STRUCTURAL INTERPRETATION OF THE CHEROKEE ARCH, SOUTH

CENTRAL WYOMING, USING 3-D SEISMIC DATA AND WELL LOGS

by

Joel Ysaccis B.

ABSTRACT

The purpose of this study is to use 3-D seismic data and well logs to map the

structural evolution of the Cherokee Arch, a major east-west trending basement high

along the Colorado-Wyoming state line. The Cherokee Arch lies along the Cheyenne

lineament, a major discontinuity or suture zone in the basement. Recurrent, oblique-slip

offset is interpreted to have occurred along faults that make up the arch. Gas fields along

the Cherokee Arch produce from structural and structural-stratigraphic traps, mainly in

Cretaceous rocks. Some of these fields, like the South Baggs – West Side Canal fields,

have gas production from multiple pays.

The tectonic evolution of the Cherokee Arch has not been previously studied in

detail. About 315 mi2 (815 km2) of 3-D seismic data were analyzed in this study to better

understand the kinematic evolution of the area. The interpretation involved mapping the

Madison, Shinarump, Above Frontier, Mancos, Almond, Lance/Fox Hills and Fort Union horizons, as well as defining fault geometries. Structure maps on these horizons show the general tendency of the structure to dip towards the west.

The Cherokee Arch is an asymmetrical anticline in the hanging wall, which is mainly transected by a south-dipping series of east-west striking thrust faults. The interpreted thrust faults generally terminate within the Mancos to Above Frontier interval, and their vertical offset increases in magnitude down to the basement. Post-Mancos

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intervals are dominated by near-vertical faults with apparent normal offset. They seem to extend up to the near the surface and affect rocks as young as Eocene in age.

From the analyzed data, two possible episodes of deformation are indicated: early thrusting and subsequent dip-slip and/or wrench movement. The main thrusting occurred during the Upper Cretaceous (between Mancos and Above Frontier interval). The wrenching and dip slip offset occurred during deposition of the Tertiary section. The thrust-fold system of the Cherokee Arch correlates in time with the Laramide Orogeny

(Late Cretaceous through mid-Eocene).

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TABLE OF CONTENTS

ABSTRACT …………………………………………………………………………… iii

TABLE OF CONTENTS ……………………………………………………..……….. v

LIST OF FIGURES …………………………………………………………….…….. vii

LIST OF TABLES ……………………………………………………………….……. xi

ACKNOWLEDGEMENTS ………………………………………………….………. xii

1. INTRODUCTION ………………………………………………………..………… 1

1.1 Research Objectives .……………………………………………..………….. 1

1.2 Previous Work ………………………………………………………………. 2

1.2.1 Regional Tectonics …………………………………………...……. 2

1.2.2 Local Tectonics ……………………………………………………. 5

1.3 Study Area and Data Set ………………………………………………...…. 12

1.3.1 3-D Seismic Data Set …………………………..………………… 17

1.3.2 Digital Well Logs ………………………………………...………. 17

1.4 Research Contributions …………………………………………….………. 19

2. GEOLOGIC SETTING …………………………………………………..………. 20

2.1 Stratigraphy ………………………………………………………...………. 20

2.2 Important Structural Conceptual Models ………………………...………… 29

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2.3 Structural Geology …………………………………………………………. 29

2.3.1 Regional Structure ……………………………………..………… 29

2.3.2 Local Structure …………………………………………………… 35

2.4 Petroleum Geology ……………………………………...…………………. 37

3. SEISMIC DATA ANALYSIS ………………………………..…………………… 39

3.1 Methods ……………………………………………………………….……. 39

3.2 Results ……………………………………………………………...………. 41

3.2.1 Synthetic Seismograms ……………………………...…………… 41

3.2.2 Horizons …………………………………………………..……… 48

3.2.3 Faults ………………………………………………………...…… 65

3.2.4 Time Structure Maps ……………………………………...……… 66

3.2.5 Time Slices …………………………………………………..…… 74

3.2.6 Isochron Maps ………………………………………….………… 74

3.3 Discussion …………………………………………………………..……… 90

4. CONCLUSIONS AND RECOMMENDATIONS ……………………………….. 98

4.1 Conclusions ……………………………………………………..………….. 98

4.2 Recommendations ………………………………………………………….. 99

REFERENCES ………………..……………………….…………………………..… 100

CD ROOM …………………………………………….……………………. Back Pocket

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LIST OF FIGURES

Figure 1.1. Structural orientation of the thrust belt orogen, arches and basins of the Laramide foreland …..……………..………………..…………………………… 4

Figure 1.2. Geologic map of Precambrian rocks of southern Wyoming and northern Colorado ……………………………………………………………...……………. . 6

Figure 1.3 Structure map: Cherokee Ridge Arch ……………………………………….. 7

Figure 1.4 Tectonic features of the Greater Green River Basin ……………...…..…….. 8

Figure 1.5 Deepest wells used for calibration purposes, South Baggs- West Side Canal field ….……………………………………………………..…….. 10

Figure 1.6 Location of the study area, and other Lewis Shale Consortium studies ..…... 11

Figure 1.7 Seismic base map on the top of the Almond for the South Baggs- West Side Canal field ………..…………………………………………………….. 13

Figure 1.8 Seismic line showing significant horizons picked on South Baggs-West Side Canal survey………………………………………………….….. 14

Figure 1.9 Seismic line B-B’ showing a flower structure observed by Hull (2001)…… 15

Figure 1.10 Shaded relief map showing location of study area…..…….…………..…... 16

Figure 1.11 Index map of northwestern Colorado and south-central Wyoming showing study area and gas fields …………….…………………………...….……. 18

Figure 2.1 Stratigraphic chart showing Phanerozoic nomenclature for the state of Wyoming ..…..………………………..………………………...... 21

Figure 2.2 Chronostratigraphic column and type log.…..…………..…..…….….…….. 27

Figure 2.3 Structure map of southwestern Wyoming and adjacent areas …....……...... 30

Figure 2.4. The location of the study area is shown in relation to the Interior Cretaceous Seaway ………………………………………………..……………….. 32

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Figure 2.5. Contractional amalgamation boundaries indicating major northeast-striking tectonic provinces. …………………………..…………… 33

Figure 2.6 Interpreted Landsat image of the Cherokee Ridge Arch ……...……………. 36

Figure 3.1 Seismic base map showing the location of seismic lines …..……....………. 40

Figure 3.2 Synthetic seismogram for the Celsius Bogey Draw 1 well ……………...... 44

Figure 3.3 Synthetic seismogram for the Haystack Unit 4 well …………….…………. 45

Figure 3.4 Synthetic seismogram for the South Baggs Unit 26 well …………………... 47

Figure 3.5 Interpreted seismic line L-L’ …………………………...…………………... 49

Figure 3.6 Horizon calibration: comparison of Cherokee Arch survey vs. South Baggs survey ………………………………………………………………… 51

Figure 3.7 Horizon calibration: comparison of Cherokee Arch survey vs. Powder Mountain survey ………………………………………………….……….. 52

Figure 3.8 Interpreted seismic line K-K’ ………………………………….…………… 54

Figure 3.9 Interpreted seismic line A-A’ ………………………………………………. 55

Figure 3.10 Interpreted seismic line B-B’ ………………………..………….…………. 56

Figure 3.11 Interpreted seismic line C-C’ ..……...………………..……………………. 57

Figure 3.12 Interpreted seismic line D-D’ …………………………..…………………. 58

Figure 3.13 Interpreted seismic line E-E’ …………...…………………………………. 59

Figure 3.14 Interpreted seismic line F-F’ …………...…………………………………. 60

Figure 3.15 Interpreted seismic line G-G’ …………..…………………………………. 61

Figure 3.16 Interpreted seismic line H-H’ …………..…………………………………. 62

Figure 3.17 Interpreted seismic line I-I’ …………….…………………………………. 63

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Figure 3.18 Interpreted seismic line J-J’ …………….…………………………………. 64

Figure 3.19 Time structure map of the Madison horizon ………………..…………….. 67

Figure 3.20 Time structure map of the Shinarump horizon …………….……………… 68

Figure 3.21 Time structure map of the Above Frontier horizon …...……….…..……… 69

Figure 3.22 Time structure map of the Mancos horizon …………………..…………… 70

Figure 3.23 Time structure map of the Almond horizon ………………………………. 71

Figure 3.24 Time structure map of the Lance/Fox Hills horizon ………………...……. 72

Figure 3.25 Time structure map of the Fort Union horizon ………………..…………... 73

Figure 3.26 Time slice map at 3608 ms (two way travel time) ……………...………… 75

Figure 3.27 Time slice map at 2352 ms (two way travel time) ……………...………… 76

Figure 3.28 Time slice map at 1848 ms (two way travel time) …………..…….……… 77

Figure 3.29 Time slice map at 1624 ms (two way travel time) …….……….….……… 78

Figure 3.30 Time slice map at 1504 ms (two way travel time) …………....….……….. 79

Figure 3.31 Isochron map from Shinarump – Madison ………..………………………. 82

Figure 3.32 Isochron map from Above Frontier – Shinarump ………..……………….. 83

Figure 3.33 Isochron map from Mancos – Above Frontier ……………………...…….. 84

Figure 3.34 Interpreted seismic line E-E’ showing intervals of major thickness change……………………………………………………………... 86

Figure 3.35 Isochron map from Almond – Mancos ……………………………………. 88

Figure 3.36 Isochron map from Lance/Fox Hills – Almond ………………..…………. 89

Figure 3.37 Isochron map from Fort Union – Lance/Fox Hills ………………...……… 91

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Figure 3.38 Percentage of thickening vs. time (Ma) …………..……………………….. 95

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LIST OF TABLES

Table 3.1 Average interval velocity based on the South Baggs Unit 26 synthetic seismogram ………………………..………………………………...….... 81

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ACKNOWLEDGEMENTS

A project such as this could not have been completed without the help,

consideration and understanding of many people:

¾ I thank Dr. Neil Hurley, my advisor, for his endless help and technical guidance in this research and throughout my studies at CSM. His comments helped to focus my thinking and made me critically evaluate the ideas presented here.

¾ My gratitude goes to Drs. Charles Kluth and Robert Benson for their valuable collaboration. They provided important input, advice and suggestions at different stages of the project that allowed me to make this product better.

¾ I acknowledge PDVSA for providing financial support.

¾ John Young, Ed Blott and Joel Scoville kindly provided data and advice for use in this project.

¾ I would also like to express my deepest appreciation to Charlie Rourke for her willingness to help and make things go nice and smooth, even during hard times.

¾ I appreciate the support from fellow students in the Lewis Shale

Consortium, especially Angel Gonzalez and Ira Pasternack.

¾ I wish to thank Yosmary, for being a loving and understanding wife, and my charming children: Angélica, Angela and Joel Gabriel. Without their kindness, love

and moral support, I could not have undertaken this endeavor.

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¾ Finally, thanks go to all of my family and friends in Venezuela who have supported and encouraged me.

xiii 1

CHAPTER 1

INTRODUCTION

This study involves a 3-D seismic interpretation of a complex area in the southeastern portion of the Greater Green River basin in Wyoming and Colorado. The area is heavily faulted, and is associated with a prominent basement uplift. Stratigraphic complexities include prograding deep-water sand lobes of the Dad member of the Lewis

Shale (Upper Cretaceous). Gas fields occur within the study area, and exploration and development drilling is being actively pursued. This chapter provides a statement of research objectives, a summary of previous work, and a statement of research contributions.

1.1 Research Objectives

The purpose of this study is to use 3-D seismic data and well logs to map and interpret the structural evolution and geometry of the Cherokee Arch, a major east-west trending basement high along the Colorado-Wyoming state line. Specific objectives include:

• Generate synthetic seismograms from wells.

• Map the structural geometry within the data set. Pick 7 horizons and faults.

• Establish timing of the structure by analyzing fault terminations and geometries.

• Document stratigraphic thickness changes and relate this to fault timing.

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1.2 Previous Work

1.2.1 Regional Tectonics

The Laramide orogen began during the Campanian (Late Cretaceous, 80 Ma) and

extended through the Mid-Eocene (~38 Ma). Variable structural orientations in the

Laramide foreland have prompted numerous hypotheses concerning the orientation(s) of stresses. These hypotheses generally fall into two categories. The first hypothesis is a single, extended phase of SW-NE compression that caused both dip-slip and oblique-slip offset and fault-propagation folds (Wise, 1963; Sales, 1968; Stone, 1969; Blackstone, 1990;

Paylor and Yin, 1993; Erslev, 1993; and Molzer and Erslev, 1995). In this model, northwest-southeast trending structures are horizontally shortened by dip-slip fault movement and associated folding, whereas the east-west structures show a high amount of left lateral oblique-slip movement. This model has growing support, primarily due to numerous outcrop measurements of fault-slip surfaces by Paylor and Yin (1993) and

Molzer and Erslev (1995).

The second proposed model suggests that changes in the maximum horizontal shortening direction existed through the Late Cretaceous (east-west) to Eocene (north- south) (Gries, 1983; Chapin and Cather, 1983). This model may be more appropriate in the southern Rockies. New fault data from New Mexico (Erslev, 2001) suggests multi-stage, multi-directional shortening. Both models can explain the gross structural orientation of the ranges and basins.

3

Figure 1.1 shows the structural orientation of the thrust belt orogen, arches and basins of the Laramide foreland with the local maximum horizontal shortening directions.

Zoback and Zoback (1989) showed that the present-day stress orientation in the western continental U.S. is in a northwest direction. Rahmat (2000) used borehole breakout analysis to show a northwest (N30-40°W) direction of maximum present-day stress in wells located in T10N R93W, just southeast of the study area. Minton (2002), Witton

(1999), and Sunnetcioglu (2002) had similar results to the north of the study area. Lorenz

(1995) concluded that Laramide compressional forces are more significant to the regional stress than east-west Sevier compression.

The Wind River Range has a northwest-southeast trending family of structures, as does the Casper Arch. This is the dominant structural grain in central and northern

Wyoming. However portions of the Granite Mountains and Casper Mountains are east- west trending. Gries (1983) cited evidence that two separate and distinct phases of deformation occurred, where the north-northwest structures formed first and were later overprinted by the east-west oriented structures. In contrast, Paylor and Yin (1993) and

Molzer and Erslev (1995) inferred oblique slip on east-west structures, and dip slip on the northwest-southeast oriented structures.

Several east-west and northwest structural trends of Precambrian age have been noted as being prominent in basement rocks of the Wyoming foreland (Thomas, 1971;

Brown, 1984). The most significant structural trend in the study area is the Cheyenne Belt, which represents the southern boundary of the Wyoming Province and the northern

4

Study Area

Figure 1.1 Structural orientation of the thrust belt orogen, arches and basins of the Laramide foreland. Red arrows indicate local maximum horizontal shortening directions. From Erslev (2001).

5

boundary of the Colorado Proterozoic Province. The Cheyenne Belt (Figure 1.2) has been

interpreted as an Early Proterozoic collision zone that marks the boundary between the

Archean craton of the southern Wyoming Province and accreted Early Proterozoic island-

arc terranes to the south (Hills and Houston, 1979; Karlstrom and Houston, 1984;

Duebendorfer and Houston, 1986).

During the late Cretaceous and early Tertiary, the Rocky Mountain foreland was fractured by deep-rooted reverse and thrust faults that uplifted broad blocks of Precambrian

basement rocks. This thick-skinned style of deformation is characteristic of the classic

“Laramide Orogeny” (Late Cretaceous through mid-Eocene). Based on the fault geometry

and the stratigraphic record analysis, the studied thrust-fold system of the Cherokee Arch

correlates in time with this orogenic event.

1.2.2 Local Tectonics

The Wyoming Geological Association (1979) and Doelger (1995) published maps

(Figures 1.3 and 1.4) that show the main tectonic features of the Greater Green River

Basin. Parker and Bortz (2001) discussed the discovery well in the West Side Canal Field,

Carbon County, Wyoming, and Moffat County, Colorado. They also explained the

reservoir stratigraphy, source rocks, and the structural and stratigraphic trapping

mechanisms for the field. The West Side Canal discovery was conceived as a structural

prospect. Pre-discovery seismic data failed to correctly map the structural closure because the effect of abnormally low velocity in shallow gas-saturated reservoirs was not

6

Figure 1.2 Geologic map of Precambrian rocks of southern Wyoming and northern Colorado. After Karlstrom and Houston (1984).

7 1 Mile ast). Contour interval is 100 ft (30 m). From the From ast). Contour interval is 100 ft (30 m). okee Ridge Arch. Datum – Lower Ft. Union Marker (West). – Lower Ft. Union Marker (West). okee Ridge Arch. Datum Datum – Lower Lewis Marker (E Datum Geological Association (1979). Wyoming

Figure 1.3 Figure Structure map: Cher

N N

8

N 50 km sin. The Cherokee . ) 1995 ne Bow Wrench. Other complex ne Bow Wrench. ( er g eater Green River Ba is shown. From Doel g faultin Arch is designated as the Medici as the designated Arch is Figure 1.4 Figure Tectonic features of the Gr

9

recognized. Subsequent drilling confirmed the existence of the structure and led to the development of several fields along the Wyoming-Colorado border. Figure 1.5 is the structural cross-section used as input for stratigraphic correlation and determination of the depths of key geologic markers.

This thesis is part of the Lewis Shale Consortium at the Colorado School of Mines

(Hurley et al., 2000). Figure 1.6 shows the location of some of the previous work in relation to the study area for this thesis.

Several theses done at the Colorado School of Mines pertain to the present study.

These are:

Rahmat (2000) studied borehole image logs to the southeast of this study area.

She interpreted paleocurrent directions and structural geology.

Zainal (2001) studied relationships in sandstone and bentonite beds in the Sand

Wash basin that identified sediment supply directions and provided time lines in the lower Lewis Shale.

Hull (2001) conducted research utilizing a 3D seismic survey that is located to the east of the study area. His interpretation of the seismic data indicates tectonic movement during deposition. He showed faulting and changes in bedding thickness that indicated the structural influence of the Cherokee Arch on sedimentation, particularly on the lower

Lewis interval.

Hull (2001) stated that local structural features observed within his data set “are interpreted to be the result of deformation related to left-lateral movement on a

10

Figure 1.5 Deep wells from South Baggs-West Side Canal field. The synthetic seismogram was generated from digital log data in well South Baggs Unit 26 (right). After Parker and Bortz (2001).

11

STUDY AREA

Figure 1.6 Inset shows the regional location of the study area. The location of borehole images (FMI), core, 3D seismic survey, and other Lewis Shale Consortium studies are also indicated. After Minton (2002).

12

subsurface wrench fault.” Additionally, he said that although it was difficult to determine, there is one case a lateral offset of 7 mi (11 km). His work suggested a strike- slip regime. Due to the importance of this work, some key figures from Hull’s (2001) thesis are shown in this section (Figures 1.7, 1.8, and 1.9).

Minton (2002) produced two regional cross sections. Cross section correlations in the northwest cross section (A-A’) indicate dip directions to the south and southwest in the Washakie basin, and thinning of the Lewis Shale to the southwest. The southeast cross section (B-B’) indicates dip directions to the northeast in the Sand Wash basin and to the southwest in the Washakie basin.

Maraj (M. S. thesis in progress) is investigating the role that fractures play in the production of the Cepo-Powder Mountain fields using cores and borehole image logs.

Her study is north of and adjacent to this study. She will build a 3-D structural model to help resolve the effects of structure as a hydrocarbon trapping mechanism, and potentially provide new targets zones for exploration by getting a sense of the sand-body geometry.

1.3 Study Area and Data Set

The study area, located in south-central Wyoming near the town of Baggs,

Wyoming, is situated along the Cherokee Arch. This basement high separates the

Washakie basin from the Sand Wash basin (Figure 1.10).

13

Figure 1.7 Seismic base map on the top of the Almond time structure showing the locations of seismic lines. Color scale is two way time (msec). After Hull (2001).

14

Figure 1.8 Seismic line A – A’ showing significant horizons picked within the data set. Orange lines are interpreted faults. Positive amplitudes are in blue, negative amplitudes are in red and zero crossings are in white. Yellow time markers are shown every 100 ms. Figure 1.7 is the index map that shows the location of this line. After Hull (2001).

15

Figure 1.9 Seismic line B-B’ showing a flower structure observed by Hull (2001). Positive amplitudes are shown in blue, negative amplitudes are shown in red, and zero crossings are shown in white. Figure 1.7 is the index map that shows the location of this line. After Hull (2001).

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Figure 1.10 Shaded relief map showing location of study area. After Thelin and Pike (1991).

17

1.3.1 3-D Seismic Data set

The study involves approximately 315 mi2 (815 km2) of 3-D seismic data in

portions of Sweetwater and Carbon Counties, Wyoming, T12-13N and R93-96W, and

Moffat County, Colorado (Figure 1.11).

The seismic data set was used to study in detail the main structural features within

the area. The 3-D seismic data set has been loaded into a Silicon Graphics workstation

with Landmark interpretation software. All seismic data interpretations and

manipulations were performed utilizing a variety of Landmark interpretation software

products.

1.3.2 Digital Well Logs

Sonic and density logs were used to produce synthetic seismograms. Gamma ray

and resistivity curves were used to identify log tops within the stratigraphic column. Ed

Blott, a consultant in Denver with extensive Rocky Mountain experience in generating synthetics seismograms, provided 16 digital well logs (LAS files). The recommendations

that he gave to Hull (2001) about using the Ormsby wavelet with band pass filters of 10-

20-40-60 Hz and 5-10-60-70 Hz were taken into account. This study used Syntool,

which is a different application, so an equivalent filter with approximately the same band

pass filter was applied.

At an early stage of the project, and once the available well data were plotted, the

difficulty in calibrating the deepest horizons was identified as well as the fact that most of

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R98W R96W R94W R92W

WASHAKIE BASIN

T 14 N

South Baggs- West Side Canal CHEROKEE RIDGE ARCH Baggs T WYOMING State 12 COLORADO Line N Little Snake

Powder Wash Four Mile Creek

T SAND WASH BASIN 10 6 Miles N

Figure 1.11 Index map of northwestern Colorado and south-central Wyoming showing study area and gas fields. The internal blue square shows the position of the interpreted 3-D seismic data. The positions of the synthetic seismograms are represented by the wells in green. The arrows give the projection path of the two deepest wells used for calibration of the deeper reflectors. The dark red and black squares give the position of the South Baggs and Powder Mountain 3-D surveys, respectively.

19

the wells with sonic and density curves were located in the north part of the survey or footwall of the Arch.

1.4 Research Contributions.

Among the major contributions of this research are:

¾ Seven horizons were interpreted in a large (315 mi2; 815 km2) 3-D seismic

volume. The selected seismic horizons were: Madison, Shinarump, Above

Frontier, Mancos, Almond, Lance/Fox Hills, and Fort Union. Time-structure

maps on the interpreted seismic surfaces were generated, as well as time slices

close to the average position of the top of the interpreted horizons, and isochron

maps between the horizons mapped.

¾ Subsurface reflection seismic and borehole data provide a comprehensive data

base for structural analysis of basement-involved folds (thrust folds) of the

Cherokee Arch. There appear to be two episodes of deformation, early thrusting

(Upper Cretaceous) and subsequent wrenching and/or dip-slip offset that appears

to affect rocks as young as Eocene in age.

¾ There is little or no evidence of the Ancestral Rocky Mountain deformation event

(Late Mississippian to Early Permian) in the stratigraphic record at the Cherokee

Arch, based upon interval isochron thickening trends for different ages of

sediments.

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CHAPTER 2

GEOLOGIC SETTING

2.1 Stratigraphy

This section has been modified from Zahm (2002) and Minton (2002). During the

Cambrian, the complex Precambrian basement of Wyoming was a low relief-platform which accumulated a thin cratonic sedimentary cover (Boyd, 1993). The Transcontinental

Arch was a major paleotectonic feature that influenced the depositional history on the

Rocky Mountain shelf, as indicated by stratigraphic thickness and facies variations associated with progressive west-to-east marine transgression during the Cambrian and

Early Ordovician (Sloss, 1963). However, the fact that the Arch was a paleotectonic feature is indicated by the absence of Silurian deposits (Gibbs, 1972; Peterson and Smith,

1986). The oldest known Paleozoic unit deposited during this long-lived marine transgression is the Cambrian Flathead Sandstone (Figure 2.1). The Flathead is overlain by various shallow-marine deposits with numerous disconformities (Snoke, 1993). By the early Devonian, much of Wyoming was emergent with only lenticular estuarine deposits preserved locally (Dott and Batten, 1981).

During the Mississipian, extensive carbonate deposition was prevalent throughout the cordillera. Mississippian rocks are thick (1000 ft or 305 m) along the western and northern borders of the state (Peterson and Smith, 1986). Widespread emergence of much of the continental interior at the close of deposition of the Madison Limestone caused

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Figure 2.1 Stratigraphic chart showing Phanerozoic nomenclature for the state of Wyoming. After Snoke (1993). This study uses the nomenclature for Central – South Central Wyoming.

22

extensive erosion and karst processes. The cause of the emergence is problematic, but studies of subtle facies changes and angular unconformities in the Madison of northern

Wyoming suggest that uplift of the Ancestral Rockies may have played a role in the emergence (Simmons and Scholle, 1990; Sonnenfeld, 1996). The Madison was deposited in shallow-water shelf environments and consists of limestone and dolomite (Craig, 1972;

Rose, 1977).

During the Pennsylvanian and Permian, the southern margin of North America experienced a collisional event manifested by the Ouachita-Marathon orogenic belt (Kluth and Coney, 1981). The Amsden Formation is an important unit that delineates the effects of this orogeny throughout Wyoming (Mallory, 1963). The Amsden thickens away from uplifted units. Most of the effects of the Ancestral Rockies in Wyoming occurred in the southern portion of the state (Mallory, 1963; Sando et al., 1975).

During the Late Pennsylvanian to Early Permian, the Tensleep Formation was deposited as a basal marine sandstone, grading upward into coastal dunes sets separated by thin interdune carbonates. In western Wyoming, the Permian Phosphoria (Park City)

Formation was deposited as a phosphatic shale, phosphorite, and bedded chert that grades eastward into a shallow-shelf marine sequence that contains limestone, dolomite, and chert.

Organic-rich members of the Phosphoria are especially significant as source rocks for hydrocarbons.

Distinct coloration identifies the Triassic units of Wyoming. The terrigenous red beds of the Chugwater Group were deposited in a muddy coastal plain. The Shinarump is

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composed of sandstone, commonly conglomeratic and with red shale beds; it is of fluviatile

channel deposition and overlies the Moenkopi Formation. The Shinarump ranges from 40

to 300 ft (12 m to 90 m) in thickness in Northwest Colorado. Between the Triassic and

Jurassic, a significant unconformity developed in Wyoming (Downs, 1949). Overlying this

unconformity are the basal gypsum, anhydrite, claystone and siltstone units of the Gypsum

Springs Formation. In addition, a lower limestone unit occurs within this unit and a fine- grained clastic unit contains gypsum. These rocks were deposited during a period of marine evaporitic deposition, probably on sabkhas. Overlying the Gypsum Springs is the

Jurassic Sundance Formation. The Sundance Formation was deposited in the shallow

Sundance Sea (Kocurek and Dott, 1983). This transgression was an early stage of a gradual worldwide rise in sea level that reached a maximum during the Late Cretaceous

(Vail et al., 1977). The Sundance is characterized by distinct light tan sandstones interbedded with green glauconitic sandstones and oolitic limestones (Paylor and Yin,

1993).

During the Late Jurassic, a major interruption occurred throughout the Rocky

Mountain region. During this period, the marine shelf was transformed into a nonmarine

environment with deposition of the fluvial Morrison Formation. Although sea level was

still rising (Vail et al., 1977), increased siliciclastic debris was being deposited, probably

related to increased orogenic activity to the west (Brenner, 1983). Deposition of the

Morrison is characterized by varicolored continental deposits of mud, sand, gravel, and

lenses of lacustrine limestone.

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During the Cretaceous, the Western Interior Seaway developed and extended from the Arctic Ocean to the Gulf of Mexico (Kauffman, 1977). Evolution of the Western

Interior Cretaceous basin is chronologically linked with orogenic events along its western margin (Jordan, 1981). The Cretaceous rocks are chiefly shale, siltstone and sandstone with occasional conglomerates (McGookey et al., 1972).

Conformably overlying Lower Cretaceous rocks are the Upper Cretaceous

Mowry Shale and Frontier Formation. The Frontier is composed of a marine to nonmarine sequence of sandstone and shale that represents a transgressive and regressive cycle of deposition. In western Wyoming, nonmarine and nearshore marine siliciclastic rocks grade eastward into offshore marine siliciclastic and carbonate units

(Merewether, 1983). Unconformities in the Frontier are indicative of transgressions and regressions as well as structural deformation. The Frontier is thickest in the western part of the Green River basin (>1,100 ft; 335 m) and thins to the east to about 550 ft

(168 m) in the Laramie Basin. Conformably overlying the Frontier is the marine Upper

Cretaceous Steele Shale and equivalent rocks, the Mancos, Cody, Baxter, and Hilliard

Shales.

West of the Washakie basin, along the Rock Springs uplift, the Mesaverde Group is comprised of four members. From bottom to top, they are the Blair Formation, Rock

Springs Formation, Ericson Formation, and Almond Formation (Weimer, 1960). The

Ericson is nonmarine and the other formations are nonmarine to marginal marine. The

25

Almond is a fluvial, inter-deltaic, and marginal marine deposit that lies conformably beneath the Lewis Shale. On the eastern flank of the Washakie basin, the Mesaverde

Group consists of the Allen Ridge Formation, the Pine Ridge Formation, and the Almond

Formation (Baars et al., 1988). The Allen Ridge formation is nonmarine to marginal marine, the Pine Ridge Formation is nonmarine, and the Almond Formation is nonmarine to marginal marine. In the Sand Wash basin, the Mesaverde Group is comprised of the

Iles Formation and above that, depending on location, either the Almond Formation or the Williams Fork Formation. The Mesaverde Group conformably underlies the Lewis

Shale (Weimer, 1960).

The Lewis Shale in the Washakie and Sand Wash basins is primarily a marine siliciclastic deposit with relatively minor calcareous and bentonitic beds. The marine environment was part of the North American Cretaceous Interior Seaway, also known as the Western Cretaceous Interior Seaway. This shallow epeiric seaway covered a large part of North America from the Gulf of Mexico through Canada. The Lewis Shale was deposited during the Maastrichtian Bearpaw transgression and regression of the North

American Cretaceous Interior Seaway (Winn et al., 1987). The siliciclastic sediments resulted from erosion of the adjacent Cordilleran highland (Molenaar and Rice, 1988) and from early Laramide structures such as the Lost Soldier anticline (Winn et al., 1987).

Deposition of the Lewis Shale in the study area persisted for about 2.4 Ma and spans four Maastrichtian ammonite range zones (Pyles, 2000). The four ammonite zones are, from oldest to youngest, Baculites eliasi, Baculites baculus, Baculites grandis, and

26

Baculites clinolobatus (Roehler, 1990). The length of time of the transgression and

regression suggests that the Lewis Shale was a third-order sequence (Pyles, 2000). Van

Wagoner et al. (1990) stated that a third-order sequence is composed of parasequences

and parasequence sets that contain no internal unconformities. There are therefore no

higher order sequences within this definition of a third-order sequence. They used the

term composite sequence to describe a sequence with a third-order duration that is

composed of higher order sequences.

In the Sand Wash and Washakie basins, the Lewis Shale overlies the Mesaverde

Group and underlies the Fox Hills Sandstone. The Lewis Shale is about 2000 to 2600 ft

(600-800 m) thick over most of the study area, but thins to the west and southwest to a

thickness of 800 ft (250 m).

Figure 2.2 shows the stratigraphic column of the area and a typical gamma ray

log for the Lewis Shale. The figure also depicts the sharp contact between the Almond

and the Lewis Shale and the gradational contact of the Lewis Shale and the Fox Hills

Formation (Witton, 1999).

The Lewis Shale is informally divided into three members: the lower shale, the

middle Dad Sandstone, and an upper shale member. Figure 2.2 shows a typical well log of the Lewis Shale along with the stratigraphic column of the area. The Dad Sandstone is the primary exploration target within the Lewis Shale as indicated by the relatively clean

gamma ray curve. Hydrocarbon production from the shaly upper and lower Lewis is rare.

Above the Lewis Shale, the Fox Hills is a regressive shallow marine sandstone

27

Figure 2.2 Chronostratigraphic column after Baars et al. (1988). Type log is the Barrel Springs 7-22 (Sec 22-16N-93W) after Witton (1999). Drafted by Minton (2002).

28

that may contain marine shale and isolated coals (Cronoble, 1969). The Fox Hills is stratigraphically equivalent to part of the Lewis Shale (Pyles, 2000). These formations commonly interfinger and the contact between the two is gradational in nature. The top of the Lewis Shale on well logs is therefore not a precise log inflection, but a coarsening upward transition. Lack of a formally defined reference section in the area has led to inconsistent selection of formation boundaries (Pyles, 2000). The Lance Formation conformably overlies the Fox Hills Formation. The Lance consists of interbedded carbonaceous shale, coal, siltstone, and sandstones (Weimer, 1960). The Fox Hills to

Lance transition is conformable, and there is some interfingering between the marginal marine facies of the Fox Hills and the nonmarine sediments of the Lance.

Beginning during the Late Cretaceous and continuing into the Tertiary, many of the present-day structural features became more prominent, with the result that many sediment sources were local. As a consequence, the stratigraphic units are less continuous and more variable than older rocks in terms of lithology and facies relationships. Tertiary rocks in the province consist of conglomerates, sandstones, siltstones, shales, limestones, and coals that were deposited in fluvial to lacustrine environments. Throughout most of southwestern Wyoming, lower Tertiary rocks include the Paleocene Fort Union Formation. The Fort Union is in general a coal-bearing unit and is lithologically similar to underlying Cretaceous rocks. The Fort Union grades upward into the Eocene Wasatch Formation which in turn intertongues upward with the

Eocene Green River Formation.

29

2.2 Important Structural Conceptual Models

In The Rocky Mountain foreland, two important conceptual models for

individual Laramide structures have emerged: the fold-thrust model (Berg, 1962; Brown,

1988), and the thrust-fold model (Blackstone, 1940; Stone, 1984). In the fold-thrust

model, an initial stage of penetrative folding in both cover and basement rock is followed

by thrust faulting, which breaks throughout the steepened forelimb of the fold. In the

thrust-fold model, thrusting and folding occur simultaneously, with initial thrust

displacement in the basement transforming into folding in the overlying sedimentary

cover. The basement thrust faults propagated into the sedimentary cover to replicate the

fold-then-fault sequence of deformation seen in the cover strata. The hanging wall of the

thrust can rotate if the basement fault is curved (Brown, 1984; Erslev, 1986) to form the

tilted block geometries characteristic of many Rocky Mountain structures.

2.3 Structural Geology

2.3.1 Regional Structure

Much of this description is modified from Minton (2002). The study area is located in the southeastern portion of the Greater Green River basin within two sub- basins known as the Washakie basin and the Sand Wash basin. Figure 2.3 shows a tectonic map of the study area and the surrounding region. The Washakie and Sand Wash basins are bounded by the Rock Springs uplift and the Uinta uplift on the west, the

Wamsutter Arch to the north, and the Sierra Madre uplift to the east. The Cherokee Arch

30

Figure 2.3 Structure map of southwestern Wyoming and adjacent areas. Structure is on the Precambrian basement in kilometers below sea level. The location of the general study area is indicated by the pink square. After Baars et al. (1988). Approximate location of lineaments after Karlstrom et al. (1988). Drafted by Minton (2002).

31

separates the Washakie basin from the Sand Wash basin.

The Greater Green River basin is a part of the Western Interior foreland basin that flanked the cratonic side of a foreland thrust belt (Jordan, 1981). The foreland basin was created as a result of the Sevier and Laramide tectonic activity that was active from the

Jurassic through the earliest Eocene (Baars et al., 1988). Changes in the style of subduction of the Farallon plate under the North American plate formed a complex variation of thrusting and subsidence in the region (Cross, 1986). In south-central Wyoming, the

Laramide uplift began during the early part of the Maastrichtian. Evidence of early

Laramide compression in the area is seen at the Lost Soldier anticline (Weimer, 1960;

McGookey et al., 1972; Reynolds, 1976). This uplift caused erosion of the Lower Lewis

Shale and Mesaverde Group and also formed an embayment in the Cretaceous seaway. The location of the Lost Soldier anticline and the embayment are shown in Figure 2.4.

Structural influences in the study area include fabric inherited from Early

Proterozoic time. At that time, the super-continent of Laurentia was being assembled in an area that would become North America (Krugh, 1997). After the Canadian Shield was assembled, a wide swath of young crustal material was sutured to the Canadian Shield’s southern margin in a series of compressional amalgamations (Karlstrom et al., 1998).

Figure 2.5 shows the location and age of the major accretion boundaries.

The Cheyenne Belt crosses the study area in an east-west direction, sub-parallel to and north of the Cherokee Arch. The Cheyenne Belt is the suture zone that separates the

32

Figure 2.4 The location of the study area is shown in relation to the Interior Cretaceous Seaway during Baculites clinolobatus time, approximately 69.4 Ma. After McGooky et al. (1972). Note the embayment caused by Laramide compression and uplift of the ancestral Lost Soldier Anticline. Drafted by Minton (2002).

33

Figure 2.5 Contractional amalgamation boundaries (1.8 to 1.65 Ga) indicating major northeast-striking tectonic provinces. The Cheyenne Belt is an Archean- Proterozoic suture that crosses the study area (green box) in an east-west direction. Present day Precambrian outcrops are shaded in blue. After Karlstrom et al. (1998). Drafted by Minton (2002).

34

Wyoming Province from the Colorado Province. This accretionary boundary was formed between 1.79 and 1.75 Ga (Snoke, 1997). The basement rocks in the study area are

Archean in age north of the Cheyenne Belt and Proterozoic in age south of the Cheyenne

Belt. The Cheyenne Belt geosuture is composed of a system of mylonite zones that are from 0.4 to 4.4 mi (0.25 to 7.1 km) wide where exposed (Snoke, 1997). The suture zone may have created a weakness in the crust that has been reactivated at various times during the geologic past, but direct linkage of movement or reactivation of the Cheyenne Belt and the Cherokee Arch has not been established in the literature (Hull, 2001).

A factor that may influence sedimentation differences between the Washakie and the Sand Wash basins is the difference in crustal thickness between the two provinces.

This could affect subsidence rates due to sediment loading. According to seismic studies by the USGS, the crust in the Wyoming Province is thin (in the range of 37 to 41 km), whereas the Colorado Province crust is on the order of 48 to 54 km thick (Snoke, 1997).

Differences in crustal thickness may have resulted in dissimilar styles of Laramide structures in the Wyoming and Colorado Provinces (Allmendinger et al., 1982).

Seismic data interpreted by Hull (2001) indicate that the Cherokee Arch structure was active during deposition of the lower Lewis Shale and became increasingly less active during the middle and upper Lewis Shale deposition.

Structure can be an important component of hydrocarbon production from the

Lewis Shale. The geologic structure in the area of the 3-D seismic survey studied by Hull

(2001) is interpreted to be a complex positive flower-type structure indicative of lateral

35

movement along a wrench fault with compression at a bend in the fault. The structure is at

a bend in the projected path of the Cheyenne Belt. A flower-type structure is therefore

consistent with reactivation of the suture zone (Hull, 2001). Surface lineaments at the

Cherokee Arch are south of the projected line of the Cheyenne Belt (Krugh, 1987). The dip

angle of the Cheyenne Belt in this area has been modeled to be about 55º to 60º, dipping to the south (Allmendinger et al., 1982). Assuming a south dip along the suture zone and a later near-vertical strike-slip fault in younger sedimentary rocks, the surficial expression of later faulting would be expected to shift to the south and be vertically above the basement weakness.

2.3.2 Local Structure

The study area is dominated by a distinct lineament on Landsat imagery that measures 71 mi (114 km) in length and defines a feature that has been termed the

Cherokee Ridge Arch (Figure 2.6). Many of the surface and subsurface structural features observed along the Cherokee Ridge have been interpreted to be the result of deformation caused by right-lateral movement on a subsurface wrench fault (Bader,

1987).

The lineament on Landsat imagery associated with the Cherokee Ridge has been interpreted to be the surface expression of a primary, master right-lateral wrench fault.

Its geographic location is in the approximate location of a previously mapped major subsurface east-west trending fault that cuts Cretaceous and Tertiary horizons (Bader,

36

Study Area

Figure 2.6 Interpreted Landsat image of the Cherokee Ridge Arch. Blue line represents the Colorado-Wyoming border. White lines are Bader (1987) interpretations. The blue square represents the study area. (After Bader, 1987).

37

1987).

Previous studies have documented the surface and subsurface expression of faults and folds in the area. In general, subsurface structural trends are similar to those observed on the surface. However, it has been observed by Cronoble (1969) and Bader

(1987) that faults tend to decrease in abundance with depth – perhaps due to the presence of flower structures.

2.4 Petroleum Geology

Reservoirs in the Cherokee Arch area include the Jurassic Nugget Sandstone,

Lower Cretaceous Dakota Sandstone, Upper Cretaceous Williams Fork Formation,

Almond Formation, Lewis Shale sandstones, Lance Formation, Paleocene Fort Union

Formation, and Eocene Wasatch Formation. Porosity ranges from 10 to 30 percent and permeability ranges from 0.1 to 500 mD (Cardinal and Hovis, 1979; Cronoble, 1969,

Parker and Bortz, 2001).

Reservoir thickness is highly variable, ranging from 10 to 40 ft (3 to 12 m).

Cretaceous reservoirs in the deeper part of the play area are low-permeability reservoirs.

Source Rocks and Geochemistry: Although there are no oil or gas analyses from fields in the play area, it is likely that the oils are sourced from Cretaceous rocks and the gases are sourced from Cretaceous and older rocks. Based on thermal maturity mapping

38

(Pawlewicz et al., 1986; Merewether et al., 1987), Cretaceous and older rocks are

thermally mature to overmature with respect to the oil window.

Timing and Migration: It is likely that structural traps were in existence when

Cretaceous source rocks entered the oil window (0.6% vitrinite reflectance). Older source

rocks, such as the Permian Phosphoria Formation, may have passed through the oil

window (1.3 % vitrinite reflectance) prior to the formation of structural traps.

Traps and Seals: The trapping mechanism for nearly all accumulations is

structural. Existing fields are anticlinal folds that are commonly faulted. Impermeable shales and/ or faults provide the seals. Depth of occurrence is 2,000 to 15,000 ft (610 to

4,572 m).

Exploration status and resource potential: This area is maturely explored. Deep drilling to the Mississippian Madison Limestone at depths of about 23,000 ft (7,010 m) has not encouraged the hope of any pre-Cretaceous reservoirs. Discovery of any fields larger than 1 MMBO is unlikely. However, it is likely that gas fields larger than 6 BCF

will be discovered. The gas potential is probably greatest in pre-Cretaceous reservoirs in

buried structural traps.

39

CHAPTER 3

SEISMIC DATA

3.1 Methods

The 3-D seismic data interpretation was performed using Seisworks – 3D seismic interpretation software. After the computation of synthetic seismograms, the formation tops of interest were tied to the seismic data. Formation tops were then interpreted within the seismic data set.

Several main tasks were performed in this section:

• Interpret key horizons and faults on 315 mi2 (815 km2) of 3-D seismic data.

• Create time structure maps to determine the structural complexity and the main

fault trends within the data set.

• Construct isochron maps to evaluate the timing of structural movements.

• Relate tectonics to sedimentation in the stratigraphic column.

Figure 3.1 is the 3-D seismic base map that shows the locations of seismic lines

illustrated in this thesis. The horizons and faults were picked every 16 to 32 lines, and in

some cases, depending on the structural complexity, the spacing was reduced to every 8

lines.

Some system resource limitations were faced while carrying out this work.

Specifically, the ZAP program, used to extend the picking of the horizons to the non- 40 K’ L’ 3.4 KM J’ I’ H I J H I F G triangulated faults. The red circles circles red The faults. triangulated cation of seismic lines. The different B’ C’ D’ E’ F’ G’ H’ show the position of 2 out of four synthetic seismograms. In green are In green are synthetic seismograms. four 2 out of of show the position the wells with digital log curve information. colored straight lines represent the represent the straight lines colored A B C D E D C A B A’ Figure 3.1 Figure showing the lo base map Seismic

K L N N

41

interpreted seismic lines did not work. Therefore the extrapolation of the picking and

map construction was done by interpolating in between the interpreted seismic lines.

3.2 Results

3.2.1 Synthetic Seismograms

Synthetic seismograms were used to correlate well log tops to horizons within the

3D seismic volume. A synthetic seismogram is an artificial seismic reflection record that is manufactured by assuming that a waveform travels through an assumed model (Sheriff,

1991). Synthetic seismograms are computed from wellbore data and are then correlated

to seismic data over a particular zone of interest. A synthetic seismogram is the

fundamental link between well data and seismic data, and it is the main tool (along with a

VSP, if available) that allows geological picks to be associated with reflections in the seismic data. If a VSP is available for a particular well, a synthetic seismogram is not needed. The VSP directly measures both time and depth to a formation of interest. The wellbore and seismic data are compared to determine formation tops as well as reflection characteristics.

Using the sonic and density log combinations available for a particular well, a wavelet is convolved with the reflection coefficient to create a synthetic seismic trace.

The normal-incidence reflection coefficient for a rock contact is an important quantity.

Sheriff (1991) defined it as "the ratio of the amplitude of the displacement of a reflected 42

wave to that of the incident wave." Mathematically, the reflection coefficient can be expressed as

where:

ρ = rock density

υ = rock velocity

1 = parameters above the interface

2 = parameters below the interface

Synthetic seismograms are normally created using specialized software. The user may be unaware of the process that creates them. Listed below are the steps necessary to create a synthetic seismogram.

• Edit the sonic and density logs for bad intervals.

• Calculate vertical reflection times.

• Calculate reflection coefficients, Ro.

• Combine the last two items to create a reflection coefficient time series.

• Convolve the reflection coefficient series with the wavelet.

In this study, the synthetic seismograms were created using Syntool, a Landmark 43

application. Most wells drilled in the study area had shallow Cretaceous reservoirs as the main targets. This made it difficult to map the older horizons.

From the well data provided for this study (Figure 1.11), only a very few wells

(Figure 3.1) contained the density and sonic log combination necessary to create synthetic seismograms. Figure 3.2 and 3.3 show synthetic seismograms for the Celsius

Bogey Draw 1 (Sec. 14 T13N-R95W) and the Haystack Unit 4 (Sec. 6 T13N-R96W) wells. These two wells represent the deepest wells that were found within the study area.

They were used to calibrate the shallower Cretaceous horizons, specifically the top of the

Almond Formation.

To solve the problem of identifying deeper seismic reflectors, a search for deepest wells was performed and only two wells were found that are close enough to be projected into the seismic dataset. The South Baggs Unit 26 (Sec. 3 T12N-R92W) well was used as a pseudo well in the survey area because its geographic position was altered. In Figure

1.11, it is possible to see that this well is located near the eastern edge of the survey, about 6 mi (9.6 km) away. This well was projected onto the hanging wall of the structure at a site that avoids the edge of the seismic data. This well helped identify the deep reflections because the rock units that produce prominent reflections are widespread and maintain nearly constant thickness.

Figure 1.5 shows a structural cross section that includes the deep well, South

Baggs Unit 26, that was used for calibration in this project. The well is located in the

South Baggs-West Side Canal field. This structural cross section shows approximate 44

Time/Depth (ms)/(ft) Seismic Tie GR RD DT RHOB

Lance/Fox Hills Lewis Shale

Asquith Almond

Figure 3.2 Synthetic seismogram for the Celsius Bogey Draw 1 (Sec. 14 T13N- R95W) well and its seismic tie.

45

Time/Depth Seismic Tie (ms)/ (ft) GR ILM DT RHOB

Lance/Fox Hills Lewis Shale

Asquith Almond

Figure 3.3 Synthetic seismogram for the Haystack Unit 4 (Sec. 6 T13N-R96W) well and its seismic tie.

46

depth and thickness of the deposited formations. The formation tops shown were used as

initial inputs for geologic correlation. Figure 3.4 shows the synthetic seismogram in the

South Baggs Unit 26 well. The Almond synthetic top, because it is the most reliable, was

matched to the seismic. The other picks occur approximately at the named formation

tops.

This preliminary calibration was corroborated by a second deep well, known as

the Powder Wash Deep Well No. 1 (Sec.29 T12N-R97W), with a total depth of 22,092 ft.

This well is located near the southwestern corner of the study area (Figure 1.11). The data from this well was not initially available. The company that owns the information agreed to release it only under a strict privacy agreement. Therefore, the synthetic

seismogram and the comparison of the Powder Wash Deep well against the South Baggs

Unit 26 well will be discussed, although no figures will be shown. After editing the

density curve for a more appropriate synthetic generation, verifying the geologic tops,

and confirming that the well was nearly vertical, we proceeded to the comparison.

Unfortunately, there was not a perfect match. The horizon identified as Frontier in the synthetic of the South Baggs Unit 26 was high (~200 msec or ~ 1550 ft) in comparison to the Powder Wash Deep well. The Basement pick was nearly equivalent to the position of the Madison in the Powder Wash Deep well. A decision had to be made.

It was decided to give more credit to the Powder Wash Deep well information due to the closer proximity to the survey. Also, faulting is noted in the South Baggs Unit 26 well 47

Time/Feet Seismic Tie (ms)/ (ft) GR Sonic Density

Fort Union

Lance/Fox Hills

Almond

Mancos

Frontier

Madison

Basement

Figure 3.4 Synthetic seismogram for the South Baggs Unit 26 well (Sec. 3 T12N- R92W) and its seismic tie. 48

(Parker and Bortz, 2001). Another point considered here was the knowledge of the

seismic character of these two markers in the Rockies. Frontier and Madison represent two high impedance peaks that are regionally widespread. In an east-west section (Figure

3.5), the Frontier and Madison, which show a strong character in the South Baggs Unit 26 synthetic, do not behave as good reflectors when followed to the west.

3.2.2 Horizons

A total of 7 horizons were interpreted in the 3-D seismic volume. The horizons range from the Tertiary Fort Union horizon to the Basement. This means that the entire vertical section was considered in this study.

Horizon interpretations were made, at a minimum, every 16 lines in the inline and crossline directions for every horizon. Because of the large size of the survey, I was unable to interpret every line. In complex areas, I used a closer spacing than 16 lines.

These seismic horizons are characterized by their good continuity, and high amplitude or

high impedance picks.

The stratigraphic identification of these horizons was based not only on synthetic

seismograms and correlations with well information, but also by comparison to work

done by Hull (2001) and Maraj (M.S. thesis, in progress). Hull (2001) worked with a 3-D

survey (29 mi2 or 74 km2) located near the southeastern corner of this study, and Maraj is

currently working with the Powder Mountain 3-D seismic (105 mi2 or 269 km2), which overlaps the northwestern side of this study (Figure 1.11). Even though these two 49

West East L L’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

6.3 KM

Figure 3.5 Interpreted seismic line L-L’. Scale is in two way time (msec). Figure 3.1 is the index map.

50

seismic surveys use a different datum, the time depths of their interpreted shallow horizons are similar (+/- 180 ms difference), and the seismic characters are similar, which made the correlations more reliable.

Figure 3.6 illustrates the comparison of the horizon picking of this work with the

South Baggs survey. Hull’s (2001) work was restricted to the Lance/Fox Hills and

Almond. Figure 3.7 shows the comparison to the Powder Mountain survey. In both comparisons, the nomenclature of the original work was kept. We can see, for example, in Figure 3.7 that the position of my horizon labeled as Fort Union matches the Lance A in the Powder Mountain study. This type of discrepancy is commonly found in a project that integrates inconsistent data. During the Late Cretaceous and continuing into the

Tertiary, the stratigraphic units are less continuous because many of the present-day structures became more prominent and many sediment sources were local. As a consequence, the geologic identification of the formations or markers is more difficult and depends on the experience of the interpreter and the amount of data available. The geologic nomenclature used here is in agreement with the stratigraphic interpretation published by Parker and Bortz (2001).

Deeper in the vertical section, the calibration and picking of the older sequences became more complicated owing to the scarcity of deep wells, the increased vertical offset of the interpreted thrust faults, and the fact that the Cherokee Arch crosses the entire study area. It was difficult to select a zone to jump across the Arch, but the use of correlation polygons, arbitrary seismic lines, availability of the two other surveys (Baggs 51 East Almond Almond Fox Hills-Lance Peak Fox Hills-Lance s 3-D gg ey vs. South Baggs South Ba of Cherokee Arch surv of Mancos Mancos Fort Union Fort Union Lance/Fox Hills Almond survey (Hull, 2001). The index map shows the approximate location of the two shows the approximate survey (Hull, 2001). The index map sections. seismic Cherokee Arch 3-D Figure 3.6 Figure Horizon calibration: comparison West

52

Powder Mountain 3-D

Fort Union

Lance A

Fox Hills

Almond

Rock Spring

Cherokee Arch 3-D

Fort Union

Lance/Fox Hills

Almond

Mancos

Figure 3.7 Horizon calibration: comparison of Cherokee Arch survey vs. Powder Mountain survey. The index map shows approximate location of the overlapping seismic sections.

53

and Powder Mountain) and similar seismic character allowed me to extend the picking with confidence.

An example of where the seismic character correlation plays an important role in the identification of the markers on each side of the structure is shown in two east-west seismic sections: Figure 3.5 (hanging wall side) and Figure 3.8 (footwall side). The horizon identified as the Madison correlates very well in both seismic sections.

The selected seismic horizons, in order of decreasing age, were the following:

Madison, Shinarump, Above Frontier, Mancos, Almond, Lance/Fox Hills, and Fort

Union. The stratigraphic identification of the horizons older than Almond relies on the use of the South Baggs Unit 26 synthetic seismogram, the geologic top information in the cross section (Figure 1.5) published by Parker and Bortz (2001), and the Powder Wash

Deep synthetic seismogram. The well was hung on the Almond horizon, which is the most reliable pick within the relatively shallow horizons. The Madison Formation in the deep sequences also worked as a good seismic pick due to its proximity to the basement.

Figures 3.8 through 3.18 show examples of interpreted seismic lines that have been prepared to illustrate examples of different parts of the survey. For confidentiality concerns, locations are described only in general terms and the line and trace numbers have been removed.

By comparing the time-structure maps and the seismic lines, it is clear that thrust faults are restricted to the deeper sequences. Thrust faults, represented in the interpreted sections by red, gold, green and pink lines, generally terminate within the Mancos to 54

West East K K’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

6.3 KM

Figure 3.8 Interpreted seismic line K-K’. Scale is in two way time (msec). Figure 3.1 is the index map.

55

North South A 10o A’

20o

30o

60o V.E =2.6 Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.9 Interpreted seismic line A-A’. Scale is in two way time (msec). The vertical exaggeration (V.E) was estimated using the Powder Wash Deep Well No. 1 synthetic seismogram. This V.E also applies to the following seismic sections (Figures 3.10-3.18 and 3.34). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 56

North South B B’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.10 Interpreted seismic line B-B’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 57

North South C C’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.11 Interpreted seismic line C-C’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 58

North South D D’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.12 Interpreted seismic line D-D’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 59

North South E E’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.13 Interpreted seismic line E-E’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 60

North South F F’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.14 Interpreted seismic line F-F’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 61

North South G G’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.15 Interpreted seismic line G-G’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map.

62

North South H H’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.16 Interpreted seismic line H-H’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 63

North South I I’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.17 Interpreted seismic line I-I’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 64

North South J J’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Above Frontier

Shinarump

Madison

3.15 KM

Figure 3.18 Interpreted seismic line J-J’. Scale is in two way time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 65

Above Frontier interval, and increase in magnitude down to the basement where the vertical offset can be more than 750 ms (5,600 ft or 1,707 m). On the other hand, the post-Mancos intervals are dominated by near-vertical faults with apparent normal offset.

Most of these faults terminate above the Fort Union. The presence of these high-angle faults is more evident as we move upward to the shallower strata. They seem to extend vertically up to near the surface, and rocks as young as Eocene are folded.

3.2.3 Faults

The fault interpretations within the seismic volume were made at every 16 to 32 traces (dip direction). In some cases, the spacing was modified to every 8 traces. The parameter controlling this spacing was the structural complexity of the zone affected by faulting. Whenever I approached the Cherokee Arch, the spacing between interpreted seismic lines was adjusted due to the higher density of faults.

The fault interpretation focused on the larger faults. Smaller faults that affected only one interval were not picked. Basement-involved faults had primary importance, and faults seen in the cover strata had secondary importance.

Different techniques were used to track the faults as accurately as possible. These techniques are:

1) Start with the use of two distant traces, find correlative faults, correlate them, and perform a triangulation of the fault. Here the software basically constructs a preliminary fault plane through the two distant traces. As result of this procedure, for 66

traces between the chosen ones, there will be a dotted line showing where the fault plane

cuts that section. The dotted lines served as a guide to draw the position of the fault.

Adjustments were needed, based on the real position of the offsets in each section.

2) Use several seismic section displays. This involved keeping open different seismic sections; interpreted and uninterpreted, so that it was possible to see the work done on the previous sections and maintain consistency in the interpretation.

3) Use time slices. The time slices permitted better control of the trajectory of the faults and showed their position and geographic distribution at different time depths. A word of caution is needed here, and the interpreter must be careful not to confuse changes of slope or folds in the time slices with faults. Time slices must be compared to the seismic sections to establish whether the fault exists or not.

3.2.4 Time Structure Maps

Figures 3.19 to 3.25 show time-structure maps on the interpreted seismic surfaces.

The color convention used for all time-structure maps is that magenta and dark blue represent a relative structural high and dark red and yellow represent a relative structural low. Dark red, linear features are interpreted faults that are presented in appropriate time slices. Figure 3.23 compares seismic to well-top structure.

The time structure maps (Figures 3.19 to 3.25) show the general tendency of the beds to dip towards the west. The deepest point is found in the northwestern corner

(footwall side) of the study area and the shallowest point is found in the southeastern 67

5 2 36 *

N N

* 3140 M M K K

4 4 . . 3 3

0 7 38 phic points. Tooth *

00 4 3 * at selected geogra at selected

90 horizon. Scale is in two-way time (msec). The (msec). horizon. Scale is in two-way time * 41

3600 00 * * 44 numbers represent two-way travel times travel times two-way represent numbers are on the upthrown side of faults. marks Figure 3.19 Figure of the Madison structure map Time

68

N N

M M * 2930 K K

4 4 . . 3 3 * 3670 * 3150 travel times at selected geographic at selected geographic times travel * 3920 thrown side of faults. p inarump horizon. Scale is in two-way time horizon. Scale is in two-way time inarump * 4140 * 3370 oints. Tooth marks are on the u oints. Tooth marks p (msec). The numbers represent two-way represent two-way The numbers (msec). Figure 3.20 Figure of the Sh structure map Time * 3470

69

N N 3060 *

M M 2650 K K

* 4 4 . . 3 3 3280 * is in two-way time is in two-way time travel times at selected geographic travel times 2825 * thrown side of faults. Frontier horizon. Scale p 3540 * 3760 * 3000 oints. Tooth marks are on the u oints. Tooth marks * (msec). The numbers represent two-way The numbers (msec). p Figure 3.21 Figure of the Above structure map Time

70

0 7

3 N N * 2

0 M M 4 K K

0 4 4 . . * 2 3 3

90 two-way time (msec). (msec). two-way time * 21

800 times at selected geographic points. times horizon. Scale is in * 2 3030 * The numbers represent two-way travel The numbers are on the downthrown side of faults. Tic marks

0 7 5 * 2 Figure 3.22 Figure of the Mancos structure map Time

71

W W 3 3 9 9 / /

N

N

1 1 1 W 1 W 3 3 9 9 / / N N 4 4 1 1

W W 4 4

9 9

/ /

N N W W 1 1 4 4 1 1 9 9 / / N N 4 4 1 1 es at selected geographic points. Tic es at selected geographic . In this figure the time structure map is structure map . In this figure the time ond calculated from well data. Contour ond calculated from

izon. Scale is in two-way time (msec). The (msec). izon. Scale is in two-way time

W

W

5 5 9 W 9 W / / 5 5 N 9 N 9 / / 1 1 1 N 1 N 4 4 1 1

W W

6 6 9 9 / / W W 6 6 N N 9 9 1 1 / / 1 1 N N 4 4 1 1 yellow numbers represent two-way travel tim marks are on the downthrown side of faults marks of Alm to the structure map compared interval is 400 ft subsea. Figure 3.23 Figure hor of the Almond structure map Time

72

N N * 1680

M M * 1300 K K

4 4 . . 3 3 * * 1500 travel times at selected geographic travel times * 2170 ox Hills horizon. Scale is in two-way time in Scale is horizon. ox Hills downthrown side of faults. * 2350 (msec). The numbers represent two-way The numbers (msec). are on the points. Tic marks Figure 3.24 3.24 Figure of the Lance/F structure map Time

73

N N

M M K K

4 4 . . 3 3 1080 * * 1580 travel times at selected geographic travel times 1835 1300 * Union horizon. Scale is in two-way time Union horizon. Scale is in two-way time * downthrown side of faults. 2050 * 1550 * points. Tic marks are on the points. Tic marks (msec). The numbers represent two-way The numbers (msec). Figure 3.25 Figure of the Fort structure map Time

74

corner (hanging wall side).

3.2.5 Time Slices

As stated before, the time slices helped to control and adjust the vertical trajectory

of the faults. Figures 3.26 to 3.30 are examples of time slice maps at different two-way

travel times. Time slices from 3D data provide a horizontal view of the entire 3D data set.

The extent of the Cherokee Arch at different times can be easily seen. The time slices

show that the Arch becomes better defined as depth increases. From this view, it is easy

to demonstrate that from the middle of the survey toward the western side, the

complexity of the Arch increases. Towards the eastern side, the strain partitioning is

mainly distributed on the gold and pink thrust faults, as evidenced by the seismic sections

(Figures 3.15 to 3.18).

For illustration purposes, time slices were taken at about the average position of

the top of each of the interpreted horizons. However, only one time slice (at 3608 ms)

was chosen here to display the structural complexity of the Above Frontier, Shinarump

and Madison horizons because the increasing vertical separation with depth makes it difficult to choose a specific time slice in any of those horizons.

3.2.6 Isochron Maps

Isochron maps were created between the horizons mapped. The color convention used for all isochron maps is that dark red represents a relative thick, which grades up to 75 N

M M K K

4 4 . . 3 3

Tooth marks are on the

s are on the downthrown side of Shinarump, and Madison levels Shinarump, Reverse Fault Normal Fault approximately at Above Frontier, approximately in the southern part of survey. upthrown side of faults. Tic mark faults. faults. . Figure 3.26 Figure This depth is (two-way travel time). slice at 3608 ms Time .

76

M N M N K K

4 4 . . 3 3 faults.

l in the southern part of (two-way time). This depth is (two-way time). approximately at the Mancos leve approximately survey. Tic marks are on the downthrown side of survey. Tic marks

Figure 3.27 Figure Slice Map: 2352 ms Time Reverse Fault Normal Fault

77

N N

M M K K

4 4 . . 3 3 faults. faults.

in the southern part of (two-way time). This depth is (two-way time). . Tic marks are on the downthrown side of . Tic marks y approximately at the Almond level approximately surve

Reverse Fault Normal Fault Figure 3.28 Figure Slice Map: 1848 ms Time

78

N N

M M K K

4 4 . . 3 3 faults. faults.

s level in the southern part of (two-way time). This depth is (two-way time). approximately at the Lance/Fox Hill approximately are on the downthrown side of the survey. Tic marks

Reverse Fault Normal Fault Figure 3.29 Figure Slice Map: 1624 ms Time

79

N N

M M K K

4 4 . . 3 3 faults. faults.

(two-way time). This depth is (two-way time). approximately at the Fort Union level in southern part of approximately are on the downthrown side of survey. Tic marks

Reverse Fault Normal Fault Figure 3.30 Figure Slice Map: 1504 ms Time

80

dark blue and magenta, indicating a relative thin. In this section, the following estimations or conversions from time to depth (or thickness) at different horizons are made based on Table 3.1. This table was constructed using the average interval velocities found in the South Baggs Unit 26 synthetic seismogram.

Figure 3.31 is the isochron map between the Shinarump and Madison. This map shows the relative homogeneous thickness of this sequence. There is an average 20 ms difference (~156 ft or 48 m) between the sides of the structure, with more sediment deposited on the hanging wall side. This difference represents a percentage of thickening of 8%, considering an average total thickness of 250 ms (~1950 ft or 595 m).

The isochron map between the Above Frontier and Shinarump (Figure 3.32) displays a slightly different pattern. The thickness is not as homogeneous as the previous one, and there is a more marked general thickening towards the west. In this sequence, the thickness difference between the north and south side increases towards the west.

There is about a 40 ms (~300 ft or 91 m) difference near the top of the anticline. The average thickness on the downthrown side is about 380 ms (~2850 ft or 869 m) and the average thickness on the upthrown side is about 360 ms (~2700 ft or 823 m). Therefore, more sediment was deposited on the downthrown side. The estimated average percentage of thickening for the Above Frontier-Shinarump interval is 11%.

The isochron map between the Mancos and Above Frontier (Figure 3.33) shows a considerable change from the two previous isochron maps. More sediments were 81

Intervals: Time (msec) Meas. Depth (feet) Dif. time (msec) Dif. depth (feet) Shinarump 2962 15409 243 1895 1 msec = 7.8 ft Madison 3205 17304 7798.4 ft/sec

Above Frontier 2622 12873 340 2536 1 msec = 7.5 ft Shinarump 2962 15409 7458.8 ft/sec

Mancos 1992 8576 630 4297 1 msec =6.8 feet Above Frontier 2622 12873 6820.6 ft/sec

Almond 1681 6541 311 2035 1 msec = 6.5 ft Mancos 1992 8576 6543.4 ft/sec

Lance/Fox Hills 1275 4257 406 2284 1 msec = 5.6 ft Almond 1681 6541 5625.6 ft/sec

Ft. Union 898 2256 377 2001 1 msec = 5.3 ft Lance/Fox Hills 1275 4257 5307.7 ft/sec

Table 3.1 Average interval velocity: Based on the South Baggs Unit 26 synthetic seismogram.

82 3.4 KM * 230 * 250 * 240 * 260 The numbers show the isochron values at selected geographic points. The numbers Figure 3.31 Figure (msec). – Madison. Scale is in two-way time from Shinarump Isochron map N

83 3.4 KM * 330 * 370 lues at selected geographic points. r – Shinarump. Scale is in two-way time time r – Shinarump. Scale is in two-way * 390 * 370 * 415 (msec). The numbers show the isochron va The numbers (msec). Figure 3.32 Figure Above Frontie Isochron map from N

84 3.4 KM * 690 * 545 is in two-way time is in two-way time * 530 lues at selected geographic points. * 675 * 615 Frontier. Scale * 700 * 570 (msec). The numbers show the isochron va The numbers (msec). Figure 3.33 Figure Mancos – Above from Isochron map

N

85

deposited on the footwall side (more accommodation space) as the deformation event

occurred. Some representative isochron values found on this side of the Arch are 675 ms

and 700 ms (~4590 and 4760 ft or 1400 and 1451 m respectively), whereas some of the

values found on the hanging wall side are 530, 570 and 600 ms (~3604, 3876 and 4080 ft

or 1098, 1181, and 1244 m respectively). There is an average of 145 ms (~986 ft or 301

m) difference between the hanging wall and the footwall sides, and the average

percentage of thickening estimated for this interval was 21%, assuming 690 ms (~4692 ft

or 1430 m) as the average total thickness of the interval.

Because one of the objectives of this investigation is to evaluate the timing of

structural movements, a more detailed analysis of this interval was performed. The

isochron map from the Mancos-Above Frontier shows that more sediment was deposited

on the footwall side as the faults (red and/or gold) grew. This interval was narrowed

down to better establish where the major thickening occurred within the sequence, and to get a more accurate timing of the structure. If we look in detail at the seismic section E-

E’ (Figure 3.34) and the isochron map between the Mancos and Above Frontier, it is apparent that the red fault grew post Above Frontier (Upper Cretaceous). Two other surfaces were drafted for illustration purposes. One of them was identified as “Near-

Niobrara,” based on the synthetic seismogram. The selected seismic section shows that the red fault was growing some time during deposition of the Niobrara Formation. If the age dating of the Niobrara and Above Frontier can be determined, then it would be possible to establish the time span during which the structure was growing. 86

North South E E’

Ft. Union

Lance/Fox Hills

Almond

Mancos

Near Niobrara 168ms Î 1142 feet (74 Ma) Above Frontier

Shinarump (~88 Ma) 212ms Î 1442 feet Madison (96 Ma)

Figure 3.34 Interpreted seismic line E-E’. Scale is in two way travel time (msec). The circles are horizon intersection points with crossing lines. Figure 3.1 is the index map. 87

Cushman (1994), in his palynostratigraphic study of the Mancos Shale, showed

an age range of 74 – 96 Ma (million years). Assuming a constant stratal accumulation

rate, it is possible to state that the red fault was growing during roughly approximately 8

Ma (88 to 96 Ma). The rate of uplift is equivalent to the footwall thickness minus the

hanging wall thickness, divided by 8 Ma. The uplift rate is 37.5 ft/Ma (11.43 m/Ma or

0.01143 m/1000 years). This value is extremely low, knowing that the Rocky Mountains rose at the rate of 1 to 10 m/1000 years (Webster, 2003)

Figure 3.35 is the isochron map between the Almond and Mancos. In this interval, the relatively constant thickness is regained. The dark blue zone is affected by

faults that may have caused some thinning in the strata due to local extension. Some

representative isochron values found in this interval are 325 ms (~2113 ft or 644 m) and

300 ms (~1950 ft or 594 m). There is an average of 15 ms (~98 ft or 30 m) difference

between the hanging wall and the footwall sides, and the average percentage of

thickening estimated for this interval is 5%, assuming 315 ms (~2048 ft or 624 m) as the

average total thickness of the interval.

Figure 3.36 is the isochron map between the Lance/Fox Hills and Almond. This

upper Cretaceous interval exhibits gradual thickening towards the northeastern part of the

study area. The values range from 300 ms (~1680 ft or 512 m) in the southwestern

corner to 400 ms (~2240 ft or 683 m) in the northeastern corner of the study area. There

is an average of 10 ms (~56 ft or 17 m) difference between the hanging wall and the 88 * 325 * 315 3.4 KM * 300 on values at selected geographic * 325 * 300 Mancos. Scale is in two-way time Mancos. Scale is in two-way time * 275 * 305 * 340 oints. p (msec). The numbers show the isochr The numbers (msec). * 305 * 370 * 280 Figure 3.35 Figure Almond - from Isochron map N

89 * 400 3.4 KM * 380 * 370 lues at selected geographic points. s - Almond. Scale is in two-way time Scale is in two-way time s - Almond. * 360 * 350 * 375 * 335 * 350 * 330 (msec). The numbers show the isochron va The numbers (msec). * 300

Figure 3.36 Figure Lance/Fox Hill Isochron map from N

90

footwall sides, and the average percentage of thickening estimated for this interval is 3%,

assuming 380 ms (~2128 ft or 649 m) as the average total thickness of the interval.

Figure 3.37 is the isochron map between the Fort Union and Lance/Fox Hills. In

this interval, the thickening is restricted to the footwall side, or northern half of the study

area. There is an average of 15 ms (~80 ft or 24 m) difference between the hanging wall

and the footwall sides, and the average percentage of thickening estimated for this

interval is 4%, assuming 360 ms (~1908 ft or 582 m) as the average total thickness of the

interval.

3.3 Discussion

A thick-skinned thrust-fold model is in excellent agreement with the analyzed

data set of this study. This model appears to be consistent along the entire survey. Two sets of faults can be recognized. Thrust faults essentially occur in the deeper sequences

and affect the basement. The second set of faults are near-vertical faults with apparent

normal offset. They tend to be rooted in older thrust faults, and they terminate in the

Tertiary section above the Fort Union.

From the time slices and the interpreted seismic sections, it can be established that the thrusting is less severe towards the east. As displacement decreased on the red fault, displacement increased on the gold fault. This implies that the two faults are kinematically linked. The red and gold faults are a linked pair that both grew at the same

time. Another point to note is that most of the interpreted thrusts do not reach higher than 91 * 335 * 345 * 360 3.4 KM * 335 * 325 Scale is in two-way time two-way time Scale is in * 340 * 310 lues at selected geographic points. * 325 rt Union – Lance/Fox Hills. * 320 * 285 (msec). The numbers show the isochron va The numbers (msec). * 300 * 250 Figure 3.37 3.37 Figure Fo from Isochron map

N

92

the Mancos horizon. The explanation for this observation is based on the thought that the

Mancos Shale attenuated the vertical extension of these reverse faults. Minor thrusting after the Mancos may represent local movement.

The large data set handled in this study images the Cherokee Arch exceptionally well. Comparing it with the data used by Hull (2001), it is clear that he was studying the eastern projection of the hanging wall of the structure. Some of the basement-involved thrust faults and fault-propagation folds interpreted in this work can be seen at the border of the northern part of the South Baggs 3-D seismic survey. The Parker and Bortz (2001) structural cross section (Figure 1.5) is also located (outside the study area) at the eastern extension of the hanging wall. The faults labeled as Cherokee Arch actually represent the near-vertical faults visible in the interpreted north-south seismic sections shown in this study.

These are examples of the contributions of this larger study, which sheds more light on the improved understanding of this particular structure. The newly acquired and wide-ranging information could possibly explain better the kinematic and geologic evolution of a particular area. Hull’s (2001) interpretation was plausible with the limited information available that he used, as well as Parker and Bortz (2001).

The Archean-Proterozoic suture that crosses the study area, called the Cheyenne

Belt, is a zone of penetrative deformation generated during an oblique collision. Due to the geographic position of the Cherokee Arch, it is very likely that this structure inherited the old fabric of the Cheyenne Belt. 93

It is also important to report the occurrence of seismic reflectors on the footwall side of the structure, particularly below the Madison horizon. These seismic reflectors, best observed on the north-south seismic sections (Figures 3.9 to 3.18), may be equivalent to Precambrian sediments. This observation is important because little is known about Wyoming, Colorado and Utah during the Late Proterozoic. Nearly all the evidence has been culled from the geologic record. Near the study area, two important

Precambrian sedimentary remnants were preserved, the Uinta Mountain Group and the

Uncompahgre Formation. They may correlate with the seismic reflectors noted below the Madison.

From the geometry mapped here, it is not necessary to interpret strike-slip motion.

However, strike-slip offset may be present in the faults that terminate in the Tertiary section. Some of these faults may have a surface expression captured by the Landsat photo interpreted by Bader (1987). A comparison between Figure 2.6 and the shallowest time slice map produced here (Figure 3.30) shows a good alignment and similarity of the interpreted near-vertical faults with the faults interpreted by Bader (1987). He associated his faults with a master right-lateral wrench fault. Additional evidence for strike-slip motion is the significant strike-slip faulting documented in some other east-west arches formed in the Rockies (for example, the Uinta Mountains, Owl Creek Mountains, and

Casper Mountain). The northeast-southwest regional compression may have caused oblique thrusting with a component of strike-slip offset. 94

The timing of the faults and the rate of uplift can be recognized by analyzing the stratigraphic thickness changes. The generated isochron maps, in this case, represent a valuable tool to evaluate when the structure was growing. However, changes in the stratigraphic record are not necessarily associated with deformation events. Besides the alternative of structural thinning over growing faults, there are others considerations like unconformities, regional thinning unrelated to structural uplift, or thinning due to compaction. We also need to account for velocity changes along the seismic survey that could affect the computation of the isochron maps.

Figure 3.38 was prepared to graphically illustrate the percentage of thickening of each one of the intervals analyzed in this work vs. time. From the observations of the graph, in conjunction with the interpreted seismic sections and isochron maps, we can decide when the stratigraphic record was affected by structural relief. In Figure 3.38, the ages established for the formations are approximate (GSA, 2003).

The isochron maps from the Shinarump-Madison and Above Frontier-Shinarump show evidence of slight thickness changes. The estimated percentages of thickening on these intervals were 8 and 11%, respectively (Figure 3.38). Between the Madison and

Above Frontier horizons, several episodes of sedimentation, unconformities, erosion, and uplift occurred during a time span of ~230 Ma.

Based on the observation of the isochron map between the Shinarump and

Madison, it is difficult to determine if uplift of the late Paleozoic Ancestral Rocky

Mountains could have affected the study area. Apparently, for the study area, there was 95

% Thickening 50 Tertiary Cenozoic Fort Union Lance/Fox Hills Almond Mancos

90 Above Frontier

130 Cretaceous

170 Mesozoic Jurassic

210

Time (Ma) Shinarump

250 Triassic

290

Madison Paleozoic 330 0 20406080100

Figure 3.38 Percentage of thickening vs. time (Ma). This figure compares the thickness of particular seismic intervals from the hanging wall (south) to the footwall (north) side of the survey.

96

not important structural relief or growth during the late Paleozoic. Practically the same

criteria apply to the isochron map between the Above Frontier and Shinarump. The

hypothesis is that the small changes observed in the stratigraphic record should not be

fully attributed to structural relief, knowing that there are other elements that could have

played an important role in modifying this thickness.

The isochron map from the Mancos-Above Frontier gives an average percentage

of thickening of 21% from the hanging wall to the footwall (Figure 3.38). In this Upper

Cretaceous interval, it is possible to state that the structure was actually growing based on

the isochron map (Figure 3.33) and seismic sections. There is an abrupt change in thickness from one side of the structure to the other.

In the isochron maps from the Almond-Mancos and Lance/Fox Hills-Almond, there is no important regional thinning due to structural growth. These two sequences

appear to be homogeneous in thickness. Their average percentages of thickening were

5% and 3%, respectively.

The isochron map between the Fort Union-Lance/Fox Hills shows slight evidence

of fault growth because of the marked differences in thickness between the hanging wall

and the footwall side of faults (Figure 3.37). Apparently the faults (pink, green and

related faults) continued to grow after deposition of the Lance/Fox Hills Formations,

even though the estimated percentage of thickening of the interval (4%) is not a good

indicator of this growth. Possible strike-slip motion should be taken into account. 97

In conclusion, from the analyzed data, two possible episodes of deformation are indicated here: early thrusting and possible subsequent dip-slip and/or wrench movement.

The thrusting mainly occurred during the Upper Cretaceous (between Mancos and Above

Frontier interval). The wrenching and dip-slip offset occurred primarily during deposition of the Tertiary section. Some of the younger faults apparently result from reactivation of the older thrust faults.

98

CHAPTER 4

CONCLUSIONS AND RECOMMENDATIONS

4.1 Conclusions:

The purpose of this study was to interpret a 3-D seismic survey of 315 mi2 (815

km2), generate a structural model, and investigate the timing of the fault movement for

the Cherokee Arch. A total of 7 horizons were interpreted in the 3-D volume. The

selected seismic horizons were: Madison, Shinarump, Above Frontier, Mancos, Almond,

Lance/Fox Hills, and Fort Union. Time-structure maps on the interpreted seismic

surfaces were generated, as well as time slices close to the average position of the top of

the interpreted horizons, and isochron maps between the horizons mapped.

• Subsurface reflection seismic and borehole data provide a comprehensive data

base for structural analysis or kinematic interpretation of the basement-involved

thrust-generated folds (thrust folds) of the Cherokee Arch. The thrust-fold term

implies that the kinematic genesis is, first basement thrusting, followed by thrust-

generated folding in the overlying sedimentary rocks. Thrusting and folding may

have occurred at nearly the same time.

• There appear to be two episodes of deformation, early thrusting (Upper

Cretaceous) and subsequent wrenching and/or dip-slip offset that appears to affect

rocks as young as Eocene in age (Tertiary). 99

• The effect of pre-existing basement weaknesses is commonly a controlling factor

in Laramide deformation. The Cheyenne Belt may have played that role for the

Cherokee Arch.

• The Cherokee Arch is an asymmetric anticline. The hanging wall is mainly

transected by a south-dipping series of east-west trending thrust faults.

• There is little or no evidence of the Ancestral Rocky Mountain deformation event

(Late Mississippian to Early Permian) in the stratigraphic record at the Cherokee

Arch, based upon interval isochron thickening trends for different ages of

sediments.

• The studied thrust-fold system of the Cherokee Arch correlates in time with the

“Laramide Orogeny” (Late Cretaceous through mid-Eocene).

4.2 Recommendations

• Evaluate the relationship of the production of the fields of this area relative to

their position on the structure.

• Build a 3D fault framework using 3D visualization software. The fault framework should provide data on the geometric relationship of the faults in 3D.

100

REFERENCES

Allmendinger, R. W., J. A. Brewer, L. D. Brown, S. Kaufman, J. E. Oliver, R. S. Houston, 1982, COCORP profiling across the Rocky Mountain front in southern Wyoming, Part 2: Precambrian basement structure and its influence on Laramide deformation: GSA Bulletin, v. 93, p. 1253-1263. [COCORP: Consortium for Continental Reflection Profiling]

Asquith, D. O., 1970, Depositional topography and major marine environments, late Cretaceous, Wyoming: AAPG Bulletin, v. 54, p. 1184-1224.

Baars, D. L., B. L. Bartleson, C. E. Chapin, B. F. Curtis, R. H. De Voto, J. R. Everett, R.C. Johnson, C. M. Molenaar, F. Peterson, C.J. Schenk, J. D. Love, I. S. Merin, P. R. Rose, R. T. Ryder, N. B. Waechter, L. A. Woodward, 1988, Basins of the Rocky Mountain region: in Sloss, L. L., ed., Sedimentary cover-North American craton: U.S., The Geology of North America, Vol. D-2, GSA, p. 109-165.

Bader, J.W., 1987, Surface and subsurface relations of the Cherokee Ridge Arch south- central Wyoming: Unpublished M.S. thesis, San Jose State University, San Jose, California, 68 p.

Berg, R.R., 1962, Mountain flank thrusting in Rocky Mountain foreland, Wyoming and Colorado: AAPG Bulletin, v. 46, p. 2019-2032.

Boid, D.W., 1993, Paleozoic history of Wyoming: in Snoke, A.W., Steidmann, J.R., and Roberts, S.M., eds., Geology of Wyoming, Wyoming State Geological Survey Memoir 5, p. 164 – 187.

Blackstone, D.L., Jr., 1940, Structure of the Pryor Mountains, Montana: Journal of Geology, v. 48, p. 590-618.

Blackstone, D.L., Jr., 1990, Rocky Mountain Foreland structure exemplified by the Owl Creek Mountains, Bridger Range and Casper Arch, Central Wyoming: Wyoming Geological Association 41st Field Conference, p. 151-166.

Brenner, R.L., 1983, Late Jurassic tectonic setting and paleogeography of the western interior, North America: in Reynolds, M.W., and Dolley, E.D., eds., Mesozoic paleogeography of the west-central United States, Rocky Mountain 101

Paleogeography Symposium 2: The Rocky Mountain Section, SEPM, Denver, Colorado, p. 101-116.

Brown, W.G., 1984, A reverse fault interpretation of Rattlesnake Mountain anticline, Big Horn basin, Wyoming: The Mountain Geologist, v. 21, p. 31-35.

Brown, W. G., 1984, Basement involved tectonics – foreland areas: AAPG, Continuing Education Course Note Series 26, 92 p.

Brown, W.G., 1988, Deformation style of Laramide uplifts in the Wyoming foreland, in Schmidt, C.J., and Perry, W.J., Jr., eds., Interaction of the Rocky Mountain foreland and the Cordilleran thrust belt: GSA Memoir 171, p. 53-64.

Cardinal, D.F., and Hovis, W.F., 1979, Little Snake field, Wyoming oil and gas fields symposium Greater Green River Basin: Wyoming Geological Association, Annual Field Conference, p. 221-223.

Craig, L.C., 1972, Mississippian System, in Mallory, W.W., ed., Geologic Atlas of the Rocky Mountain Region: Rocky Mountain Association of Geologists, Denver, p. 100-110.

Cronoble, J. M., 1969, Geology of the South Baggs – West Side Canal gas field, Carbon County, Wyoming and Moffat County, Colorado: Unpublished M.S. Thesis, Colorado School of Mines, Golden, Colorado, 46 p.

Cronoble, J. M., 1969, South Baggs-West Side Canal gas field, Carbon County, Wyoming and Moffat County, Colorado: in Barlow, J. A., ed., Wyoming Geological Association Guidebook, 21st Annual Field Conference, p. 129-137.

Cross, T. A., 1986, Tectonic controls of foreland basin subsidence and Laramide style deformation, western United States: in Allen, P. A., and Homewood, P., ed, Foreland basins: International Association of Sedimentologists Special Publication 8, p. 15-39.

Cushman, R.A., Jr., 1994, Palynostratigraphy of the upper Cretaceous Mancos Shale in western Colorado: Unpublished Ph.D. Disertation, Colorado School of Mines, Golden, Colorado, 668 p.

Doelger, M., 1995, Geologic framework and future development of major gas plays of the Greater Green River Basin: Geological characterizations of the Cretaceous gas plays in the Greater Green river basin, Transaction of the Consortium for 102

emerging gas resources in the Greater Green River Basin, GRI/DOE/IPAMS, November 6, Denver, Colorado, p. 49-79.

Dott, R.H., Jr., and Batten, R.L., 1981, Evolution of the Earth (3rd edition): New York, McGraw-Hill Book Company, 573 p.

Downs, G.R., 1949, Mesozoic rocks of the northern Powder River basin, Wyoming: Wyoming Geological Association, Fourth Annual Field Conference Guidebook, p. 46-51.

Duebendorfer, E. M., and Houston, R. S., 1986, Kinematic history of the Cheyenne belt: A Proterozoic suture in southeastern Wyoming: Geology, v. 14, p. 171-174.

Erslev, E.A., 1986, Basement balancing of Rocky Mountain foreland uplifts: Geology, v. 14, p. 259-262.

Erslev, E.A., and Rogers, J.L., 1993, Basement-cover geometry of Laramide fault- propagation folds in Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States: GSA Special Paper 280, p. 125-146.

Erslev, E. A., Hennings, P.H., and Zahm, C.K., 2001, Kinematics and structural closure of basement-involved anticlines in the central Rocky Mountains Petroleum Province (abs.): AAPG Bulletin, v. 85, No. 13.

Gibbs, F.K, 1972, Silurian System : in Mallory, W.W., eds., Geologic atlas of the Rocky Mountains region: Rocky Mountain Association of Geologists, Denver, Colorado, p. 86-89.

Gries, R., 1983, North-south compression of Rocky Mountain foreland structures: in Lowell, J. D., ed., Rocky Mountain foreland basins and uplifts, Rocky Mountain Association of Geologists, p. 9-32.

GSA, “Geologic time scale.” Online posting. September 15, 1997. Virtual Geology. 4 Sept 2003

Hamzah, S., 2001, Subsurface correlation of the Lewis Shale in the Washakie and Great Divide Basins, Wyoming: Unpublished M. S. Thesis, Colorado School of Mines, Golden, Colorado, 120 p.

103

Haun, J. D., 1961, Stratigraphy of post-Mesaverde Cretaceous rocks, Sand Wash basin and vicinity, Colorado and Wyoming: in Wiloth, G. J., ed., Wyoming Geological Association Guidebook, Symposium on Late Cretaceous Rocks of Wyoming, p. 116-124.

Hills, F. A., and Houston, R. S., 1979, Proterozoic tectonics of the central Rocky Mountains, North America: Contributions to Geology, v. 17, p. 89-109.

Hull, E. A., 2001, 3-D seismic interpretation of tectonics and sedimentation within the upper Cretaceous Lewis Shale and Fox Hills sandstone Baggs, Wyoming: Unpublished M. S. Thesis, Colorado School of Mines, Golden, Colorado, 73 p.

Jordan, T. E., 1981, Thrust loads and foreland basin evolution, Cretaceous, Western United States: AAPG Bulletin, v. 65, p. 2506-2520.

Karlstrom, K. E., Flurkey, A. J., and Houston, R. S., 1983, Stratigraphy and depositional setting of the Proterozoic Snowy Pass Supergroup, south eastern Wyoming: Record of an early Proterozoic Atlantic-type cratonic margin: GSA Bulletin, v. 94, p. 1257-1274.

Karlstrom, K. E., and Houston, R. S., 1984, The Cheyenne belt: analysis of a Proterozoic suture in southern Wyoming: Precambrian Research, v. 25, p. 415-446.

Karlstrom, K. E., and E. D. Humphreys, 1998, Persistent influence of Proterozoic accretionary boundaries in the tectonic evolution of southwestern North America: Interaction of cratonic grain and mantle modification events: in Boyd, D.W., J.A. Lillegraven, A. W. Snoke, E. Jennings, and K. E. Karlstrom, eds., Rocky Mountain Geology, v. 33, No. 2, p. 161-179.

Kauffman, E.G., 1977, Geological and biological overview: Western Interior Cretaceous basins, in Kauffman, E.G., editor, Cretaceous facies, faunas, and paleoenvironments across the Western Interior basin: The Mountain Geologist, v. 14, p. 75-99.

Kluth, C.F., and Coney, P.J., 1981, Plate tectonics of the Ancestral Rocky Mountains: Geology, v. 9, p. 10-15.

Kocurek, G., and Dott, R.H., Jr., 1983, Jurassic paleogeography and paleoclimate of the central and southern Rocky Mountain region: in Reynolds, M.W., and Dolly, E.D., eds., Mesozoic paleogeography of the west-central United States, Rocky 104

Mountain Paleogeography Symposium 2, The Rocky Mountain Section, Society of Economic Paleontologists and Mineralogists, Denver Colorado, p. 101-116.

Krugh, K. A., 1987, U-Pb thermochronologic constraints on the early Proterozoic tectonic evolution of the Hartville uplift, southeast Wyoming: Unpublished M. S. Thesis, University of Wyoming, Laramie, Wyoming, 163 p.

Lorenz, J. C., 1995, Stresses and fractures in the Frontier Formation, Green River Basin, predicted from basin-margin tectonic element interactions: in Jones, R. W., ed., Resources of Southwestern Wyoming: Wyoming Geological Association Guidebook, p. 45-60.

Mallory, W.W., 1963, Pathfinder uplift of Pennsylvanian age in southern Wyoming: U.S. Geological Survey Professional Paper 450-E, p. E57-E60.

McGookey, D. P., J. D. Haun, L. A. Hale, H. G. Goodell, D. G. McCubbin, R. J. Weimer, and G. R. Wulf, 1972, Cretaceous System: in W. W. Mallory, ed., Geologic Atlas of the Rocky Mountain Region: Rocky Mountain Association of Geologists, Denver CO, p. 190-228

Merewhether, E.A., Krystinik, K.B., and Pawlewicz, M.J., 1987, Thermal maturity of hydrocarbon-bearing formations in southwestern Wyoming and northwestern Colorado: U.S. Geological Survey Miscellaneous Investigation Map I-1831

Merewhether, E. A., 1983, The Frontier Formation and mid-Cretaceous orogeny in the foreland of southwestern Wyoming: The Mountain Geologist, v. 20, p. 121-138.

Minton, G., 2002, Subsurface study of the Lewis Shale in the Southern Washakie and Sand Wash Basins using borehole images, core, well log and seismic data: Unpublished M.S. Thesis, Colorado School of Mines, Golden, Colorado, 220 p.

Molenaar, C. M., and D. D. Rice, 1988, Cretaceous rocks of the Western Interior Basin: in Sloss, L. L., ed., The Geology of North America, Vol. D-2, Sedimentary Cover-North American Craton: U.S., GSA, p. 77-82.

Molzer, P.C., and Erslev, E.A., 1995, Oblique convergence during northeast-southwest Laramide compression along the east-west Owl Creek and Casper Mountain Arches, Central Wyoming: AAPG Bulletin, v. 79, p. 1377-1394.

Parker, J. M., and Bortz, L. C., 2001, West Side Canal gas field, Carbon county, Wyoming: The Mountain Geologist, v. 38, p. 211-224. 105

Pawlewicz, M.J., Lickus, M.K., Law, B.E., and Dickinson, W.W., 1986, Thermal maturity map showing depth to 0.8 percent vitrinite reflectance in the Greater Green River Basin, Wyoming, Colorado, and Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF-1890.

Paylor, E.D. and Yin, A., 1993, Left-slip evolution of the Owl Creek Fault system, Wyoming, during Laramide shortening: in Laramide Basement deformation in the Rocky Mountain Foreland of the Western United States, Schmidt, C.J., Chase, R.J., and Erslev, E.A., eds., GSA Special Paper 280, p. 229-242.

Peterson, J.A., and Smith, D.L., 1986, Rocky Mountain paleogeography through geologic time, in Peterson, J.A., editor, Paleotectonics and sedimentation in the Rocky Mountain region, United States: AAPG Memoir 41, p. 3-19.

Picard, M.D., 1997, Mesozoic history of Wyoming: in Jones, R.W. and Harris, R.E., eds., Wyoming State Geological Survey Public Information Circular 38, p. 73-105.

Pyles, D. R., 2000, A high frequency sequence stratigraphic framework for the Lewis Shale and Fox Hills Sandstone, Great Divide and Washakie Basins, Wyoming: Unpublished M.S. Thesis, Colorado School of Mines, Golden, Colorado, 261 p.

Rahmat, Nor Ainulhuda Mazniz Nor, 2000, Borehole image analysis of the Cretaceous Lewis Shale, Sand Wash Basin, Moffat County, Colorado: Unpublished M.S. Thesis, Colorado School of Mines, Golden, Colorado, 156 p.

Roehler, H. W., 1990, Stratigraphy of the Mesaverde Group in the central and eastern Greater Green River basin, Wyoming, Colorado, and Utah: U.S Geological Survey Professional Paper 1506-A14, 52 p.

Rose, P. R., 1977, Mississippian carbonate shelf margins, western United States, in Rocky Mountain thrust belt geology and resources: Wyoming Geological Association 29th Annual Field Conference, p. 155-172.

Reynolds, M. W., 1976, Influence of recurrent Laramide structural growth on sedimentation and petroleum accumulation, Lost Soldier area, Wyoming: AAPG Bulletin, v. 60, p. 12-33.

Sando, W.J., Gordon, M., Jr., and Dutro, J. T., 1975, Stratigraphy and geologic history of the Amsden Formation (Mississippian and Pennsylvanian) of Wyoming: U.S. Geological Survey Professional Paper 848-A, 83 p. 106

Sheriff, R. E., 1991, Encyclopedic Dictionary of Exploration Geologists. 3rd Ed.: Tulsa, Society of Exploration Geophysicists.

Simmons, P.A., and Scholle, P.A., 1990, Late Paleozoic uplift and sedimentation, northeast Bighorn basin, Wyoming: in Spencht, R.W., ed., Wyoming Sedimentation and Tectonics, Wyoming Geological Association Guidebook, 41st Annual Field conference, p. 39-56.

Sloss, L.L., 1963, Sequences in the cratonic interior of North America: GSA Bulletin, v. 74, p. 93-114.

Snoke, A. W., 1997, Geologic history of Wyoming within the tectonic framework of the North American cordillera: Wyoming State Geological Survey Public Information Circular, 36 p.

Snoke, A. W., J.R. Steidtmann, and S.M. Roberts, 1993, Geology of Wyoming: The Geological Survey of Wyoming, Memoir 5, 937 p.

Sonnenfeld, M.D., 1996, An integrated sequence stratigraphy approach to reservoir characterization of the Lower Mississippian Madison Limestone, emphasizing Elk Basin Field, Bighorn Basin, Wyoming and Montana: Unpublished Ph.D Dissertation. Colorado School of Mines, Golden, Colorado, 438 p.

Stone, D.S., 1969, Wrench faulting and Rocky Mountain tectonics: The Mountain Geologist, v. 6, p. 67-79.

Stone, D.S., 1984, The Rattlesnake Mountain, Wyoming, debate: A review and critique of models: The Mountain Geologist, v. 21, p. 37-46

Sunnetcioglu, M., 2002, Acoustic and resistivity borehole image log interpretation, Barrel Springs Field, Carbon County, Wyoming: Unpublished M. S. Thesis, Colorado School of Mines, Golden, Colorado, 110 p.

Suryanto, D., 2003, 3-D geologic model of the Lewis Shale in the Great Divide and Washakie Basins, Wyoming: Unpublished M.S. Thesis, Colorado School of Mines, Golden, Colorado, 254 p.

Thelin, G. P. and Pike, 1991, Landforms of the coterminous United States: Digital shaded relief portrayal. U.S. Geological Survey, Denver, Colorado, Map.

107

Thomas, G. E., 1971, Continental plate tectonics: southwest Wyoming: in Renfro, A.R., ed., Wyoming Geological Association Guidebook, 23rd Annual Field Conference, p. 103-124.

Vail, P.R., Mitchum, R.M., Jr., and Thompson, S., III, 1977, Global cycles of relative sea level, in Payton, C.E., ed., Seismic stratigraphy – applications to hydrocarbon exploration: AAPG Memoir 26, p. 83-97.

Van Wagoner, J. C., R. M. Mitchum, Jr., K. D. Campion, and V. D. Rahmanian, 1990, Siliciclastic sequence stratigraphy in well logs, cores and outcrops: AAPG Methods in Exploration Series, No. 7, 55 p.

Webster, C.L. “Geochronometers: How accurate are they?” Online posting. Winter 1990. Geoscience reports. 30 Aug. 2003

Weimer, R. J., 1960, Upper Cretaceous stratigraphy, Rocky Mountain area: AAPG Bulletin, v. 44, p. 1-20.

Weimer, R. J., 1961, Uppermost Cretaceous rocks in central and southern Wyoming, and northwest Colorado: Wyoming Geological Association Guidebook, 16th Annual Field Conference, p. 17-28.

Weimer, R. J., 1970, Rates of deltaic sedimentation and intrabasin deformation, Upper Cretaceous of Rocky Mountain region: in Morgan, J. P., ed., Deltaic sedimentation modern and ancient, SEPM Special Publication 15, p. 270-292.

Winn Jr., R. D., M. G. Bishop, and P. S. Gardner, 1987, Shallow-water and sub-storm- base deposition of Lewis Shale in Cretaceous Western Interior seaway, south- central Wyoming: AAPG Bulletin, v. 71, p. 859-881.

Wise, D.U., 1963, Keystone faulting and gravity sliding driven by basement uplift of the Owl Creek Mountains, Wyoming: AAPG Bulletin, v.47, p. 586-598.

Witton, E. M., 1999, Outcrop and subsurface characterization of the Lewis Shale, Carbon County, Wyoming: Unpublished M. S. Thesis, Colorado School of Mines, Golden, Colorado, 219 p.

Zahm, C.K., 2002, 3D Strain and basement-involved folding, Thermopolis Anticline, Wyoming: Unpublished Ph.D. Disertation, Colorado School of Mines, Golden, Colorado, 242 p.

108

Zainal, Z., 2001, Subsurface geology and petroleum system of the lower Lewis Shale in the Sand Wash basin, northwest Colorado: Unpublished M. S. Thesis, Colorado School of Mines, Golden, Colorado, 191 p.

Zoback, M. L., and Zoback, M. D., 1989, Tectonic stress field of the continental U.S.: in Pakiser, L. C. and Mooney, W. D., eds., Geophysical framework of the Continental US: GSA Memoir 172, Boulder, Colorado, p. 523-539.