U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2016–3030

June 2016 Printed on recycled paper

National and Global Petroleum Assessment

Assessment of Continuous (Unconventional) Oil and Gas Resources 

in the Late Cretaceous Mancos Shale of the Piceance Basin, 

Uinta-Piceance Province, Colorado and Utah, 2016

Using a geology-based assessment methodology, the U.S. Geological Survey assessed technically recoverable mean resources of 

74 million barrels of shale oil, 66.3 trillion cubic feet of gas, and 45 million barrels of natural gas liquids in the Mancos Shale of 

the Piceance Basin in Colorado and Utah.

Introduction

The U.S. Geological Survey (USGS) 

completed a geology-based assessment 

of the continuous (unconventional) oil 

and gas resources in the Late Cretaceous 

Mancos Shale within the Piceance Basin 

of the Uinta-Piceance Province (fig. 1). 

The previous USGS assessment of the 

Mancos Shale in the Piceance Basin was 

completed in 2003 as part of a comprehensive assessment of the greater UintaPiceance Province (U.S. Geological 

Survey Uinta-Piceance Assessment Team, 

2003). Since the last assessment, more 

than 2,000 wells have been drilled and 

completed in one or more intervals within 

the Mancos Shale of the Piceance Basin 

(IHS Energy Group, 2015). In addition, 

the USGS Energy Resources Program 

drilled a research core in the southern 

Piceance Basin that provided significant 

new geologic and geochemical data that 

were used to refine the 2003 assessment 

of undiscovered, technically recoverable 

oil and gas in the Mancos Shale.

Geologic Summary of the Mancos 

Shale in the Piceance Basin

The Late Cretaceous Mancos Shale 

was deposited within a foreland basin 

setting during the initial development 

and ultimate extension of the Cretaceous 

Western Interior Seaway. At its maximum extent, the Western Interior Seaway 

spanned from the present-day Arctic 

Ocean in the north to the Gulf of Mexico 

in the south, to central Utah in the west, 

and as far east as western Minnesota. 

Figure 1. Map of the Uinta-Piceance Province and extent of six Mancos Shale assessment 

units. (AU, assessment unit)

The central part of the foreland basin 

was later segmented into multiple Rocky 

Mountain basins, including the Piceance 

Basin, during the Laramide orogeny. 

The Mancos Shale in the Piceance Basin 

represents more than 29 million years of 

deposition (Cobban and others, 1994). 

The unit is more than 4,000 feet thick 

throughout most of the basin and is dominated by mudrocks with varying amounts 

of terrigenous siliciclastic and biogenic 

calcite deposited in an offshore marine 

0 25 50 MILES

0 25 50 KILOMETERS

Source: National Park Service

Vernal

COLORADO

UTAH

Douglas Creek arch

Gunnison

Grand

Junction

Glenwood

Springs

COLORADO

UTAH

WYOMING

ARIZONA NEW 

MEXICO

IDAHO

Area

of study

Uinta-Piceance Province (part)

Distal Frontier Conventional AU

Lower Mancos Tight Gas AU

Lower NiobraraTight Gas AU

Upper Niobrara Shale Gas AU

EXPLANATION

Upper Niobrara Shale Oil AU

Upper Mancos Tight Gas AU

–109° –108° –107°

40°

39°
setting. The percentage of biogenic 

calcite in the Mancos Shale is highest in 

the lower half of the formation (Johnson 

and Rice, 1990). Organic matter is also 

highest in the lower half, consisting of 

Type II and mixed Type II/III kerogen 

with as much as 4.9 weight percent total 

organic matter (Johnson and Rice, 1990; 

Hawkins and others, 2012). In the upper 

half of the Mancos Shale, discontinuous 

siltstone and sandstone bodies represent 

distal to proximal shoreface deposits 

(Kirschbaum, 2003). 

Geologic Model for Assessment

The primary hydrocarbon source 

rock for the Mancos/Mowry Total 

Petroleum System (TPS) is the Mancos 

Shale. Hydrocarbons generated in the 

Mancos Shale have migrated into tight 

(that is, continuous) reservoirs within 

the formation and into conventional 

reservoirs both above and below the 

formation. Tight reservoirs within the 

Mancos Shale include (1) offshore 

marine siltstones and sandstones in the 

basal members of the Mancos Shale; 

(2) sandstones and siltstones of the distal 

shoreface to tidally influenced Prairie 

Canyon Member (Cole and others, 

1997), including the Mancos “B” zone 

of Kopper (1962); and (3) shoreface 

sandstones in the upper Mancos Shale 

and Late Cretaceous Iles Formation. 

Mancos Shale gas also migrated into conventional reservoirs in the Piceance Basin 

along the Douglas Creek arch (Lillis and 

others, 2003). Hydrocarbons were also 

generated and retained within the calcareous, organic-rich Niobrara Member in the 

lower part of the Mancos Shale.

Tight gas in the younger, shallower 

Mancos Shale intervals is produced 

primarily from vertical and directional 

wells in which the reservoirs have been 

hydraulically fractured. The tight-gas 

production is reported as commingled 

production with the overlying Williams 

Fork Formation. Tight gas and continuous oil and gas in the older and deeper 

intervals of the Mancos Shale are produced mostly from horizontal wells that 

have been hydraulically fractured.

Assessment Units

The Late Cretaceous Mancos Shale 

was assessed previously as a single 

interval. The assessment combined all 

of the continuous accumulation resources 

within the Mancos and Mowry Shales 

into one set of three assessment units 

(AUs) that addressed the hydrocarbon 

resources in the entirety of the UintaPiceance Province (Kirschbaum, 2003). 

This current assessment of the Mancos 

Shale differs from the previous assessment in two ways: (1) it only assesses 

the Mancos Shale within the Piceance 

Basin and (2) the Mancos Shale strata 

are subdivided vertically into separate 

AUs. Each AU differs in geological 

characteristics that control hydrocarbon 

Assessment input data–Continuous AUs

Lower Mancos Tight Gas AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 50,000 1,550,000 4,651,000 2,083,667

Average drainage area of wells (acres) 100 160 220 160

Success ratio (%) 30 60 90 60

Average EUR (BCFG) 0.08 0.6 1.2 0.622

AU probability 1.0

Lower Niobrara Tight Gas AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 50,000 2,000,000 4,651,000 2,233,667

Average drainage area of wells (acres) 100 160 220 160

Success ratio (%) 60 85 95 80

Average EUR (BCFG) 0.4 0.7 1.7 0.753

AU probability 1.0

Upper Niobrara Shale Gas AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 50,000 1,000,000 3,800,000 1,616,667

Average drainage area of wells (acres) 100 160 220 160

Success ratio (%) 60 85 95 80

Average EUR (BCFG) 0.5 0.8 2.6 0.901

AU probability 1.0

Upper Niobrara Shale Oil AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 5,000 1,000,000 2,000,000 1,001,667

Average drainage area of wells (acres) 100 160 220 160

Success ratio (%) 10 30 75 38.33

Average EUR (MMBO) 0.01 0.03 0.08 0.032

AU probability 0.95

Upper Mancos Tight Gas AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 2,220,000 2,800,000 3,742,000 2,914,000

Average drainage area of wells (acres) 10 20 40 23.33

Success ratio (%) 80 92 94 88.67

Average EUR (BCFG) 0.15 0.4 0.65 0.408

AU probability 1.0

Table 1. Key assessment input data for five continuous assessment units in the Mancos 

Shale, Piceance Basin.

[AU, assessment unit; %, percent; EUR, estimated ultimate recovery per well; BCFG, billion cubic feet of gas; 

MMBO, million barrels of oil. The average EUR input is the minimum, median, maximum, and calculated mean. 

Shading indicates not applicable]

accumulation type and extent, the type 

of drilling and completion techniques 

applied, and the distribution of estimated 

ultimate recovery volumes of producing 

wells. These key factors influenced the 

input data used to assess the technically 

recoverable hydrocarbons in the Mancos 

Shale (table 1).

Five continuous AUs were defined 

in the Piceance Basin Mancos Shale 

for this assessment of the Mancos/

Mowry TPS: (1) Lower Mancos Tight 

Gas AU, (2) Lower Niobrara Tight 

Gas AU, (3) Upper Niobrara Shale 

Gas AU, (4) Upper Niobrara Shale 

Oil AU, and (5) Upper Mancos Tight 

Gas AU (fig. 1). One conventional 

AU, the Distal Frontier Conventional 

AU, was also identified but was not 
quantitatively assessed. The Distal 

Frontier Conventional AU is hypothetical 

and defined by the presence of the 

Frontier Sandstone where it interfingers 

with the Mancos Shale as determined by 

examination of well logs.

The Lower Mancos Tight Gas 

AU includes tight-gas reservoirs in the 

Mowry Shale (where present) and in 

the Mancos Shale above the Dakota 

Sandstone and below the Niobrara 

Member of the Mancos Shale. The AU 

boundary is defined by the outcrop of 

the Mancos Shale to the north, east, and 

south, and along the axis of the Douglas 

Creek arch to the west (fig. 1).

The Lower Niobrara Tight Gas 

AU includes the bioturbated siliciclastic interval in the basal 250–400 feet 

of the Niobrara Member. The geographic 

extents of the Lower Niobrara Tight Gas 

AU and the Lower Mancos Tight Gas AU 

are the same.

The Upper Niobrara Shale Gas 

AU and Upper Niobrara Shale Oil AU 

include the portion of the Niobrara 

Member that is rich in organic matter 

and biogenic calcite (Hawkins and 

others, 2012). The AU boundaries were 

drawn using mapped vitrinite reflectance 

values from the base of the Mancos 

Shale. Vitrinite reflectance values 

between 0.6 percent and 1.35 percent 

(Peters and Cassa, 1994) were used to 

define the Upper Niobrara Shale Oil AU. 

Vitrinite reflectance values greater than 

1.35 percent (Peters and Cassa, 1994) 

were used to define the Upper Niobrara 

Shale Gas AU.

The Upper Mancos Tight Gas AU 

consists of proximal to distal shoreface 

deposits and offshore marine sandy 

siltstones in the upper Mancos Shale and 

the Iles Formation above the Niobrara 

Member and below the Williams Fork 

Formation. Gas production from the 

Upper Mancos Tight Gas AU is commonly reported as commingled with production from the overlying Williams Fork 

Formation. The Upper Mancos Tight Gas 

AU boundary was drawn on the outcrop 

of the Iles Formation.

Resource Summary

The U.S. Geological Survey (USGS) 

assessed undiscovered, technically 

recoverable continuous (unconventional) 

oil and gas resources in the Mancos 

Shale of the Piceance Basin (table 2). 

The USGS estimated mean volumes of 

66.3 trillion cubic feet of gas, 74 million 

barrels of oil, and 45 million barrels 

of natural gas liquids in the Mancos/

Mowry Total Petroleum System. All of 

the undiscovered hydrocarbon resources 

quantitatively assessed are continuous 

(unconventional) reservoirs. The 

Distal Frontier Conventional Assessment 

Unit was not quantitatively assessed.

Total petroleum system (TPS) 

and assessment units (AUs)

AU 

probability

Accumulation 

type

Total undiscovered resources

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Mancos/Mowry TPS

Lower Mancos Tight Gas AU 1.0 Gas 1,266 4,343 10,297 4,875 1 3 10 4

Lower Niobrara Tight Gas AU 1.0 Gas 2,482 7,837 16,366 8,441 1 6 16 7

Upper Niobrara Shale Gas AU 1.0 Gas 1,875 6,360 15,659 7,268 1 5 15 6

Upper Niobrara Shale Oil AU 0.95 Oil 0 64 175 74 0 71 218 86 0 4 14 5

Upper Mancos Tight Gas AU 1.0 Gas 28,498 43,773 69,129 45,661 6 21 44 23

Total undiscovered 

continuous resources 0 64 175 74 34,121 62,384 111,669 66,331 9 39 99 45

Distal Frontier Conventional AU Not quantitatively assessed

Total undiscovered continuous

and conventional resources 0 64 175 74 34,121 62,384 111,669 66,331 9 39 99 45

Table 2. Assessment results for continuous and conventional oil and gas resources in the Mancos Shale, Piceance Basin.

[TPS, total petroleum system; AU, assessment unit; MMBO, million barrels of oil; BCFG, billion cubic feet of gas; NGL, natural gas liquids; MMBNGL, million barrels of 

natural gas liquids. Results shown are fully risked estimates. For gas accumulations, all liquids are included under the NGL category. F95 represents 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. Shading indicates not applicable]

Outcrop of the Niobrara Member of the Mancos Shale known as The Castle, near Delta, 

Colorado. Photograph by Joshua Hicks, U.S. Geological Survey.
Mancos Shale Assessment Team

Sarah J. Hawkins (Task Leader, shawkins@usgs.gov), Ronald R. Charpentier, 

Christopher J. Schenk, Heidi M. Leathers-Miller, Timothy R. Klett, Michael E. Brownfield, 

Tom M. Finn, Stephanie B. Gaswirth, Kristen R. Marra, Phuong A. Le, Tracey J. Mercier, 

Janet K. Pitman, and Marilyn E. Tennyson

For Additional Information

Assessment information can be accessed at the USGS Energy Resources Program Web site 

at http://energy.usgs.gov.

Acknowledgments

The authors thank David Noe 

(Colorado Geological Survey), 

Gus Gustason (Enerplus Resources), Mark 

Kirschbaum (USGS, retired), and Steve 

Sonnenberg and Kathy Emme (Colorado 

School of Mines) for all their assistance and 

guidance throughout this work.

References

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Fouch, T.D., and Obradovich, J.D., 

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in the western interior of the United 

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G.C., 1997, The Prairie Canyon Member, 

a new unit of the Upper Cretaceous 

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