This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Denver, Colorado, USA, August 2014.

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1 URTeC: Upper Cretaceous Niobrara Chalk in Buck Peak Field, Sand Wash Basin, NW Colorado: Depositional Setting, Lithofacies, and Nanopore Network Robert G. Loucks* and Harry D. Rowe, Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas Copyright 2014, Unconventional Resources Technology Conference (URTeC) DOI /urtec This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Denver, Colorado, USA, August The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper without the written consent of URTeC is prohibited. Summary The Upper Cretaceous Niobrara Chalk in the Sand Wash Basin is characterized by having more terrigenous components than the Niobrara Chalk further to the east. This difference in lithology affects reservoir quality and the potential of the chalk as a matrix-producing reservoir. The degraded reservoir does not appear productive as a shaleoil reservoir, but may be productive as a shale-gas reservoir in the deeper and hotter parts of the Sand Wash Basin. The major objective of this paper is to present a preliminary characterization of the Niobrara Chalk as a shale-gas system in the northwest Sand Wash Basin. Introduction The Upper Cretaceous Niobrara Chalk (figs. 1, 2) is a gas- and oil-producing trend that covers several states in the northern Rocky Mountain area. Longman et al. (1998) and Finn and Johnson (2002) demonstrated that the lithofacies of this unit vary vertically and especially regionally. They stated that the unit becomes more clay rich to the west and pinches out into the Hilliard and Baxter Shales. Associated with this regional lithofacies transition is a pore-network change wherein micropores dominate the cleaner chalks to the east, and nanopores dominate the impure chalks to the west. This transition of pore types, however, has not been documented and characterized. The documentation and characterization of this nanopore network in the western area of Niobrara production is an important objective of this study. Stage Age (M.Y.) Central Southwestern Western Central Texas Western Alabama Interior Arkansas A B C D Maastrichtian Campanian Santonian Coniacian Niobrara Pecan Gap Austin Saratoga Annona Ozan Demopolis Arcola Upper Cretaceous Interior Seaway Turonian Cenomanian 93.9 Fairport Greenhorn Eagle Ford Buda A B C D Gulf of Mexico Figure 1. Upper Cretaceous stratigraphic section emphasizing chalks. Ages according to IUGS (July 2012). Chart partly based on Bottjer (1986).

2 URTeC: Mancos Sh Buck Peak bench GR Core 1 Conductivity Core 2 Tow Creek bench 6600 Core Wolf Mtn bench Core Lower Niobrara Carlile Sh Core 5 Figure 2. Atlantic Richfield No USA wireline log with stratigraphic units. Specific research objectives include (1) providing an overview of the depositional setting of the chalks in the western Niobrara producing area, (2) defining the lithofacies and associated mineralogy, and (3) describing the characteristics of the nanopore network in the marly chalk. The rock data for this study come from five 50-ft cores spaced throughout the 1240-ft Niobrara section of the Atlantic Richfield No USA well (fig. 2). General Depositional Setting The Niobrara Chalk was deposited in the Western Interior Seaway during Coniacian and Santonian times (figs. 1, 3) (Longman et al., 1998). The chalk is age-equivalent to the Austin Chalk on the Upper Cretaceous shelf in Texas (fig. 1). Cross sections from Finn and Johnson (2002) show the Niobrara transitioning into shales toward the west. They also show carbonate-rich sections that are stratigraphically interbedded with more terrigenous material.

3 URTeC: Figure 3. Upper Cretaceous paleogeographic map. From Blakey (2013). Insert showing paleoenvironments as modified from Roberts and Kirschbaum (1995). During Coniacian and Santonian times, the Western Interior Seaway was open between the proto-arctic Ocean and the Gulf of Mexico (fig. 3). The open seaway produced regional bottom currents generated by the exchange of cold and warm waters between these areas (Parrish et al., 1984). Depositional Model and Lithofacies Based on descriptions in the literature of the Atlantic Richfield No USA well cores, a general depositional model was developed and is presented in figure 4. This model represents the upper four cores noted in figure 2. As shown in figure 3, the seaway was bordered to the east and west by continental deposits. According to a paleogeographic map published by Roberts and Kirschbaum (1995), the location of the No USA well borders the edge of the terrigenous-rich western part of the seaway (fig. 3). Their paleogeographic map also shows active volcanism to the west. Figure 4. General depositional model for the Niobrara Chalk in the Sand Wash Basin, showing deposition profile, stratified water column, and depositional processes.

4 URTeC: The strata in the upper four cores in the No USA well (figs. 2, 5 10) were deposited in an anaerobic to dysaerobic bottom environment based on high total organic carbon (TOC); relative lack of burrow traces; wellpreserved laminations; and dominance of coccoliths, Inoceramus, and copepod-pellet allochems. The anaerobic to dysaerobic bottom environment implies a stratified water column below the oxygen-minimum boundary. The stratigraphically lowest core for this well (figs. 2, 9) is highly bioturbated with no hydrodynamic structures preserved, indicating oxygenated bottom-water conditions in a nonstratified body of water. Fabric/Structure Mineral Composition Textural Components Poorly laminated Calcite Volcanic ash Calcite beef Fracture Inoceramus Globigerinids Horizontal burrows Figure 5. Core description legend. Dolomite Silt Siliciclastic mud Pyrite Thin-section location Measured Section / Well: Atlantic Richfield No USA Stratigraphic Interval: Niobrara Chalk ( ft) 6270 Fabric / Structure Mineral Composition Textural Components (Including Porosity) 50% 50% Quartz grains Laminations produced are mainly clay by flattened pellets to silt size % Calium (Carbonate Proxy) Location: Moffat Co.. Colorado Logged By: Loucks Date: Sept % Silica (Mud Proxy) Fossils % TOC Calcite beef Muscovite 6330 Figure 6. Core 1 description.

5 URTeC: Measured Section / Well: Atlantic Richfield No USA Stratigraphic Interval: Niobrara Chalk ( ft) Fabric / Structure This section of core is better laminated Mineral Composition Textural Components (Including Porosity) 50% 50% % Calium (Carbonate Proxy) Location: Moffat Co.. Colorado Logged By: Loucks Date: Sept % Silica (Mud Proxy) Fossils % TOC Bottom-current ripples Figure 7. Core 2 description. Measured Section / Well: Atlantic Richfield No USA Stratigraphic Interval: Niobrara Chalk ( ft) Fabric / Structure This section of core is better laminated Mineral Composition Textural Components (Including Porosity) 50% 50% % Calium (Carbonate Proxy) Location: Moffat Co.. Colorado Logged By: Loucks Date: Sept % Silica (Mud Proxy) Fossils % TOC Bottom-current ripples Figure 8. Core 3 description.

6 URTeC: Measured Section / Well: Atlantic Richfield No USA Stratigraphic Interval: Niobrara Chalk ( ft) Fabric / Structure This section of core is better laminated Mineral Composition Textural Components (Including Porosity) 50% 50% % Calium (Carbonate Proxy) Location: Moffat Co.. Colorado Logged By: Loucks Date: Sept % Silica (Mud Proxy) Fossils % TOC Bottom-current ripples Figure 9. Core 4 description. Measured Section / Well: Atlantic Richfield No USA Stratigraphic Interval: Niobrara Chalk ( ft) Location: Moffat Co.. Colorado Logged By: Loucks Date: Sept Fabric / Structure Mineral Composition Textural Components Fossils (Including Porosity) % Calium (Carbonate Proxy) % Silica (Mud Proxy) % TOC % 50% Fault 7460 Abundant calcite fracture fill in fault zone 7470? Figure 10. Core 5 description.

7 URTeC: Depositional processes of the upper four cores include suspension settling of globigerinids, coccospheres, and copepod fecal pellets (fig. 4). Terrigenous sediments were introduced into the deeper basin by dilute turbidites and mud plumes. Bottom currents reworked the fine-grained sediments, resulting in laminations, starved ripples, and subtle scour surfaces. Because the bottom was anoxic to dysaerobic, bioturbation was extremely rare. Depositional processes for the stratigraphically lowest core (core 5) were the same, but with aerobic bottom conditions, bioturbation destroyed all the sedimentary structures as well as the copepod fecal pellets. Figure 11(a) shows the mineralogy in cores 1, 2, 4, and 5 to be predominantly calcite, with 10 20% dolomite. Terrigenous material ranges from 20 to 50%. Core 2 is 50 60% terrigenous components; therefore, cores 1, 2, 4, and 5 are classified as chalky marl, and core 2 is classified as a calcareous terrigenous mudstone (fig. 11[a]). It is interesting and important to note that clay-size material is mainly quartz and clay minerals. Silt and very finegrained sand are common (figs. 10[a], [b]). A(a) Clay All groups 50 Calcareous mudstone 50 Pure chalk Chalky marl Calcite and dolomite 50 Quartz (b) B Illite Pyrite (c) C OM Illite Calcite Quartz Dolomite Chlorite Quartz Calcite pellets Albite Albite Quartz Phosphate OM Phosphate Pyrite Dolomite Figure 11. (a) Mineralogy based on XRD analysis. (b) 6947 ft Ar-ion milled sample: EDX chemical map showing calcite chalk pellets with micropores, quartz, and albite clay-size silt. (c) EDX chemical map showing clay- to silt-size quartz in a clay mineral matrix.

8 URTeC: Cores 1, 3, and 4 (figs. 6, 8, 9) have essentially the same general lithofacies, which can be characterized as laminated, globigerinid, pyritic, terrigenous, mud-rich, dolomitic lime wackestones (chalky marl) (figs. 12, 13). Other megafossils are Inoceramus (fig. 12[c]) and oyster fragments. Coccoliths are abundant, as shown by FSEM analysis (fig. 12[b]). A very common allochem is flattened "chalk pellets" (term of Longman et al., 1998) (figs. 13[a], [b], [c]) that produce many of the laminae. These coccolith-rich pellets are thought to be produced by pelagic copepods (Hattin, 1975). Only a few rare horizontal burrows were noted. Laminations can be diffused to very sharp (figs. 13[c], [d]). Some laminations may be very low-amplitude starved ripples (fig. 13[c]). Several layers are concentrations of Inoceramus and/or oyster fragments and are attributed to debris flows (fig. 12[c]). Core 4 contains several volcanic ash beds composed of quartz, mica, and clay (figs. 14[a], [b]). Core 2 (figs. 2, 7) is similar to cores 1, 3, and 4 but contains more terrigenous material and is a calcareous terrigenous mudstone. Figure 12. (a) Thin-section photomicrographs showing fauna and textures. Globigerinids are much more common than benthic forams. (b) SEM photomicrograph of matrix containing coccoliths and clay. (c) Poorly laminated Niobrara with layer of Inoceramus shells deposited by gravity-flow currents.

9 URTeC: Figure 13. (a) Compacted coccolith lime pellets are very common in the Niobrara. Flattened pellets emphasize laminations. (b) Fecal pellets contain peloids. (c) Irregular laminae, which may be a product of bottom-current ripples. (d) Even parallel laminae resulting from suspension or dilute turbidite deposition. (e) Burrowed facies of core 5. (f) Thin-section scan showing bioturbation in core 5.

10 URTeC: Figure 14. (a) Core slab with a thin volcanic ash bed. (b) Thin section of volcanic ash bed shows quartz, clay, and biotite. Photograph taken with polarized light. (c) Fractured core sample from fault zone in core 5. (d) Calcite-filled fractures with some remaining pores. Core 5 (figs. 2, 10) is interpreted to be cut by a fault based on the rapid change in lithofacies, fauna abundance, and a zone of fractures. Above the fault, the lithofacies is similar to the lithofacies seen in cores 1 through 4. Beneath the fault is a very different lithofacies that is composed of interbedded, highly burrowed, globigerinid/inoceramus, pyritic, terrigenous, mud-rich, dolomitic lime wackestones and packstones (chalky marl) (figs. 13[e], [f]). The burrows are Chondrites and probable Thalassinoides. The burrowing has disrupted all laminations, and Longman et al. (1998) suggested that the bioturbation also disaggregated the chalk pellets, which are absent from the lithofacies. Handheld XRF analysis was undertaken on the cores in order to develop the chemostratigraphic record (see core descriptions in figs. 5 10). Several of the major elements are good proxies for marine versus terrigenous deposition: calcium for the marine biogenetic contribution and silica for the terrigenous contribution. The curves show variations in carbonate (calcium) and terrigenous mud (silica) that XRD analysis did not resolve. The handheld XRF

11 URTeC: analysis corroborates the terrigenous mud richness of core 2 (fig. 7). The fault in core 5 is highlighted by a higher calcium content that corresponds to calcite cement in the fault zone related fractures (figs. 14[c], [d]). The depositional setting of the laminated lithofacies is interpreted to be a quiet-water anoxic setting, below the storm wave base (fig. 4). Low-oxygen bottom-water conditions inhibited burrowing and precluded fauna such as Inoceramus from living there. Globigerinids, which lived in the shallower water column, were deposited by suspension and later reworked by weak bottom currents. The copepod fecal pellets were also produced in the water column. The laminated lithofacies shows a high average TOC content of 2.4%, as opposed to the burrowed facies, with an average TOC content of 0.9% (fig. 15), which supports anoxic bottom-sediment conditions. A paleotopographic model (fig. 3) presented by Roberts and Kirschbaum (1995) shows that the area of the Niobrara core used in this study was in the central part of the Western Interior Seaway, which favored carbonate deposition, but was also fairly close to the area of terrigenous mud input. The map also shows volcanic activity to the west that accounts for the volcanic ash beds noted (figs. 14[a], [b]). 30 All data Mean TOC = 2.21% n =249 Laminated 15 Burrowed TOC % 4 Figure 15. Total organic carbon analysis. In the lower left figure, the population on the left is from the burrowed facies, and the population on the right is from the laminated facies. The histograms to the far right show TOC by cored interval.

12 URTeC: Pore Network Pore networks were investigated in this core with (1) blue-dye impregnated polished thin sections, using a petrographic scope equipped with UV light; and (2) polished thin sections and Ar-ion milled samples, using a fieldscanning electron microscope (FSEM). The blue-dye impregnated polished thin sections did not show any visible micropores. FSEM was necessary to actually see any pores, as they are generally less than 2 microns and most are in the nanopore-size range. Pollastro and Scholle (1986) presented an evolution of pores in the eastern part of the Niobrara play area, where the chalks are lower in terrigenous content. Their chart shows the evolution of poorly lithified pure chalk with porosities in the 40 50% range, dropping to less than 10% porosity between 6000 and 7000 ft in well-cemented chalks. In contrast to this analysis, the Niobrara in the western area is rich in terrigenous content, associated pores are smaller, and porosity (not measured) appears to be only several percent. Figure 16 shows FSEM microphotographs of Ar-ion milled samples, using the pore classification of Loucks et al. (2012). Initial analysis shows that the dominant pore types are intraparticle pores in chalk pellets (between coccolith hash) (figs. 13[b], 16[c]). Other pore types include intraparticle pores in clay flocculates (fig. 16[a]), intraparticlepore fluid inclusions, intraparticle pores in pyrite framboids, organic matter pores (figs. 16[b], [d]), and grain-edge interparticle pores (fig. 16[d]). Figure 16. FSEM Ar-ion milled photomicrographs. (a) Intraparticle pores in clay platelets. (b) Organic matter pores. (c) Intraparticle pores within a pellet. (d) Organic matter pores.

13 URTeC: Vincelette and Foster (1992) stated that the Niobrara producing pore network in the Buck Peak Field is composed of fractures, which are partly filled with calcite cement (figs. 14[c], [d]). The present analysis of the Niobrara pore network in the field indicates that the matrix pores would add little to production, especially because the area is an oil reservoir. Further to the west, where R o values are higher (Finn and Johnson, 2002), these pore types could be important for shale-gas reservoirs. Conclusions The Niobrara Chalk in the Sand Wash Basin is not a true chalk, but chalky marl. It does contain a nano- to micropore system that might produce gas in the deeper and more mature part of the basin. However, reservoir quality would be very low, as is typical of shale-gas systems. Acknowledgements Funds were provided by the Carbonate Reservoir Characterization Research Laboratory at the Bureau of Economic Geology. The paper was edited by Stephanie Jones and formatted by Jones and Jamie Coggin at the Bureau of Economic Geology. Publication was authorized by the Director, Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin. References Blakey, R., 2011, Colorado Plateau Geosystems, Inc., Western North America Series: Bottjer, S. J., 1986, Campanian-Maastrichtian chalks of southwestern Arkansas: petrology, paleoenvironments and comparison with other North American and European chalks: Cretaceous Research, v. 7, no. 2, p Finn, T. M., and Johnson, R. C., 2002, Niobrara total petroleum system in the southwestern Wyoming Province: Chapter 6 of Petroleum Systems and Geologic Assessment of Oil and Gas in the Southern Wyoming Province, Wyoming, Colorado, and Utah, U.S. Geological Survey Digital Data Series DDS-69-D, p Hattin, D. E., 1975, Petrology and origin of fecal pellets in upper Cretaceous strata of Kansas and Saskatchewan: Journal of Sedimentary Research, v. 45, no. 3, p Longman, M. W., Luneau, B. A., and Landon, S. M., 1998, Nature and distribution of Niobrara lithologies in the Cretaceous western interior of the Rocky Mountain region: The Mountain Geologist, v. 35, no. 4, p Loucks, R. G., Reed, R. M., Ruppel, S. C., and Hammes, U., 2012, Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores: AAPG Bulletin, v. 96, no. 6, p Parrish J. T., Gaynor, G. C., and Swift, D. J. P., 1984, Circulation in the Cretaceous Western Interior Seaway of North America, a review, in Stott, D. F. and Glass, D. J., eds., The Mesozoic of middle North America: Canadian Society of Petroleum Geologists, Memoir 9, p Pollastro, R. M., and Scholle, P. A., 1986, Exploration and development of hydrocarbons from low-permeability chalks an example from the Upper Cretaceous Niobrara Formation, Rocky Mountain region, in Spencer, C. W., and Mast, R. F., eds., Geology of tight gas reservoirs: AAPG Studies in Geology, no. 24, p Roberts, L. N. R., and Kirschbaum, M. A, 1995, Paleogeography of the Late Cretaceous of the western interior of middle North America coal distribution and sediment accumulation: U.S. Geological Survey Professional Paper 1561, 115 p. Vincelette, R. R., and Foster, N. H., 1992, Fractured Niobrara of northwestern Colorado, in Schmoker, J. W., Coalson, E. B., and Brown, C.A., eds., Geological studies relevant to horizontal drilling examples from western North America: Rocky Mountain Association of Geologists Guidebook, p