Chapter 6 Niobrara Total Petroleum System in the Southwestern Wyoming Province
By Thomas M. Finn and Ronald C. Johnson Volume Title Page
Chapter 6 of Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah By USGS Southwestern Wyoming Province Assessment Team
U.S. Geological Survey Digital Data Series DDS–69–D
U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director
U.S. Geological Survey, Denver, Colorado: Version 1, 2005
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Manuscript approved for publication May 10, 2005
ISBN= 0-607-99027-9 Contents
Abstract ……………………………………………………………………………………… 1 Introduction …………………………………………………………………………………… 1 Acknowledgments …………………………………………………………………………… 1 Structural and Tectonic Setting ……………………………………………………………… 3 Depositional Setting ………………………………………………………………………… 3 Niobrara Formation …………………………………………………………………………… 3 Niobrara Total Petroleum System …………………………………………………………… 9 Source Rocks ………………………………………………………………………………… 9 Reservoir Rocks ……………………………………………………………………………… 9 Seal Rocks …………………………………………………………………………………… 9 Traps ………………………………………………………………………………………… 11 Thermal Maturity ……………………………………………………………………………… 12 Hydrocarbon Generation ……………………………………………………………………… 12 Migration Summary …………………………………………………………………………… 14 Events Chart ………………………………………………………………………………… 14 Assessment Units in the Niobrara Total Petroleum System ………………………………… 17 Niobrara Continuous Oil Assessment Unit (AU 50370361) ……………………………… 17 Niobrara Continuous Gas Assessment Unit (AU 50370362) ……………………………… 19 Assessment Results of Undiscovered Resources …………………………………………… 21 Niobrara Continuous Oil Assessment Unit (AU 50370361) ……………………………… 21 Niobrara Continuous Gas Assessment Unit (AU 50370362) ……………………………… 21 References …………………………………………………………………………………… 23 Appendix A. Data form used for evaluating the Niobrara Continuous Oil Assessment Unit, Niobrara Total Petroleum System, Southwestern Wyoming Province, Wyoming, Colorado, and Utah …………………………………………………………………………………… 26
Figures
1. Index map of Southwestern Wyoming Province …………………………………………… 2 2. Map showing Paleographic reconstruction of the Western Interior Basin ………………… 4 3. Regional stratigraphic cross sections of Upper Cretaceous rocks ………………………… 5 4. Type log and subsurface cross section of the Niobrara Formation ………………………… 6 5. Correlation chart of Cretaceous and lower Tertiary rocks ………………………………… 7 6. Isopach map of the Niobrara Formation …………………………………………………… 8 7. Map of the Niobrara Total Petroleum System ……………………………………………… 10 8. Plots of Rock-Eval results ………………………………………………………………… 11 9. Structure contour map of Sand Wash Basin ……………………………………………… 12
III 10. Thermal maturity map of the Niobrara Formation ………………………………………… 13 11. Burial-history curves based on vitrinite reflectance ……………………………………… 15 12. Burial-history curves based hydrous-pyrolysis kinetic models …………………………… 16 13. Events chart ……………………………………………………………………………… 17 14. Map of the Niobrara Continuous Oil Assessment Unit (AU 50370361) …………………… 18 15. Map of the Niobrara Continuous Gas Assessment Unit (AU 50370362) …………………… 20 16. Estimated ultimate recovery graphs for the Continuous Oil Assessment Unit (AU 50370361) ……………………………………………………………………………… 22
Table
Table 1. Summary of assessment results ……………………………………………………… 23
IV Niobrara Total Petroleum System in the Southwestern Wyoming Province
By Thomas M. Finn and Ronald C. Johnson
Abstract eastern and southern flanks of the basin where thermal maturi- ties are less than 1.35 percent Ro and generally are considered The Niobrara Total Petroleum System (TPS) is a self- to be within the oil window. The continuous gas assessment sourced system that produces oil and natural gas from frac- unit is hypothetical and is located in the deeper portions of the tured carbonate rock reservoirs in the Upper Cretaceous basin where thermal maturities are greater than 1.35 percent Niobrara Formation and equivalent rocks of the Southwestern Ro. The mean volume estimate of undiscovered oil resource Wyoming Province. The Niobrara TPS encompasses the for the Niobrara continuous oil assessment unit is 103.6 mil- area of eastern and southeastern Greater Green River Basin lion barrels of oil. The hypothetical Niobrara continuous gas of southwestern Wyoming and northwestern Colorado. The assessment unit was not quantitatively assessed because of a Niobrara and equivalents were deposited during a major Late lack of production data and (or) a suitable analog. Cretaceous marine transgressive cycle that created conditions favorable for the deposition of fine-grained marine carbonate rocks and the preservation of organic matter. The Niobrara Introduction ranges in thickness from 900 to 1,800 feet and consists mainly of interbedded organic-rich shale, calcareous shale, and marl. The Southwestern Wyoming Province (SWWP) (5037) The hydrocarbon source beds contain predominantly Type-II is a large sedimentary and structural basin that formed during oil-prone organic matter with total organic carbon contents the Laramide orogeny (Late Cretaceous through Eocene). The ranging from 0.85 to 2.75 weight percent. Thermal maturities SWWP covers approximately 23,000 mi2 and occupies most of range from less than 0.60 percent R (vitrinite reflectance) o southwestern Wyoming, parts of northwestern Colorado, and a along the eastern and southern flanks of the basin to greater small area of northeastern Utah (fig. 1). The basin is structur- than 1.35 percent R in deeper parts of the basin. Because of o ally bounded on the west by the Wyoming thrust belt, on the the fine-grained nature of the Niobrara, petroleum production north by the Wind River and Granite Mountains uplifts, on the is dependant on fractures that develop in hard, brittle, carbon- east by the Rawlins, Sierra Madre, and Park uplifts, and on ate-rich zones. These brittle calcareous reservoirs are overlain, the south by the Uinta Mountains and Axial Basin uplift. The underlain, and (or) interbedded with soft, ductile marine shales Niobrara Total Petroleum System (TPS) (503703) produces that inhibit migration and seal the hydrocarbons within the primarily oil from fractured, calcareous-rich shales, shaley fractured reservoir zones. Burial-history reconstructions and limestones, and marls from the Upper Cretaceous Niobrara petroleum generation models show that the Niobrara entered Formation or equivalent rocks, in the eastern portions of the the oil window between 72 and 67 million years ago. Along Greater Green River Basin (GRRB). the shallow flanks of the basin the Niobrara remains in the oil window. With continued subsidence and burial in the deeper portions of the basin, the Niobrara reached thermal maturities sufficient to crack oil to gas. Acknowledgments The Niobrara TPS is subdivided into two assessment units based on thermal maturity: a continuous oil assessment The authors thank R.M. Pollastro and C.J. Schenk of unit and a continuous gas assessment unit. The assessment the U.S. Geological Survey (USGS) National Oil and Gas units are characterized as “continuous-type” accumulations Assessment team for discussions regarding the assessment of because they have the following characteristics: (1) no down- the Niobrara Total Petroleum System. We would also like to dip hydrocarbon/water contact, (2) little or no water produc- thank Troy Cook (USGS) for interpretation of production data, tion, (3) abnormally pressured, and (4) production independent and providing graphs of EURs; P.H. Nelson and Joyce Kibler of structural closure. The continuous oil assessment unit is an (USGS) for providing drill-stem test and pressure data; L.N. established assessment unit located in the updip parts of the Roberts (USGS) for providing key maps, and burial history 1 2 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
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Figure 1. Index map of southwestern Wyoming, northeastern Utah, and northwestern Colorado showing the location of the South- western Wyoming Province, structural configuration, intrabasin uplifts, and subbasins. Structure contours drawn on top of the Mesaverde Group. Contour interval = 1,000 feet. Niobrara Total Petroleum System in the Southwestern Wyoming Province 3 and petroleum-generation models; and P.G. Lillis and M.D. Barge platform to the west flank of the Rock Springs uplift Lewan (USGS) for providing geochemical data and numerous and separates the Green River Basin (sometimes referred to as discussions regarding organic geochemistry and source rocks. the Bridger Basin) on the south from the Hoback Basin to the This manuscript was reviewed by R.M. Pollastro and M.J. north. East of the Rock Springs uplift, the east-west-trending Pawlewicz of the USGS, and their suggestions and comments Wamsutter arch extends from the eastern flank of the Rock are greatly appreciated. Springs uplift to south of the Rawlins uplift and separates the Great Divide Basin in the northeast corner of the SWWP from the Washakie Basin. The Cherokee ridge roughly parallels the Colorado-Wyoming State line and separates the Washakie Structural and Tectonic Setting Basin on the north from the Sand Wash Basin on the south (fig. 1). The western margin of the SWWP is formed by the Wyoming thrust belt (fig. 1), an easterly bulging salient of the Sevier orogenic belt that began to form in latest Jurassic or earliest Cretaceous time and ended in the Eocene (Arm- Depositional Setting strong and Oriel, 1965). The western margin is an area that is characterized by imbricate, gently west-dipping, east-directed During Late Cretaceous time, a broad epicontinental decollement-style (thin-skinned) thrust faults that involve seaway, the Western Interior Seaway, extended from the Arctic only sedimentary rocks, and north-trending folds (Royse and Ocean to the Gulf of Mexico. The seaway (fig. 2) inundated others, 1975). A broad, asymmetric foreland basin developed an asymmetric foreland basin that was bordered on the west east of the Sevier orogenic belt in response to tectonic and by the tectonically active highlands of the Sevier orogenic sediment loading (Jordan, 1981), and during much of the Cre- belt that supplied sediment to eastward-flowing streams. The taceous Period the foreland basin was partially to completely eastern shore was on the stable cratonic platform that sup- flooded by the Western Interior Seaway (fig. 2). The seaway plied little sediment (Molenaar and Rice, 1988; Williams and formed in response to the presence of the subsiding foreland Stelck, 1975). During much of Late Cretaceous time, the sea basin and periodic eustatic rise in sea level. By the close of advanced and retreated across the western part of the fore- the Cretaceous Period (Maastrichtian) the western shoreline of land basin, resulting in a complex pattern of intertonguing the seaway retreated eastward as the foreland basin gradually marine and nonmarine deposits. The nonmarine deposits are filled in with sediments derived from the eroding uplifts of the represented by eastward-thinning clastic wedges of sandstone, Sevier orogenic belt (fig. 2). The end of the Cretaceous also siltstone, shale, and coal. The marine deposits are represented marks the onset of the Laramide orogeny, a period of crustal by westward-thinning tongues of marine shale and siltstone instability and compressional tectonics in the foreland basin (fig. 3). In the eastern and central sediment-starved parts of the that continued into the Eocene. During the Laramide orogeny, seaway, clastic input was minimal, creating conditions favor- the foreland basin was fragmented into numerous smaller able for carbonate deposition. In the eastern and southeastern basins that were each flanked by rising basement-cored uplifts. portions of the SWWP, carbonate-rich sediments are repre- The smaller basins, such as the Greater Green River Basin, sented by the Niobrara Formation and equivalent rocks in the subsided rapidly and were depocenters for the accumulation of lower parts of the Mancos and Steele Shales. thick lacustrine and continental sediments. Present-day structure of the SWWP is largely a result of Laramide compressional deformation that is characterized Niobrara Formation by basement-involved thrust faults (thick-skinned), reverse faults, wrench faults, and strongly folded and faulted anti- The Niobrara Formation and laterally equivalent rocks clines and synclines. The north, south, and eastern margins of were deposited during a period of high eustatic sea level and the basin are formed by Precambrian basement-cored uplifts crustal subsidence in the Western Interior Seaway, result- that in many places have overridden the sedimentary basin ing in a major marine transgression and conditions favorable section along high-angle basement-involved (thick-skinned) for carbonate deposition. In the eastern part of the seaway thrust faults (Ryder, 1988). Several major intrabasin uplifts are where clastic input was minimal, chalks and limestone are present in the SWWP and include the Moxa arch, La Barge the principal lithologies of the Niobrara in the Denver Basin platform, Sandy Bend arch, Rock Springs uplift, Wamsutter of eastern Colorado (Pollastro and Scholle, 1986). In the arch, and Cherokee ridge (fig. 1). Several of these intraba- SWWP, the Niobrara and equivalent rocks reflect a gradual sin uplifts subdivide the SWWP into smaller structural and westward decrease in chalk and other carbonate components topographic subbasins. The most prominent of these uplifts is and an increase in siliciclastic sediments because of its closer the north-south-trending Rock Springs uplift, a doubly plung- proximity to the detrital source, the thrust belt to the west. ing asymmetric anticline that divides the SWWP into nearly In the SWWP, the Niobrara Formation and equivalent rocks equal halves. West of the Rock Springs uplift, the northwest- consist of interbedded organic-rich shale, calcareous shale, southeast-trending Sandy Bend arch extends from the La and marl, with minor amounts of sandstone, siltstone, lime- 4 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
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Niobrara Total Petroleum System in the Southwestern Wyoming Province 5
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Figure 3a,b. Generalized regional stratigraphic cross sections of restored Upper Cretaceous rocks. A–A’ extends east-west across southern Wyoming, and B–B’ extends north-south from northern Wyoming to northwestern Colorado. Cross sections show the complex intertonguing relationship between marine and nonmarine deposits. Rocks of fluvial origin are shown in red; coastal plain, green; marine-shoreline, shelf, and slope sandstones and siltstones, yellow; marine shale, gray; calcareous-rich marine deposits are shown in blue. Modified from Roehler, 1990. 6 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
Trigg #1-5X (a) sec. 5, T. 6 N., R. 86 W.
induction gamma Mancos Shale Texas Buck Knolton 14 Peak Malco bench sec. 8, T. 4 N., R. 91 W. Tennessee 1 Kowach Richfield James 1 Mancos sec. 25, T. 6 N., R. 9 W. 1-A-Govt. sec. 25, T. 6 N., R. 86 W. Shale sec. 14, T. 5 N., R. 88 W. Cabeen Bradley 5 oil 0
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Figure 4. Type log (a) and subsurface cross section (b) of the Niobrara Formation in northwestern Colorado. Marine sandstones are shown in yellow, clay-rich marine shale in gray. Calcareous-rich zones that are more prone to fracturing are highlighted in light blue. Type log modified from Vincelette and Foster, 1992. Subsurface cross section modified from Haskett, 1959. Heavy vertical black bars indicate oil-producing zones. Location of type log and line of section are shown in figure 7. stone, and chalk (Vincelette and Foster, 1992; Longman and surface but is identified by a distinctive high-resisitivity kick others, 1998). A 200- to 400-ft-thick zone directly overlying on electric well logs (fig. 4; also Finn and Johnson, Chapter the Frontier Formation has been identified by Vincelette and 14, this CD–ROM, their pl. 1). Regional studies by Longman Foster (1992) as the Carlile Shale (fig. 4a). This zone, identi- and others (1998) show that the thickness of the Niobrara and fied on well logs by a lower resistivity response than adjacent equivalent rocks varies from less than 100 ft in South Dakota units, consists of clay-rich, noncalcareous marine shale and is to more than 1,800 ft in west-central Wyoming, with a range included with the Niobrara in this study. This conforms with of thickness in the eastern portions of the Southwestern Wyo- usage by numerous authors including Haskett (1959) (fig. 4b), ming Province of 900 to 1,800 ft (fig. 6). This prominent west- Kucera (1959), and Hale (1961). To the west and north, typical ward thickening of the Niobrara is due to the large influx of Niobrara facies grades into noncalcareous marine shale, sand- siliciclastic sediments derived from actively eroding highlands stone, and siltstone of the Baxter Shale (fig. 3a; also Finn and of the thrust belt to the west. Johnson, Chapter 14, this CD–ROM, their pl. 1). The west- The age of the Niobrara is considered to be Coniacian ern limit of this facies change occurs in the deeper, sparsely to Santonian (Hale, 1961), and Bader and others (1983) show drilled portions of the Sand Wash, Washakie, and Great Divide that the upper part of the Niobrara corresponds to the ammo- Basins; however, the limits of the Niobrara calcareous facies nite zone of Desmoscaphites bassleri. This species, which is of the Niobrara are not well mapped. late Santonian, also occurs in a marker bed several hundred The Niobrara is underlain by the Frontier Formation feet below the top of the Airport Sandstone Member of the and overlain by a thick sequence of marine shale. In the Sand Baxter Shale in exposures on the Rock Springs uplift (Smith, Wash Basin in Colorado, the thick overlying marine section is 1965). A similar age is reported by Smith (1961) for the basal named the Mancos Shale; in the Washakie and Great Divide part of the Lazeart Sandstone Member of the Adaville Forma- Basins of Wyoming it is named the Steele Shale (fig. 5). The tion in the thrust belt near Kemmerer, Wyoming. basal contact of the Niobrara is well defined and coincides with the uppermost sandstone of the underlying Frontier Formation. The upper contact is transitional with the overlying Mancos or Steele Shales and is not well defined in the sub- Niobrara Total Petroleum System in the Southwestern Wyoming Province 7
Green River Great Divide Rock Springs Sand Wash and Hoback and AGE Basins and uplift Basin Moxa arch Washakie Basins
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Figure 5. Generalized correlation chart of Cretaceous and lower Tertiary rocks in the Southwestern Wyoming Province. The stratigraphic interval contained in the Niobrara Total Petroleum System is highlighted in light blue. Modified from Ryder, 1988. 8 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
111° 108 107° 110° 109° °
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Figure 6. Isopach map showing the distribution and thickness of the Niobrara Formation in the Southwestern Wyoming province. Thickness in feet. Contour interval = 300 feet. Modified from Longman and others, 1998. Niobrara Total Petroleum System in the Southwestern Wyoming Province 9
kerogen. The Type- III kerogen was most likely derived from Niobrara Total Petroleum System terrestrial sources along the western shoreline, and (or) from cavings from the overlying Mancos Shale. TOC values for the (503703) same 28 Niobrara samples ranged from 0.85 to 2.75 weight percent (fig. 8a) with an average of 1.85 weight percent. For The Niobrara Total Petroleum System (TPS) (503703) samples with Tmax values less than 435 (fig. 8a), TOC content produces self-sourced oil and natural gas from low-permeabil- increased and ranged from 1.1 to 2.75 weight percent with an ity, fractured, carbonate-rich rocks in the Upper Cretaceous average of 2.06 weight percent. Niobrara Formation and from its stratigraphic equivalents in the lower parts of the Mancos and Steele Shales. The Niobrara TPS encompasses the eastern one-third of the SWWP and covers approximately 8,400 mi2 of the Sand Wash, Washakie, Reservoir Rocks and Great Divide Basins (fig. 7). The eastern and southeastern boundaries of the TPS are defined by the base of the outcrop The lithology of the Niobrara Formation and equiva- of the Niobrara or equivalent rocks at the base of Mancos or lent strata in the eastern part of the province is composed of Steele Shales. The southern boundary is the Axial Basin uplift interbedded calcareous shale, shaley limestone, and marls. The (fig. 1). The western boundary is poorly defined because few fine-grained nature of these lithologies results in little matrix wells have penetrated the Niobrara in the deeper portions porosity or permeability, therefore production is dependent on of the Sand Wash, Washakie, and Great Divide Basins. The fractures. Vincelette and Foster (1992) have identified several western boundary is defined here to coincide with subsurface discrete zones of calcareous shale, shaley limestone, or marl mapping presented by Barlow and others (1994). Oil produc- informally referred to as the Wolf Mountain, Tow Creek, and tion in this area appears to be restricted to the Niobrara, and Buck Peak benches that account for the majority of Niobrara geochemical data indicate no other reservoirs contain Niobrara production (fig. 4). These benches have greater carbonate sourced oil (P.G. Lillis, oral commun., 2002,). Thus, strati- content than adjacent, more shaley beds and form hard, brittle graphically, the Niobrara TPS is constrained to the Niobrara zones that promote fracturing and produce good reservoirs. Formation and equivalent rocks (fig. 5). Reservoir zones typically range from 50 to 200 ft in thickness, but in some wells may be as much as about 400 ft thick. Detailed fracture studies from surface and subsurface rocks in northwest Colorado show that the fractures in the Source Rocks Niobrara are mineralized and contain either calcite, quartz, or gypsum (Vincelette and Foster, 1992; Saterdal, 1955). These Regional studies by Landon and others (2001) and vein-filling minerals may partially line or completely plug Meissner and others (1984) indicate that organic matter in fracture systems and reduce permeability. Vincelette and Fos- the Niobrara Formation is Type-II, oil-prone kerogen. The ter (1992) found that many fractures associated with Laramide sediments were deposited during a major marine trangressive structures were completely filled by calcite, which resulted in cycle known as the Niobrara cyclothem (Kauffman, 1977) poor reservoir performance. In contrast, the best production during which highstand conditions persisted. This resulted in is from reservoirs with fractures associated with Neogene-age basin deepening that produced anoxic conditions favorable to extensional features where calcite cements line, rather than the preservation of organic matter. According to Landon and plug, fractures within productive reservoirs (Vincelette and others (2001), the richest source rocks are in the Denver Basin Foster, 1992). Vincelette and Foster also found that many of with total organic carbon (TOC) contents ranging up to about the productive fractures associated with Laramide structures 8 weight percent in areas that were sediment-starved along that were initially plugged with calcite vein material were the foreland basin. Westward into south-central Wyoming the reopened by later Neogene extension, particularly in areas TOC content decreases to 2.14 weight percent (Landon and where preexisting fractures were oriented with structures others, 2001) because influx of siliciclastic sediments from the formed by post-Laramide extension. In these cases, fractures western area diluted the organic matter. Vincelette and Foster were reopened along preexisting zones of weakness and lined, (1992) report that up to 700 ft of the Niobrara has source-rock but not completely filled, by calcite. potential in northwestern Colorado. Rock-Eval data for 28 Niobrara samples (U.S. Geological Survey Organic Geochemistry Database) from well cuttings from northwestern Colorado and south-central Wyoming Seal Rocks were plotted on a modified van Krevelen diagram (Espit- alie and others, 1977), a graph of S2 vs. %TOC (Langford A thick section (up to several thousand feet) of soft, and Blanc-Valleron, 1990), and a hydrogen index vs. Tmax noncalcareous Mancos or Steele Shale provides a good qual- diagram (Bordenave and others, 1993) (fig. 8). These plots ity regional seal over the Niobrara Formation within the TPS indicate that the majority of the samples are Type-II, oil- (Finn and Johnson, Chapter 14, this CD–ROM, their plate 1). prone kerogen with some mixing from Type-III, gas-prone In addition, hard, brittle reservoir zones within the Niobrara 10 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
111° 108 107° 110° 109° °
T 40 Teton N
T Fremont 43° 35 N Natrona
P in e d a l Hoback e a n Basin t T ic 30 l in N Sublette e
La Barge platform
Lincoln
T T 25 Southland Royalty 25 N 1 Eagles Nest N S a nd 42° y B 2 e Great Divide 4 n ,0 d 0
t 2 0
a f Basin 0 r i ,0
ch l 00 Carbon p
M u
o
x
a
s
T g Sweetwater T
20 n
i 20
N n r Wamsutter N i a p rch s
S
a
B 0 0
0 a , Western limit of Niobrara Formation 6 0
r 1 0 k
c from Barlow and others (1994) ,0 c
Uinta h r 2 o 1
e 0 R v 0 i ,0 8 Koch Expl. 4 R T , 1 Adobe Town 0 T 15 0 15 0 N Washakie N n 2 e 4 ,0 Basin e 0 r 2 0 0 , G 0 0 0 WY R 85W 41° R 115W R 110W R 105W R 100W R 95W R 90W idge 0 Cherokee r 0 0 , 6 1 1 T 6, Daggett 00 10 Summit 0 N 2 0,000 Sand Wash Basin Uintah Duchesne 00 Jackson 12,0 UT Texas Pacific Type log 1 Bear 8 ,000 Routt T Moffat 5 N Cross section Southwestern Wyoming Province 0 0 0 shown in figure 4 , CO 4 Outcrop of Niobrara Formation Grand and equivalent strata
40° Niobrara TPS 503703 Rio Blanco
Location of well used in burial history
0 10 20 30 40 Miles
T 5 S Garfield Eagle
Figure 7. Map of the Southwestern Wyoming Province showing the extent of the Niobrara Total Petroleum System, major structural elements, location of type log and cross section, and location of wells used in burial reconstructions. Contours represent the approximate depth in feet to the base of the Niobrara Formation (modified from Kirschbaum and Roberts, Chapter 5, this CD–ROM). Contour interval = 2,000 feet. Niobrara Total Petroleum System in the Southwestern Wyoming Province 11 )
K 9 C
O (a) R
S 8
M I A
R I
G 7
/ I N O
B 6 II R A C 5 III O R D
Y 4 H S M
A 3 R G I L L
I 2 M (
2 1 S 0 Figure 8. Plots of data from 0 1 2 3 Rock-Eval analysis of kerogen of PERCENT TOTAL ORGANIC CARBON 28 Niobrara Formation samples, USGS Organic Geochemistry 900
T y I Database. The samples are p e (b) 900 (c) from well cuttings from north- 800 I western Colorado and south- 700
central Wyoming (the samples T 750 y p e from northwestern Colorado I 600 I X are from the Sand Wash Basin X E E II D
D 600 N
and the northern part of the N I 500 I N Piceance Basin), (a) S vs. total N
2 E E G organic carbon, (b) hydrogen G
O 450 400 O R R
index vs. Tmax, and (c) modified D D Y Y H van Krevelen diagram. Majority 300 H of samples indicate a Type-II 300 oil-prone kerogen with some 200 Type-III. Black dots represent 150 samples with a Tmax < 435, red III 100 e yp dots represent samples with a T III Tmax ≥ 435. 0 0 400 450 0 50 100 150 200 Tmax OXYGEN INDEX
are overlain, underlain, and (or) interbedded with relatively ductile, clay-rich marine shales that are lower in carbon- Vincelette and Foster (1992), fracture production occurs along ate content than the adjacent reservoirs (fig. 5). These soft, the crestal parts of anticlines as well as down both the steep plastic zones are not as prone to fracturing and tend to seal the and gentle flanks where production is independent of structural hydrocarbons within the fractured reservoirs (Mallory, 1977; closure. These fractures formed in response to folding and Pollastro, 1992). faulting of brittle reservoir rocks during Laramide compres- sional tectonics. Additional production comes from fractures or fracture swarms that are associated with faults and fault sys- tems formed during post-Laramide Neogene extension (Vince- Traps lette and Foster, 1992); the best producing wells intersect or are in close proximity to normal faults. According to Hansen Most Niobrara production is from fractured reservoirs (1986) post-Laramide extension began early in the Miocene associated with Laramide-age folding (fig. 9) and, according to and continued into the Quaternary at a diminishing rate. 12 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
o 107 R 90W Syncline R 85W T 10 Anticline N
Faults, bar and ball on downthrown side 0 0 0 0 0 Structure contours drawn on 0 0 , 0 0 0 S.L. the Dakota Ss. 8 , 0 - 0 , . 6 , Contour level = 1,000 ft. - 2 L 4 - . Moffat - Oil well S Gas well Routt -6,000 4 ,0 0 2 0 , -4, 0 000 0 0 - 2, 00 0 S .L T . S .L 2 5 2 . , 0 ,0 0 0 0 S N 2 . 0 ,00 L. 0 0 0 ,0 4 S.L. 0 00 6, 4,000
Figure 9. Structure contour map on the Dakota Sandstone in the southern part of the Sand Wash Basin showing the relationship of Niobrara Formation production to fold axes and faults. Heavy red line is the Southwestern Wyoming Province boundary. Sources of structural data are Tweto (1976, 1979) and Barlow and Haun, Inc. (1997) Abbreviation S.L., sea level. Thermal Maturity to initiate the thermal cracking of oil to gas is not well known but is generally considered to be greater than 1.35 percent R (Law, 2002); therefore, Niobrara gas accumulations are Thermal maturity mapping of the Niobrara from vitrin- o predicted for those areas where the R is greater than 1.35 per- ite reflectance (R ) data shows a general increase in maturity o o cent. Direct measurements of thermal maturity were unavail- westward into deeper parts of the basin. R values range from o able for Niobrara samples; therefore, R data were collected less than 0.60 percent along the flanks of the basin, to greater o from the overlying coal-bearing Mesaverde Group and Lance than 1.35 percent in the deeper portions of the Sand Wash, and Fort Union Formations (Law, 1984; Pawlewicz and others, Washakie, and Great Divide Basins (fig. 10). Thermal matu- 1986; Merewether and others, 1987; and Pawlewicz and Finn, rity generally reflects the structural configuration of the basin 2002). These data were plotted on a R versus depth graph and and indicates that thermal maturation trends are related to the o a visual best-fit line was drawn through the points and extrapo- structural development of the basin. The 0.60 percent R con- o lated to the Niobrara horizon (fig. 10). tour is the lower limit of oil generation from Type-II organic matter and geographically defines the pod of mature source rock. Areas that are less than 0.60 percent Ro are considered thermally immature, and the generation of oil or thermogenic Hydrocarbon Generation gas is not expected. The “oil window” for Type-II organic mat- ter is commonly considered to be in the range of 0.60 to Burial-history curves using PetroMod1D Express (ver- 1.35 percent Ro (Hunt, 1996). Thus, Niobrara oil accumula- sion 1.1) (see Roberts and others, Chapter 3, this CD–ROM, tions are predicted for those areas where the Ro is between for detailed discussion of petroleum generation models, and 0.60 and 1.35 percent. The level of thermal maturity needed parameters used to generate burial and thermal histories) Niobrara Total Petroleum System in the Southwestern Wyoming Province 13
107 111° 110° 109° 108° °
T 40 Teton N
T Fremont 43° 35 N Natrona
Sublette
T 30 N
Lincoln
T T 25 25 N N
42°
Carbon
Rawlins T Sweetwater T 20 20 N N Western limit of Niobrara Formation 0 .8 from Barlow and others (1994) 0 Uinta
T T 15 15 N N
0 .6 0 WY R 85W 41° R115W R110W R105W R100W R95W R90W
T Daggett 10 Summit N 35 1. Uintah Jackson Duchesne UT 0 .6 0 Routt T Moffat 0.80 5 N Southwestern Wyoming Province 60 CO 0. Outcrop of Niobrara Formation Grand and equivalent strata
40° Niobrara TPS 503703 Rio Blanco Gas well Oil well
0 10 20 30 40 Miles T 5 S Garfield Eagle
Figure 10. Eastern part of the Southwestern Wyoming Province showing variations in levels of thermal maturity based on
vitrinite reflectance (Ro) for the Niobrara Formation and equivalent rocks. 14 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah were generated for three wells to model petroleum-generation that oil is stable at higher maturities. The results of model- history of Niobrara source rocks. The Texas Pacific #1 Bear, ing by Roberts and others (Chapter 3, this CD–ROM) using located along the shallow southeastern flank of the Sand Wash hydrous-pyrolysis kinetic parameters of Tsuzuki and others Basin (fig. 7), is located in sec. 26, T. 7 N., R. 89 W. The Koch (1999) indicate that oil cracking begins at an Ro of about Exploration #1 Adobe Town unit, located in the deep central 1.72 percent, gas generation peaks at around 2.39 percent and part of the Washakie Basin (fig. 7), is in sec. 20, T. 15 N., R. ends around 2.79 percent. The end of the gas-generation phase 97 W. The Southland Royalty #1 Eagles Nest, located in the occurs at a transformation ratio of 0.99, meaning there is virtu- deep northern part of the Great Divide Basin (fig. 7), is located ally a 100-percent reaction and almost no oil left to transform in sec. 29, T. 25 N., R. 91 W. Two types of modeling were to gas. performed on these wells to predict timing and type of hydro- Based on the hydrous-pyrolysis kinetic models, timing carbon generation. They are (1) time-temperature modeling of oil generation in the #1 Bear (fig. 12a) well shows that the based on vitrinite reflectance and (2) kinetic modeling based Niobrara began generating oil at about 70 Ma, remained in on time-temperature integrated with the results of hydrous- the oil window until about 43 Ma, but did not reach thermal pyrolysis experiments. maturities sufficient to crack oil to gas. Oil generation began Time-temperature modeling predicts critical levels of in the 1 Adobe Town well (fig. 12b) around 68 Ma and ended thermal maturity based on vitrinite reflectance by integrating by 60 Ma. Continued burial in the deeper part of the Washakie burial and thermal history and time (fig. 11). The burial- Basin elevated maturities to a level sufficient to crack oil to history curve for the #1 Bear well (fig. 11a) indicates that the gas at around 54 Ma, and with additional burial the Niobrara base of the Niobrara was buried to a depth of approximately passed through the gas window by 48 Ma. Oil generation in 7,500 ft and entered the oil window at about 72 Ma. The top the #1 Eagles Nest well (fig. 12c) began at 66 Ma and ended of the Niobrara passed through the oil window at maximum by 59 Ma. Gas generation from thermal cracking of oil began burial of 12,500 ft and entered the oil cracking phase, which around 54 Ma and continued until about 24 Ma. ended when regional uplift and cooling occurred at about 5 Ma. The burial-history curve for the #1 Adobe Town well (fig. 11b) shows that the Niobrara was buried to a depth of about 9,000 ft when entering the oil window at 71 Ma. With Migration Summary continued subsidence and the accumulation of about 6,500 ft Landon and others (2001) believe that given the fine- of additional sediment along the deep axis of the Washakie grained lithology, low matrix porosity and permeability of the Basin, the Niobrara passed through the oil window at 57 Niobrara, and the integrity of the overlying and interbedded Ma and reached an R of 1.35 percent, the level of thermal o seals, hydrocarbons have not migrated far from where they maturity generally considered sufficient to crack oil to gas. were generated. Figure 10 shows that most of the oil produc- The burial-history curve for the #1 Eagles Nest well (fig. tion from the Niobrara in the SWWP is in areas where the 11c) shows a burial history similar to the #1 Adobe Town Niobrara is thermally mature with respect to oil generation well. In this well the Niobrara was buried to about 11,000 ft (0.60 percent to 1.35 percent R ), with few wells producing and entered the oil window at about 67 Ma. With additional o in areas where the Niobrara is immature or marginally mature accumulation of 7,500 ft of sediments in the deep trough of (R <0.60 percent). This relationship of thermal maturity and the Great Divide Basin, the Niobrara passed through the oil o hydrocarbon production also indicates that most of the oil window by 56 Ma, when the thermal cracking of oil to gas was that has been produced was generated in place and has not then initiated. migrated great distances into less mature reservoirs. Kinetic models were also generated for these same wells and used to reconstruct the maturation history of Type-II oil-prone kerogen (fig. 12). Kinetic modeling by Roberts and others (Chapter 3, this CD–ROM) uses time and temperature Events Chart integrated with results of laboratory hydrous-pyrolysis experi- ments to predict timing and type of hydrocarbon genera- The events chart (fig. 13) summarizes the essential ele- tion. Modeling by Roberts and others (this CD–ROM) using ments of source rock, reservoir rock, seal rock, and overburden hydrous-pyrolysis kinetic parameters of Lewan and Ruble rock and the processes of generation, migration, accumulation, (2002) suggests that oil generation for Type-II kerogen begins and trap formation that are essential to form petroleum accu- at an Ro of 0.68–0.69 percent, peaks at around 0.92 percent, mulations. Deposition of source, reservoir, and intra- and ends at an Ro of 1.13 to 1.17 percent. The bottom of the formational seal rocks of the Niobrara Formation in the oil window occurs at a transformation ratio of 0.99, meaning SWWP occurred during Late Cretaceous time (88–84 Ma). there is essentially no kerogen left to convert to oil. This was followed by deposition of several thousand feet The level of thermal maturity at which oil is cracked to of Mancos Shale, and equivalent Steele Shale, forming a gas is uncertain but is generally thought to be greater than thick, overlying regional seal. Based on burial-history and an Ro of about 1.35 percent (Law, 2002); however, hydrous- petroleum-generation curves, the amount of overburden rock pyrolysis experiments by Tsuzuki and others (1999) indicate required to reach thermal maturation levels sufficient to gener- Niobrara Total Petroleum System in the Southwestern Wyoming Province 15
AGE (Ma) 100 80 60 40 20 0 MESOZOIC (part) CENOZOIC Cretaceous (part) Paleogene Neogene Q 0 Lance Fm )
T Lewis Sh E E F 5,000 Mesaverde Gp N I ( H
T Mancos Sh P E 10,000 %R range D o Niobrara Fm
L 0.60 - 1.35 undifferentiated A
I Mesozoic rocks Phosphoria Fm R 1.35 - 2.00 U
B Lower 15,000 Pennsylvanian (a) rocks
AGE (Ma) 100 80 60 40 20 0 MESOZOIC (part) CENOZOIC Cretaceous (part) Paleogene Neogene Q 0
) Eocene
T 5,000 rocks E E F 10,000 Fort N I
( Union Fm Lance Fm H 15,000
T Lewis Sh
P %R range o Mesaverde Gp E
D 20,000 0.60 - 1.35 Mancos Sh L A
I 1.35 - 2.00 25,000 Niobrara R Frontier Fm
U undifferentiated Mesozoic rocks B 30,000 Phosphoria (b) Fm
AGE (Ma) 100 80 60 40 20 0 MESOZOIC (part) CENOZOIC Q Cretaceous (part) Paleogene Neogene 0 Eocene rocks )
T 5,000
E Fort
E Union Fm F 10,000 N I
( Lance Fm
H 15,000 T Lewis Sh P
E %Ro range Mesaverde Gp
D 20,000
L 0.60 - 1.35 Steele Sh A I Niobrara Fm
R 25,000 1.35 - 2.00 Frontier Fm Mowry Sh
U Jurassic and Lower Triassic rocks Cretaceous rocks B 30,000 Phosphoria (c) Fm
Figure 11. Burial-history curves showing levels of thermal maturity based on vitrinite reflectance for the (a) Texas Pacific #1 Bear, (b) Koch Exploration #1 Adobe Town, and (c) Southland Royalty #1 Eagles Nest. Location of wells is shown in figure 7. Burial-history curves were constructed using the program PetroMod1D Express (version 1.1). 16 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
AGE (Ma) 100 80 60 40 20 0 MESOZOIC (part) CENOZOIC Cretaceous (part) Paleogene Neogene Q 0 Lance Fm
) Lewis Sh T E E
F 5,000 Mesaverde Gp N I ( H
T Mancos Sh P E
D 10,000 Niobrara Fm L
A Mesozoic and I Paleozoic rocks R undifferentiated U B 15,000 (a)
AGE (Ma) 100 80 60 40 20 0 MESOZOIC (part) CENOZOIC Cretaceous (part) Paleogene Neogene Q 0
) Eocene 5,000 T rocks E E
F 10,000
N Fort I
( Union Fm
H Lance Fm
T 15,000 Lewis Sh P
E Mesaverde Gp
D 20,000 L Steele Sh A I
R 25,000 Niobrara Fm
U Mesozoic and Paleozoic rocks B undifferentiated 30,000 (b)
AGE (Ma) 100 80 60 40 20 0 MESOZOIC (part) CENOZOIC Cretaceous (part) Paleogene Neogene Q 0 Eocene rocks )
T 5,000
E Fort
E Union Fm F 10,000 N I ( Lance Fm H
T 15,000 Lewis Sh P
E Mesaverde Gp D 20,000 L Steele Sh A I Niobrara Fm R 25,000
U Mesozoic and Paleozoic rocks B undifferentiated 30,000 (c)
Figure 12. Burial-history curves showing maturation history of Type-II kerogen based on hydrous-pyrolysis kinetic models for the (a) Texas Pacific #1 Bear, (b) Koch Exploration #1 Adobe Town, and (c) Southland Royalty #1 Eagles Nest. Location of wells is shown in figure 7. Burial-history curves were constructed using the program PetroMod1D Express (version 1.1). Green represents the oil window. Red represents the gas-generation phase from the thermal cracking of oil. Niobrara Total Petroleum System in the Southwestern Wyoming Province 17
150 125 100 75 50 25 0 Geologic time scale MESOZOIC CENOZOIC (m.y.b.p.) 1 6 2 4 5 4 Petroleum 4 Cretaceous Paleogene Neogene O P l a system events i g Early Late l . Eocene Mioc. Po .
Kn Km Kmv Tfu Twgr undiff. undiff. ROCK UNIT Kle Kl SOURCE ROCK RESERVOIR ROCK SEAL ROCK OVERBURDEN ROCK Neogene Laramide orogeny extension TRAP FORMATION oil generation from Type II kerogen N ION- ON- ATIO RAT RATI UMUL gas generation from cracking of oil GENE MIG ACC COMMENTS PRESERVATION onset of thermal cracking of oil CRITICAL MOMENT
Figure 13. Events chart showing the relationship of essential geological elements and processes for the Niobrara Total Petroleum System and assessment units. Kn, Niobrara Formation; Km, Mancos or Steele Shale; Kmv, Mesaverde Group; Kle, Lewis Shale; Kl, Lance Formation; Tfu, Fort Union Formation; Twgr, Wasatch and Green River Formations; undiff., undifferentiated Oligocene or Miocene deposits.
Assessment Units in the Niobrara Total Petroleum System (503703) ate oil occurred at about 72 Ma. Critical overburden deposition An assessment unit (AU) is a mappable volume of rock was from the Mancos Shale and equivalent rocks and several within a Total Petroleum System that contains known or pos- thousand feet of the Mesaverde Group. Along the shallow tulated oil and gas accumulations that share similar geologic flanks of the basin, the Niobrara remained in the oil window characteristics (Klett and others, 2000). The Niobrara Total until about 10–5 Ma when regional uplift and erosion occurred Petroleum System (503703) is subdivided into two AUs based (preservation time). With continued subsidence and the addi- on levels of thermal maturity: (1) a continuous oil assessment tional accumulation of 6,500 to 7,500 ft of sediments of the unit (AU 50370361), and (2) a continuous gas assessment unit Lewis Shale, Lance Formation, and the Fort Union Formation (AU 50370362). in the deeper parts of the basin, the Niobrara passed through the oil window at approximately 59 Ma, reaching thermal maturities greater than 1.35 percent R and initiating thermal o Niobrara Continuous Oil Assessment Unit cracking of oil to gas (critical moment). Because mainly oil is produced from Niobrara reservoirs that are thermally mature (AU 50370361) with respect to oil (R 0.60 to 1.35 percent) and Niobrara- o The Niobrara Continuous Oil Assessment Unit (AU sourced oil is not found in other reservoir rocks, it appears 50370361) is an established AU that produces mainly oil from that little or no migration has taken place. Fractures that form self-sourced, organic-rich fractured carbonate-rock reservoirs. the reservoirs in brittle, calcareous-rich zones of the Niobrara It encompasses approximately 4,600 mi2 and occupies the developed as a result of Laramide compressional tectonics, southern and eastern flanks of the Sand Wash Basin in Colo- and during post-Laramide Neogene extension. These fractured rado and the eastern flanks of the Washakie and Great Divide carbonate-rock reservoirs are associated with anticlinal, syn- Basins in Wyoming (fig. 14). The eastern boundary of the clinal, and monoclinal folds, and fault zones. 18 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
111° 107° 110° 109° 108°
T 40 Teton N
T Fremont 43° 35 N Natrona
P in e d a le Hoback a n Basin t T ic 30 l Sublette in N e La Barge platform
Lincoln
T T 25 25 N N S an dy 42° 2 B Great Divide 4 e ,0 n 0 d 0 t 20 a f Basin ,
i 0 r l
ch 00 Carbon p
M u
o
x a Sweetwater
s T g T 20 n 20
i
N n r Wamsutter N i ar p ch s
S a
B 0 0 0 , a Western limit of Niobrara Formation 6 0 1
r 0 k
c from Barlow and others (1994) ,0 c
h 2
Uinta r 1 o e 0
R 0 v 0 i , 8 4 R , T 0 T 15 0 15 0 N Washakie N n 2 e 4 ,0 Basin e 2 0 r 0 0 ,0 G 0 0 WY R 85W 41° R 115W R 110W R 105W R 100W R 95W R 90W idge 0 Cherokee r 0 0 , 6 1 1 60 T Daggett 00 10 Summit N 20,000 Sand Wash Basin Uintah Duchesne 00 Jackson 12,0 UT 8 ,000 Routt T Moffat 5 N Southwestern Wyoming Province 0 0 0 , Outcrop of Niobrara Formation CO 4 and equivalent strata Grand
Niobrara TPS 503703
40° Niobrara Continuous Oil Rio Blanco AU 50370361
Gas well Oil well
T 0 10 20 30 40 Miles 5 S Garfield Eagle
Figure 14. Southwestern Wyoming Province showing the areal extent of the Niobrara Continuous Oil Assessment Unit (AU 50370361) and location of producing wells. Contours show approximate depth to the base of the Niobrara Formation (modified from Kirschbaum and Roberts, Chapter 5, this CD–ROM). Contour interval = 2,000 feet. Niobrara Total Petroleum System in the Southwestern Wyoming Province 19
Continuous Oil AU, coinciding with the TPS boundary, is the future discoveries in this oil AU would most likely be “sweet base of the outcrop of the Niobrara Formation or stratigraphic spots” formed by fractures associated with Laramide com- equivalent strata at the base of the Mancos or Steele Shales; pressional features, including faults, anticlinal and synclinal the southern boundary is the Axial Basin uplift (fig. 1). The trends, regional lineament features, and faulting associated western boundary is defined by the 1.35 percent Ro contour, with post-Laramide Neogene extension. which is generally considered the bottom of the oil window and the level of thermal maturity at which oil cracks to gas (Law, 2002; Hunt, 1996). This AU is restricted stratigraphi- cally to the Niobrara and equivalent rocks because there is no Niobrara Continuous Gas Assessment Unit geochemical evidence of Niobrara-type oils being produced (AU 50370362) from other reservoir rocks, indicating little or no migration has taken place. This is due to the presence of overlying and The Niobrara Continuous Gas Assessment Unit (AU interbedded thick plastic shales that seal fractured Niobrara 50370362) is a hypothetical assessment unit that is believed reservoirs and provide a barrier to migration. to have the potential to produce natural gas from fractured The oil accumulations in the Niobrara and equivalent carbonate-rich reservoirs in the Niobrara Formation and strata are of the continuous type (unconventional) using the equivalent rocks. The gas is self-sourced and believed to criteria established by Schmoker (1996) and have the follow- have originated from the thermal cracking of self-sourced ing characteristics: oil within the Niobrara Formation. The gas AU encompasses approximately 3,800 mi2 of the western half of the Niobrara TPS and is located in the deeper downdip parts of the Sand 1. Lack of downdip water contacts (Vincelette and Foster, Wash, Washakie, and Great Divide Basins (fig. 15). The 1992; Mallory, 1977). eastern boundary of the gas AU is defined by the 1.35 percent 2. Little or no water production (Vincelette and Foster, Ro contour, the level of thermal maturity that is commonly 1992; Cummings, 1961). believed to be the bottom of the oil window and the level at which oil begins to thermally crack to gas (Law, 2002; Hunt, 3. Production extending downdip off structure into 1996). The southern boundary of the gas AU is defined by the synclinal areas (Vincelette and Foster, 1992). Axial Basin uplift (fig. 1). The western boundary of the AU is the western limit of the Niobrara facies in the subsurface as 4. Abnormally pressured (either high or low). Vincelette mapped by Barlow and others (1994) and coincides with the and Foster (1992) report a pressure gradient for the western boundary of the Niobrara TPS. Good-quality top seals Buck Peak field of 0.285 psi/ft calculated from drill- are provided by the soft plastic shales of the overlying Mancos stem tests. Unpublished pressure data reports pressure and Steele Shales that act as a barrier for vertical migration; gradients for fields in the Sand Wash Basin range from thus, the gas AU is restricted stratigraphically to the Niobrara 0.25 to nearly 0.40 pounds per square inch per foot interval. (P.H. Nelson and J. Kibler, U.S. Geological Survey, The Niobrara Continuous Gas Assessment Unit (AU oral commun., 2003). 50370362) is characterized as a continuous-type (unconven- 5. Production independent of structural closure (Vince- tional) accumulation with the characteristics of an indirect lette and Foster, 1992; Mallory, 1977). basin-centered gas system (BCGS) as defined by Law (2000, 2002). Some of the characteristics of the Niobrara Continuous 6. Reservoirs in close association with source rocks. Gas Assessment unit that are attributes of an indirect BCGS include: The Niobrara Continuous Oil Assessment Unit is lightly explored with the first oil discovery at Tow Creek in 1924 1. Oil-prone source rock. (Ogle, 1961). Since then, according to the IHS Energy Group (2001) production database, 337 wells have penetrated the 2. Pressure mechanism – oil cracking. Niobrara section, with approximately 150 producing from the 3. Top of BCGS greater than 1.3–1.4 percent R . Niobrara. Producing intervals range in depth from 1,200 to o nearly 10,000 ft. According to Vincelette and Foster (1992), 4. Abnormally pressured. Reservoir pressures reported most production to date is associated with the Laramide-age for the Twin Buttes field in the northern part of the anticlinal and monoclinal folds (fig. 9), and many of these Great Divide Basin indicate a pressure gradient for the accumulations (“sweet spots”) were discovered while drilling Niobrara reservoir of 0.28 psi/ft (Kendell, 1979). known structures for deeper conventional reservoir horizons. Due to the underpressured nature of the Niobrara reservoirs, 5. Good quality top seal. it is suggested that many potential producers were bypassed because conventional drilling muds may have caused forma- Little exploration has taken place within this gas AU tion damage or masked shows in the Niobrara. Potential for because throughout most of the AU the top of the Niobrara 20 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
111° 107° 110° 109° 108°
T 40 Teton N
T Fremont 43° 35 N Natrona
P in e d a Hoback le Basin a n t T ic 30 l Sublette in N e La Barge platform
Lincoln Twin Buttes T T 25 25 N N S an dy 42° 2 B Great Divide 4 e ,0 n 0 0 d t 20 f Basin ,
a i 0 rc l 00 Carbon
h p
M u
o
x a Sweetwater
s T T 20 g
n 20
i
N n
r Wamsutter N i arc p h s
S
a
B 0 0 0 ,
a k Western limit of Niobrara Formation 6 0 c 1 r 0
c 0
o from Barlow and others (1994) ,
h r 2
Uinta R 1 e 0 0 v 0 i , 8 4 R , T 0 T 15 0 0 15 N Washakie N n 2 e 4 ,0 e 0 Basin r 2 0 0 , G 0 0 0 WY R 85W 41° R 115W R 110W R 105W R 100W R 95W R 90W Cherokee ridge 0 Shell 0 0 Creek , 6 1 T 1 6, Daggett 00 10 Summit 0 N 20,000 Sand Wash Basin Uintah Duchesne 00 Jackson UT 12,0
8 ,000 Routt T Moffat 5 N
0 Southwestern Wyoming Province 0 0 , CO 4 Outcrop of Niobrara Formation Grand and equivalent strata
Niobrara TPS 503703 40°
Niobrara Continuous Gas Rio Blanco AU 50370362
Gas well
0 10 20 30 40 Miles T 5 S Garfield Eagle
Figure 15. Southwestern Wyoming Province showing the areal extent of the Niobrara Continuous Gas Assessment Unit (AU 5037062) and location of producing wells. Contours show approximate depth to the base of the Niobrara Formation (modified from Kirschbaum and Roberts, Chapter 5, this CD–ROM). Contour interval = 2,000 feet. Niobrara Total Petroleum System in the Southwestern Wyoming Province 21 occurs at depths ranging from 12,000 to greater than and synclinal trends, faults and fault zones, and plunges of 22,000 ft in the deep troughs of the Sand Wash, Washakie, and folds, identified from published geologic and structure maps Great Divide Basins (fig. 15). According to IHS Energy Group (Love and Christiansen 1985; Tweto, 1976, 1979; Barlow and (2001) database, only two fields produce or have produced Haun, Inc., 1997). These areas would most likely occur in natural gas from single wells in the Niobrara or equivalent areas where thermal maturities range from 0.60 to 1.35 percent strata: (1) the Twin Buttes field located in the northern part of Ro. The maximum area would include additional unidenti- the Great Divide Basin, and (2) the Shell Creek field located fied subtle structures and regional lineaments (Maughan and along the southern flank of the Cherokee ridge (fig. 15). Perry, 1986; Thomas, 1971). The minimum area accounts for During the late 1970s, the Twin Buttes field produced natural fractures that contain vein-filling minerals, thus resulting in a gas from what is now a single shut-in well in the Niobrara at loss of permeability and nonproductive reservoirs (Vincelette depths greater than 13,000 ft. Cumulative production in this and Foster, 1992). well was nearly 11 MMCF with no oil or water production Graphs showing the estimated ultimate recovery (EUR) reported. The Shell Creek field currently produces natural distribution for Niobrara wells are shown in figure 16. Figure gas from a single well in the Niobrara at depths greater than 16a shows the distribution for all wells, and figure 16b shows 15,000 ft, with a cumulative production through 1999 of the distribution by discovery thirds. The number of wells in approximately 226 MMCF, 3,694 barrels of water, and no oil. each discovery third is the same, but the time span represented Potential for future discoveries in this AU would most likely for each third may be different. The median EUR for all wells be “sweet spots” associated with fracture development located is 55,000 barrels of oil (BO) with a maximum of 1,600,000 along major structural elements such as the Cherokee ridge BO. Median EURs for the first, second and third discov- and the Wamsutter arch. ery thirds are 150,000, 60,000, and 20,000 BO (Appendix), respectively. Lower EURs for the second and third thirds most likely reflects interference among infill wells drilled at closer than optimal spacing. Assessment Results of Undiscovered The minimum, median, and maximum total recovery Resources per cell for untested cells having potential for additions to reserves over the next 30 years is 1,000 BO, 80,000 BO, and 1,600,000 BO, respectively (Appendix). The median of 80,000 BO is lower than that of the historical first third of 150,000 Niobrara Continuous Oil Assessment Unit BO because we suggest that many of the best locations were (AU 50370361) drilled early, but is higher than the median for the second and third discovery thirds because we believe that if new wells are The Niobrara Continuous Oil Assessment Unit (AU drilled outside areas of established production,they will not 50370361) covers approximately 4,600 mi2 in the eastern por- experience interference problems. tion of the SWWP (fig. 14). Input data for the assessment are The minimum, median, and maximum area per cell of shown on the FORSPAN ASSESSMENT MODEL FORM in untested cells having the potential for additions to reserves the Appendix. The minimum, median, and maximum areas, in the next 30 years is 40, 160, and 400 acres, respectively. in acres, for the AU are 2,622,000, 2,914,000, and 3,205,000 This wide range in drainage areas reflects the variable produc- acres, respectively. The reason for the uncertainty is because tion that can be typical of many fractured reservoirs. Variable the position of the 1.35-percent Ro isotherm, the western production in these wells is due to the rapid lateral changes boundary of the AU, is based on sparse Ro data. The AU is in fracture intensity caused by lithologic variations, fault inten- mostly untested with only 337 wells penetrating the Niobrara sity, and tectonic setting (Vincelette and Foster, 1992). section for a median percentage of 2 percent of the total AU Tabulated results for the Niobrara Continuous Oil Assess- area tested. Of the 337 tested cells, 133 were identified as pro- ment Unit for undiscovered oil, gas, and natural gas liquids ducers for a historical success ratio of 40 percent. This success that have potential for additions to reserves in the next 30 ratio might have possibly been higher, but due to the under- years are summarized in table 1. pressured nature of the Niobrara reservoirs it is believed that many potential producers were bypassed because conventional drilling muds may have caused formation damage or masked Niobrara Continuous Gas Assessment Unit (AU shows in the Niobrara. 50370362) The minimum, median, and maximum percentages of the untested AU area that has potential for additions to reserves The Niobrara Continuous Gas Assessment Unit (AU in the next 30 years are 2, 5, and 8 percent, respectively (see 50370362) was not quantitatively assessed because of the lack Appendix). The median area represents potential fracture of production data or a suitable analog. (“sweet spots”) development associated with known anticlinal 22 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
10,000
1,000 ) L B B
M 100 ( R U E
10
(a) 1 0 10 20 30 40 50 60 70 80 90 100 Percentage of wells
10,000 1st third ? 2nd third 3rd third ? 1,000 ) L B B M
( 100 R U E
10
(b) 1 0 10 20 30 40 50 60 70 80 90 100 Percentage of wells
Figure 16. Estimated ultimate recovery (EUR) for wells in the Niobrara Continuous Oil Assessment Unit (AU 50370361): (a) EUR distribution for all wells, (b) EUR distribution by discovery thirds. Discovery thirds refers to the division into three equal parts of the number of wells drilled in the AU. The wells were ordered by completion date and then divided into three equal or nearly equal num- bers of wells in order to investigate how the EURs have changed with time. The 2.5 and 5.5 MMBBL points indicated by a (?) on the curve for the 1st discovery third are most likely multiple wells listed under a single lease name and probably do not represent EURs for a single well. Niobrara Total Petroleum System in the Southwestern Wyoming Province 23
Table 1. Summary of assessment results for the Niobrara Total Petroleum System.
[MMBO, million barrels of oil. BCFG, billion cubic feet of gas. MMBNGL, million barrels of natural gas liquids. Results shown are fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas liquids) category. F95 denotes a 95-percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. CBG denotes coalbed gas. Shading indicates not applicable]
Total Petroleum Systems Total undiscovered resources (TPS) Field Oil (MMBO) Gas (BCFG) NGL (MMBNGL) and Assessment Units (AU) type Niobrara TPS Oil 66.90 100.50 151.00 103.60 34.90 59.10 99.90 62.20 1.90 3.50 6.50 3.70 Niobrara Continuous Oil AU Gas 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Niobrara Continuous Gas AU Gas Not quantitatively assessed Total continuous resources 66.90 100.50 151.00 103.60 34.90 59.10 99.90 62.20 1.90 3.50 6.50 3.70
References
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Basin-center gas symposium, Oct. 6, 2000, Rocky Mountain Association of Geologists, Program with abstracts, 8 p. Pawlewicz, M.J., and Finn, T.M., 2002, Vitrinite reflectance data for the Greater Green River Basin, southwestern Wyo- Law, B.E., 2002, Basin-centered gas systems: American ming, northwestern Colorado, and northeastern Utah: U.S. Association of Petroleum Geologists Bulletin, v. 86, no. 11, Geological Survey Open-File Report 02–339, 15 p., 1 table, p. 1891–1919. 2 figures. Lewan, M.D., and Ruble, T.E., 2002, Comparison of petro- Pollastro, R.M., 1992, Natural fractures, composition, cyclic- leum generation kinetics by isothermal hydrous and non- ity, and diagenesis of the Upper Cretaceous Niobrara isothermal open-system pyrolysis: Organic Geochemistry, Formation, Berthoud field, Colorado, in Schmoker, J.W., v. 33, p. 1457–1475. 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Pollastro, R.M., and Scholle, P.A., 1986, Exploration and Smith, J.H., 1961, A summary of stratigraphy and paleontol- development of hydrocarbons from low-permeability ogy, Upper Colorado and Montana Groups south-central chalks—An example from the Upper Cretaceous Niobrara Wyoming, northeastern Utah, and northwestern Colorado, Formation, Rocky Mountain Region, in Spencer, C.W., and in Wiloth, G.J., ed., Symposium on Late Cretaceous rocks, Mast, R.F., eds., Geology of tight gas reservoirs: American Wyoming and adjacent areas: Wyoming Geological Asso- Association of Petroleum Geologists Studies in Geology, ciation 16th Annual Field Conference, p. 101–111. no. 24, p. 129–141. Smith, J.H., 1965, A summary of stratigraphy and paleontol- Roberts, L.N.R., and Kirschbaum, M.A., 1995, Paleogeogra- ogy, Upper Colorado and Montana Groups, south-central phy of the Late Cretaceous of the Western Interior of middle Wyoming, northeastern Utah, and northwestern Colorado, North America—Coal distribution and sediment accumula- in DeVoto, R.H., and Bitter, R.K., eds., Sedimentation of tion: U.S. Geological Survey Professional Paper 1561, 115 p. Late Cretaceous and Tertiary outcrops, Rock Springs uplift: Wyoming Geological Association 19th Field Conference, Roehler, H.W., 1990, Stratigraphy of the Mesaverde Group in p. 13–26. the central and eastern Greater Green River Basin, Wyo- ming, Colorado, and Utah: U.S. Geological Survey Profes- Thomas, G.E., 1971, Continental plate tectonics—Southwest sional Paper 1508, 52 p. Wyoming, in Renfro, A.R., ed., Symposium on Wyoming tectonics and their economic significance: Wyoming Geo- Royse, F., Jr., Warner, M.A., and Reese, D.L., 1975, Thrust logical Association Guidebook, p. 103–123. belt structural geometry and related stratigraphic problems, Wyoming−Idaho−northern Utah, in Bolyard, D.W., ed., Tsuzuki, N., Takeda, N., Suzuki, M., and Yoki, K., 1999, The Deep drilling frontiers of the central Rocky Mountains: kinetic modeling of oil cracking by hydrothermal pyrolysis Rocky Mountain Association of Geologists Guidebook , experiments: International Journal of Coal Geology, v. 39, p. 41–54. p. 227–250. Ryder, R.T., 1988, Greater Green River Basin, in Sloss, L.L., Tweto, Ogden, 1976, Geologic map of the Craig 10 ✕ 20 ed., Sedimentary cover—North American craton: The quadrangle, northwestern Colorado: U.S. Geological Survey Geological Society of America, Decade of North American Miscellaneous Investigations Series Map I–972, scale Geology, p. 154–165. 1:250,000. Saterdal, A., 1955, Tow Creek Oil Field Routt County, Colo- Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geo- rado, in Ritzma, H.R., and Oriel, S.S., eds., Guidebook to logical Survey, scale 1:500,000. the geology of northwest Colorado: Intermountain Associa- tion of Petroleum Geologists and Rocky Mountain Associa- Vincelette, R.R., and Foster, N.H., 1992, Fractured Niobrara of tion of Geologists, p. 110–112. northwestern Colorado, in Schmoker, J.W., Coalson, E.B., and Brown, C.A., eds., Geological studies relevant to hori- Schmoker, J.W., 1996, Method for assessing continuous-type zontal drilling—Examples from western North America: (unconventional) hydrocarbon accumulations, in Gautier, Rocky Mountain Association of Geologists Guidebook, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., p. 227–239. eds., 1995 national assessment of United States oil and gas resources—Results, methodology, and supporting data: U.S. Williams, G.D., and Stelck, C.R., 1975, Speculations on the Geological Survey Digital Data Series DDS–30, release 2, Cretaceous paleogeography of North America, in Caldwell, 1 CD–ROM. W.G.E., ed., The Cretaceous System in the Western Interior of North America: Geological Association of Canada Spe- cial Paper 13, p. 1–20. 26 Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah
Appendix A. Data form used for evaluating the Niobrara Continuous Oil Assessment Unit, Niobrara Total Petroleum System, Southwestern Wyoming Province, Wyoming, Colorado, and Utah.
FORSPAN ASSESSMENT MODEL FOR CONTINUOUS ACCUMULATIONS--BASIC INPUT DATA FORM (NOGA, Version 7, 6-30-00)
IDENTIFICATION INFORMATION Assessment Geologist:… T.M. Finn and R.C. Johnson Date: 8/26/2002 Region:…………………… North America Number: 5 Province:…………………. Southwestern Wyoming Number: 5037 Total Petroleum System:. Niobrara Number: 503703 Assessment Unit:………. Niobrara Continuous Oil Number: 50370361 Based on Data as of:…… IHS Energy Group, 2001, NRG 2001 (data current through 1999), Wyoming Oil and Gas Conservation Commission Notes from Assessor…..
CHARACTERISTICS OF ASSESSMENT UNIT
Assessment-Unit type: Oil (<20,000 cfg/bo) or Gas (>20,000 cfg/bo) Oil What is the minimum total recovery per cell?… 0.001 (mmbo for oil A.U.; bcfg for gas A.U.) Number of tested cells:.………… 337 Number of tested cells with total recovery per cell > minimum: ……... 133 Established (>24 cells > min.) X Frontier (1-24 cells) Hypothetical (no cells) Median total recovery per cell (for cells > min.): (mmbo for oil A.U.; bcfg for gas A.U.) 1st 3rd discovered 0.15 2nd 3rd 0.06 3rd 3rd 0.02
Assessment-Unit Probabilities: Attribute Probability of occurrence (0-1.0) 1. CHARGE: Adequate petroleum charge for an untested cell with total recovery > minimum …… 1.0 2. ROCKS: Adequate reservoirs, traps, seals for an untested cell with total recovery > minimum. 1.0 3. TIMING: Favorable geologic timing for an untested cell with total recovery > minimum……….. 1.0
Assessment-Unit GEOLOGIC Probability (Product of 1, 2, and 3):………...... ……. 1.0
4. ACCESS: Adequate location for necessary petroleum-related activities for an untested cell with total recovery > minimum ……………………………………………………………… 1.0
NO. OF UNTESTED CELLS WITH POTENTIAL FOR ADDITIONS TO RESERVES IN THE NEXT 30 YEARS
1. Total assessment-unit area (acres): (uncertainty of a fixed value) minimum 2,622,000 median 2,914,000 maximum 3,205,000
2. Area per cell of untested cells having potential for additions to reserves in next 30 years (acres): (values are inherently variable) calculated mean 173 minimum 40 median 160 maximum 400
3. Percentage of total assessment-unit area that is untested (%): (uncertainty of a fixed value) minimum 97 median 98 maximum 99
4. Percentage of untested assessment-unit area that has potential for additions to reserves in next 30 years (%): ( a necessary criterion is that total recovery per cell > minimum) (uncertainty of a fixed value) minimum 2 median 5 maximum 8 Niobrara Total Petroleum System in the Southwestern Wyoming Province 27
Appendix A. Data form used for evaluating the Niobrara Continuous Oil Assessment Unit, Niobrara Total Petroleum System, Southwestern Wyoming Province, Wyoming, Colorado, and Utah.—Continued Assessment Unit (name, no.) Niobrara Continuous Oil, Assessment Unit 50370361
TOTAL RECOVERY PER CELL
Total recovery per cell for untested cells having potential for additions to reserves in next 30 years: (values are inherently variable) (mmbo for oil A.U.; bcfg for gas A.U.) minimum 0.001 median 0.08 maximum 1.6
AVERAGE COPRODUCT RATIOS FOR UNTESTED CELLS, TO ASSESS COPRODUCTS (uncertainty of fixed but unknown values) Oil assessment unit: minimum median maximum Gas/oil ratio (cfg/bo)………………………...……. 300 600 900 NGL/gas ratio (bngl/mmcfg)………………….…. 30 60 90
Gas assessment unit: Liquids/gas ratio (bliq/mmcfg)….…………..……
SELECTED ANCILLARY DATA FOR UNTESTED CELLS (values are inherently variable) Oil assessment unit: minimum median maximum API gravity of oil (degrees)…………….…………. 35 39 45 Sulfur content of oil (%)………………………...… 0.02 0.1 0.5 Drilling depth (m) ……………...…………….…… 610 1500 2750 Depth (m) of water (if applicable)……………….
Gas assessment unit: Inert-gas content (%)……………………….....….. CO2 content (%)………………………………..….. Hydrogen-sulfide content (%)……………...……. Drilling depth (m)…………………………………. Depth (m) of water (if applicable)……………….
Success ratios: calculated mean minimum median maximum Future success ratio (%).. 60 50 60 70
Historical success ratio, tested cells (%) 40
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