U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3011

March 2009

Oil Shale Assessment Project Fact Sheet

Nahcolite Resources in the Green River Formation, 

Piceance Basin, Northwestern Colorado

Printed on recycled paper

Introduction

The U.S. Geological Survey (USGS) 

recently completed an assessment of 

in-place nahcolite (NaHCO3) resources in 

the Piceance Basin, northwestern Colorado (fig. 1). Nahcolite is present in the 

oil shale deposits of the Parachute Creek 

Member of the Eocene Green River 

Formation. It occurs as disseminated 

aggregates, nodules, bedded units of 

disseminated brown crystals, and white 

crystalline beds associated with dawsonite (NaAl(OH)2

CO3) and halite (NaCl). 

The nahcolite-bearing facies are divided 

into an unleached part containing the 

nahcolite and halite, which is estimated 

to be as much as 1,130 ft thick, and an 

upper leached part several hundred feet 

thick containing minor nahcolite aggregates and nodules. Locally, thick beds of 

halite and brown fine-grained nahcolite 

lie in the depocenter of the basin (figs. 1 

and 2), but thin laterally away from the 

basin center and grade into beds of white, 

coarse-grained nahcolite. In the central 

part of the study area, the top of the 

nahcolite-bearing rocks range in depth 

from about 1,300 to 2,000 ft.

Dissolution of water-soluble minerals, mostly nahcolite and halite, in the 

upper part of the nahcolite-bearing facies 

has created a collapsed leached zone 

as much as 580 ft thick that consists of 

laterally continuous units of solution 

breccia and fractured oil shale containing 

solution cavities. The top of the leached 

zone is not yet defined in the basin, but 

it probably extends into the A groove in 

the upper part of the Parachute Creek 

Member (fig. 2).

Using a geology-based assessment methodology, the U.S. Geological Survey estimated an in-place resource of 43.3 billion short tons 

of nahcolite (sodium bicarbonate) in the oil shale deposits of the Parachute Creek Member of the Eocene Green River Formation in 

the Piceance Basin, northwestern Colorado. More than 69 percent, or 29.9 billion short tons, of the estimated nahcolite resource is 

found in the R–4, L–5, and R–5 oil shale zones.

108° 30' 108°00'

40°00'

39° 30'

T 99 W T 98 W T 97 W T 96 W T 95 W

Colorado

Index map

0 12 MILES

70

Pice ance

WhiteRiver

White River 64

64

13

13

139

139 Meeker

Rangely

Rifle

Rio Blanco

DeBeque

Parachute

City or town

Area underlain by

 oil shale rocks

EXPLANATION

County line

Core hole with

 nahcolite analyses

Colorado River

Creek

Creek

Parachute Creek

MESA

GARFIELD

RIO BLANCO

Roan

Boundary of nahcolite-

 bearing zone

Boundary of saline zone—

 Includes dawsonite and halite

C0002

T. 2 N.

T. 1 N.

T. 1 S.

T. 2 S.

T. 3 S.

T. 4 S.

T. 5 S.

T. 8 S.

T. 7 S.

T. 6 S.

R. 102 W. R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W. R. 94 W.

Figure 1. Map of northwestern Colorado showing locations of nahcolite-bearing core 

holes, the inferred boundaries of the nahcolite-bearing zone and saline zone of Pitman and 

others (1989), and generalized outcrops of the oil shale-bearing rocks in the Parachute Creek 

Member of the Eocene Green River Formation in the Piceance Basin.
The presence of nahcolite is a 

concern for the surface retorting and 

in-place oil shale processes because 

nahcolite decomposes to natrite (Na2

CO3) 

or soda ash, carbon dioxide, and water 

at temperatures near 100°C during 

the two recovery processes. In addition, nahcolite is considered a leasable 

mineral and thus must not be destroyed 

during processes used to recover oil 

from oil shale. Nahcolite is a potential 

raw material for the manufacture of a 

variety of basic industrial chemicals (for 

example, baking soda), but the mineral 

has perhaps received more attention for 

its potential use in removing sulfur dioxide from industrial stack gases. Because 

nahcolite seems to be a potentially valuable coproduct of oil shale, it becomes 

important to know where and in what 

abundance it occurs in the oil shale.

Methodology

In this assessment, a spatial interpolation and extrapolation method for 

generating resource maps and computing 

resource volumes was used—the Radial 

Basis Function (RBF) in ArcGIS GeoStatistical Analyst (Environmental Systems 

Research Institute, Inc. (ESRI), Redlands, 

Calif., 2006, v. 9.2).

The RBF method in GeoStatistical 

Analyst is an exact interpolator; it will 

honor all data points and not introduce 

errors at those locations. Where it is 

important for the modeling method to 

honor the measured values, RBF can also 

extrapolate values above or below the 

actual values away from the data point 

locations. The final resource models for 

each nahcolite-bearing zone were created 

using a sampling method containing 

ten moving window sectors with fifteen 

neighbors in each sector, except for the 

L–5 zone where the model was limited to 

ten neighbors or data points.

Although the RBF method does not 

generate as complete an error estimate 

as kriging, it does give the difference 

between the predicted and measured value 

for each control point, and it does give an 

overall difference value for an entire oil 

shale zone model. To obtain the difference between the predicted value and the 

measured value, the RBF method predicts 

a value at a given control point from the 

nearest control points (in this assessment either 10 or 15) without knowing 

the actual value measured at that control 

point. That predicted value is then compared with the measured value, and the 

difference between the two is calculated.

Nahcolite resource models using 

weight-percent nahcolite data were 

created using the RBF, which interpolated (and extrapolated) surfaces using 

a multiquadratic gridding method. Once 

the models were created, we quantified total tonnages of nahcolite by using 

ESRI Spatial Analyst’s Zonal Statistics 

function. That is, tonnages could be 

determined by township using the nahcolite resource model (208.71 ft = 63.615-m 

cell size) and a township grid that had 

been clipped with an outcrop file derived 

from the base of the Mahogany zone of 

the Green River Formation for both the 

nahcolite resources in the oil shale zones 

and the total in-place nahcolite within the 

inferred extent of the nahcolite-bearing 

interval (figs. 1 and 3).

100 80 60 40 0 2020 40 60 80 100

Oil in GPT; Nahcolite in wt. %

Wolf Ridge Minerals Corp.,

Dunn 20–1, C0002

Sec 20 T. 1 S., R. 98 W.

1,500

2,000

2,500

R–3

L–3

R–4

L–4

R–5

Zone

DEPTH IN FEET

A groove

Mahogany zone

B groove

Rich oil shale

zone R–6

Rich oil shale

zone R–5

Rich oil shale

zone R–4

Rich oil shale

zone R–1

Rich oil shale

zone R–0

Rich oil shale

zone R–3

Rich oil shale

zone R–2

Stratigraphic nomenclature for oil shale

zones (Donnell and Blair, 1970;

Cashion and Donnell, 1972) Parachute Creek Member in part Garden Gulch Member in part Green River Formation

Lean oil shale zone L-2

Lean oil shale zone L–1

Lean oil shale zone L–0

Lean oil shale

zone L–4

Lean oil shale zone L-3

Lean oil shale

zone L–5

EXPLANATION

Oil-yield histogram

 (in gallons per ton)

Saline bed (halite

 and nahcolite)

Nahcolite-rich bed

Nahcolite histogram

 (in weight percent)

Figure 2. Plot showing oil-yield (gallons per ton, GPT) and nahcolite (weight 

percent, wt. %) histograms and stratigraphic nomenclature for oil shale zones 

(Donnell and Blair, 1970; Cashion and Donnell, 1972) for the Wolf Ridge Minerals 

Corp., Dunn 20–1 core hole, north-central Piceance Basin, Colorado.
R 96 W R 95 W

R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W

T

8

S

T

7

S

R 93 W

T

6

S

T

5

S

T

4

S

T

3

S

T

9

S

R 94 W

R 101 W R 100 W R 99 W R 98 W R 97 W

T

2

S

T

1

S

T

1

N

R 96 W R 95 W

T

2

N

438,963,008 625,540,992

39,181,500

5,501,810,176 4,088,640,000

400,094,016

958,769,984

12,936,299,520 8,513,489,920

624,356,992

6,055,349,760 3,110,690,048

108°30' 108°00'

40°00'

39°30'

0 3 6 12 MILES

0 5 10 2.5 KILOMETERS

EXPLANATION

Top of Mahogany zone

Base of Parachute Creek Member

Inferred extent of nahcolite

 depocenter

Colorado

Index map

Nahcolite in-place resource by township

 (in short tons) 4,088,640,000

Figure 3. Map showing in-place nahcolite (NaHCO3) resource volumes (in short tons) in the lower and middle parts of the 

Parachute Creek Member of the Eocene Green River Formation by township, Rio Blanco County, northwestern Colorado. Inferred 

extent of nahcolite depocenter outlined in red.
Resource Summary

Based on the methodology outlined above, the U.S. Geological Survey 

(USGS) assessed the nahcolite resources 

in the lower and middle parts of the Parachute Creek Member of the Green River 

Formation in the north-central part of the 

Piceance Basin. In some areas, nahcolite 

is economically minable as a separate 

commodity. However, because most of the 

nahcolite forms in variable sized aggregates scattered through oil shale beds, 

in most areas it is subeconomic unless 

recovered as a coproduct of oil shale 

extraction. Oil shale zones R–3, R–4, L–4, 

and R–5, from 150 to about 370 ft thick, 

contain the largest amounts of nahcolite 

by weight (fig. 2); in-place estimates are 

shown by township in figure 3 and listed 

in table 1. Total amounts of nahcolite in 

the Parachute Creek Member are extrapolated from estimated mineral contents of 

58 core holes (fig. 1). The total in-place 

resource is estimated at about 43.3 billion 

short tons (table 1).

For Additional Information

Supporting geologic studies of 

nahcolite-bearing units, assessment units, 

oil shale and nahcolite analyses, and the 

methodology used in assessing the nahcolite resources in the Parachute Creek 

Member of the Green River Formation 

in the Piceance Basin are in progress. 

Assessment results are available at the 

USGS Central Energy Team website: 

http://energy.cr.usgs.gov/other/oil_shale/.

Contact Information

For further information contact 

Michael E. Brownfield (mbrownfield@

usgs.gov), Ronald C. Johnson 

(rcjohnson@usgs.gov), Jesse G. Self 

(jself@usgs.gov), or Tracey J. Mercier 

(tmercier@usgs.gov).

References

Cashion, W.B., and Donnell, J.R., 

1972, Chart showing correlation of 

selected key units in the organic-rich 

sequence of the Green River Formation, Piceance Creek basin, Colorado, 

and Uinta Basin, Utah: U.S. Geological Survey Oil and Gas Investigations 

Chart OC–65.

Donnell, J.R., and Blair, R.W., Jr., 1970, 

Resource appraisal of three rich oil 

shale zones in the Green River Formation, Piceance Creek basin, Colorado: 

Colorado School of Mines Quarterly, 

v. 65, no. 4, p. 73–87.

Pitman, J.K., Pierce, F.W., and Grundy, 

W.D., 1989, Thickness, oil-yield, 

and kriged resource estimates for 

the Eocene Green River Formation, 

Piceance Creek basin, Colorado: U.S. 

Geological Survey Oil and Gas Investigations Chart OC–132, 6 sheets with 

4-page text.

Table 1. In-place nahcolite resources and tons per acre, estimated by township, in the nahcolite-bearing interval of the 

Parachute Creek Member of the Eocene Green River Formation, Piceance Basin, Rio Blanco County, northwestern Colorado.

[All values calculated by ESRI’s ArcGIS GeoStatistical Analyst extension with a Radial Basis Function (RBF) Multiquadratic Gridding Function; 

1 acre = 4,047 m]

Township Acres Area (m2) Estimated tons 

per acre

Estimated in-place resource, 

in short tons

T. 1 N. R. 97 W. 16,671 67,465,300 186,593 3,110,690,000

T. 1 N. R. 98 W. 16,193 65,530,900 373,948 6,055,350,000

T. 1 S. R. 96 W. 4,844 19,603,000 128,893 624,360,000

T. 1 S. R. 97 W. 24,450 98,945,900 348,200 8,513,490,000

T. 1 S. R. 98 W. 23,844 96,493,500 542,541 12,936,300,000

T. 1 S. R. 99 W. 4,881 19,752,800 196,429 958,770,000

T. 2 S. R. 96 W. 9,719 39,331,500 41,166 400,090,000

T. 2 S. R. 97 W. 23,478 95,012,400 174,148 4,088,640,000

T. 2 S. R. 98 W. 20,507 82,989,100 268,289 5,501,810,000

T. 3 S. R. 96 W. 3,956 16,009,400 9,904 39,180,000

T. 3 S. R. 97 W. 12,838 51,953,700 48,726 625,540,000

T. 3 S. R. 98 W. 8,613 34,855,700 50,965 438,960,000

Total 169,994 687,943,200 43,293,190,000