I NATURAL FRACTURES in the NIOBRARA FORMATION

NATURAL FRACTURES IN THE NIOBRARA FORMATION, BOULDER TO LYONS, COLORADO

Michael Douglas Collins

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ABSTRACT

Naturally fractured reservoirs have become an important target for petroleum exploration, specifically with the increasing use of horizontal drilling and multistage fracking. In the Denver Basin the Niobrara Formation is a resource play consisting of chalks and marls in which natural fractures aid in hydrocarbon deliverability within the reservoir units. Several models have been put forth in understanding the fractures including: Laramide tectonics, Neogene extensional tectonics, solution of evaporates, hydrocarbon generation, and regional stress patterns. This study will help to examine the controls on natural fractures that exist within the different units of the Niobrara

Formation and how they can aid in hydrocarbon production.

From outcrop analysis of jointing, the ideal σ1 results indicate a sub-horizontal

Laramide compression with an average attitude of 14/071. Evidence of normal faulting indicated reactivation of previous induced shear fractures by separated calcite filled walls. Two main joint sets were observed and measured throughout the outcrop study areas with J1 systematic joints averaging 248/ 76 and later J2 joints averaging 162/75.

The vertical stylolites formed perpendicular to J1 joints and Laramide slip direction with an average stylolitic pole to plane of 11/077.

Data from the bulk mineralogy, total organic content, fracture mapping and geological characterization of the units within the study interval, together indicate that the chalk units have a relatively high ratio of calcite to siliciclastics, low TOC wt. % and low average fracture spacing in comparison to the marls.

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The structural events that caused the rock failure within the Niobrara Formation are based on syn-Laramide compression and later post-Laramide de-pressuring and un- roofing.

TABLE OF CONTENTS

ABSTRACT ...... iii LIST OF FIGURES ...... vii LIST OF TABLES ...... xi ACKNOWLEDGEMENTS ...... xii CHAPTER 1 INTRODUCTION ...... 1 1.1 Importance ...... 1 1.2 Objectives ...... 2 CHAPTER 2 BACKGROUND ...... 3 2.1 Field Area Location ...... 3 2.2 Regional Geology ...... 3 2.2.1 Denver Basin ...... 5 2.2.2 Front Range Structure ...... 5 2.3 Niobrara Formation ...... 11 2.3.1 Stratigraphy & Depositional Environment ...... 12 2.3.2 Source Rock ...... 15 2.3.3 Reservoir Rocks ...... 16 CHAPTER 3 PREVIOUS WORK...... 17 3.1 Laramide Tectonic and Kinematic Models ...... 17 3.2 Mechanical and Fracture Stratigraphy ...... 21 CHAPTER 4 METHODOLOGY ...... 25 4.1 Field Work ...... 25 4.2 Measured Section and Sampling ...... 25 4.3 Fracture Maps ...... 26 4.4 Fracture Scanlines ...... 31 4.5 Advantages and Biases of Field Techniques ...... 34 CHAPTER 5 DETAILED FRACTURE ANALYSIS ...... 37 5.1 Stylolite Data Analysis ...... 37 5.2 Joint Data Analysis ...... 41 5.3 Niobrara Formation Rock Mechanics ...... 41

5.4 Conclusions from Fracture Analyses ...... 44 5.5 Researcher Comparison ...... 46 CHAPTER 6 MECHANICAL STRATIGRAPHY ANALYSIS ...... 50 6.1 Measured Section ...... 50 6.2 Bulk Mineralogy ...... 55 6.3 Thermal Maturity ...... 58 6.4 Fracture Maps ...... 65 6.5 Fracture Scanlines ...... 70 6.6 Conclusions from Mechanical Stratigraphy Analysis...... 76 CHAPTER 7 CONCLUSIONS ...... 79 7.1 Concluding Results ...... 79 7.2 Recommendations for Future Work...... 81 REFERENCES ...... 83 APPENDIX A STYLOLITES ...... 93 APPENDIX B BULK MINERALOGY ...... 95 APPENDIC C THERMAL MATURITY ...... 97 APPENDIX D FRACTURE MAPS ...... 99 APPENDIX E JOINT FRACTURES ...... 104 CD/DVD ...... POCKET

LIST OF FIGURES

Figure 2.1: Location of study area: (a) Denver Basin regional map with locations of major fields; (b) Close up of outcrop location along the Colorado Front Range (modified from Sonnenberg, 2011)...... 4 Figure 2.2: Niobrara Petroleum System; (a) Structural contour map of the Precambrian Basement and locations of Niobrara within the Denver basin and (b) Cretaceous Stratigraphic column for the basin. Dashed lines indicate locations of Niobrara source rocks (modified from Sonnenberg, 2011)...... 6 Figure 2.3: Simplified geologic map of the eastern margin of the Front Range south of Boulder, Colorado. Smoothed rose diagrams show the trend of the ideal σ1 axis trends for three major domains (modified after Selvig, 1994)...... 7 Figure 2.4: (a) Surface geology and location map of Rocky Flats seismic line along Coal Creek Canyon. (b) Rocky Flats seismic line showing surface geology and subsurface geologic interpretation (modified after Ebasco, 1993)...... 8 Figure 2.5: Paleoenvironment of the Early Niobrara Formation (Fort Hays) and the location of the Western Interior Cretaceous Seaway within the southern Rockies. Red outline is the location of the Denver Basin within the seaway (modified from Mallory, 1972)...... 13 Figure 2.6: On the left, a stratigraphic column of the Niobrara Formation with its relation to sea level change. On the right a type log of the Upper Cretaceous stratigraphy including the Niobrara Formation in the Denver Basin (modified from Longman et al., 1998; Barlow, 1985 and Kauffman 1985)...... 14 Figure 2.7: Niobrara Van Krevelen diagram. The Niobrara source rocks are Type II kerogens that are based on recorded Rock-Eval Data. HI values decrease significantly with increasing burial depth and thermal maturity. Data for plot is from Rice (1984); Barlow (1985); Pollastro and Martinez (1985) after Sonnenberg and Weimer (1993)...... 15 Figure 2.8: Porosity versus depth plot for the Niobrara chalks from the Denver Basin (from Lockridge and Scholle, 1978; Precht and Pollastro, 1985). Dashed Line above is a comparison to European chalks. Difference in porosity is likely due to burial history indicating that the Niobrara was buried deeper than its current depth...... 16 Figure 3.1: Tectonic map of the Laramide foreland basins indicating the trends of major and minor Laramide arches. The blue arrows indicate average shortening directions from published and unpublished minor fault data collection (modified from Allen, 2009; Erslev and Koenig, 2009)...... 19 Figure 3.2: Average fracture intensity (fractures/meter) by stratigraphic member for the Austin Chalk and the mechanical stratigraphy (modified from Corbett et al., 1987)...... 24

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Figure 4.1: Rope and chalk system used to aid in the creating a mapping window for fracture maps on bedding planes...... 27 Figure 4.2: Fracture map of the Fort Hays Limestone bedding surface at Six Mile Fold. The fracture map produces results on fracture attributes (i.e. intensity, density & mean trace length)...... 28 Figure 4.3: Example of a circular window with mapped fracture traces. (a) red dots are fracture trace intersection (n) with the circle. (b) Refers to the blue dots that are trace endpoints (m) within the circle (modified after Rohrbaugh et al. 2002 and Blake, 2009)...... 29 Figure 4.4: (a) Schematic diagram of conversion from fracture trace orientation to trend and plunge. (b) The highlighted green column indicates a conversion of the rake angle based on the strike and dip of the bedding plane in which the fracture was measured...... 30 Figure 4.5: Location at Six Mile Fold, C bench of the Niobrara Formation. Outcrop photo of a fracture scanline in cross sectional view. Note that measure tape is parallel to bedding plane. Scan line used to measure the distance between each fracture for a given cross section, in order to produce results of average fracture spacing...... 32 Figure 4.6: Location at Six Mile Fold, C bench of Niobrara Formation. Outcrop photo of data collection of a fracture scanline, including fracture strike, dip and spacing. 33 Figure 4.7: Schematic diagram indicating fracture scanlines in cross section and a fracture map. Fracture scanlines measured on perpendicular wall faces to intercept sister joint set. Drawing not to scale ...... 34 Figure 5.1: The mechanisms behind concentration gradients during dissolution creep. Material is dissolved from surfaces with high resolved compressive stress and diffuses to sites of lower compressive stress, as in a pressure shadow or window (modified from Davis and Reynolds, 1984)...... 38

Figure 5.2: Stylolites forming perpendicular to σ1 and systematic joint sets. (a) Thrust faulting, (b) shear (strike-slip) faulting, (c) compressional strain (no faulting) (modified from du Rochet, 1981)...... 38 Figure 5.3: (a) Equilateral stereonet plot of the stylolitic planes, (b) rose diagram of stylolitic planes, (c) contoured plot (CI 3%) of poles to stylolitic planes, and (d) color key to separated contoured intervals...... 39 Figure 5.4: (a) Photo of a vertical stylolite cross cutting an open extensional fracture with marked σ1 and σ3 directions. (b) Cross section view through a vertical stylolite in relation to a calcite filled fracture. Samples gathered from Fort Hays Limestone in the CEMEX Limestone Quarry Lyons, Colorado...... 40 Figure 5.5: Joint data that were collected from Six Mile Fold and CEMEX Limestone Quarry. (a) Equilateral stereonet plot of the J1 and J2 planes, (b) Equilateral stereonet plot of the J1 and J2 poles, (c) rose diagram of J1 and J2 planes, (d) contoured plot (CI 3%) of poles to planes of the J1 and J2 joints...... 42

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Figure 5.6: Outcrop photo of C Chalk bench at the CEMEX Limestone Quarry depicting the J2 joint set terminating into the systematic J1 joint set...... 43 Figure 5.7: Secor’s (1965) model of the role of fluid pressure in jointing. (a) A low differential stress load existing between σ1 and σ3. Unloading through pressure release allows for the stress to pass all the way to the tensile failure envelope, and the rock breaks by mode I failure. (b) In this case differential stress is greater between σ1 and σ3 and when unloading occurs the stress circle collides with the parabolic window and transitional tensile failure occurs. (c) Differential stress is yet even greater and the stress circle does not make it to the tensile failure envelope and the failure occurs in a preferred direction in relation to principle confining pressure. (d) Differential stress is greater through the increase of principle confining pressure, which indicates that rock failure will occur before then tensile envelope is reached. (Pc = principle confining stress, θ = angle between rock failure and σ1, DS = differential stress, σ1* = greatest principal effective stress, σ3* = least principal effective stress) (modified from Secor 1965)...... 45 Figure 5.8: Map sketch of the principle horizontal stress field orientations gathered from vertical stylolites and faults and measured fractures. Data collected from the Fort Hays Limestone, Six Mile Fold Boulder Colorado (modified from Weimer, 1996)...... 48 Figure 6.1: Measured section of the Niobrara Formation at the CEMEX Limestone Quarry with interpreted sedimentary structures. Section measured from surface down to bottom of quarry based on available exposures...... 52 Figure 6.2: Outcrop photos for measured section of varying units within the Niobrara Formation at the CEMEX Limestone Quarry. (F1) Fort Hays Limestone, (F2) C- Marl Unit with carbonate stringers, (F3) C-Marl Unit with chalk stringers, (F4) C- Chalk Unit with sharp over lying contact, (F5) B-Chalk Unit, (F6) A-Marl Unit and A-Chalk with fracture swarming...... 53 Figure 6.3: Set of outcrop photos of the Niobrara Formation at the CEMEX Limestone Quarry, Lyons Colorado. (a) Side wall view of upper section to top of quarry. (b) Middle section indicating easterly dipping strata. (c) Lower section of formation including chalk marl stacked sequence, Fort Hays Limestone, and Codell Sandstone...... 54 Figure 6.4: A constructed mineralogical geocolumn log through the Niobrara Formation at the CEMEX Limestone Quarry. Geocolumn is compared to the described lithology and interpreted sedimentary structures of the measured section...... 57 Figure 6.5: A schematic developed and modified by the Niobrara Consortium for classifying cretaceous marine stratigraphy of the Niobrara Formation...... 58 Figure 6.6: Pyrite found throughout the chalk and marls of the Niobrara Formation. (a) Bedding lined pyrite. (b) Nodular pyrite. Pictures taken within the C-Chalk Units of the Niobrara Formation at the CEMEX Limestone Quarry...... 59

Figure 6.7: Kerogen quality plot for samples collected along the measured section of the Niobrara Formation at the CEMEX Limestone Quarry Lyons, Colorado. Samples located in the organic lean zone are the chalk and Fort Hays samples which indicate that they are not the source rocks...... 61 Figure 6.8: Modified Van Krevelen diagram indicating that the samples are TYPE II kerogens except for the outliers which are the carbonates that have high oxygen indexes...... 62 Figure 6.9: Thermal maturity of the source rocks within the measured section of the Niobrara Formation, CEMEX Limestone Quarry, Lyons Colorado...... 63 Figure 6.10: A TOC wt. % log through the Niobrara Formation at the CEMEX Limestone Quarry Lyons, Colorado. TOC wt. % log is compared to the described lithology and the bulk mineralogy...... 64 Figure 6.11: Locator map for fracture maps at Six Mile Fold field site location...... 65 Figure 6.12: Fracture Maps gathered from the Fort Hays Limestone bedding surface at the Six Mile Fold field site location. Key: Green – J1 joint set, Purple – J2 Joint set, Orange – possible J3 joint set...... 66 Figure 6.13: Stereonet plots of the plunge and trend of the four fracture maps taken at Six Mile Fold Boulder, Colorado...... 67 Figure 6.14: Geologic map of Six Mile Fold with interpreted contacts between units and location of cross section...... 69 Figure 6.15: Cross section through Six Mile Fold with interpreted fault slip...... 70 Figure 6.16: Spacing difference plots with fracture number on the y-axis ( fracture sample count within the scanline) and fracture spacing on the x-axis (distance between each fracture). (a) Plot for the B-Chalk unit adjacent to fold hinge at Six Mile Fold, Boulder Colorado. (b) Plot for the B-Chalk unit at CEMEX Limestone Quarry Lyons, Colorado...... 71 Figure 6.17: Rabbit Mountain Anticline and the location of CEMEX Limestone Quarry. (a) Geologic map of the Rabbit Mountain Anticline. (b) Geologic cross-section through ideal location of CEMEX Limestone Quarry and Rabbit Mountain Anticline. (c) Picture of Rabbit Mountain showing steep east limb and planar southwest dipping limb (modified from Matthews et al., 1975)...... 73 Figure 6.18: A constructed average fracture spacing log through the Niobrara Formation at the CEMEX Limestone Quarry. Fracture spacing log is compared to the described lithology and interpreted sedimentary structures of the measured section and calculated TOC wt. %...... 74 Figure 6.19: Fracture Swarm found within the C-Marl Unit of the Niobrara Formation at the CEMEX Limestone Quarry Lyons, Colorado...... 75 Figure 6.20: Constructed mechanical stratigraphy through the Niobrara Formation at the CEMEX Limestone Quarry. Mechanical stratigraphy is based off of the described lithology, bulk mineralogy, thermal maturity, and average fracture spacing...... 78

LIST OF TABLES

Table 6.1: Results of calculations for intensity, density and mean trace length for the four fracture maps gathered at Six Mile Fold Boulder, Colorado. (Units – feet) ...... 68 Table 6.2: Average fracture spacing for scanlines taken at both field locations with same orientation including calculated standard deviation ...... 72

ACKNOWLEDGEMENTS

First and foremost, I would like to thank my advisor, Dr. Steve Sonnenberg for always having an open door and the patience to answer my never-ending questions. I would also like to express my thanks to my committee members, Dr. Bruce Trudgill and

Dr. John Humphrey for their support and suggestions over the past two years. This thesis was made possible through their guidance and encouragement.

I am grateful to the Niobrara Oil and Gas Consortium for funding my project and providing data. Specific thanks to Pat Fisher for having me properly trained and granting me access to the CEMEX Limestone Quarry for field work.

I very much appreciate the discussions and input from Professor Emeritus Dr.

Robert Weimer on the structural evolution of the Colorado Front Range and for all of his input and efforts on the project. Thank you to my undergraduate advisor Dr. Whitney

Autin for inspiring and encouraging me to become a petroleum geologist. To Kurt

Miller, my industry mentor, I appreciate all the wisdom and guidance for my career in the oil and gas industry.

I would like to thank the Colorado School of Mines Department of Geology and

Geological Engineering for accepting me into the Masters program, and for providing me with an excellent education and endless opportunities. Specifically, I would like to express my thanks to Marilyn Schwinger and Debbie Cockburn, who always had the answers to all my questions.

To all of my friends at the Colorado School of Mines, thank you for your encouragement, support and friendship. More specifically, Craig Kaiser for all of his xii

help with field work interpretation and David Thul for his input and efforts on thermal maturity analysis.

I will forever be grateful to my family for all their support. Thank you to my parents Jim and Connie Collins, for providing me with every opportunity to pursue my dreams, no matter what they were. Thank you to my brother and sister in law, Dan and

Kim, my brother Joe, and to Benny, my favorite yellow lab, who always listened to what

I had to say no matter what. I have made it this far in pursuing my dream of being a

Petroleum Geologist because of their continued support and encouragement. Drill, baby drill!!!

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CHAPTER 1

INTRODUCTION

1.1 Importance

Naturally fractured reservoirs are an important target for petroleum exploration, specifically with the increase use of new technologies such as horizontal drilling and multistage fracture stimulation. Fractures lend themselves to being migration pathways for hydrocarbons to the wellbore and they tend to help in recovery of hydrocarbons from reservoirs. Corbett et al. (1987) stated that for optimum success in exploration an adequate characterization of a fracture system based on lithology, rock properties, structural position, and tectonic history is needed.

Recent drilling activity in the southern Rockies has been targeting the marine mud rocks of the Late Cretaceous Western Interior Seaway, specifically the Niobrara

Formation. The key to successful drilling exploration within these chalk plays has been locating and predicting the brittle zones where natural fractures are abundant. The

Niobrara has been successfully produced from many fields within the Western Interior

Cretaceous basin.

The Colorado Front Range exposure of the Niobrara Formation provides an opportunity to study and map the fracture system, determine fracture properties and to differentiate between the varying fracture generations. This study aims to fully characterize the natural fracture networks within the Niobrara Formation. Integration of outcrop field work quantifying the fracture network with comparison to the rock properties provides insight into the fracture heterogeneities and their varying systems. A

simple structural model provides an overview of the interaction between fractures and the stratigraphy of the Niobrara Formation along the Colorado Front Range.

This information will be used to determine the effects of fluid movement through the fracture system of a reservoir model. This will help with better understanding the complexities and variations within the Late Cretaceous carbonate reservoirs in the

Colorado Rockies. These results can be used to enhance methods of hydrocarbon recovery from the reservoir and to help determine the best potential targets.

1.2 Objectives

The purpose of this study is to determine controls on natural fracture variability within

the Niobrara Formation.

The goals of this study include:

1) Investigate the fracture spacing, intensity, density, and mean trace length in

outcrop.

2) Determine the relationship between natural fractures and tectonic setting,

lithology, and mineralogy.

3) Produce results that can subsequently be used to aid in generation of a reservoir

geomodel, incorporating the effect of fluid flow through the fracture network.

CHAPTER 2

BACKGROUND

The geologic history of the Niobrara Formation is complex and has been the focus of many studies in the southern Rocky Mountain region. Several studies have been completed on the structure and mechanics of the Niobrara Formation and this chapter presents an overview of the, regional and local geology, and an overview of the Niobrara stratigraphy and depositional environment.

2.1 Field Area Location

The outcrop location of this study is located along the Colorado Front Range between Boulder and Lyons. The field sites are approximately five to ten miles north of

Boulder Colorado on the east side of the Southern Rockies (Figure 2.1). The two field site locations includes Six Mile Fold (a compressional structural feature), and the

CEMEX cement quarry, with flanking beds dipping off to the east towards the center of the basin. Six Mile Fold is located on Boulder City and Boulder County open space, and requires a permit for access. The CEMEX Limestone Quarry is mixed for limestone from the Fort Hays member of the Niobrara Formation, for production of cement. Access to the quarry is granted upon completion of MSHA safety training and approval from the quarry operator.

2.2 Regional Geology

The Front Range geology is complex in both its tectonic history as well as its stratigraphy. The stratigraphy across the Denver Basin is fairly correlative during the

Cretaceous, although there are many localized tectonic features that cause variation

(a) (b)

(c)

(a) 50 Miles N

(b) N 2 Miles

CEMEX Quarry 6 Mile Fold

Figure 2.1: Location of study area: (a) Denver Basin regional map with locations of major fields; (b) Close up of outcrop location along the Colorado Front Range (modified from Sonnenberg, 2011).

This section describes the stratigraphic and structural history of the Denver Basin and

Colorado Front Range, with an emphasis on the Niobrara Formation

2.2.1 Denver Basin

The Denver Basin is considered one of the largest sedimentary basins within the

Rocky Mountain Region. The foreland basin was structurally deformed during the

Laramide Orogeny and the Rocky Mountain uplift. The basin is considered to be asymmetric with a steeply dipping left flank close to the foothills and a more gently dipping eastern flank towards Kansas and Nebraska (Figure 2.2). Within the Denver

Basin there are multiple source rocks that were deposited during what was known as the

Western Interior Cretaceous (WIC) seaway, including the Niobrara Formation (Longman et al., 1998). This was a major transgression that represents the maximum sea-level highstand, during Cretaceous deposition. During the sea-level transgression large volumes of marine shales were deposited and are key source rocks in the Cretaceous. At maximum transgressions (highstands) large amounts of coccolith-rich and planktonic foraminifera-rich carbonate deposition accumulated carbonate sediments (chalks) that are interbedded among the shale intervals. The chalk-rich carbonate facies varies westward through the basin due to siliciclastic sediments input from the orogenic belt (Weimer,

1978).

2.2.2 Front Range Structure

The Colorado Front Range is a compressional uplift that is a result of the deformation driving the Laramide Orogeny. Due to the variation in deformation induced by the Laramide Orogeny on the Rockies, only the structure for the city locations of

Golden and Boulder will be discussed.

50 Miles N

Figure 2.2: Niobrara Petroleum System; (a) Structural contour map of the Precambrian Basement and locations of Niobrara within the Denver basin and (b) Cretaceous Stratigraphic column for the basin. Dashed lines indicate locations of Niobrara source rocks (modified from Sonnenberg, 2011). The surficial geology along the Front Range near the city of Boulder consists of a series of exposed cretaceous units’ adjacent to faulted Precambrian basement (Figure

2.3). The major features of the Laramide Orogeny include: uplift and rotation of the basement block giving a monocline form; overturning of the lip of the block by folding; faulting and folding over the Denver basin block; and, the Eocene erosional surface.

At the base of the rotated block is the Basin Margin Fault which is thought to be an eastward branch of the Golden Fault zone (Weimer, 1996). From an east-west seismic section six miles to the south at Deer Creek indicate an abrupt change from

Figure 2.3: Simplified geologic map of the eastern margin of the Front Range south of Boulder, Colorado. Smoothed rose diagrams show the trend of the ideal σ1 axis trends for three major domains (modified after Selvig, 1994). undeformed basin strata to steeply dipping strata of the deformed zone (Hu, 1993). A similar relation was seen from seismic sections in the Rocky Flats north of Golden

(Domoracki, 1986; Ebasco team, 1993) (Figure 2.4). A few hundred feet of section is cut out by movement on the Basin Margin Fault. From outcrop the surface trace of the fault is placed at the change from steep to shallow dips within the Denver Formation, a relationship observed along the central Front Range (Weimer, 1996). According to

Weimer (1996), the Golden Fault disappears and the Basin Margin Fault becomes the

(a)

(b)

Figure 2.4: (a) Surface geology and location map of Rocky Flats seismic line along Coal Creek Canyon. (b) Rocky Flats seismic line showing surface geology and subsurface geologic interpretation (modified after Ebasco, 1993).

break along which the main block rotation and uplift occurred. The Basin Margin Fault may not exist all the way from the Precambrian basement to the surface, based on the amount of slip recognized in seismic. Likely, the few hundred feet of throw that exist along the fault is not enough accommodation for the amount of growth folding that is occurring during deformation. Regional fractures patterns represent regional tectonic events, and are usually long and form orthogonal sets (Nelson, 1987; Tiab and

Donaldson, 2004).

The main regional even that impacted fracture development along the Colorado

Front Range is the Laramide Orogeny. There are several theories as to how the Laramide

Orogeny affected the mountain building of the Rockies.

The two theories include hot spot/mantle plume theory and the shallow subduction theory. The first idea is that a hot spot develops over a mantle plume and the plate moves over that location gathering heat which causes upwelling and expansion

(Conner and Harrison, 2003). The North American plate was thought to have moved over a hot spot, which was previously covered by an oceanic plate. The plate moved over the hot spot formed by the mantle plumes and the heat partially melted the continental crust. This theory gives explanation as to why the Rockies are so wide. The scattered volcanoes that occur on the oceanic plate are an effect of the hot spots and are widely accepted as the theory to their formation.

The second theory is that crustal deformation resulting in uplift arched domes, basins and large anticlines is believed to be the result of shallow subduction of the

Farallon plate beneath the North American plate (Conner and Harrison, 2003). As the angle of subduction decreased the Farallon plate subducted nearly horizontally beneath

parts of the North American plate. This is evident by the lack of volcanoes in the western interior of the United States and was characterized by NE compressional strain, due to the uplift of the North American plate (Conner and Harrison, 2003). Conner and Harrison

(2003) stated that shallow subduction movement in the horizontal direction is due to the hot spots, which caused the density of the continental plate to decrease allowing for it to travel a greater distance creating the wider mountain range.

The continued compressions along convergent boundaries at the end of the

Cretaceous period caused large faults in western North America. As previously stated the faults that occur along the Front Range have been mapped deep into the Precambrian rocks of the continent and created basement uplifts (Weimer, 1996). The basement uplifts are important because they determine when the fault deformation occurred, thus giving support to the shallow subduction theory. So in reference to the study area the

Golden and Basin Margin faults are interpreted as foreland fold and thrust belt that allowed for uplift and un-roofing of the overlying layers, releasing the confining pressure.

The Front Range uplift and associated deformations have been demonstrated in structural cross sections and have been discussed by many authors over the past 50 years

(e.g. Boos and Boos, 1957; Berg, 1962; Grose, 1972; Davis and Young, 1977; Tweto,

1983; Jacob, 1983; Erslev, 1993, Hembre and TerBest, 1997). Most authors believe that the uplift is a result of east-directed stress. There have been multiple published cross sections across the east flank of the Front Range through the years, indicating varying interpretations of dip angles and locations of faults. There have been interpretations of the Golden Fault and the Basin Margin Fault to have been low-angle thrust and to high

angle reverse fault. It is likely that the fault plane dips are variable along the entire Front

Range.

In summary the formation of the Front Range structure is due to the deformation during the Laramide Orogeny. The Laramide dates during the Pierre deposition (70 to 71

Ma), with the intrusive volcanism occurring 66 to 64 Ma, which includes the Cretaceous-

Tertiary boundary. The main phase of deformation occurred in the early Paleocene extending into the early to Middle Eocene (64 to 50 Ma). The Laramide Orogeny ceased movement before the formation of the late Eocene erosional surface.

2.3 Niobrara Formation

The Niobrara Formation has been productive in the Rocky Mountain region for many years, mainly from structural traps. Recently with combined technologies of horizontal drilling and multistage fracture stimulation and the concept of basin centered oil and gas accumulations, many areas are being drilled to determine if the Niobrara is a resource play. A resource play is an area of continuous hydrocarbon saturation located in the deeper part of the basins. The reservoirs within the Niobrara Formation are low permeability chalks and marls. The source beds are thermally mature in the deeper part of the varying Laramide basins. In the deeper part of the basins, specifically the Denver

Basin, matrix porosity is low (< 8.0%) and natural fracture occurrence is important for production. The natural fractures are important in controlling “sweet spots” and can form from several causes. Several models have been put forth in understanding the fractures including: Laramide tectonics, Neogene extensional tectonics, solution of evaporates, hydrocarbon generation, and regional stress patterns. High matrix porosity is present within the Niobrara Formation in the shallow biogenic gas accumulations of eastern

Colorado and Western Kansas (Rice, 1975, 1984; Rice and Claypool, 1981; Lockridge,

1977; Lockridge and Scholle, 1978; Lockridge and Pollastro, 1988; Kelso et al., 2006).

The porosity decreases with increasing burial depth as a result of compaction and cementation.

The Niobrara Formation has been productive from multiple fields throughout the

Rocky Mountain region. Some of these include: Wattenberg, Beecher Island and Silo

Field (Figure 2.2). It is a tight reservoir in the deeper parts of the basin and requires horizontal drilling and multi-stage hydraulic fracturing. The Niobrara petroleum system is present throughout most of the Rocky Mountain Region and is prospective in varying locations.

2.3.1 Stratigraphy & Depositional Environment

The Upper Cretaceous Niobrara Formation, was deposited in the WIC seaway 82-

89 Ma (Figure 2.5). It is Campanian in age and is a part of the Colorado Group (Figure

2.6). The formation is approximately 300 feet thick and is broken into two members; the

Fort Hays Limestone and Smoky Hill Member. The dominant lithologies within the basin are limestone (chalks) that are interbedded with calcareous shales (marls)

(Longman et al., 1998).

Within industry the three large chalk zones of the Smoky Hill Member have been sub-divided into the A, B, and C chalks (Figure 2.6). These limestone chalk facies are composed of coccolith-rich fecal pellets (that come from pelagic copepods), inoceramid and oyster shell fragments, as well as clay and quartz cements (Lockridge and Scholle,

1978; Sonnenberg and Weimer, 1993). The chalks are thought to be deposited in normal marine conditions with a well-mixed and oxygenated water column.

200 Miles

Figure 2.5: Paleoenvironment of the Early Niobrara Formation (Fort Hays) and the location of the Western Interior Cretaceous Seaway within the southern Rockies. Red outline is the location of the Denver Basin within the seaway (modified from Mallory, 1972).

Figure 2.6: On the left, a stratigraphic column of the Niobrara Formation with its relation to sea level change. On the right a type log of the Upper Cretaceous stratigraphy including the Niobrara Formation in the Denver Basin (modified from Longman et al., 1998; Barlow, 1985 and Kauffman 1985). These chalk zones are an influx of warm Gulfian currents moving from the south into the WIC seaway at highstand sea levels (Longman and Luneau, 1998). Between the chalks are the interbedded marly shale cycles that are affected by siliciclastic input from the western orogenic belt. The interbedded layers are thought to be controlled by sea- level regression and the input of terrestrial sediment. The terrestrial influence brings in siliciclastic input that increases the rate of sedimentation allowing for burial of the organics deposited from the coccolith pellets making the shaly marls the source beds for the chalky reservoir zones of the Niobrara Formation.

2.3.2 Source Rock

The Niobrara is a potential significant self-sourced resource play, which contains reservoir rocks, rich source rocks, and abundant seals acting as trapping mechanism

(Sonnenberg, 2011). Recently it has been targeted as a major resource play in which its organic rich fractured chalk benches act as reservoirs for type II and III kerogens

(Meissner et al. 1984; Sonnenberg and Weimer, 1993; and Landon et al. 2001). Rice

(1984) indicated organic rich beds within the formation, with TOC values ranging from

0.43 to 5.8 wt. %. Rock-Eval analysis has been performed by several authors (Figure

2.7) (Rice, 1984; Pollastro and Martinez, 1985) and indicated that the source is from marine kerogens that increase in maturity with depth.

000

Figure 2.7: Niobrara Van Krevelen diagram. The Niobrara source rocks are Type II kerogens that are based on recorded Rock-Eval Data. HI values decrease significantly with increasing burial depth and thermal maturity. Data for plot is from Rice (1984); Barlow (1985); Pollastro and Martinez (1985) after Sonnenberg and Weimer (1993).

2.3.3 Reservoir Rocks

The reservoir rocks within the Niobrara Formation have undergone both mechanical, and chemical compaction and cementation. They are poor reservoirs ranging in porosities from 1-10% and permeabilities in the millidarcies. Dewatering, mechanical compaction, grain and fossil breakage and reorientation reduce the porosity and permeability within the reservoir rocks (Longman et al., 1998; Precht and Pollastro,

1985) (Figure 2.8). The density porosity log verses depth indicates that at 1000 feet porosity is approximately 45% and at 7000 feet the porosity is at 8-10%. Known production exists in the east with shallow biogenic gas accumulations (approximately less than 3500 ft) which has been considered to be produced based on high matrix porosity

(Rice, 1975, 1984; Rice and Claypool, 1981; Lockridge, 1977; Lockridge and Scholle,

1978; Lockridge and Pollastro, 1988; Kelso et al., 2006). In the deeper part of the basin, the Niobrara chalk zones are buried to depths of 7000-8000 feet with very low porosity and permeability. These locations are where the Niobrara Formation is exploited as a potential resource play within the Southern Rockies.

Figure 2.8: Porosity versus depth plot for the Niobrara chalks from the Denver Basin (from Lockridge and Scholle, 1978; Precht and Pollastro, 1985). Dashed Line above is a comparison to European chalks. Difference in porosity is likely due to burial history indicating that the Niobrara was buried deeper than its current depth.

CHAPTER 3

PREVIOUS WORK

The Niobrara Formation has had a large amount of research completed throughout the WIC seaway. The focus of research has been on understanding the stratigraphy, depositional environment, the source/reservoir and structural deformation. The research that has been completed thus far on the stratigraphy within the WIC basin, and ranges from regional allostratigraphic correlation to lithology variations (Weimer, 1960; Scott and Cobban, 1964; Kauffman, 1977; Hattin and Siemers, 1978; Sonnenberg and Weimer,

1981; Hann, 1981; Barlow, 1985, 1986; Rodriguez, 1985; Longman et al., 1998; Landon et al., 2001). There has been several works done on the source rocks within the Niobrara

Formation, trying to understand thermal maturity. Rock-Eval work was done by Rice

(1984) and Pollastro (1985) to gather geochemical data to help understand the organic content of the source beds within the Formation. Other source rock research has been completed in order to gain a better understanding of resistivity vs. depth and increasing thermal maturity (Longman et al., 1998; Landon et al., 2001; Sonnenberg and Weimer,

1993). There has been completed work on the reservoirs of the Niobrara Formation to help understand chemical and mechanical compaction in relation to the reservoir properties (Precht and Pollastro, 1985; Pollastro and Scholle, 1986).

3.1 Laramide Tectonic and Kinematic Models

Foreland basins are a result of the movement along faults, and are affected by the processes of mass transport by erosional, crustal thickening, and regional subsidence

(Jordan and Flemings, 1991; DeCelles and Giles, 1996; Catuneanu, 2004). The evolution of a foreland basin is held in their building of the stratigraphic units, and the kinematic

structure of faulting, folding and the basement strata associated with foreland deformation processes (Conner and Harrison, 2003). Understanding previous researchers’ observations and remarks on Laramide, post-Laramide tectonic models and subsurface fracture studies will help in better understanding the fracture mechanisms of the Niobrara Formation within the Denver Basin.

Since the 1960s there has been a diverse amount of hypotheses attempting to explain fault strikes and fold trends of the Rocky Mountain foreland basin. The main school of thought is that horizontal compression is main source of deformation in the

Laramide Orogeny. Compelling modern geological, geophysical and kinematic data clearly indicates the dominance of horizontal slip and compression during the Laramide

Orogeny (Stone, 1984, 2005; Erslev, 1986, 1993, 2005; Holdaway, 1998; Erslev et al.,

2004; Erslev and Larson, 2006).

There has been much discussion on Laramide tectonics including the kinematics of the orogeny (Wise, 1963; Sales, 1968; Stone, 1969), possible rotation of the Colorado

Plateau (Gries, 1983; Karlstrom and Daniel, 1993) and plate tectonics to Laramide structural trends (Bird, 1988, 1998; Saleeby, 2003).

Allen (2009) mentioned that the Laramide arches trends vary greatly from N-S

(Front Range), NE-SW (Hartville Arch), NW-SE (Wind River Arch) to E-W (Unita, Owl

Creek arches) (Figure 3.1). Stone (1969) attributed the NW trending arches to being a product of compressive “wrench faults”. Stone’s (1969) interpretation and varying arch direction gave direction into the basis for the rotation hypotheses. Gries (1983) argued that the varying arches formed as a result of counterclockwise rotation of compressive stress, whereas Bird (1988, 1998) indicated that there was a 15º clockwise rotation of

125ft

Figure 3.1: Tectonic map of the Laramide foreland basins indicating the trends of major and minor Laramide arches. The blue arrows indicate average shortening directions from published and unpublished minor fault data collection (modified from Allen, 2009; Erslev and Koenig, 2009).

Laramide compression. The exact direction of rotation is still not understood but more recent studies have focused on fault data and its relation to ideal compression axes in the

Rocky Mountains (Gregson and Erslev, 1997; Holdaway, 1998; Ruf and Erslev, 2000;

Erslev, 2001; Erslev et al., 2004; Magnanai et al., 2005; Erslev and Larson, 2006). Erslev and Koenig (2009) performed a minor fault analysis throughout the Rocky Mountains and indicated an average slip of (067/01) and compression of (067/02), which indicates a unidirectional direction of shortening and compression. This coincides with the principle deformation processes behind the Laramide Orogeny.

Post-Laramide fracture mechanisms are commonly associated with the Rio

Grande Rift. Tweto (1980) described the rift as a continuous graben system that extends from New Mexico through the San Luis valley to Leadville, CO. Aldrich et al. (1986) observed NW-SE dikes in the southwestern U.S. and indicated that the least principal stress directions were perpendicular to the dike trends and thus parallel with regional extension trends and dual stages of extension. Kellog (1999) examined the close proximity of Neogene normal faults with Laramide thrust faults, and suggested that both fault types were genetically linked reactivations. McMillan et al. (2006) hypothesized that possible local extension is associated with the Rio Grande Rift projecting into the central Rockies.

Subsurface studies in the Denver Basin have produced hypotheses from data sets including geophysical well logs, 2D and 3D seismic surveys, and well-to-well communication maps. Weimer (1980) indicated that stratigraphic thicks and thins were associated with paleostructures which were active during the Cretaceous. The paleostructures were thought to have been controlled by recurrent basement fault-block

movement along the Transcontinental Arch (Weimer, 1978, 1980; Sonnenberg and

Weimer, 1981). Davis and Weimer (1976) concluded that normal faulting of the

Niobrara Formation was recurrent movement during the Laramide as a result of growth folding.

Prichett (1993) concluded from his detailed fault mapping study of the Sooner

Field 3D that potential fault mechanisms included regional basement wrench faulting and regional uplift coinciding with sedimentation. Weimer (1996) agreed with Prichett

(1993) that wrench fault zones in the Denver Basin explains primary NE-SW fault systems, with added secondary synthetic and antithetic faults compartmentalize

Cretaceous reservoirs. Longman et al. (1998) investigated the Niobrara formation across the Denver Basin using well log data and revealing results that support basement fault block movement during the Laramide Orogeny.

From more modern techniques, Birmingham (1998) was able to use Formation

Micro-Imaging (FMI) to indicate that there are sub-vertical and listric normal faults in the

Niobrara Formation and Codell Sandstone. Allen (2009) indicated that Noble Energy

Inc. in 2008, performed a borehole breakout study on Wattenberg Field and concluded that the lack of horizontal stress, the presence of N-S borehole breakouts and “frac into” directions adjacent to the Colorado Front Range suggest that Laramide compression ended and was replaced by de-pressuring during regional uplift and erosion of overburden, or by post-Laramide extension.

3.2 Mechanical and Fracture Stratigraphy

Studies on mechanical and fracture stratigraphy show that a link exists among facies, sedimentary cycles, diagenesis and fracturing. Understanding this link is

fundamental for characterizing fluid flow in natural fractured reservoirs, especially carbonate zones. The fracture density and intensity is strongly controlled by heterogeneities of rock properties between and within facies, which in turn are determined by sedimentary textures and, dominantly, by the combination of sedimentary and diagenetic facies (Naccio et al., 2005). The control played by the mechanical facies distribution across the stratigraphic succession may justify strong variations in the fracture density within the same sedimentary cycle. The mechanical stratigraphy of a given rock strata allows for interpretation of kinematic deformation processes. Cardozo et al. (2005) used kinematics and mechanical modeling of the Rip Van Winkle (SE New

York, USA) and La Zeta (SW Mendoza, Argentina) anticlines to illustrate the influence of mechanical stratigraphy and initial stress states on kinematic deformation of fault propagation folds.

Identification of mechanical and fracture stratigraphy leads to a clearer understanding of fracture patterns and more accurate prediction of fracture attributes away from the wellbore. Mechanical stratigraphy is the process of subdividing stratified rocks into discrete mechanical units defined by properties such as tensile strength, elastic stiffness, brittleness, and fracture mechanics. Fracture stratigraphy subdivides a rock into different fracture units according to some extent of intensity, density or some other measureable observed attribute (Lauback et al., 2009). Mechanical stratigraphy is the by-product of depositional composition and structure, and chemical and mechanical changes overlaid on rock composition, texture, and interfaces after deposition (Naccio et al., 2005; Lauback et al., 2009). Fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure. In a rock body the mechanical property

changes reflect diagenesis and fractures evolve with loading history, therefore mechanical stratigraphy and fracture stratigraphy do not always coincide with each other

(Lauback et al., 2009). Ideally a rock bodies’ mechanical properties agree with the measured fracture intensity and density of the given unit. Though, quantified observations of fracture properties are not always completed or their accuracy is low

(Lauback et al., 2009). Specifically in subsurface studies, current mechanical stratigraphy is generally measurable (XRD, XRF, SRA, RockEval, compression stress test analysis, outcrop & core descriptions), but because of inherent limitations of sampling, fracture stratigraphy is commonly incompletely known. Formation Micro

Imaging (FMI) works well for measuring fractures for a given well bore, but if the well misses the fracture swarm within the subsurface, then the fractures will never be encountered. Therefore a thorough fracture study of outcrop field locations can aid in putting together a model for understanding the natural fracture pattern for a given stratigraphic unit.

The Upper Cretaceous Niobrara Formation on the western edge of the Front

Range has low measured matrix porosity. The natural fracture conductivity is thought to be the pathway for hydrocarbon migration. Since fractured chalks have been known to be extremely prolific reservoirs (Corbett et. al. 1987) it is important to understand these natural fracture systems when possible. Previous work has been done in trying to comprehend the fracture systems within the Niobrara Formation (Weimer, 1980; Davis,

1985; Campbell et al., 1992; Vincellette and Foster, 1992, Erslev and Koenig, 2009,

Allen, 2009). Research was completed as to why the fracture networks exist within the formation, including Laramide deformation and Neogene extension, with less work

completed as to how the fractured networks aid in hydrocarbon production for the formation within such locations as the Greater Wattenberg Field.

Similar work was completed on fractured reservoirs from the time equivalent

Austin Chalk in central Texas, trying to dissolve its mechanical stratigraphy (Corbett et al., 1987; Wiltschko et al., 1991). Corbett separated the Austin Chalk into its own mechanical units based on counting fractures from the outcrop, cores, FMI data and deformed relative samples to delineate its mechanical behavior (Figure 3.2). The work that was completed on the Austin Chalk gave further insight into the separation of the mechanical units and how they play a role in the effective porosity of the reservoirs and hydrocarbon production.

Figure 3.2: Average fracture intensity (fractures/meter) by stratigraphic member for the Austin Chalk and the mechanical stratigraphy (modified from Corbett et al., 1987).

CHAPTER 4

METHODOLOGY

4.1 Field Work

This project incorporates multiple field excursions over a period of 3 months from

November 2011 through February 2012 in which data were collected from two field sites.

The field excursions focused on the data collection of the natural fractures patterns that exist within the outcrops. This includes the collecting and building of fracture maps, horizontal scan lines and measured sections, and sampling for bulk mineralogy, organic richness, and thermal maturity.

The study area contains two field sites that contain the Niobrara Formation along the west flank of the Colorado Front Range. A total of four fracture maps and 10 scanlines were generated from Boulder to Lyons, Colorado on the Niobrara bedding surfaces and side walls, within its given units. There were 265 fractures, 16 stylolites as well as 18 representative samples gathered throughout the units for source rock and mineralogy analysis, as well as noted observations of other structural features such as, fold hinges, and pencil cleavage planes. All structural orientations gathered from field work in this study will be reported using the right-hand-rule (RHR) (i.e. Strike/Dip

000/00, Plunge/Trend 00/000).

4.2 Measured Section and Sampling

The measured section was taken at CEMEX Limestone Quarry with a total thickness of 320ft. Due to the fact that the quarry is excavated down in tiers the wall faces are extremely steep to almost vertical; therefore, the physical section was measured from the top of the quarry to the lowest point in the pits (Codell Sandstone). The

measured section was described based on the varying units within the formation, and observations were noted on distinguishing features such and fracture swarms, calcite cementation, shell fragments, smell, color, texture, etc.

Within the measured section 18 samples were collected from each of the varying facies within the Niobrara Formation. There are nine units within the measured section and two samples were gathered at each. A pick-axe was used to expose strata along the wall faces so that the samples would not be affected by weathering. The rock samples were then extracted from the fresh wall faces for further lab analysis (i.e., XRD, SRA).

Each of the samples from the nine units were broken down to size and run through a source rock analyzer (SRA) located in the Department of Geology and Geological

Engineering at the Colorado School of Mines for results on thermal maturity and total organic content (TOC) and X-ray diffraction (XRD) at the Mineral Lab located in Golden

Colorado for results on bulk mineralogy.

The results from the measured section and samples taken from the CEMEX

Limestone Quarry are used to compare against the measured fracture spacing, intensity, and density and mean trace length to help better understand the mechanical stratigraphy of the Niobrara Formation.

4.3 Fracture Maps

A rope and chalk system was used to build circular mapping windows for each of the four fracture maps (Figure 4.1). Window sizes ranged from 6 feet to 10 feet depending upon the size of exposures. At each of the locations, bedding plane strike and dip were gathered as well as the orientation measurements for the major fractures, and their characteristics were noted (open closed, composition of fill, etc.) (Figure 4.2).

3 feet

Figure 4.1: Rope and chalk system used to aid in the creating a mapping window for fracture maps on bedding planes.

Circular windows were used to reduce sampling bias (Mauldon et al., 2001;

Rohbaugh et al., 2002) and are oriented such that north is facing up (Figure 4.2). The fracture intersections with the circle (n) and fracture endpoints within the circle (m) were calculated and used to determine fracture density, intensity and mean trace length (Figure

4.3). The intensity refers to the fracture length per unit areas and has units of m-1

(Equation 3.1), and density is the number of fractures per unit area and has units of m-2

(Equation 3.2). Mean trace length refers to the mean trace length for individual fractures in a sampling area (Equation 3.3). Where, (n) is the number of intersections with the circle, m is the number of endpoints within the circle, and (r) is the radius of the circle.

3 feet

Figure 4.2: Fracture map of the Fort Hays Limestone bedding surface at Six Mile Fold. The fracture map produces results on fracture attributes (i.e. intensity, density & mean trace length).

Fracture Intensity = n/4r (Equation 3.1)

Fracture Density = m/2πr2 (Equation 3.2)

Mean Trace Length = (πr/2)(n/m) (Equation 3.3)

The length of the radius of the circular scanlines has a significant impact on the accuracy of intensity, density and mean trace length (Rohrbaugh et al., 2002). To

accurately calculate these values, the radius of the circle needs to be greater than the average

(a) (b)

Figure 4.3: Example of a circular window with mapped fracture traces. (a) red dots are fracture trace intersection (n) with the circle. (b) Refers to the blue dots that are trace endpoints (m) within the circle (modified after Rohrbaugh et al. 2002 and Blake, 2009). fracture block size. A fracture block size is based on the number of fractures per unit area. A smaller circular window will over-estimate density and under-estimate mean trace length. Therefore it is pertinent to gather an understanding of the fracture spacing per area so that a proper radius is selected for the circular scanline. The calculation accuracy increases as the radius increases.

Each fracture within the fracture maps was given an orientation and length. Each of the endpoints for the fractures were recorded as either terminated against circle (C) terminated against a continuing fracture (F), or a natural termination (E).

The fracture maps are located on slightly inclined bedding planes, therefore the recorded orientation does not specifically represent the trend of the fracture trace. The measurements gathered are used to calculate the rake (angle between the horizontal and

any linear feature) of the trace fracture. The value gathered from the rake angle and the bedding plane strike and dip, is plugged into a spreadsheet to yield the trend and plunge of the fracture trace.

Figure 4.4: (a) Schematic diagram of conversion from fracture trace orientation to trend and plunge. (b) The highlighted green column indicates a conversion of the rake angle based on the strike and dip of the bedding plane in which the fracture was measured.

In Figure 4.4a an example is given where a fracture measurement is recorded on a given bedding plane that strikes 005˚ and dips 20˚. The right hand rule rake angle of

the fracture relative to the strike direction of the bedding plane is calculated to be 125˚.

The values are then plugged into the spreadsheet that coverts the rake according to the right-hand rule, if necessary, and calculated the trend and plunge of the fracture trace

(Figure 4.4b). With this calculation an assumption is made that the fractures are perpendicular to bedding. Through field observations of the outcrops this assumption is not always correct, though most of major fracture joint sets are perpendicular to bedding.

The orientation of the fracture trace as well as length was used to generate stereonets and rose diagrams in Dips 2010 version 5.108 software and graphs in Microsoft Excel 2010.

The results from this study will be used to better understand the mechanical stratigraphy and natural fracture pattern of the Niobrara Formation along the Colorado

Front Range.

4.4 Fracture Scanlines

Fracture scanlines are used for quantifying fracture spacing in cross section. A measure tape was used to measure the spacing between the fractures for the given units within the formation (Figure 4.5). The scanlines ranged from 30 to 150 feet in length on pending the size of the outcrop exposure. At each of the locations, bedding plane strike and dip were gathered as well as the orientation measurements were recorded for the major fractures, and their characteristics were noted (open closed, composition of fill, etc.) (Figure 4.6).

Measure tape was laid out so that it is parallel to bedding strike. This is to ensure that fracture spacing is measured perpendicular to bedding. The distance between the fractures (g) is averaged for the total count of the fractures in the scanline in order to

determine the average fracture spacing. Fracture spacing refers to the average spacing of fractures for a given cross sectional distance and has units of m/1 (Equation 3.4).

15ft

Figure 4.5: Location at Six Mile Fold, C bench of the Niobrara Formation. Outcrop photo of a fracture scanline in cross sectional view. Note that measure tape is parallel to bedding plane. Scan line used to measure the distance between each fracture for a given cross section, in order to produce results of average fracture spacing. Average Fracture Spacing = (Σg/t) (Equation 3.4)

Where, (g) refers to the distance between each fracture and (t) refers to the total number fractures for the given scanline.

The length of the scanline and wall face selection is important when trying to quantify fractures in cross section. The more fracture that are intercepted within the scanline the more accurate and precise the results will be. The distance of the scanline is

important because as stated above (section 4.4) the scanline must be large or long enough to encompass the fracture block spacing. The longer the scanlines the more likely hood of intercepting repetition of the fracture pattern in cross section.

Figure 4.6: Location at Six Mile Fold, C bench of Niobrara Formation. Outcrop photo of data collection of a fracture scanline, including fracture strike, dip and spacing. Each of the cross sectional scanlines were measured on two perpendicular wall faces if possible based on exposure for each of the given units within the formation

(Figure 4.7). This is to make sure that each joint set is intercepted. Measuring fractures on bedding plane brings out the varying joint sets therefore, perpendicular wall faces should be used when quantifying cross sectional scanlines. The orientation of the fracture trace as well as length was used to generate stereonets rose diagrams and fracture

spacing graphs in Dips 2010 version 5.108 software and graphs in Microsoft Excel 2010.

Fracture Map

Fracture Scan Line

1m 2m 3m 4m 5m 6m 7m

Figure 4.7: Schematic diagram indicating fracture scanlines in cross section and a fracture map. Fracture scanlines measured on perpendicular wall faces to intercept sister joint set. Drawing not to scale The results from this study will be used to understand the natural fracture pattern of the Niobrara Formation along the Colorado Front Range. The fracture scanlines where gathered in different members of the Niobrara Formation at multiple field locations, so that a comparison can be made between the natural fractures spacing and the lithology and mineralogy of the given unit in order to determine its mechanical stratigraphy.

4.5 Advantages and Biases of Field Techniques

The quantification of fracture parameters such as spacing, density, intensity, and

mean trace length aids in the assessment of hydrocarbon flow and storage in fractured reservoirs. Ideally the fractures are the hydrocarbon deliverability to the well bore and do not contribute to the total reservoir storage capacity. The relation to what is seen in the outcrop in terms of fracture spacing and orientation can only be projected into the subsurface based on relative values. This is because there exact deformation forces that are acting upon the Front Range do not coincide with the tectonic forces that are acting on the interior of the basin. Though it can be complicated in deciding what approach to incorporate in a reservoir model. The mechanical stratigraphy is important to the reservoir model but as Lauback et al., (2009) stated, understanding the fracture properties are usually lacking. The two models offered thus far; fracture maps and scanlines encompass these differing parameters to quantify fractures in outcrop.

Fracture maps are ideal for trapping the total fracture orientations because of their ability to limit sampling bias (Mauldon et al., 2001; Rohrbaugh et al., 2002). The fracture maps are also created on the bedding surface so they allow the field geologist to gain a better understanding of the paring joint sets. Fracture maps also increase in accuracy as sampling radius and counts increase. This method has advantages over the fracture scanlines, which provide rapid estimates of fracture intensity and spacing (Priest and Hudson, 1981; LaPointe and Hudson, 1985; Becker and Gross, 1996). The advantages are that straight scanlines are orientation bias, length bias, censoring, and pattern heterogeneity bias (Terzaghi, 1965; Baecher and Lannet, 1978; Priest and Hudson

1981; LaPointe and Hudson, 1985; Priest 1993; Mauldon and Mauldon, 1997). Fracture spacing is an important parameter to industry when it comes to building reservoir models.

The advantages to outcrop fracture spacing per given unit of length is that it can be

compared to subsurface horizontal image logs fracture spacing. The orientation bias is very important in fracture scanlines, because if the field geologist does not have an understanding of all fracture sets in the outcrop then they may be missed all together in the data collections (Figure 4.7). That is why it is important to find cross sectional outcrops that intercept all joint sets within the given unit.

As well as quantifying fractures in outcrop the measured values from stylolites and compressional features will aid in interpretations of the paleo-stress that caused the deformation. All data collected will be compared against each other and to the natural geology, to try and understand how tectonic setting, lithology and mineralogy are controlling natural fracture pattern of the Niobrara Formation.

CHAPTER 5

DETAILED FRACTURE ANALYSIS

Fracture data that were gathered from the field were analyzed to determine the direction of shortening along the Colorado Front Range, estimate the regional Laramide stresses, and identify regional joints and calcite filled fracture preferred orientation.

Summary of the data is presented as stereonets and roseplots for all, stylolites and joints to help in the understanding fracture mechanisms affecting the Niobrara Formation at surface outcrop, and are a useful resource with comparison to subsurface fractures sets.

5.1 Stylolite Data Analysis

The idea behind a stylolite is that it is a pressure-solution feature that occurs due to compression (Davis and Reynolds, 1984). Ideally it is process of dissolution creep in which there is selective removal, transport and reprecipitation of material through fluid films along grain boundaries or pore fluids between grains (Figure 5.1). Material is removed through dissolution and migrates from high to low stress locations. Most features are at high angles to the greatest principle compressive stress (σ1). This indicates that the strike direction of the solution feature is perpendicular to σ1 during its formation

(Du Rochet, 1984) (Figure 5.2). Ideally highly soluble minerals will dissolve preferentially to those with lower solubilities. Impure carbonates are a great example of a rock body that can be susceptible to dissolution creep. Calcite typically dissolves more readily than quartz, clays and iron-manganese oxides (Andrews and Railsbak, 1997).

Therefore a carbonate rich unit like the Niobrara Formation would be susceptible to dissolution creep while buried at depth in the subsurface, which could be an indication of principle compressive stress.

2) Material diffuses toward sites with σ 1 lower concentration

1) Material σ3 σ dissolves, creating 3 high concentration fluid

3) Material deposited σ1 in pressure shadow (window)

Figure 5.1: The mechanisms behind concentration gradients during dissolution creep. Material is dissolved from surfaces with high resolved compressive stress and diffuses to sites of lower compressive stress, as in a pressure shadow or window (modified from Davis and Reynolds, 1984).

σ1

(b) σ (a) 2

σ σ 3 3 σ 1

σ1

σ 1 σ σ 3 3

(c)

σ 1

Pressure dissolution stylolites (n=16) (Appendix A) were collected from the Six

Mile Fold and CEMEX Limestone Quarry, in the Fort Hays Limestone and overlying

Smoky Hill member of the Niobrara Formation (Figure 5.3). The vertical stylolites consistently have an N-S strike, with a steep dip to the west and have an average attitude of 167/79 (Figure 5.4).

The bedding oblique stylolites can be used as a possible indicator for horizontal shortening with axes normal to the plane of stylolite formation (Twiss and Moore, 1993).

From the data collected the average stylolitic pole to plane is 11/077, which has a close match with the Laramide shortening direction from the average joint strike attitudes

(248/76).

(a) (b)

Figure 5.3: (a) Equilateral stereonet plot of the stylolitic planes, (b) rose diagram of stylolitic planes, (c) contoured plot (CI 3%) of poles to stylolitic planes, and (d) color key to separated contoured intervals.

σ1 (a)

σ1 (b)

Figure 5.4: (a) Photo of a vertical stylolite cross cutting an open extensional fracture with marked σ1 and σ3 directions. (b) Cross section view through a vertical stylolite in relation to a calcite filled fracture. Samples gathered from Fort Hays Limestone in the CEMEX Limestone Quarry Lyons, Colorado.

5.2 Joint Data Analysis

From the principle data collected thus far, there is one major joint set that exist within the Niobrara Formation at the field locations. This occurs in both a compressional feature at Six Mile Fold and in a slightly deformed feature of the CEMEX Limestone quarry. 265 joint attitudes were collected from the various members of the Niobrara

Formation between Six Mile Fold and the CEMEX Limestone Quarry Boulder Colorado

(Figure 5.5). The data collected for joint analysis is a combination of the fractures measured in the scanlines fracture maps and other observed joints. The presence of a terminating relationship indicates an ENE to E-W systematic J1 joint set and a secondary

NNW to NS J2 joint set. The J1 joints were mainly calcite cemented with a steep dip angle and an average attitude of 248/76, which paralleled the local right-lateral and left- lateral shear fractures. The J1 joints bisected the conjugate shear fractures at most outcrop locations. The initial observations were not clear as to whether the J1 joints sets were a strike-slip fault with no identifiable slickenlines. The shear fractures that did not have any identifiable slip could be easily misinterpreted if they were reactivated by regional extension.

J2 joints were commonly seen terminating into J1 joints (Figure 5.6). The J2 joints paralleled bedding strike and dip with an average attitude of 162/75. Most of the J2 joints were perpendicular to J1 joints and were not calcite cemented which is an indication that they are not of the same deformation forces that created the J1 joints (Figure 5.5).

5.3 Niobrara Formation Rock Mechanics

The interpretation behind the mechanics of the deformation process that created the J1 and J2 joints are thought to be similar with the difference being that the J1 joints

Figure 5.5: Joint data that were collected from Six Mile Fold and CEMEX Limestone Quarry. (a) Equilateral stereonet plot of the J1 and J2 planes, (b) Equilateral stereonet plot of the J1 and J2 poles, (c) rose diagram of J1 and J2 planes, (d) contoured plot (CI 3%) of poles to planes of the J1 and J2 joints. formed from increased pressures within the subsurface, whereas the J2 joints are evident by de-pressuring from unloading. The J1 joints attitude is comparable to the Laramide stress orientation and formed from compressive stress with the combination of fluid expulsion. Secor’s (1965) model on fluid pressure explains the variation of pressure release of a rock body on varying scenarios of differential stress. Increasing pore pressure on a rock body increases the forces of expulsion which act differentially towards the confining forces of the rock. This is the same principle that is occurring when a rock

J2 J1

1ft

Figure 5.6: Outcrop photo of C Chalk bench at the CEMEX Limestone Quarry depicting the J2 joint set terminating into the systematic J1 joint set. body is uplifted and brought to the surface. The confining pressure of the rock body is decreasing do to unloading. Modern hydraulic fracking is another example of driving a rock body to lowest pressure by introducing a confining fluid. If you increase the pore pressure of the rock body by introducing frack fluid then your respective values for σ1 and σ3 would each be reduced by the value of the fluid pressure driving the stress circle to the left toward the field of tensional failure decreasing the pressure on the rock body causing a fracture to occur. The goal is to drive the rock body to the lowest principle stress in order to make it easier to achieve failure, or the creation of a fracture. Therefore an interpretation is made that the J1 joint set propagated parallel to Laramide stress with the aided help of fluid expulsion from hydrocarbon maturation.

The mechanics behinds the uplift of the Front Range and the J2 joint set is interpreted to be caused the variation of the fracture pattern to occur. The lithostatic load

of the overlying formations was removed and de-pressuring occurs. What is happening is the principle stress which is vertical or lithostatic load is decreasing as uplift is occurring.

The point at which principle stress changes from vertical to horizontal is when tensile failure can occur. In the terms of a Mohr Column circle, the idea is to drive the circle to the left until it reaches the tensile failure envelope (Figure 5.7a). The joint sets are fairly orthogonal to each other, which indicate that differential stress between σ1 and σ3 is small enough that the stress circle can be driven into collision with the tensile failure envelope.

This means that θ (the angle at which failure occurs in relation to principle confining stress) will be approximately 90˚, or perpendicular to principle confining stress. This indicates that σ1 and σ3 where fairly equal in value in horizontal direction and σ2 is in the vertical direction. If the differential stress is higher the stress circle will intercept the parabolic failure envelope and transitional tensile failure will occur and the joint set will have bifurcating angles that are fairly shallow (Figure 5.7b). If the differential stress is even larger then it requires less de-pressuring of lithostatic load and rock failure can occur at greater depths and will develop shear fractures that are roughly 45˚ to principle confining stress within the subsurface (Figure 5.7c). If σ1 is steadily increased or an increasing confining pressure then the differential stress will be greater and rock failure can occur with little to no decrease in confining pressure (Figure 5.7d). Therefore the J2 joint set occurred during uplift allowing for low differential stress creating tensile mode I failure (Figure 5.7a)

5.4 Conclusions from Fracture Analyses

Ideally the solution stylolites that occurred in the outcrops happened at burial depth. There is not enough confining pressure to develop the stylolites at the surface.

Figure 5.7: Secor’s (1965) model of the role of fluid pressure in jointing. (a) A low differential stress load existing between σ1 and σ3. Unloading through pressure release allows for the stress to pass all the way to the tensile failure envelope, and the rock breaks by mode I failure. (b) In this case differential stress is greater between σ1 and σ3 and when unloading occurs the stress circle collides with the parabolic window and transitional tensile failure occurs. (c) Differential stress is yet even greater and the stress circle does not make it to the tensile failure envelope and the failure occurs in a preferred direction in relation to principle confining pressure. (d) Differential stress is greater through the increase of principle confining pressure, which indicates that rock failure will occur before then tensile envelope is reached. (Pc = principle confining stress, θ = angle between rock failure and σ1, DS = differential stress, σ1* = greatest principal effective stress, σ3* = least principal effective stress) (modified from Secor 1965).

Therefore the Niobrara Formation was buried at depth when the stylolites would have formed. The stylolites formed perpendicular to minor faults and J1 joints indicating the principle horizontal stress that resulted in their formation is the Laramide shortening

event (σ1). Normal fault data were highly variable with no real evidence of slip except for slight separation of calcite filled joints.

The structural events that caused the rock failure within the Niobrara Formation are based on Laramide compression (J1) and un-roofing events occurring during uplift

(J2). Abutting relationships show that the older systematic J1 joints have an average orientation of 250/76, whereas the younger orthogonal non-systematic J2 joint set had an average orientation of 162/75. J1 joint strikes are slightly deviated from the average

Laramide σ1 trends which indicate that the differential stress had to be fairly low during rock failure. The presence of the NNW striking J2 joints does not fit with the Laramide shortening directions and indicates a post-Laramide tectonic development, likely do to de-pressuring from un-roofing.

5.5 Researcher Comparison

Eric Erslev and his research students at CSU have performed similar research in trying to determine the Laramide shortening directions and fractures analysis along the

Colorado Front Range. In looking closer at the stylolite orientation Allen (2010) measured stylolites along the Colorado Front Range and determined an average attitude of 174/74 which he concluded that the J1 joint set crosscuts vertical stylolites. Weimer

(1996) came to the same conclusion on stylolite direction 175˚ measured at Six Mile

Fold, Boulder Colorado. For this research both cross-cutting relationship where observed at outcrop and in hand sample.

Erslev and Larson (2006) built Laramide models along the Front Range in which they measured 7,837 slickensided minor faults and concluded that there is a predominance of horizontal shortening during the Laramide Orogeny. Their calculations

concluded that an average slip and maximum compression axes have average trends of

077-267. Later in Erslev and Koenig, (2009) they collected minor fault data (n = 21,129) from a variety of pre-Laramide units to calculate an average slip 01/067, and maximum compressive stress of 02/067. Allen (2010) backed this up further with a calculated average orientation of Laramide fault slip direction to be 14/086 based on a best fit α angles (+/- 1˚) using Orient (Vollmer, 1992) and SELECT (Erslev, 1998). This conclusion on Laramide shortening direction is further supported with Holdaway’s

(1998) shortening direction of 11/079 and Weimer’s (1996) work along the Front Range in which he concluded a principle horizontal stress of 065 (Figure 5.8). The results on

Laramide shortening directions gathered from this study compare well with the previous work competed. In comparison to the offsetting nonsystematic J2 joint set, Allen (2010) came to the same conclusion that unroofing and depressuring allowed for extensional fractures to form perpendicular to the systematic J1 joint sets. These results work within the rock mechanics model allowing for extensional mode I failure (Figure 5.7).

In comparison to the subsurface fracturing is likely at different spacing. The J1 joint set is most likely to be encountered within the subsurface. This is based on the evidence of Laramide shortening (σ1) and vertical stylolite cross-cutting relationships.

Ideally subsurface analysis will reveal that there are multiple episodes fracturing that are going to occur. From subsurface analysis Allen (2010) indicated that the J2 joint set strikes are consistent with “frac into” orientations and borehole breakouts observed on the west side of the Denver Basin by Birmingham (2001) and Richter (B. Richter, personal communication, 2009). Noble Energy Inc. stress strain analysis in the Wattenberg Field and Denver Basin from 2008 to 2009 indicates a varied arrangement of SHmax directions.

Figure 5.8: Map sketch of the principle horizontal stress field orientations gathered from vertical stylolites and faults and measured fractures. Data collected from the Fort Hays Limestone, Six Mile Fold Boulder Colorado (modified from Weimer, 1996). This is due to the stresses of post-Laramide regional extension in the direction of

Laramide compression. The fracture investigation by Noble Energy Inc. using FMI north of Wattenberg Field indicated multiple fracture orientations with varying cross- cutting relationships. The variation in fracture strikes and the relative timing suggest multiple mechanisms are present in the subsurface and at outcrop. 3D seismic surveys in the Denver Basin show complex networks of faulting throughout the upper Cretaceous strata.

In a mechanical aspect an interpretation is made that the fracture density and intensity will vary between the center and margins of the basins. In specific to the

Denver Basin the fracture density and intensity will be higher at the west margin than at

basin center and east margin. This is interpreted to be the effects of the deformation from the Laramide Orogeny along the Front Range and western basin margin. In comparing surface fractures gathered at outcrop to fractures buried at depth in the subsurface in a potentially overpressured situation would have to be relative to location. The fractures that are at the surface do not have the constraints of lithostatic load acting upon them therefore their fracture mechanics would not be the same as in an overpressured situation.

The variation in fracture spacing between the different units in comparison to mineralogy is relative to the subsurface. The average fracture spacing density and intensity within the marl units will likely be higher than in the chalk units. This is due to the ductility of the rock which is based upon its mineralogy and elasticity. Therefore a good understanding of the lithology and bulk mineralogy is important for targeting the relatively more fractured zones within the formation. Determining the more fractured units will aid in the hydrocarbon deliverability to the well bore. With the advance in stimulation and treatment techniques to the well bore to help increase effective permeability, the interception of the natural fractures is going to be important to deliverability.

CHAPTER 6

MECHANICAL STRATIGRAPHY ANALYSIS

The fracture data that were collected from the field sights were analyzed and interpreted in order to determine the mechanical and fracture stratigraphy of the

Cretaceous Niobrara Formation. Summary of the data as fracture maps, scanlines, stereonets and roseplots for all fracture data along bedding planes and cross section wall faces in order to determine the fracture spacing, intensity, density, and mean trace length.

These results will help in the understanding of fracture mechanics that are affecting the

Niobrara Formation at surface outcrops, and can be useful information in building a reservoir model for subsurface fractured reservoir compartments.

6.1 Measured Section

From the principle data collected thus far a measured section was created for the

Niobrara Formation (Figure 6.1). The measured section for this study is based on the gathering data from the CEMEX Limestone Quarry. From the results thus far there are multiple chalk and marl units within the formation.

Starting from the base moving up stratigraphically is the Fort Hays Limestone member. The Fort Hays Unit is approximately 16ft thick, light grey, brittle limestone unit (Figure 6.2_F1). It is severely fractured with evidence of orthogonal joint sets.

Within the unit at the quarry there is vertical solution stylolites, bioturbation (shell fragments), bedding and nodular pyrite. The J1 joint set (248/76) was noted to be calcite cemented within the Fort Hays Limestone. The overlying C-Marl Unit is approximately

148ft thick light grey calcareous marlstone. Within the unit are varying one to three foot thick chalk stringers with one thick 12ft chalk unit that are inter-bedded with marlstones

(Figure 6.2_F2, F3). The unit has evidence of fracture swarms that are calcite cemented, scattered shell fragments, small one to two inch ash beds and both bedding and nodular pyrite. Above the C-Marl Unit is the 16ft thick calcareous C-Chalk Unit (Figure 6.2_F4).

The unit is a grey calcareous chalk that is highly fractured (cemented) with evidence of organic rich marlstone stringers, vertical stylolites, bioturbation, bedding and nodular pyrite. The C-Chalk Unit forms a sharp contact with the overlying B-Marl Unit. The overlying B-Marl Unit is approximately 82ft thick marlstone. It is a fissile organic rich unit that has evidence of carbonate stringers, little bioturbation, bedding and nodular pyrite. The fractures found within the unit look to have conductivity between each other due to water seepage. Large scale shell fragments were recovered from unit approximately 25 inch width within the unit. Overlying unit is the 16ft B-Chalk Unit

(Figure 6.2_F5). The unit is a gray to light black argillaceous chalk that has cemented joint fractures, thin marlstone stringers and minor fossil fragments. The A-Marl Unit is a

30ft organic rich dark gray to black marlstone. The marlstone is fissile along bedding planes, with calcite lined fractures, bedding and nodular pyrite. The Upper A-Marl Unit is a dark gray to black organic rich marlstone. There is evidence of fracture swarms that are calcite cemented, minor faulting, and a carbonate rich stringer at the top of the section

(Figure 6.2_F6). Above the A-Marl Unit is the A-Chalk Unit which is approximately

10ft thick light gray to beige chalk that has evidence of small organic stringers bedding lined pyrite and through going fractures in a fracture swarm (Figure 6.2_F6). The measured section is summarized as a 320ft thick unit that has slightly dipping flanking beds of marl and chalks of varying lithology that are stacked upon each other (Figure

6.3).

Figure 6.1: Measured section of the Niobrara Formation at the CEMEX Limestone Quarry with interpreted sedimentary structures. Section measured from surface down to bottom of quarry based on available exposures.

30ft F1 5ft F2

5ft 10ft F3 F4

5ft F5 15ft F6

Figure 6.2: Outcrop photos for measured section of varying units within the Niobrara Formation at the CEMEX Limestone Quarry. (F1) Fort Hays Limestone, (F2) C-Marl Unit with carbonate stringers, (F3) C-Marl Unit with chalk stringers, (F4) C-Chalk Unit with sharp over lying contact, (F5) B-Chalk Unit, (F6) A-Marl Unit and A-Chalk with fracture swarming.

(a)

15ft

(b)

25ft

(c)

15ft

Figure 6.3: Set of outcrop photos of the Niobrara Formation at the CEMEX Limestone Quarry, Lyons Colorado. (a) Side wall view of upper section to top of quarry. (b) Middle section indicating easterly dipping strata. (c) Lower section of formation including chalk marl stacked sequence, Fort Hays Limestone, and Codell Sandstone. 54

6.2 Bulk Mineralogy

The data sampling from outcrop was used to develop a mineralogical geocolumn log constructed through the measured section to help determine the bulk mineralogy of the Niobrara Formation in comparison to the described lithology (Figure 6.4) (Appendix

B). The names of the given units within the measured section were gathered from a classification scheme adopted by the CSM Niobrara Consortium (Figure 6.5). The issue with this classification scheme with the samples collected from XRD analysis is that it does not take into consideration quartz with its classification. Therefore the quartz and clay percentages were combined to sum a total of the discharge sediment that is being transported off the orogenic belt. That was supplemented into the classification scheme for the “clay” percentages to keep naming consistent.

The amount of calcite that is found throughout the measured section is fairly consistent accept in the chalk benches, it can be seen in the A, B, C units and the Fort

Hays Limestone. The amount of calcite in the chalk benches ranges from 64% in the lower C-Chalk Unit up to 95% in the Fort Hays Limestone. Whereas in the marlstone the calcite percentages range from 29% in the A-Marl Unit to 58% in the C-Marl Unit. The larger percentage of calcite found in the C-Marl could be due to the carbonate stringers found within the unit (Figure 6.2_F2). The amount of quartz and clay found within the units are as consistent as calcite except posing the opposites trends in the chalks compared to the marls (Figure 6.4). Quartz ranges from 5% in the Fort Hays Limestone to 15% in the Lower C-Chalk Units, whereas it ranges from 16% in the C-Marl Unit to

30% in the A-Marl Unit. This consistency can also be seen in the amount of clay within the measured section. The clay content ranges from 3% in the Upper C-Chalk Unit to

10% in the A-Chalk. In the marls the clay content ranges from 16% in the C-Marl Unit to 30% in the A-Marl. The remaining percentages of the measured section consist of pyrite, plagioclase and other unidentified minerals that could not be detected by the XRD machine.

The interpretation of the bulk mineralogy within the measured section is comparable to the described lithology and previous works in depositional environment.

The amount of calcite vs. clay and quartz are an inverse relationship within the given units. This is because the amount of carbonate that is being produced in the WIC seaway was raining out consistently and it is a fluctuation of siliciclastics and clays that controls the variation in lithology. The more siliciclastics and clays brought into the system will choke out the carbonates and deposit marlstone. Whereas when there is little input of siliciclastics and clays into the system from the orogenic belt then the carbonate can dominate the lithology depositing chalks. There is a fairly high amount of clays and siliciclastics found throughout the section. This is likely due to the close proximity to the orogenic belt. The closer to the orogenic belt the greater amount of sediment received during uplift and erosion. A possible explanation to the amount of pyrite found in the rock could be due to the production of sulfur from the source rocks bonding with free iron in solution thus forming bedding and nodular pyrite (Figure 6.6). The plagioclase that is found within the measured section is likely unweathered clays minerals. Again this is due to the close proximity to the orogenic belt, making it so there was not enough time for the plagioclase to weather into clay. Overall the bulk mineralogy is consistent with the depositional model and described lithologies by previous researchers.

Figure 6.4: A constructed mineralogical geocolumn log through the Niobrara Formation at the CEMEX Limestone Quarry. Geocolumn is compared to the described lithology and interpreted sedimentary structures of the measured section.

Figure 6.5: A schematic developed and modified by the Niobrara Consortium for classifying cretaceous marine stratigraphy of the Niobrara Formation. 6.3 Thermal Maturity

The data that was collected thus far indicates that there are multiple source rock units within the Niobrara Formation (Appendix C). The samples indicate that the kerogen quality is of TYPE II – TYPE III oil-gas-prone with multiple samples in the organic lean zone (Figure 6.7). Interpretations as to why the kerogen quality is TYPE

II – TYPE III oil-gas-prone are because they are mature “cooked” source rocks.

Therefore the actual source rock quality is likely a TYPE II oil prone source rock. The modified Van Krevelen plot also suggests its kerogen itself is TYPE II, with fairly low hydrogen indexes (Figure 6.8). Based on the hydrogen index and the Tmax from the S2 peak the source rocks of the Niobrara Formation at the CEMEX Limestone Quarry are

(a)

1 inch

(b)

1 inch

Figure 6.6: Pyrite found throughout the chalk and marls of the Niobrara Formation. (a) Bedding lined pyrite. (b) Nodular pyrite. Pictures taken within the C-Chalk Units of the Niobrara Formation at the CEMEX Limestone Quarry.

thermally mature in peak to post peak oil generation (Figure 6.9). The TOC wt. % also correlates with the chalk-marl sequence distribution described in the measured sections and the bulk mineralogy from the XRD analysis (Figure 6.10). The TOC wt. % in the chalks range from as low as 0.42 wt. % in the Fort Hays Limestone to 3.79 wt. % in the lower C-Chalk Unit. Whereas in the marlstone the TOC wt. % ranges from 2.2 wt. % in the lower B-Marl Unit to 5.56 wt.% in the lower C-Marl Unit. The slight jump in TOC wt. % in the C-Chalk Unit is likely due to some of the marlstone stringers being organic rich. Overall the C-Marl Unit is the most organic rich with the overall highest ranges of

TOC wt. %.

The Interpretation of the thermal maturity is that the source rocks and the reservoir rocks of the formation correlate with the chalk-marl sequences. Overall the systems marlstone source rocks are thermally mature TYPE II kerogens that are in the peak to post peak oil generation window and are charging the reservoirs of the chalk units. A conclusion can be drawn that the thermal maturity and source TOC wt. % have a direct relationship with clay content and an inverse relationship with the amount of calcite present within a sample. The higher percentage of calcite found within a rock sample the lower percentage of TOC wt. %. Exact geologic reasons as to why the organics of the Niobrara Formation are stored within the marlstones are not exactly understood. Possible interpretations are that the siliciclastics are coming off the orogenic belt and creating an anoxic environment in which organic material is being preserved.

Another interpretation is that the siliciclastics that are coming off the orogenic belt are actually choking out the carbonate that is raining out in the water column and trapping a higher concentration of organics upon deposition.

TYPE I TYPE II oil-prone oil-prone 16 usually usually

lacustrine marine

12 Mixed TYPE II -III oil-gas-prone

Organic Lean TYPE III

6 gas-prone REMAINING HYDROCARBON POTENTIAL (S2, mg HC/g rock) HC/g mg (S2, POTENTIAL HYDROCARBON REMAINING

2 TYPE IV inert

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

TOTAL ORGANIC CARBON (TOC, wt.%)

1000

900 l

TYPE I

800

700 II

600 TYPE II

500

400 HYDROGEN INDEX (HI, mg HC/gmg HYDROGEN (HI, INDEX TOC) 300

200 III TYPE III

100 IV TYPE IV

0 0 10 20 30 40 50 60 70 80 90 100

OXYGEN INDEX (OI, mg CO2/g TOC)

Figure 6.8: Modified Van Krevelen diagram indicating that the samples are TYPE II kerogens except for the outliers which are the carbonates that have high oxygen indexes.

1000 Immature Mature Postmature

TYPE I oil-prone 900 usually lacustrine Oil

Window Condensate Condensate

800

Wet Wet Zone Gas

700

TYPE II oil-prone usually marine 600

500

400 TYPE II-III

oil-gas-prone HYDROGEN HYDROGEN INDEX ( mg HI, HC/g TOC) 300

200 TYPE III gas-prone

Dry Gas Window

100

TYPE IV inert 0 400 425 450 475 500 Tmax (oC)

Figure 6.9: Thermal maturity of the source rocks within the measured section of the Niobrara Formation, CEMEX Limestone Quarry, Lyons Colorado.

Figure 6.10: A TOC wt. % log through the Niobrara Formation at the CEMEX Limestone Quarry Lyons, Colorado. TOC wt. % log is compared to the described lithology and the bulk mineralogy.

6.4 Fracture Maps

Four fracture maps were gathered at the Six Mile Fold field area (Figure 6.11,

6.12) (Appendix D). The fractures maps were gathered from bedding surfaces of the Fort

Hays Limestone. The results of the fracture maps indicate that there are three joint sets within the member (Figure 6.12, 6.13).

200ft

Figure 6.11: Locator map for fracture maps at Six Mile Fold field site location.

(1) (2)

3 Feet

(3) (4)

3 Feet 3 Feet

Figure 6.12: Fracture Maps gathered from the Fort Hays Limestone bedding surface at the Six Mile Fold field site location. Key: Green – J1 joint set, Purple – J2 Joint set, Orange – possible J3 joint set.

The green fractures set gathered from the fracture maps are consistent with J1 joints measured both at Six Mile Fold and the CEMEX Limestone Quarry. The purple fractures striking north south are consistent with other measured J2 joints at different outcrop locations. There is a third joint set (orange) intercepted within all four fracture maps. This third set was not encountered within the fracture scanlines at the

(1) (2)

(3) (4)

Figure 6.13: Stereonet plots of the plunge and trend of the four fracture maps taken at Six Mile Fold Boulder, Colorado.

CEMEX Limestone Quarry. The third joint set may exist within the other units throughout the formation but they were not intercepted as frequently within the scanlines.

This is largely due to the outcrop bias and availability of exposure.

In looking at the results of the fractures maps there can be noted variation amongst results based on given location of outcrop (Table 6.1). The results indicate that

intensity and density tend to trend inversely with mean trace length. This makes sense in that if the intensity and density decrease within a given fracture map then the overall trace length will increase (Table 6.1). Reasons as to why there is variations amongst the maps within the same unit, is likely due to levels of deformation based on compressional stress. Six Mile Fold is a set of megascopic low angle south plunging, open and closed anticline-syncline folds (Figure 6.14, 6.15). Due to the fact that it is a compressional system indicates that there are going to be varying degrees of deformation. From interpretation of the geology the fractures maps are located along exposures that are tapering off of a fold limb of the syncline and transitioning into the fold nose of the anticline. Fracture intensity and density increases for a given rock body when in closer proximity to the fold hinge and will decrease the further away from source. Therefore the higher fracture intensity and density found in fracture map one compared to fracture map four is due to the transition in and out of a fold limb.

Table 6.1: Results of calculations for intensity, density and mean trace length for the four fracture maps gathered at Six Mile Fold Boulder, Colorado. (Units – feet)

Variables Map 1 Map 2 Map 3 MAP 4 Intensity(1/ft) 2.000 2.000 1.300 1.200 Density(1/ft2) 1.024 0.794 0.384 0.358 Mean Trace Length(L/ft2) 3.906 5.040 6.771 6.696

From these results of the fracture maps a conclusion can be drawn that fracture intensity density and mean trace length of the Fort Hays Limestone are partially controlled by the severity and proximity to deformation. Conclusions would have been drawn on the other units of the Smoky Hill member of the Niobrara Formation at the given field sites if exposure would have allowed it.

Figure 6.14: Geologic map of Six Mile Fold with interpreted contacts between units and location of cross section.

Figure 6.15: Cross section through Six Mile Fold with interpreted fault slip.

6.5 Fracture Scanlines

Ten scanlines were created to determine the fracture spacing for each of the given units (Appendix E). The two field sites Six Mile Fold and CEMEX Limestone Quarry have variable fractures spacing (Figure 6.16, Table 6.2). At Six Mile Fold the average fracture spacing is 1.19ft with a standard deviation of 0.50ft and at the CEMEX

Limestone Quarry the average fracture spacing is 5.7ft with a standard deviation of 2.8ft

(Table 6.2). There are multiple explanations as to why the standard deviation is high for the average fracture spacing; the bias of wall face selection, total distance of scanline, and what is being counted as fracture. As stated in chapter 4.6 the bias of where you select your wall face is going to have a large affect on the fracture spacing, which determines what fracture orientations are going to be measured. The greater the distance the scanline spans the higher the accuracy of the average fracture spacing. These factors are aiding in the control of the standard deviation on the average fracture spacing.

Variation in the fracture spacing from the two field sites is largely due to the severity of deformation. As stated in the previous section (Chapter 6.4) Six Mile Fold is

(a)

(b )

Figure 6.16: Spacing difference plots with fracture number on the y-axis ( fracture sample count within the scanline) and fracture spacing on the x-axis (distance between each fracture). (a) Plot for the B-Chalk unit adjacent to fold hinge at Six Mile Fold, Boulder Colorado. (b) Plot for the B-Chalk unit at CEMEX Limestone Quarry Lyons, Colorado.

Table 6.2: Average fracture spacing for scanlines taken at both field locations with same orientation including calculated standard deviation

a compressional anticline-synclinal fold system, in which the fracture maps indicated that intensity and density decreased the farther away from the fold hinge. The CEMEX

Limestone quarry is located on the dipping limb of the Rabbit Mountain Anticline, which is a basement cored monoclonal fold (Figure 6.17). Essentially it is a pop up block that is pushed up into the overlying sedimentary rocks, forcing them to fold over the edges of the Precambrian blocks. The shape of the block determined the shape of the folds in the overlying sedimentary strata. The CEMEX Limestone Quarry located along the planar, south west dipping limb is likely being under less compressional stress than the fold hinge at Six Mile Fold. Therefore a conclusion can be drawn that for the Niobrara

Formation the closer to the zone of deformation the lower the average fracture spacing.

The remaining fracture scanlines were gathered from the CEMEX Limestone

Quarry, to help determine the variation of fracture spacing for given units within the formation. The results of the scanlines taken at the CEMEX Limestone Quarry indicate that there is a variation amongst fracture spacing for the given units of the Niobrara

Formation (Figure 6.18).

The average fracture spacing is fairly consistent throughout the measured section except within the chalk benches and Fort Hays Limestone. The average fracture spacing within the benches ranges from 5.7ft in the A-Chalk Unit to 4.70ft in the basal Fort Hays 72

Figure 6.17: Rabbit Mountain Anticline and the location of CEMEX Limestone Quarry. (a) Geologic map of the Rabbit Mountain Anticline. (b) Geologic cross-section through ideal location of CEMEX Limestone Quarry and Rabbit Mountain Anticline. (c) Picture of Rabbit Mountain showing steep east limb and planar southwest dipping limb (modified from Matthews et al., 1975).

Figure 6.18: A constructed average fracture spacing log through the Niobrara Formation at the CEMEX Limestone Quarry. Fracture spacing log is compared to the described lithology and interpreted sedimentary structures of the measured section and calculated TOC wt. %.

Limestone. Whereas in the marlstone the average fracture spacing ranges from 9.6ft in the lower C-Marl Unit to as low as 6.0ft in the B-Marl Unit. For one of the fracture scanlines collected in the lower C-Chalk – C-Marl units there was an average fracture spacing of 3.3ft. This is a closer fracture spacing than any of the other units which is interpreted as a fracture swarm (Figure 6.19). The fracture swarm is a high frequency of fracture within a given rock strata in comparison to the average fracture spacing for that unit. The little change in fracture spacing found within the C-Chalk Unit and the surrounding marlstone, is due to the bias in wall face selection and the availability of exposure.

Figure 6.19: Fracture Swarm found within the C-Marl Unit of the Niobrara Formation at the CEMEX Limestone Quarry Lyons, Colorado. The interpretation of the average fracture spacing within the measured section is comparable to the described lithology, bulk mineralogy, thermal maturity and previous work completed on fracture studies of the Niobrara Formation and other equivalent units.

The average fracture spacing has an inverse relationship with the amount of calcite found

within the bulk mineralogy and a direct relationship with the amount of clay and quartz.

The higher amount of calcite found within a given sample of rock the lower the ductility and stress needed to cause failure and develop a fracture. The same principle holds true with the marls. An increase in the clay content within the rock sample, the higher amount of elasticity and ductility it will have which, in turn requires a higher stress in order to achieve failure. Therefore the Fort Hays Limestone and overlying chalk units are more brittle or less ductile than the interfingering marl units. The average fracture spacing throughout the measured section also has a direct relationship with TOC Wt. % (Figure

6.18). The higher TOC Wt. % in the rock sample the larger the fracture spacing. This is due to the fact that the source rocks are related to the marls which have the greatest percentage of clay, which overall raises the ductility of the rock.

6.6 Conclusions from Mechanical Stratigraphy Analysis

The fracture properties collected and/or calculated from Six Mile Fold and the

CEMEX Limestone Quarry include: orientation, intensity, density, mean trace length, average fracture spacing, thermal maturity, and bulk mineralogy. These results and interpretations are used to determine the mechanical stratigraphy and levels of fracturing related to deformation

From the results thus far the mechanical stratigraphy is broken up into: 1) lower massive brittle limestone, 2) A lower-middle ductile chalk-marl unit, 3) a middle brittle chalk unit, 4) a upper-middle ductile marl unit, 5) upper middle massive chalk unit, 6) upper ductile marl unit, and 7) an upper brittle chalk unit (Figure 6.20). The four mechanical stratigraphic units correspond to; 1) Fort Hays Limestone, 2) the Lower C-

Chalk, C-Marl, 3) C-Chalk, 4) B-Marl, 5) B-Chalk, 6) A-Marl and 7) A-Chalk units

respectively. The reasons for the separation of the mechanical units are based on the levels of deformation and its controls on the fracture intensity, density, mean trace length and average fracture spacing. The greater the deformation within the Niobrara Formation

(i.e. faulting, folding, pressure release from expulsion at depth and unroofing) the higher the fracture intensity, density, and fracture spacing will be within its given units. As well as the bulk mineralogy and described lithologies of the measured section, this indicates that there is a cyclic change in deposition of the chalk-marl sequences. This is controlled by the levels of calcite versus the siliciclastics and clay minerals within the system.

These ratios of calcite to siliciclastics and clays also correlate to the amount of TOC wt.

% found within the rock which again relates back to the elasticity and ductility of the given units. Therefore units that have high TOC wt. %, clay content, quartz content, and fracture spacing are separated into ductile mechanical units. Whereas units with higher calcite concentrations with lower TOC wt. % and lower average fracture spacing are separated into brittle (lower ductility) units.

Figure 6.20: Constructed mechanical stratigraphy through the Niobrara Formation at the CEMEX Limestone Quarry. Mechanical stratigraphy is based off of the described lithology, bulk mineralogy, thermal maturity, and average fracture spacing.

CHAPTER 7

CONCLUSIONS

7.1 Concluding Results

The natural fractures that exist within the Niobrara Formation are extensive and can be documented from outcrops along the Colorado Front Range to fracture swarms at

7000ft deep within the Denver Basin. The controls on the natural fractures within the

Niobrara Formation are bulk mineralogy, thermal maturity, and levels of deformation.

The main conclusions of this study are:

1. The bedding oblique stylolites can be used as a possible indicator for horizontal

shortening with axes normal to the plane of stylolite formation. The data

collected indicates the average stylolitic pole to plane is 11/077, which has a close

match with the Laramide slip direction from the joint analysis (248/76) measured

on the joint surfaces. The stylolites formed perpendicular to minor faults and J1

joints indicating the principle horizontal stress that occurred during their

formation is the Laramide shortening event (σ1).

2. Abutting relationships show that the older systematic J1 joints have an average

orientation of 250/76, whereas the younger orthogonal non-systematic J2 joint set

had an average orientation of 162/75. J1 joint strikes are slightly deviated from

the average Laramide σ1 trends. The presence of the NNW striking J2 joints does

not fit with the Laramide shortening directions and indicates a post-Laramide

tectonic development, likely do to de-pressuring from un-roofing.

3. The measured section is summarized as a 320ft thick unit that has slightly dipping

flanking beds of marl and chalks of varying lithology that are stacked upon each

other.

4. The amount of calcite vs. clay and quartz are an inverse relationship within the

given chalk-marl sequences and the high concentration quartz found within the

formation is due to the close proximity to the orogenic belt during deposition.

The pyrite found in the rock is due to the production of sulfur from the source

rocks bonding with free iron in solution thus forming bedding and nodular pyrite.

The plagioclase that is found within the measured section is likely unweathered

clays minerals.

5. The Niobrara Formation marlstone source rocks are thermally mature TYPE II

kerogens that are in the peak to post peak oil generation window and are charging

the reservoirs of the chalk units. The thermal maturity and source TOC wt. %

have a direct relationship with clay content and an inverse relationship with the

amount of calcite present within a sample. The higher percentage of calcite found

within a rock sample the lower percentage of TOC wt. %.

6. Fracture intensity and density tend to trend inversely with mean trace length and

the higher fracture intensity and density found in the fracture maps is due to the

transition in and out of a fold limb. The average fracture spacing of the rock units

within the formation decreases the closer towards the zone of deformation.

7. The standard deviation of the average fracture spacing is controlled by the bias of

where wall face is selected is going to have a large effect on the fracture spacing,

which determines what fracture orientations are going to be intercepted. The

greater the distance the scanline spans the higher the accuracy of the average

fracture spacing.

8. The average fracture spacing for the Niobrara Formation has an inverse

relationship with the amount of calcite found within the bulk mineralogy and a

direct relationship with the amount of clay and quartz. The higher amount of

calcite found within a given sample of rock, the lower the ductility and stress

needed to cause failure and develop a fracture. The average fracture spacing

throughout the measured section also has a direct relationship with TOC wt. %.

The higher TOC wt. % in the rock sample the larger the fracture spacing.

9. The mechanical stratigraphy is broken up into: 1) lower massive brittle limestone,

2) a lower-middle ductile chalk-marl unit, 3) a middle brittle chalk unit, 4) a

upper-middle ductile marl unit, 5) upper middle massive chalk unit, 6) upper

ductile marl unit, and 7) an upper brittle chalk unit.

7.2 Recommendations for Future Work

The following is a list of recommendations for future work to build upon the conclusions and results of this thesis:

1. Collection of additional fracture maps within the various units of the Niobrara

Formation to develop a better understanding of the influence of fracture intensity,

density and, mean trance length.

2. A greater number of exposed field locations for larger sample set to gather bulk

mineralogy and thermal maturity data, to better understand the factors controlling

the natural fractures.

3. FMI logs of laterals completed within the various units of the Niobrara Formation

to gather more information on the fracture spacing horizontally within the

subsurface

4. A data set that included borehole breakouts, within the formation to help

determine modern day stresses within the basin in relation to paleostress.

5. A modern 3D seismic acquisition across the Colorado Front Range to try and

determine fault orientation and any relative fractures that can be recognized.

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APPENDIX A STYLOLITES

Field Location Six Mile Fold Location: multiple wall faces Formation Member: Fort Hays LS

Strike Dip Pole to plane (trend) Pole to plane (plunge) 175 85 085 05 160 80 070 10 165 82 075 08 160 75 070 15 162 76 072 14 173 70 083 20 176 82 086 08

Field Location: CEMEX Quarry Outcrop Location: multiple wall faces Formation Member: Fort Hays LS, C-Chalk

Strike Dip Pole to plane (trend) Pole to plane (plunge) 175 85 085 05 180 82 090 08 177 80 087 10 170 85 080 05 165 90 075 00 180 75 090 15 175 90 085 00 180 85 090 05 178 82 088 08

APPENDIX B BULK MINERALOGY

DEPTH CALCITE QUARTZ CLAY PYRITE PLAGIOCLASE UNIDENTIFIED

316 78 12 10 0 0 0

313 80 11 9 0 0 0

305 70 14 13 2 0 0

286 29 30 30 2 5 4

276 86 8 5 1 0 0

273 91 4 4 1 0 0

260 68 16 15 1 0 0

195 55 20 19 3 3 0

175 90 5 3 0 2 0

170 68 14 16 2 0 0

100 58 13 16 3 7 3

82 58 19 19 2 2 0

76 64 16 18 1 1 0

75 64 15 19 2 0 0

48 58 16 21 0 5 0

20 52 22 20 3 0 3

8 93 5 0 0 0 2

4 95 5 0 0 0 0

APPENDIC C THERMAL MATURITY

Top Depth Sample Sample SRA S1 S2 S3 Tmax Calc. HI OI S2/S3 S1/TOC PI

(ft) Type Prep TOC (°C) % Ro *100

316 OUTCROP CRUSH 0.68 0.38 0.51 0.7 492.1 1.70 75 103 0.7 56 0.43 313 OUTCROP CRUSH 0.42 0.41 0.52 0.36 328.3 -1.00 124 86 1.4 98 0.44 286 OUTCROP CRUSH 5.28 4.2 8.86 0.57 454 1.01 168 11 15.5 80 0.32 277 OUTCROP CRUSH 1.82 1.87 2.73 0.33 453.5 1.00 150 18 8.3 103 0.41 276 OUTCROP CRUSH 2.14 2.96 3.53 0.44 455.7 1.04 165 21 8.0 138 0.46 273 OUTCROP CRUSH 2.8 2.67 4.38 0.46 453.1 1.00 156 16 9.5 95 0.38 260 OUTCROP CRUSH 3.42 3.64 5.37 0.43 460 1.12 157 13 12.5 106 0.40 195 OUTCROP CRUSH 2.3 1.29 2.98 0.25 454.7 1.02 130 11 11.9 56 0.30 175 OUTCROP CRUSH 2.68 1.81 3.68 0.33 454.3 1.02 137 12 11.2 68 0.33 170 OUTCROP CRUSH 2.2 3.19 3.17 0.32 453.3 1.00 144 15 9.9 145 0.50 100 OUTCROP CRUSH 4.15 4.58 7.8 0.36 459.1 1.10 188 9 21.7 110 0.37 82 OUTCROP CRUSH 5.22 5.71 9.08 0.27 457.9 1.08 174 5 33.6 109 0.39 76 OUTCROP CRUSH 3.79 4.07 7.01 0.22 452.4 0.98 185 6 31.9 107 0.37 75 OUTCROP CRUSH 5.56 4.73 9.45 0.44 450.2 0.94 170 8 21.5 85 0.33 48 OUTCROP CRUSH 4.91 5.88 9.11 0.31 461.7 1.15 186 6 29.4 120 0.39 20 OUTCROP CRUSH 0.75 0.57 0.81 0.16 443.9 0.83 108 21 5.1 76 0.41 8 OUTCROP CRUSH 0.51 0.56 0.78 0.4 343.2 -1.00 153 78 2.0 110 0.42 4 OUTCROP CRUSH 0.42 0.54 0.79 0.26 395.5 -1.00 188 62 3.0 129 0.41

APPENDIX D FRACTURE MAPS

Fracture Map 1 Strike 160 Dip 80 Number of Fractures 22 intersections 20 endpoints 40 radius 2.5 Intensity 2 Density 1.02 Mean Trace Length 3.91

Trend Plunge 165 14 150 15 155 10 148 15 152 12 155 10 160 15 065 22 062 18 060 20 065 18 070 19 072 17 080 16 070 18 065 20 068 19 125 15 120 10 118 12 115 14 122 12

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Fracture Map 2 Strike 003 Dip 55 number of fractures 19 intersections 20 endpoints 31 radius 2.5 Intensity 2 Density 0.7936 Mean Trace Length 5.040

Trend Plunge 162 12 160 10 158 15 155 12 154 8 062 25 068 21 063 23 072 20 075 18 070 16 068 19 070 22 065 20 070 18 120 12 115 10 122 14 118 10

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Fracture Map 3 Strike 005 Dip 35 number of fractures 11 intersections 13 endpoints 15 radius 2.5 Intensity 1.3 Density 0.384 Mean Trace Length 6.771

Trend Plunge 165 12 155 10 148 14 152 10 065 18 062 20 070 17 068 16 118 12 120 08 115 10

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Fracture Map 4 Strike 5 Dip 25 number of fractures 15 intersection 12 endpoints 14 radius 2.5 Intensity 1.2 Density 0.3584 Mean Trace Length 6.696428571

Trend Plunge 165 10 155 08 150 15 160 10 060 25 065 28 072 21 120 12 125 10 118 13 115 14 120 10 123 15 118 12 112 10

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APPENDIX E JOINT FRACTURES

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Field Location Six Mile Fold Location: Wall Face 1 Formation Member C-Chalk

Spacing Difference Count # Spacing (cm) Strike (360) Dip (90) Comments Spacing Difference (m) (Ft) 1 30 150 83 30 0.98 2 60 153 85 30 0.98 3 110 165 85 50 1.64 4 150 155 65 40 1.31 5 250 162 70 100 3.28 6 330 155 68 Micro fractures in between 80 2.62 7 370 161 68 large sets. Oxide staining 40 1.31 8 440 158 71 and an increase in dip 70 2.30 9 480 160 68 40 1.31 10 560 163 70 80 2.62 11 660 165 81 100 3.28 12 760 166 78 100 3.28 13 850 167 81 90 2.95 14 950 165 80 100 3.28 15 1000 163 75 50 1.64 16 1050 164 75 50 1.64 17 1130 155 75 80 2.62 18 1190 151 75 Pronounced Fractures 60 1.97 19 1220 155 74 30 0.98 20 1270 161 74 (J2 Fracture set) 50 1.64 21 1300 163 70 clean fractures terminating 30 0.98

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into J1 Fracture sets 2.62 22 1380 164 74 80 Spacing Difference Spacing Difference Count # Spacing (cm) Strike (360) Dip (90) Comments (cm) (Ft) 23 1450 161 76 70 2.30 24 1520 164 77 70 2.30 25 1630 163 75 110 3.61 26 1720 162 74 90 2.95 27 1780 159 75 60 1.97 28 1850 165 73 70 2.30 29 1960 160 71 110 3.61 30 2030 161 72 70 2.30 Average Strike / Average Fracture Spacing Dip 161 75 (cm) 67.7 2.22

Standard Deviation 25.4 0.83 Total Length (cm) 2030 n = 30

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Field Location: Six Mile Fold Formation Member: C-Chalk Spacing Count # Spacing (cm) Strike (360) Dip (90) Comments (cm) Spacing (ft) 1 10 250 76 10 0.33 2 25 255 73 closely spaced fractures 15 0.49 3 36 260 71 11 0.36 4 45 261 72 more brittle 9 0.30 5 66 258 81 21 0.69 6 74 255 83 8 0.26 7 85 254 85 J1 propagates through J2 11 0.36 8 91 251 85 6 0.20 9 105 250 82 Perp to pencil clevage 14 0.46 10 115 245 80 10 0.33 11 127 248 78 calcite cement in fract 12 0.39 12 135 246 79 8 0.26 13 148 251 81 13 0.43 14 156 250 83 8 0.26 15 165 248 85 9 0.30 16 180 253 81 15 0.49 17 190 256 78 calcite cement in fract 10 0.33 18 200 260 75 10 0.33 19 205 263 73 5 0.16 20 209 255 75 Iron oxide staining 4 0.13 21 221 253 71 12 0.39 22 241 251 73 20 0.66

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23 256 248 72 15 0.49 24 275 251 74 Iron oxide staining 19 0.62 Spacing Count # Spacing (cm) Strike (360) Dip (90) Comments (cm) Spacing (ft)

25 290 249 72 15 0.49 26 310 253 75 20 0.66 27 325 252 76 calcite cement in fract 15 0.49 28 341 250 73 16 0.52 29 360 250 74 calcite cement in fract 19 0.62 30 369 253 70 9 0.30 31 390 255 73 21 0.69 32 410 252 75 lots of weathering 20 0.66 33 425 254 79 15 0.49 34 436 255 81 pronounced fracture 11 0.36 35 451 259 80 15 0.49 36 463 261 81 calcite cement in fract 12 0.39 37 475 263 80 calcite cement in fract 12 0.39 38 490 261 78 15 0.49 39 503 262 79 weathering 13 0.43 40 515 261 80 12 0.39 41 528 260 75 iron staining 13 0.43 42 539 255 75 11 0.36 43 555 253 77 16 0.52 44 568 255 76 13 0.43 45 585 251 78 pronounced fracture 17 0.56 46 599 250 79 14 0.46 47 610 250 76 11 0.36

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48 628 248 74 18 0.59 49 640 244 73 mud between layers 12 0.39 Spacing Count # Spacing (cm) Strike (360) Dip (90) Comments (cm) Spacing (ft)

50 655 247 71 15 0.49 51 667 249 74 12 0.39 52 675 251 71 Perp to pencil cleavage 8 0.26 53 688 250 73 13 0.43 54 701 249 72 13 0.43 55 712 253 70 11 0.36 56 728 251 74 16 0.52 57 735 252 73 7 0.23 58 749 253 75 calcite cement in fract 14 0.46 59 763 257 76 14 0.46 60 779 255 75 16 0.52 61 795 250 73 calcite cement in fract 16 0.52 62 808 250 71 13 0.43 63 825 251 75 17 0.56 64 841 248 79 pronounced fracture 16 0.52 65 855 251 80 pronounced fracture 14 0.46 66 863 249 81 pronounced fracture 8 0.26 67 874 253 80 11 0.36 68 890 255 77 16 0.52 69 901 253 73 calcite cement in fract 11 0.36 70 925 261 75 24 0.79 71 938 263 74 13 0.43 72 951 265 71 13 0.43

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73 967 263 70 16 0.52 74 981 261 68 14 0.46 Spacing Count # Spacing (cm) Strike (360) Dip (90) Comments (cm) Spacing (ft)

75 995 258 70 14 0.46 Average Strike / Dip 254 76 Average Spacing (cm / Ft) 13.27 0.44 Fracture Per Spacing (n/cm) 0.08 0.0025 Standard Deviation 3.88 0.13 Total Length (cm) 995 n = 75

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Field Location: 6 Mile Fold Formation Member: B-Chalk

Count # Spacing (cm) Strike (360) Dip (90) Spacing Difference Spacing Difference (Ft) 1 10 225 58 10 0.33 2 50 220 65 40 1.31 3 120 230 30 70 2.30 4 180 220 65 60 1.97 5 230 210 45 50 1.64 6 255 270 67 25 0.82 7 290 265 70 35 1.15 8 330 205 65 40 1.31 9 380 240 78 50 1.64 10 420 271 55 40 1.31 11 450 265 65 30 0.98 12 500 270 55 50 1.64 13 550 240 75 50 1.64 14 600 245 64 50 1.64 15 620 240 56 20 0.66 16 640 230 70 20 0.66 17 680 235 65 40 1.31 18 710 220 65 30 0.98 19 720 235 68 10 0.33 20 770 245 65 50 1.64 21 790 235 70 20 0.66 22 810 225 65 20 0.66 23 840 235 60 30 0.98 24 890 210 55 50 1.64

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Count # Spacing (cm) Strike (360) Dip (90) Spacing Difference Spacing Difference (Ft)

25 900 205 45 10 0.33 26 940 210 55 40 1.31 27 990 220 55 50 1.64 28 1030 235 65 40 1.31 29 1060 205 55 30 0.98 30 1090 230 51 30 0.98 Average Strike / Dip 233 61 36.3 1.19 Stand Dev 15.25 0.50 Total Length (cm) 1090 n = 30

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Field Location: CEMEX Quarry Formation Member: Lwr C-Chalk Strike Spacing Difference Count # Spacing (cm) (360) Dip (90) Comments (m) Spacing Diff (Ft) 1 100 250 71 100 3.3 2 320 270 70 very close spacing 220 7.2 3 455 220 65 fracture swarm 135 4.4 4 620 225 72 165 5.4 5 710 225 70 fracture swarm 90 3.0 6 785 230 64 75 2.5 7 820 255 62 35 1.1 severely calcite 8 930 234 71 cemented 110 3.6 9 1025 242 72 95 3.1 10 1065 255 78 40 1.3 11 1150 260 75 calcite cemented 85 2.8 12 1230 250 70 80 2.6 13 1440 255 65 210 6.9 14 1560 260 62 120 3.9 15 1610 242 63 50 1.6 16 1680 255 64 70 2.3 16 1720 260 72 40 1.3 16 1800 235 75 80 2.6 Average Fracture Average Strike / Dip 246 69 Spacing 100.0 3.3 Standard Deviation 57.1 1.9 Total Length (cm) 1800 n = 16

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Field Location CEMEX Quarry Formation Member: FHLS Count # Spacing (cm) Strike (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 1 35 255 86 Frack sworm @ 100 cm 35 1.1 2 90 245 78 55 1.8 3 240 240 83 calcite cements 150 4.9 4 350 243 87 110 3.6 5 470 235 79 120 3.9 6 610 250 85 Frack sworm @ 730 cm 140 4.6 7 730 265 83 120 3.9 8 900 251 78 170 5.6 9 1000 238 87 100 3.3 10 1200 235 85 stylolites 200 6.6 11 1280 245 89 80 2.6 12 1450 249 86 stylolites 170 5.6 13 1660 235 87 210 6.9 14 1850 253 85 calcite cemented 190 6.2 15 2040 240 84 190 6.2 16 2240 240 90 stylolites 200 6.6 17 2350 242 83 110 3.6 18 2450 237 85 100 3.3 19 2660 241 85 210 6.9 20 2830 250 83 shell fragments 170 5.6 21 3050 245 85 shell fragments (large) 220 7.2 22 3250 238 83 200 6.6 23 3370 235 85 120 3.9

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Count # Spacing (cm) Strike (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 24 3500 241 83 130 4.3 25 3570 248 87 70 2.3 Average Strike / Dip 244 84 Average Fracture Spacing (cm) 142.8 4.7 standard deviation 53.1 1.7 Total Length (cm) 3570 n = 25

115

Field Location CEMEX Quarry Formation Member: B-Chalk Count # Spacing (cm) Strike (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 1 40 253 83 30 1.0 2 240 241 85 200 6.6 3 300 270 85 60 2.0 4 450 270 65 150 4.9 5 530 252 70 80 2.6 6 670 265 68 Micro fractures in between 140 4.6 7 900 258 68 large sets. Oxide staining 230 7.5 8 1010 260 71 and an increase in dip 110 3.6 9 1120 255 68 110 3.6 10 1320 250 70 200 6.6 11 1440 260 81 120 3.9 12 1710 255 78 iron staining 270 8.9 13 1810 253 81 100 3.3 14 1930 270 80 clear defined fractures 120 3.9 15 2300 251 75 370 12.1 16 2500 255 75 200 6.6 17 2760 253 75 260 8.5 18 3040 271 75 Pronounced Fractures 280 9.2 19 3300 261 74 260 8.5 20 3370 255 74 (J2 Fracture set) 70 2.3 21 3590 255 70 220 7.2

clean fractures terminating into J1 Fracture sets

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Count # Spacing (cm) Strike (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 22 3780 253 74 190 6.2

23 4000 255 76 220 7.2 Average Fracture Spacing Average Strike / Dip 257 75 (cm) 173.5 5.7 Standard Deviation 84.7 2.8 Total Length (cm) 4000 n = 23

117

Field Location CEMEX Quarry Formation Member: B-Chalk Count # Spacing (cm) Strike (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 1 40 165 83 40 1.3 2 180 170 85 140 4.6 3 250 160 85 defined fractures 70 2.3 4 330 165 65 80 2.6 5 400 170 70 70 2.3 6 450 165 68 Micro fractures in between 50 1.6 7 500 170 68 large sets. Oxide staining 50 1.6 8 610 161 71 and an increase in dip 110 3.6 9 700 165 68 90 3.0 10 830 168 70 calcite lined 130 4.3 11 900 165 81 70 2.3 12 1000 170 78 100 3.3 13 1100 168 81 100 3.3 Average Fracture Spacing Average Strike / Dip 166 75 (cm) 84.6 2.8 Stamdard Deviation 30.7 1.0 Total Length (cm) 1100 n = 13

118

Field Location: CEMEX Quarry Formation Member: A-Chalk Count # Spacing (cm) Strike (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 1 60 245 85 60 2.0 2 270 255 83 210 6.9 3 460 260 80 190 6.2 4 650 263 82 190 6.2 5 720 258 77 70 2.3 6 875 250 79 Micro fractures in between 155 5.1 7 975 248 85 large sets. Oxide staining 100 3.3 8 825 250 89 and an increase in dip -150 -4.9 9 1095 255 83 270 8.9 10 1250 260 80 155 5.1 11 1520 255 78 iron staining from pyrite 270 8.9 12 1720 248 85 200 6.6 13 1830 240 83 110 3.6 14 1965 251 89 calcite cemented 135 4.4 15 2145 245 89 180 5.9 16 2320 240 85 175 5.7 17 2560 250 80 240 7.9 18 2720 250 89 Pronounced Fractures 160 5.2 19 3100 243 85 380 12.5 20 3500 250 81 (J2 Fracture set) 400 13.1 Average Fracture Spacing Average Strike / Dip 251 83 (cm) 175 5.7 Standard Deviation 116.9 3.8

Total Length (cm) 3500 n = 20

119

Field Location CEMEX Quarry Formation Member: B-Marl Strike Count # Spacing (cm) (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 1 60 245 85 60 2.0 2 550 260 80 490 16.1 3 725 261 85 sulfur smell 175 5.7 4 900 255 78 175 5.7 5 1200 240 83 Calcite cemented 300 9.8 6 1290 251 89 90 3.0 7 1350 245 89 Calcite cemented 60 2.0 8 1650 240 85 300 9.8 9 1800 250 80 fairly pronounced fractures 150 4.9 10 1950 250 89 150 4.9 11 2000 243 85 50 1.6 12 2230 250 81 iron staining 230 7.5 13 2400 235 75 170 5.6 14 2510 255 83 110 3.6 15 2850 245 81 340 11.2 16 2950 240 76 100 3.3 Average Strike / Average Fracture Spacing Dip 248 83 (cm) 184.4 6.0 Standard Deviation 128.9 4.2

Total Length (cm) 2950 n = 16

120

Field Location: CEMEX Quarry Formation Member: A-Marl Strike Count # Spacing (cm) (360) Dip (90) Comments Spacing Difference (m) Spacing Difference (Ft) 1 60 248 82 60 2.0 2 350 261 80 290 9.5 3 550 255 75 sulfur smell 200 6.6 4 625 238 70 75 2.5 5 920 241 76 shell fragments 295 9.7 6 1150 260 77 230 7.5 7 1360 255 82 calcite cemeted 210 6.9 8 1625 245 84 calcite cemeted 265 8.7 9 1785 240 85 calcite cemeted 160 5.2 10 2000 250 82 215 7.1 11 2150 240 85 iron staining 150 4.9 12 2275 252 80 125 4.1 13 2500 250 78 225 7.4 14 2720 245 81 220 7.2 15 2960 255 75 240 7.9 16 3100 260 80 140 4.6 17 3520 245 75 Systematic J1 Joint Set 420 13.8 18 3800 260 82 280 9.2 19 4110 255 75 310 10.2 20 4350 245 80 240 7.9 21 4600 242 76 250 8.2

121

Strike Count # Spacing (cm) (360) Dip (90) Comments Spacing Difference (m) Spacing Difference (Ft) 22 4900 252 80 300 9.8 23 5200 245 75 300 9.8 Average Strike / Average Fracture Spacing Dip 250 79 (cm) 226.1 7.4 Stand Dev discluding 1st value 75.7 2.5

Total Length (cm) 5200 n = 23

122

Outcrop Location: CEMEX Quarry Formation Member: C-Chalk Strike Count Spacing (cm) (360) Dip (90) Comments Spacing Diff (m) Spacing Diff (Ft) 1 25 233 70 60 2.0 2 125 240 75 100 3.3 3 350 225 80 J1 joint set 225 7.4 4 650 230 74 300 9.8 5 725 245 72 75 2.5 6 1000 230 75 calcite filled 275 9.0 7 1050 260 80 50 1.6 8 1300 245 82 possible fault 250 8.2 9 1425 235 84 125 4.1 10 1500 233 86 75 2.5 11 1750 225 80 250 8.2 12 2000 235 72 250 8.2 13 2200 240 68 possible fault 200 6.6 14 2320 265 75 120 3.9 15 2460 240 80 140 4.6 16 2750 245 75 290 9.5 17 2900 230 74 150 4.9 18 2950 245 80 50 1.6 19 3000 250 85 50 1.6 Average Fracture Spacing Average Strike and Dip 239.5 77.2 (cm) 159.7 5.2 Stand Dev discluding 1st value 90.5 3.0 Total Length 3000

n= 19

123

Outcrop Location: CEMEX Quarry Formation Member: C-Marl Strike Count Spacing (cm) (360) Dip (90) Comments Spacing Difference (m) Spacing Difference (Ft) 1 100 233 70 100 3.3 2 820 240 75 carbonate stringers 720 23.6 3 1200 225 80 380 12.5 4 1500 230 74 organ rich 300 9.8 5 1900 245 72 400 13.1 6 2300 230 75 calcite cemented 400 13.1 7 2500 260 80 200 6.6 8 2950 245 75 organ rich 450 14.8 9 3025 255 82 iron staining 75 2.5 10 3250 260 85 carbonate stringers 225 7.4 11 3560 252 83 310 10.2 12 3710 265 87 150 4.9 13 3820 245 84 110 3.6 Average Strike and Average Fracture Spacing Dip 245 79 (cm) 293.85 9.6 Stand Deviation 178.13 5.8

Total Length 3820 n= 13

CD/DVD

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