me with something, it was nice to know that you are around and always ready for assistance.

Thanks go to Dr. David Pyles for the time we spent discussing sedimentary structures in fluvial systems and Ms. Shauna Gilbert for her patience and effort to keep the computer laboratories in top shape.

I would like to thank all of the geoscientists at Schlumberger’s Data and Consulting Services at Greenwood, Colorado. Looking back at all the work I did in your office, I think that this research would have taken much more time without the great support from Jim Urdea during my internship with Schlumberger in 2005 and long after.

Jim, thanks for the permission to get access to Geoframe and Petrel. Also, this research

would have been much more difficult without Randy Koepsell. Randy, you are a great

person and mentor. Thanks for your time and the commitment you showed to teach me

Geoframe. You made my work on image logs and Geoframe a pleasant and learning

experience. Thanks go to Adil Manzoor, Tom Becker, Steve Sturm, and Jim Benson. I would like to thank Barbara Luneau and Andie Gomez for their time and help with Petrel.

My Petrel modeling would have been nearly impossible without the help of Ekeng Henshaw. Also, I would like to thank Leonardo (Leo) Vega for writing a quick code that helped me count facies and sand bodies in my wells.

Thanks go to Dr. Matthew Pranter for his guidance and time spent to discuss the Petrel facies model. I would like to thank Nicholas Sommer for his time discussing the connectivity application and the workflow document he prepared and shared with me.

Thanks go to Dr. Zulfiquar Reza who provided the connectivity application.

Thank you for your time, comments, feedback, and revisions of my work.

Thanks go to Jennifer Brotherhood and Janice Roberts, from ExxonMobil Exploration Company, for their help during my internship in 2006.

Finally, I want to express my appreciation to my family and friends. Your love,

understanding, and support kept me going away from all of you. I would like to thank

Maya for the many and long “therapy sessions” on the phone. Also, I would like to thank

all the friends I made in Colorado. Knowing all of you made me feel at home. You made

me enjoy this great state and, specifically, the outdoors and tearing/repairing an ACL.

For my uncle and friend Akram M. Arawi.

CHAPTER 1

INTRODUCTION

Many of the Rocky Mountain hydrocarbon fields are known for their low permeability, on the order of micro and millidarcies. The Greater Natural Buttes (GNB) field in the Uinta basin (Figure 1-1) is no exception. The low permeability in many fields makes production, even that of gas, a challenging task. The low permeabilities are aggravated because the Upper Cretaceous Mesaverde Group has fluvial and shoreface deposits that are often non-continuous and lenticular in shape. Thus, it is important to better understand the occurrence of the sand bodies, their dimensions, and orientations in order to model the subsurface for production prediction and efficiency.

Wireline image logs, although less commonly acquired compared to other conventional open-hole logs, prove to be a good structural and stratigraphic tool to interpret and evaluate the Upper Cretaceous sandstone reservoirs in the Mesaverde Group. Image logs are useful to interpret sand bodies in terms of various elements/facies in a certain environment of deposition. Also, referring to studies done on sandstone outcrops of the Mesaverde Group in the Piceance basin, the dimensions and orientation of various sand facies can be used to model an area around one of the study wells.

1.1. Research Objectives

The research has two main objectives. First, the structural elements picked in the

wells (mainly fractures and borehole breakouts) are studied to determine the orientations

of the principle stress directions in the GNB field. The results are compared to the stress

map of North America (NA) and other work carried out to determine the principle stress

directions in other fields in the basin (Chapter 4). Second, the stratigraphic aspect of the

study aims to:

Figure 1 - 1: The Greater Natural Buttes field in the Uinta basin. USA map after

www.enchantedlearning.com; Uinta basin map modified after Johnson and

Roberts (2003).

1- Classify the sand bodies into various fluvial and shoreface facies or elements, and find their proportions in every well and in different zones in each well.

2- Assign facies/element dimensions based on outcrop analogs.

3- Build a subsurface 3D facies model to reflect the occurrence, distribution, orientations, and dimensions of the sand bodies.

4- Show how various infill-drilling scenarios can intersect different numbers of sand bodies, and draw conclusions on the best drilling scenario(s) to adopt for optimum production efficiency assuming that all sand facies have the same quality.

1.2. Studied Wells and Data

The Uinta basin (Figure 1-1) is an asymmetric foreland basin in NE Utah. The basin is an extension of the Piceance basin in NW Colorado. It covers an area of 9,300 mi

(24,087 km

) and is a “typical Rocky Mountain asymmetrical basin” (Osmond, 1965;

Osmond, 1968). The study area is the GNB field in the eastern part of the Uinta basin (Figure 1-1).

Four different fields (Table 1-1): the Bonanza (BON), the Kennedy Wash Federal Unit (KWFU), the Natural Buttes (NBU), and the Pawwinnee (PAW) fields are represented in the study. They are located in the central part of the Uinta basin, inside the GNB field. The studied data are four Formation Micro-Imager (FMI) logs from wells in the above mentioned fields (Table 1-1; Figure 1-2).

1.3. Previous Work

There have been numerous publications to study the principal stress orientation and stratigraphy in North America (NA) and, specifically, the Uinta-Piceance region.

Lorenz (2003) presented a relationship between fractures in the Mesaverde Group in the

Piceance basin and regional geological events. Zoback and Zoback (1989) classified the

Rocky Mountain (RM) region into various stress “provinces” and published a stress map

of the continental United States. Zoback et al. (1985) studied the relation between in-situ

Figure 1 - 2: Map view showing the relative locations of the four studied wells. No field

boundaries are specified on the map.

Table 1 - 1: Four studied wells and their locations in the Uinta basin.

stress and borehole breakouts, and Koepsell et al. (2003) interpreted FMI images of 23 different wells in the Piceance basin and showed the strike direction distribution of open natural fractures.

Luthi (2001) and Grace and Newberry (1998) presented detailed work on how to understand, process, and interpret image logs and any feature seen on them. Longman and Koepsell (2005) published detailed work done on many wells (including the ones studied in this research) in the GNB field where image logs were used to interpret environments of deposition of various sandstone formations.

The Upper Cretaceous stratigraphy was intensively studied by Johnson (2003), Hettinger and Kirschbaum (2003), and Lawton (1986) who described outcrops for sedimentary structures and paleocurrent direction determinations. The architecture and dimensions of the sand bodies in various sandstone formations of the Mesaverde Group were published by Cole and Cumella (2003; 2005) and Anderson (2005) and studied by Panjaitan (2006) and German (2006).

Petrel facies modeling done by Henshaw (2005) was reviewed as an example for fluvial facies modeling in the Uinta basin.

1.4. Methodology

The following is the methodology used to meet the study objectives, with details

presented in subsequent chapters:

1- Process the FMI logs using Schlumberger’s Geoframe software to make them ready for interpretation (Chapter 3).

2- Trace key structural elements (faults, micro-faults, open natural fractures, resistive/healed fractures, borehole breakouts, and drilling-induced fractures) and assign dip and dip azimuth for each. Classify each type of feature into a set, and determine average dip and dip azimuth for each set to infer principal stress directions (Chapter 4).

3- Compute cumulative fracture trace lengths and fracture porosities in each well (Chapter 4).

4- Trace key sedimentary features (bedding planes, cross bedding, erosional scours, and slumps) on the image logs, classify them into dip sets, and assign each sedimentary feature a dip and dip azimuth (Chapter 5).

5- Interpret sedimentary features and open-hole logs to determine and differentiate sand elements into shoreface sand, channel sand/point bars, or crevasse splays (Chapter 5).

6- Filter shale bedding planes from the rest of the data and sort them into dip domains to prepare cumulative dip plots. The change in the plot slope gradient may help to predict faults, micro-faults, and sequence boundaries that may not be clear from the image logs (Chapter 5).

7- Determine, from different shale intervals, the post-depositional dip/structure and remove it.

8- Construct dip azimuth vector plots to determine, when possible, accretion orientations in the channel sands and point bars and infer paleocurrent directions (Chapter 5).

9- Determine the proportion of every facies element in each well (Chapter 5).

10- Create a synthetic facies log in Petrel and use facies dimensions and orientations to populate a conceptual 3D model in the vicinity of well NBU-222 (Chapter 6). Well NBU-222 was chosen because its FMI image log has less gas entry and tool pull than the remaining logs. Also, we could locate, around well NBU-222, other wells with formation tops specified. This provided more control to shape the horizons separating the modeled zones in Petrel.

11- Study various infill drilling scenarios to determine the number of sand bodies

intersected in each scenario, and predict the effects of each on production and its

efficiency (Chapter 6).

1.5. Research Contributions

The main results of the research include the determination of the directions of the principle stress orientation in 4 wells. The present-day maximum horizontal stress direction (S

) was found to trend 103 degrees from north. The minimum horizontal stress (S

) was found to trend 17 degrees from north. The orientations agree with the regional trend.

Stratigraphic analysis shows that the logged intervals of the Mesaverde Group contain sand and shale in almost equal proportions. The average net-to-gross (N/G) percent in the studied wells in the Mesaverde (Upper and Lower) and the Castlegate Sandstone was found to be 55%. The sand was divided into crevasse splays, channel sands, and shoreface sands that formed 5%, 33%, and 10% of the logged intervals in the mentioned formations, respectively. Lagoonal and washover sands were encountered as well. We could infer from the results that the Upper Mesaverde Group was deposited in a fluvial environment, the Lower Mesaverde Group contained deposits from fluvial, shoreface and transition environments, and the Castlegate Sandstones was deposited in a shallow-marine environment.

The facies model built in Petrel showed that, in a depositional system similar to

the one we are studying, all the sand bodies present can be penetrated when drilling wells

at a 1.35-ac (0.005 km

) spacing; half of sand bodies may be penetrated by more than one

well. When we vary the well spacing between 20-ac (0.08 km

) and 5-ac (0.02 km

), the

percentage of sand bodies intersected will vary between 12% and 50%, respectively, of

the total sand bodies found in the subsurface. To drill at 40-ac (0.17 km

) spacing or

larger may bypass more than 90% of the subsurface sand bodies.

CHAPTER 2

REGIONAL GEOLOGY

This chapter is a review of the global tectonics of North America (NA), the regional tectonics of Utah and the Uinta basin, and the regional stratigraphy of the Uinta- Piceance region. A summary of the global tectonics that formed and shaped the craton of NA will be summarized with a focus on the main regional tectonic events that shaped the Rocky Mountain (RM) region and formed the structural features surrounding the Uinta basin. The regional stratigraphy of interest will be summarized with a focus on the producing sandstones of the Upper Cretaceous Mesaverde Group and their environments of deposition. Finally, the Mesaverde Group will be presented as a petroleum system in the Greater Natural Buttes (GNB) field area, reviewing the petroleum system elements and summarizing the GNB characteristics.

2.1. North America and Global Tectonics

The North American craton, known as Laurentia, has been coherent since 1.7 Ga and included Greenland and parts of Scotland until their separation during the Late Cretaceous. It is a group of “seven micro-continents,” called provinces, collected by orogenic activity between 2.0 and 1.8 Ga (Hoffman, 1989). Some of the provinces belonged to subducting plates during convergence or forelands during collisional orogenies. Others were parts of overriding plates at one point of their history and hinterlands during orogenies. Early Proterozoic rifting and collisional deformation controlled the dimensions of these provinces (Hoffman, 1989).

The northwestern margin of North America witnessed Precambrian orogenies

(Grenville and Racklan orogens) that were the result of “thrusting directed toward the

interior of Laurentia.” The Proterozoic ended with a continental breakup taking place at

0.8 Ga, which developed a number of aulacogens, one of which is the Uinta Mountains

(Hoffman, 1989). Bally et al. (1989) reviewed the main tectonic events that shaped NA and influenced its evolution to the configuration that we know today.

A western volcanic arc was present through the Paleozoic and Mesozoic. During the mid-Paleozoic, the western margin of NA was involved in major episodes of shortening that resulted in continent-directed thrust belts with a “locally well-developed foredeep succession containing Devono-Carboniferous sediments” (Oldow et al., 1989).

During the Late Permian (255 Ma), the western half of the supercontinent Pangea (Euramerica, Gondwana, and Siberia) formed after the ocean basin separating the first from the latter two closed. The final reconstruction of this supercontinent took shape by late Carboniferous and remained unaltered until the Early Jurassic (Bally et al., 1989).

The western margin type of NA changed from being a passive margin to an active one during the Permian and Early Triassic (Oldow et al., 1989; Johnson, 1992).

During the Middle and Upper Triassic and the Early Jurassic, a new rift system formed along the east coast of NA. This rift system was the precursor of a subsequent system that led to the development of the Gulf of Mexico and the Central Atlantic Ocean.

The first appearance of a narrow oceanic basin was seen at the beginning of the Cretaceous (144 Ma), and later the “Proto-Caribbean Ocean formed,” separating NA from South America (SA) (Late Cretaceous). At about 119 Ma (Late Neocomian to early Aptian), the Central Atlantic Ocean showed slight expansion and a small oceanic basin appeared, separating South America from Africa. During the Late Cretaceous (84 Ma, late Santonian), sea-floor spreading continued between Africa and NA. A new rifting system started developing between NA and Europe.

The Triassic to Paleocene evolution of the Cordillera created strike–slip faults that displaced previously overthrust terranes, accretionary wedges, and foreland fold belts, 3,106 mi (5,000 km) in length (Oldow et al., 1989). This created an estimated 62-124 mi (100-200 km) of shortening in the RM region (Oldow et al., 1989).

The early Oligocene (38 Ma) witnessed the opening of the Labrador Sea, the

formation of Baffin Bay, and the separation between Greenland and Norway. Also, the

leading edge of the Pacific Plate started to impinge upon the western margin of NA,

causing the San Andreas fault to develop and the Gulf of California to open. This change

in plate motion and stress distribution caused many tectonic changes in the western

United States. The craton of the Southwestern part of NA was involved in local basement uplifts of either the Paleozoic Wichita-Ancestral Rocky Mountain system or the Laramide Southern Rocky Mountains (Bally et al., 1989).

2.2. The Rocky Mountain Region Tectonics

The main tectonic events in the Rocky Mountain Region will be summarized below.

2.2.1. The Ancestral Rocky Mountains

The Ancestral Rocky Mountains (ARM) formed during a period ranging from Pennsylvanian to Early Permian between orogenies related to the Ouachita-Marathon fold belt. They represent the youngest Paleozoic structural event (Hintze, 1988; Oldow et al., 1989), and many of their structural deformations have been overprinted by the Laramide deformation. The main uplifts, believed to have been driven by the collision of NA and SA, are the Pennsylvanian Uncompahgre and Front Range uplifts of New Mexico and Colorado (Johnson, 1992). Both were reactivated during the Laramide deformation in the southern RM. Also, subsidence led to the formation of the Eagle, the northern Paradox, and the SE Oquirrh basins, the southern Wyoming shelf, and the eastern Callville shelf (Figure 2-1).

Triassic strata covering the Uncompahgre Highlands marked the end of the ancestral Rockies uplifts. Deposits were accumulating (thickening to the west) from the Triassic through the Early Cretaceous (Hintze, 1988).

2.2.2. The Sevier Orogeny

The Sevier Orogeny took place between 100-80 Ma (Stokes, 1986) and was

characterized by eastward thin-skinned (piggy-back) thrusting in response to active

subduction along the west margin of NA (Hintze, 1988; Johnson, 2003). The orogenic

Figure 2 - 1: Physiographic provinces of the Uinta-Piceance basin region. The Wasatch

line is the eastern edge of the Wasatch Plateau and Range; GNB = Greater Natural

Buttes. Modified after Raisz (1972) and Stokes (1977) in Johnson (1992).

belt crosses Utah diagonally.

With the ARM completely eroded, the sea flooded Utah from the east causing a westward-trending shoreline to become parallel to the “Meso-Cordilleran” foothills. The Cordillera became higher and wider, and the region east of the Wasatch line started to sink. Both uplift and subsidence, due to thrust loading and perhaps mantle flow as

“magmatism began to migrate inboard into the central Rocky Mountain region” as a result of the decrease in angle of subduction of the Farallon Plate during the Campanian (Decelles, 2004), were extreme. This produced a great amount of sediments to be deposited as well as rapid transport by streams and created the foreland basin east of the thrust belt (Johnson, 2003). The orogeny reached its “maximum intensity during Late but not latest Cretaceous, had its maximum effects west of the Wasatch line or along it,” and ceased to move by the end of the Cretaceous or very shortly thereafter (Stokes, 1986).

2.3.3. The Laramide Orogeny

The Laramide Orogeny is a thick-skinned thrust event with thrust faults extending deeply into the basement rocks (Johnson, 2003). This “intraforeland” basement uplift disrupted the pattern of subsidence in the foreland region and shifted the sediment accumulation eastward (Decelles, 2004). It has affected territory east of the Wasatch line (Stokes, 1986), and overlapped with the Sevier compression (Hintze, 1988) and continued right after it, from the mid-Late Cretaceous (Campanian) (80-40 My) to the early Eocene (84-66 Ma) (Stokes, 1986). The uplift was caused by a few degrees of rotation of the Colorado Plateau relative to the continental interior when the eastern Pacific floor was being subducted beneath NA (Hintze, 1988). The basement uplift was the result of a shear zone that extended into the lower crust and by medium-angle (25-35 deg) reverse faults, with no mantle involvement (Bally et al., 1989).

The direct results of this orogeny are the Rocky Mountains and the uplift of the

Uinta Mountains, which, geomorphologically, are an east-west trending anticlinal fold

belt (160 mi [257.5 km] long and 30 mi [48 km] wide). Other minor uplifts resulted as

well, such as the San Rafael Swell, Circle Cliffs, Monument, Kaibab, and Uncompahgre

uplifts (Figure 2-2). In addition to uplifts, the Laramide Orogeny created mini fluvio-

lacustrine basins by disrupting the large foreland basin created during the Late Cretaceous through the Eocene (Johnson, 2003; Stokes, 1986).

The reason why the Laramide deformation is concentrated in the central RM is perhaps its occurrence during a time of faster than usual convergence of the Farallon and the NA plates (Johnson, 2003) as well as the “the Late Cretaceous-Early Tertiary flat- slab” event (Decelles, 2004). Subsidence during the Early Cretaceous was induced by thrust loading by adjacent thrust sheets (Johnson, 2003; Bally et al., 1989). The Uinta basin was created by a “deep trough formed to the south of the southward-thrusting Laramide Uinta uplift” (Johnson, 2003).

2.2.4. The Wasatch Range

The Wasatch Range does not belong to any of the major orogenies mentioned above. It is a Tertiary mountain range believed to have originated after and caused by the Sevier Orogeny (Stokes, 1986).

Other disturbances affected Utah. These were regional uplifts known towards the end of the Cretaceous-early Tertiary period (Stokes, 1986).

In conclusion, Tertiary uplifts affected all of NA, causing the Cretaceous sea to withdraw on a regional scale (this is supported by the exposure of marine strata in the Grand Canyon of Arizona) (Stokes, 1986).

2.3. The Cretaceous Geology of Utah

Four major physiographic provinces occur in the State of Utah: the Colorado Plateau, the Middle Rocky Mountains, the Basin and Range, and the Colorado Plateau/Basin-Range (Stokes, 1986). The state is crossed by the NE-SW Wasatch line (Figure 2-1) that was a late Precambrian rift arm that widened to become the “Paleozoic Cordilleran Geosyncline” (Stokes, 1986).

The Cretaceous lasted for 78 My and was the time “of the last epicontinental sea

in Utah” (Hintze, 1988; Osmond, 1965) (Figure 2-3). During the later phase of the

Figure 2 - 2: The main structural elements that resulted from the Laramide Orogeny in

Utah. An approximate location of the GNB (Greater Natural Buttes) field is

indicated. Modified after Stokes (1986).

Figure 2 - 3: Late Cretaceous (~75 Ma) Paleogeography of the western United States. UT

= Utah and CO = Colorado. Modified after Blakey (2003).

Cretaceous, a major marine-flooding event, from north and south, divided NA into two large land masses. Towards the end of the period, the oceans withdrew from the continental interior as a result of both local and global influences. The local ones are indicated by the belt of deltas and nearshore deposits that migrated progressively eastward. The global influence was the fact that continental seas withdrew from major continents at almost the same time (Stokes, 1986).

Most of Utah’s Cretaceous rocks were deposited during the second half of the period as the prevalent flat topography (Latest Jurassic and early Cretaceous) led to mild erosion. The sediments were being deposited in both central and eastern Utah, creating a wedge of sediment reaching up to 3,000 ft (914 m) in thickness to be later eroded by the Colorado River during the Cenozoic. Cretaceous deposition (Figure 2-4) in the eastern part of Utah was accompanied by subsidence to create a total Cretaceous sediment thickness exceeding that of the remaining Paleozoic and Mesozoic sediments (Hintze, 1988).

2.4. The Uinta Basin

The Uinta basin (Figure 1-1) was part of a Cretaceous foreland basin which is “a continent-long area of downwarping that stretched from the Arctic to Mexico” (Johnson and Roberts, 2003). The basin, 100 mi (161 km) long and nearly 100 mi (161 km) wide, is asymmetrical and bordered by the Uinta Mountains to the north, the San Rafael Swell to the southwest, and the Wasatch Mountains to the west (Spencer 1987; Osmond, 1968).

It extends into the Paradox and Piceance basins to the south and east, respectively, with the Douglas Creek Arch bounding it to the east (Osmond, 1968).

The basin has occupied an “intraplate geologic setting throughout Phanerozoic

time and is mostly underlain by Phanerozoic strata” (Johnson, 1992). Thermally driven

subsidence began at 590 Ma. The development of the Antler orogenic belt to the west

resulted in an increase in subsidence rate and a change in depositional pattern in the

westernmost Uinta-Piceance basin (Johnson, 1992). The whole period was one of

submergence and subsidence in the more stable central and eastern parts of the region.

Figure 2 - 4: Outcrops of Upper Cretaceous rocks (green patches) and the main uplifts in

Utah and surrounding states. Modified after Johnson (2003).

From Mid-Late Mississippian to early Early Permian, the overlapping influences of tectonic interactions along continental margins to the west and southeast led to a complex mix of contractional, extensional, and strike-slip deformation in the region.

Erosion of the Antler belt started, and a complex heterogeneous tectonic style, characterized by local uplift, subsidence, and volcanism, was the result. The evolution of the basin during the period extending from Middle Jurassic to Early Cretaceous was a continuation of an “initial pulse of rapid, asymmetric subsidence linked to a thrusting event in eastern Nevada followed by a period of slower, more uniform subsidence and relative tectonic quiescence” (Johnson, 1992). During the Late Cretaceous, the region was “located near the western shoreline of the Western Interior seaway and within the Cretaceous Rocky Mountain Foreland basin” (Hettinger and Kirschbaum, 2003). The seaway retreated from the Uinta-Piceance region during the latest Cretaceous (Turonian to Early Campanian), fluctuated during the Campanian with an orientation between N65E and N15W, and was outside of the area completely by the Maastrichtian (Hettinger and Kirschbaum, 2003). The foreland basin was segmented by basement-cored uplifts into restricted lacustrine basins (Johnson, 1992). With the Laramide Orogeny, compressional forces controlled the structural development of the region of the Uinta basin caused by the Uinta (north) and the Uncompahgre Uplifts (south) (Cuzella and Stancel, 2006).

The basin, as we know it today, took shape as a result of relatively higher uplift of its margins than its interior, creating 3,000-6,000 ft (914-1,829 m) of relief. This development started during the Early Tertiary (Paleocene or Eocene) and was intermittent since then (Osmond, 1965). The early Tertiary, therefore, is the time for the formation of the Uinta basin, with its northern boundary formed during the Precambrian with the abrupt rise of the Uinta Mountains (Osmond, 1965; Hintze, 1988).

2.5. Regional Stratigraphy

The stratigraphic interval of interest extends from the marine Mancos Shale to the

Green River Formation, excluding both (Figure 2-5). The latter contains the main

producing sandstones in the Uinta basin and the ones penetrated by the studied wells

(Cuzella and Stancel, 2006) (Figures 2-5 and 2-6).

Figure 2 - 5: Generalized stratigraphic column of Uinta-Piceance province. Modified

after Johnson (2003).

Figure 2 - 6: Different divisions and nomenclatures of the different sandstones of the

Mesaverde Group. The red rectangle indicates the GNB field. The interval

bounded by the blue lines is referred to as the CGATE Sandstone in the text. The

Lower Mesaverde interval (MVL) is indicated by the green segment. Modified

after Hettinger and Kirschbaum (2003).

The Upper Cretaceous Mesaverde Group has a thickness ranging from 2,000 to 3,000 ft (607-914 m), and it can reach 4,000 ft (1,219 m) of “interbedded sandstones, siltstones, shale and coal,” as demonstrated by Longman and Koepsell (2005) through well data. The group was sourced from eroded sediment from the Sevier orogenic belt to the west. The sediments were shed to central and eastern Utah during repetitive cycles of regressions and transgressions of the Campanian sea (Hintze, 1988; Johnson, 2003) in the

“Western Interior Seaway” (Cole and Cumella, 2003). The sediment wedge thickened (thinning toward the east and north) as subsidence took place.

The sandstones of the Mesaverde Group vary in thickness from 2 to 50 ft (0.6-15 m) and can be found at depths ranging from 6,000 to 13,000 ft (1,829-3,962 m) (Longman and Koepsell, 2005). The group was divided differently by various authors in different basins and within different parts of the same basin. The various groupings of the different sandstones of the Mesaverde Group are shown in Figure 2-6.

In an ascending order, the formations are:

1- The Mancos Shale is black marine shale. On the logs, gas entries are commonly seen coming from the shale sections. The Mancos “B” interval is “a laterally extensive package of thin, relatively clean, very fine grained sandstones in the Mancos Shale.”

It forms the deepest interval penetrated in the wells (Longman and Koepsell, 2005).

2- The Blackhawk Formation is a sheet-like sandstone deposited in wave-dominated deltas and along strandlines downcurrent from the deltas. The change and progradation in the shoreline, distributary channel switching, and local and regional transgressions produced stacked and imbricated sequences of delta-front sandstones.

The organic-rich deposits that are normally present behind the Blackhawk Formation shoreline thin remarkably in the Natural Buttes field (Pitman et al., 1987).

3- The Castlegate Sandstone is a “massive sandstone body ranging between 150-260 ft (46-79 m) and deposited in a braided plain environment” (Cuzella and Stancel, 2006).

In general, this sandstone does not constitute main producing reservoirs in the GNB

(Cuzella and Stancel, 2006). The top of the Blackhawk Formation is not identified in

the wells, and the Castlegate Sandstone discussed to in the text includes upper

sections of the Blackhawk Formation (Figure 2-6).

4- The Buck Tongue is a shale member of the Mancos Shale deposited 68 Ma. It is “the last preserved marine incursion” in the GNB field (Cuzella and Stancel, 2006;

Longman and Koepsell, 2005).

5- The Sego Sandstone is a “shallowing-upward marine shoreline sandstone” (Longman and Koepsell, 2005). It was deposited in tidal channel, estuarine and shoreface settings (Cuzella and Stancel, 2006). 130 ft (40 m) thick, the sandstone is rarely completed in the GNB field as it lacks a lateral seal (Cuzella and Stancel, 2006). In the studied wells, the Sego Sandstone is a transitional interval containing mainly washover sand and shale, deposited in a lagoonal environment (Longman and Koepsell, 2005). The blocky Sego Sandstone is a shaly lower-shoreface sandstone that grades up into upper shoreface and marine sandstone. This sand has good reservoir characteristics but is mostly wet.

6- The Neslen Formation is a continental coal-bearing formation which is restricted to the eastern part of the Uinta basin (Pitman et al., 1987). It was deposited in coastal plains and floodplain swamps (Longman and Koepsell, 2005). It has a gradational and intertonguing contact with the underlying marginal-marine Sego Sandstone (Pitman et al., 1987). The Corcoran Member, the Cozette Member, and the Rollins Member of the Iles Formation are equivalent to the Lower Mesaverde and the Neslen Formation in the Uinta basin (Johnson, 1993; Cumella and Ostby, 2003; Koepsell et al., 2003;

Cole and Cumella, 2005). These sand members were deposited in inner-shelf, deltaic,

shoreface, estuarine, and lower coastal plains settings (Cole and Cumella, 2005). The

trend of the Corcoran and Cozette shorelines was NE-SW, whereas the Rollins

shoreline trended NNE-SSW (Cumella and Ostby, 2003; Koepsell et al., 2003). The

change in the trend may be the result of a “tectonically influenced shift in the basin

subsidence” (Cumella and Ostby, 2003; Johnson, 1993). Various regressive cycles

have led to the intertonguing of the Lower Mesaverde Sandstones with the Mancos

Shale (Johnson, 2003). This indicates that marine flooding happened very rapidly at

the end of each regressive cycle, with the Castlegate Sandstone being the oldest

regressive cycle in the Mesaverde. The shoreline is believed to have moved from the

NW to SE during those regressions, resulting in a purely fluvial environment that

dominated the area after the regressions (Johnson, 2003). In the studied wells, the

Neslen Formation is a “relatively homogeneous, non-coaly package of sandstones and shales…with consistent very low, non-spiky resistivity response. It contains 4-10 fluvial channel sandstones less than 15 ft (4.6 m) thick.” The Lower Neslen interval contains coal beds and rare thin sandstones deposited in a coastal plain (Longman and Koepsell, 2005).

7- The Farrer and Tuscher Formations, referred to as the Upper Mesaverde Group, are not differentiated in the GNB field (Cuzella and Stancel, 2006). In the Piceance basin, both sandstones are grouped into the Williams Fork Formation that is conformable with the overlying Iles Formation. The Williams Fork Formation was deposited in a non-marine environment, on a coastal plain “west of a prograding shorelines”

(Cumella and Ostby, 2003; Johnson, 1993). In the Piceance basin, the formation has an upper sand-rich interval and a lower sand-poor interval (Cole and Cumella, 2005), and a variable thickness due to truncation and variations in subsidence during deposition. It consists of interbedded sandstone, mudrock, and coal deposited in alluvial-plain, coastal-plain, and marginal-marine settings. In its lower interval, it contains many coal seams in the “Cameo-Wheeler coal zone (Figure 2-6), which consists of interstratified coal, carbonaceous to very carbonaceous mudrocks, ironstone concretion, sandstone, and rare conglomerate,” as described from cores and outcrops in the Piceance basin (Cole and Cumella, 2005). The Cameo coal zone was deposited in “paludal environments of the lower coastal plain” (Cumella and Ostby, 2003).

Above the Mesaverde, younger formations were penetrated by wells with no image log coverage. These are the Wasatch Formation and the Green River Formation.

The former is 2,000-3,000 ft (610-914 m) thick, deposited in a marginal lacustrine

environment. It is comprised of shale, siltstones, paludal limestones, and lenticular fluvial

sandstones (Cuzella and Stancel, 2006). The Green River Formation is mainly shale

interbedded with sandstone and limestone, deposited in a lacustrine setting (Cuzella and

Stancel, 2006). The lacustrine shale of the Green River Formation (the base of which is

likely time transgressive) forms a seal against upward migration of the gas from the

Mesaverde Group. This same shale is a good marker to separate the Mesaverde Group from the Wasatch Formation.

2.5.1. The Mesaverde Group in the Studied Wells

Longman and Koepsell (2005) presented a concise summary of the formations and environments of deposition encountered in the wells they studied (including our four wells) in the GNB field. They divided the wells into zones without necessarily defining accurately the formation tops. These intervals are the Castlegate Formation, the Sego Formation, the Neslen Formation, the Mesaverde Group (divided into Lower, Middle, and Upper Mesaverde), and the Mancos Shale.

The guidelines used to divide the Mesaverde Group, according to Longman and Koepsell (2005), are the presence of the thick sandstone packages with blocky GR vs. the thin fining-upward fluvial channel sandstones. Also, the presence and absence of coal beds were another criterion followed to divide the Mesaverde Group as follows (Longman and Koepsell, 2005):

1- The Upper Mesaverde has braided stream channels and a blocky GR signature.

2- The upper interval of the mid-Mesaverde contains fining-upward fluvial channel fills (very few with blocky GR) with thick floodplains. No coal is present.

3- The Middle Mesaverde is recognized by its thick braided-stream sandstones with blocky GR signature. The sandstones can be separated by thin intervening shale beds.

This is the most sand-rich part of the Mesaverde.

4- The lower fluvial interval of the Mesaverde contains fining-upward fluvial channel sandstones.

For this study, however, the formation tops in the wells, used to create horizons

for modeling in Petrel, were adopted as provided by the Kerr McGee Corporation. Two

main zones, the Upper Mesaverde and the Lower Mesaverde, are modeled in well NBU-

222 (Chapter 6). The lower zone includes what Longman and Koepsell (2005) interpreted

to be the upper Castlegate Formation, the Sego Sandstone, and the Neslen Formation.

The upper zones include what Longman and Koepsell (2005) interpreted to be “fluvial”

and “braided” deposits of the undifferentiated Tuscher and Farrer Formations.

2.5.2. Mesaverde Group Environment of Deposition

A sedimentary environment is a “geographically restricted part of the earth’s surface, which can be distinguished….by the set of physical, chemical, and biological processes which exist and characterize it.” A facies “is the sum of physical, chemical, and biological characteristics which permits to differentiate a sedimentary body from another” (Serra, 1985). The main environments of deposition encountered in the studied wells are the fluvial and shoreface environments. Both environments existed during the Upper Cretaceous when the Mesaverde Group was being deposited in the GNB field.

Sandstones from both a fluvial environment (Figure 2-7) and a shoreface environment (Figure 2-8) were seen in the studied wells.

The fluvial environment is a continental depositional environment where

sediments are carried and transported by running water to be later deposited into

lacustrine or marine basins (Galloway and Hobday, 1996). The stream systems develop

on different slopes with segments having low sinuosity (braided streams) or high

sinuosity (meandering streams). Fluvial facies, according to Galloway and Hobday

(1996), can be a combination of channel fill (lag, accretionary channel, different types of

bars), channel-margin deposits (crevasse splays, levees), and floodbasin deposits

(floodplain, backswamp, and interchannel lakes). The internal structure in each facies

depends on the geometry of the channel and the direction of water flow. Downstream bed

accretion causes deposition of longitudinal bars (Figure 2-9), which are characteristic of a

braided-stream system. Lateral accretion and the formation of point bars are the main

characteristics of high-sinuosity, meandering-stream systems (Figure 2-10). Tabular and

trough cross stratification may be rare to common in the middle to upper point-bar

succession. Also, a vertical decrease in grain size is observed. The top of an old point bar

is usually vegetated and capped by fine floodplain sediments (Galloway and Hobday,

1996). Scour surfaces are common in a fluvial system. Channel lags, coarse bed-loads,

and plant debris may be seen on top of those basal erosional surfaces.

Figure 2 - 7: Fluvial environment showing different facies identified from borehole image logs. Modified after Selley (1978) in Hallam (1981).

Figure 2 - 8: A model of nearshore sand deposits showing the locations where shoreface

sands form. Modified after Scholle and Spearing (1982) in Prothero (1990).

Figure 2 - 9: Depositional model showing the sedimentary structures in (A) longitudinal

bar and (B) transverse bar in a braided channel. Modified after Galloway and

Hobday (1996).

Figure 2 - 10: Depositional model showing the sedimentary structures in (A) an upward-

fining sequence in a mid- or downstream point bar and (B) upstream end of a

point bar. Modified after Galloway and Hobday (1996).