Coal Availability Study—Standing Rock Area, San 

Juan Basin Northwest New Mexico 

BY 

Gretchen K. Hoffman and Glen E. Jones 

Study funded through US Geological Survey Cooperative Agreement No. 

98HQAG2017- Availability of Coal Resources Menefee Formation, San 

Juan Basin, northwest New Mexico April 1, 1998-September 30, 1999

New Mexico Bureau of Geology and Mineral Resources 

Division of New Mexico Institute of Mining and Technology 

Socorro, NM 87801 

Open-file Report 540 

July, 2011 
 

Introduction 

 Coal is an important part of New Mexico’s economy and contributes substantially to the 

state’s educational funds through royalties and taxes. New Mexico produced 28.24 million st of 

coal in 1998, ranking 13th in the nation. Of this total, 11.71 million st (> 41%) is produced from 

the Lee Ranch and McKinley mines, both extracting coal from the Menefee Formation. Twothirds of the state’s tonnage is delivered to three electrical generating stations in New Mexico. 

All but a small portion of the remaining production is shipped by rail to generating stations in 

Arizona. The Lee Ranch and McKinley mines are the only mines in New Mexico with the rail 

spurs to the main Burlington Northern-Santa Fe line, allowing them to ship coal to Arizona. Lee 

Ranch delivers coal to nearby Escalante Generating Station at Prewitt, NM. McKinley sends coal 

to Salt River Project’s Coronado generating station and Tucson Electric’s Springerville power 

plant near Springerville, AZ, and to Arizona Public Service Cholla plant, near St. Johns, AZ.* 

 With the above economic and geologic factors considered, four quadrangles on the 

southeast edge of the Standing Rock field were chosen for this study (Fig. 1). The four 7.5 

minute quadrangles chosen for this study are the Seven Lakes, Kin Nahzin Ruins, Laguna 

Castillo, and Orphan Annie Rock. This 350-sq mi area along the Menefee outcrop has economic 

potential because of it proximity to the existing railroad spur and for the quality of the coals. The 

northern extent of the Lee Ranch coal mine is approximately 10 miles southeast of the study area 

and the South Hospah lease area (Texas Utilities-Chaco Energy) is within the study area. There 

is no previous coal production from this area. 

*Update 2011: This study was completed in 1999 before McKinley mine closed and Ed Segundo mine, northwest of 

Lee Ranch opened. El Segundo lease was previously South Hospah mentioned in text and shown on Fig. 1 

Geologic Setting 

Menefee exposures within the Standing Rock field trend, more or less parallel to the Late 

Cretaceous shoreline (N55oW). The Standing Rock field is within the Chaco slope and therefore the 

units dip gently N–NW into the San Juan Basin (SJB). This area is dominated by gently northdipping beds. Some high-angle faulting was delineated in the southernmost part of the study area 

during exploration drilling in the South Hospah mine lease as shown on Figure 1. 

 The southern edge of the Standing Rock field is the contact of the Point Lookout Sandstone 

and the Cleary Coal Member of the Menefee Formation. This field extends along this contact over 

portions of T.'s 16-19 N., and R.'s 9-17 W for a distance of approximately 70 miles (Fig. 1). 

Standing Rock’s eastern boundary with the San Mateo field is arbitrarily set at the north-south 

boundary between R8W and R9W. The northern boundary is the northernmost outcrop of the 

Cleary Coal Member of the Menefee Formation. The Cleary Coal Member is defined as a coalbearing sequence and the overlying Allison Member is barren of coal. The western boundary of the 

Standing Rock field is defined by the pinchout of the Point Lookout Sandstone. This is the 

southwestern extent of the Point Lookout Sandstone and beyond this northwest-southeast trending 

line, the Menefee Cleary Coal Member and the Crevasse Canyon Gibson Coal Member are 

undivided. 

Coal Geology 

The coals in the Standing Rock field are typically within a shale-dominated sequence, but 

several are overlain or underlain by siltstone or sandstone. These coals are within the Cleary 

Member of the Menefee Formation, near the base of this unit. One coal zone, the Green, is directly 

above or within a few feet of the Point Lookout Sandstone contact. Most of the coals are within 200 

ft of the Point Lookout Sandstone (Fig. 2). Six coal zones are recognized in the Cleary coal-bearing 

 

sequence in the South Hospah mine area (Chaco Energy, 1981). The zones from the base of the 

Cleary Coal Member upward are the Green, Blue, Red, Tan, Bronze, and Creme zones. The author 

added a seventh zone, Gold, recognized in this study area. 

 Many of these coal beds and zones are thin and only the three lowermost zones have any 

continuity throughout the area. Only these three coal zones are used for resource calculations in this 

study. The three coal zones are very similar in average coal seam thickness, total coal within a 

zone, and zone thickness (Fig. 3). There is great variability of seam thickness within a zone over the 

entire study area. . Total coal isopachs for the Green, Blue and Red zones (Figs. 4, 5, 6) illustrate 

the variability of thickness in the two most consistent coal zones within the study area. Average 

zone thickness varies from over 5 ft to 9 ft thick, including interburden. Each zone averages one 

seam, but the Green zone can have a maximum of five seams, the Blue three seams, and the Red 

zone a maximum of four seams. Average total thickness of the Cleary Coal Member is 125 ft. 

There is great variability in this thickness because of the lenticularity of the coals and the coal zones. 

This lenticularity is illustrated in the cross sections located on Figure 7 along strike (Figs. 8, C-C’; 9, 

D-D’) and downdip (Figs. 10, A-A’; 11, F-F’). Correlation of seams on these cross sections and for 

resource calculation is by zone rather than by coal bed. Any coal on the cross section does not 

necessarily represent the same bed on an adjacent section; rather the coals are correlated as being 

within the same zone.

 
0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

Zones (number of point)

Thickness in Feet

Seam avg 3.33 4.58 3.27

Seam max 14.00 17.00 18.10

Total coal avg 4.38 5.87 3.91

Total coal max 17.50 17.00 18.10

Interburden avg 4.16 5.95 7.33

Zone Average 6.28 9.02 5.43

Zone Max 26.5 34.5 31.6

Red (176) Blue (191) Green (197)

Figure 3. Coal zone thickness and interburden characteristics by zone. Numbers in

parenthesis are number of data points.

Coal quality 

 The apparent rank of Cleary Coal Member coals in the Standing Rock field is 

subbituminous A (Hoffman, 1996a). Weighted–average analyses on an as–received basis follows:

 Standing Rock 

 Average Std Dev. No. of Samples Average for Zones 

 (Hoffman, 1996a) (This study)

 Red (13) Blue (37) Green (14) 

Moisture (%) 17.03 1.42 27 16.06 16.44 16.74 

Ash (%) 13.32 3.35 27 11.37 13.30 13.09 

Volatile matter (%) 33.83 1.41 27 34.86 34.55 34.69 

Fixed carbon (%) 35.81 2.79 27 37.60 35.74 35.40 

Sulfur (%) 1.06 0.38 27 0.79 1.23 1.27 

Calorific value (Btu/lb) 9429 552 27 9822 9612 9755 

Lbs. of Sulfur/MMBtu 1.09 0.41 30 1.09 1.57 1.31 

 

 








Weighted averages by zone are shown above and on Figure 12. The sulfur values of the 

Blue and Green zones are high for the SJB and greater than the weighted average for the entire 

Standing Rock field. These Menefee coals would not meet the New Source Performance 

Standards of the Clean Air Act of 1.2 lbs of SO2 per million Btu (Energy Information 

Administration, 1993). Both the Green and Blue zones pyritic sulfur content is high, 0.38% of total 

sulfur for the Green and 0.61% of total sulfur for the Blue zone. Blending is necessary to lower the 

sulfur content and perhaps washing would help to remove some of the pyritic sulfur, lowering the 

total sulfur content. Lee Ranch mine blends their product in part because of the variability of the 

sulfur content. 

The Standing Rock study area coals are medium-ash, nonagglomerating, subbituminous 

coals with weighted zone averages from 11,218 to 11,403 moist, mineral matter-free Btu/lb 

(MMFBtu/lb). These rank values are higher than Fruitland coals that are mined elsewhere in the 

SJB and the ash content of these coals is lower than the Fruitland Formation coals. An average of 

the few oxide analyses available reveal the major constituent of the ash is SiO2 (52%), followed 

by AlO2 (21%), Fe2O3 (9%), CaO (5%), and MgO (1.3%). The remaining oxides are individually 

less than 1%. 

 
0

2

4

6

8

10

12

14

16

18

Percent

Ash 11.37 13.30 13.09 13.32

Moisture 16.06 16.44 16.74 17.03

Sulfur 0.79 1.23 1.27 1.06

Zones

Lbs sulfur/MMBtu 1.09 1.57 1.31 1.09

Red (13) Blue (37) Green (14)

Standing 

Rock field 

(27)

Figure 12. Weighted averages of quality analyses by zone. Numbers 

in parenthesis are number of data points. 

Available data 

The database for the Standing Rock study is a subset of the data collected and entered by 

the New Mexico Bureau of Mines (NMBMMR) into the National Coal Resource Data System 

(NCRDS). The NMBMMR has had cooperative grants with the U.S. Geological Survey (USGS) 

to enter data into NCRDS for the past 20 years. Exploration projects in the study area have 

resulted in a clustered data set (Fig. 7). Drill holes are concentrated in the surface-minable area 

and less data is available for the deep coal area except where oil and gas logs are available. Other 

sources of drill hole data are the South Hospah and Lee Ranch mine plans, Coal Resource 

Occurrence and Coal Development Potential maps (CROCDP), and NMBMMR drilling. After 

the project began, additional data were entered into the database, in particular oil and gas logs 

from NMBMMR subsurface library, to help fill in gaps. 

 
A total of 140 data points were evaluated for the four-quadrangle study area (Seven 

Lakes, Kin Nahzin Ruins, Orphan Annie Rock, and Laguna Castillo). Additionally, 142 data 

points from the quadrangles surrounding this area were used to calculate resources. 

Coal availability studies in New Mexico 

Detailed methodology 

Cleary Coal Member Menefee coals in the Standing Rock area are subbituminous; 

therefore, resource calculations are based on a minimum thickness of 2.5 ft and 1770 tons/acre ft. 

The following parameters, which conform to USGS Circular 891 (Wood et al., 1983), were used 

to calculate resources for each zone. Total coal (2.5 ft) within a zone instead of individual 

seams is the basis of resource calculations because of the lenticularity of the coals. Coals thinner 

than 2.5 ft and above or below coals meeting the thickness criteria and separated by a parting less 

than the thickness of the thinner coal are included in the calculation. The following are the 

resource criteria used in this study: 

Thickness (ft): Depth (ft): Reliability 

 2.5–5 10:1 stripping ratio Measured (1/4 mi) 

5–10 0–250 Indicated (1/4–3/4 mi) 

 10–20 250–500 Inferred (3/4–3 mi) 

 >20 500–1000 

 >1000 

Reliability categories are limited to measured, indicated and inferred. No hypothetical reliability 

category was used in this study because of the lenticularity of these coal beds. 

 Line data for the Point Lookout-Menefee contact was digitized from Beaumont (1998). 

These line data form a boundary mask between areas of coal and no coal. Data files with 

elevations for tops of coal zones were gridded in Arc Info® and overlain with a grid from the 

1:100,000 Chaco Mesa Digital Elevation Model (DEM). From this combined layer, the 

 
 

overburden maps for each zone were generated. This overburden layer supplied the zero depth 

line, or crop line, for each zone. The resulting overburden maps for the Green, Blue, and Red 

zones are illustrated in Figures 13-15. The upper depth limits for the categories are highlighted 

on these maps. 

From the original database created for the study, subset files were produced for each zone 

with latitude, longitude, total thickness, and point identification. These files included data from 

quadrangles surrounding the four-quadrangle study area. By using data from surrounding 

quadrangles, reliability categories that overlap into the study area were included in the resource 

calculations. The data files were used to produce coal thickness grids and polygon files for the 

reliability categories. The reliability polygons were gridded and each cell assigned a thickness 

from the thickness grid. By overlaying the overburden layer onto the reliability layers, the total 

area (calculated in m2 and converted to acres) for each thickness, depth, and reliability category 

were determined for the four-quadrangle area. Volumes (acre-ft) were calculated using the 

thickness attribute of the cells, and finally multiplied by 1770 tons/acre-ft to result in original 

resource tonnage for each zone. 

Land-use restrictions were digitized from the 1:100,000 Chaco Mesa topographic 

quadrangle. No mining has taken place in this area therefore; there was no mined out layer to 

subtract from the original resource. Technical restriction filters were applied to the remaining 

resource layers for each zone. Appropriate buffers, as discussed in the following section, were 

assigned to the digitized land-use restrictions. These restriction layers were consecutively 

overlain on the combined overburden, reliability, and thickness layers with the mined-out areas 

and technical restrictions to calculate the resource tonnage removed by each restriction. 



Overview of restrictions 

The following is a list of restrictions that were considered for this area. The buffers 

applied to these restrictions adhere to the New Mexico Coal Surface Mining Regulations 19 

NMAC 8.2, which follow the Federal regulations: 

Restrictions Buffer

County roads 100 ft on either side 

NM State Highway 57 and Tribal Highway 9 100 ft on either side 

Pipelines, powerlines 50 ft on either side 

Buildings, public or private 300 ft 

Chaco National Monument outlier (1/4 section) (entire area) 

Ponds 100 ft 

 Railroad spur 100 ft on either side 

Figure 16 illustrates these restrictions along with the Menefee outcrop. 

Technological restrictions 

Technical parameters that influence the resources of this study are: 

Coal too close to the surface. Coal with less than 20 ft of overburden is subtracted from the 

remaining resource estimate. Coal with less than 20 ft of overburden is removed because SJB 

coal within this interval is generally weathered or sometimes burned and can not be used for 

energy production. Most operating mines in the SJB use the greater-than-20-ft depth guideline 

for calculating mine reserves 

Coal too thin at depth. Coal beds 2.5 ft to 5 ft thick are not considered minable at depths greater 

than 250 ft. The original resources are calculated for this depth category but these results are 

removed under the technical restrictions. 

 
Land-use restrictions 

Restrictions to mining in the project area are few, but some are significant. A secondary 

state road (NM State Highway 57, Tribal Highway 9), pipelines, and powerlines transect the area 

(Figs. 1, 16). These restrictions were digitized from the 1:100,000 Chaco Mesa topographic 

quadrangle. The buildings were digitized from the 7.5-min quadrangles and the oil and gas fields 

were drawn on the Chaco Mesa 1:100,000 quadrangle from oil and gas pool maps available at 

the NMBM&MR. A railroad spur, built to connect the South Hospah mine to the main line, was 

plotted on the 1:100,000 Chaco Mesa quadrangle and digitized. The Chaco National Monument 

outlier was digitized from the 1: 100,000 quadrangle. Although archeological sites are in the 

study area, they would likely be mitigated, and were not considered. 

Standing Rock study area resources 

The original resources for the Standing Rock study area are 3.82 billion st (Table 1, 2). 

There is no mining production in this area to remove from the original resource. The 

technological restrictions, removing near surface coal (0–20 ft) and thin coal (2.5–5 ft) at 

depths greater than 250 ft removed 1.24 billion st from the total resource, the largest depletion of 

remaining resources. This restriction withdraws 32.6 % from the remaining available resource, 

only a minor amount, 8 million st, is from the 10:1 stripping ratio category. Most of the 

technological restriction applies to thin coals at depths greater than 250 ft. 

Restrictions related to pipelines, powerlines, and major roads in the study area remove 21 

million st from the remaining available resource. It is unlikely that NM State Highway 57 or 

Navajo Tribal Highway 9 would be mitigated although some of the county roads might be 

considered. Figure17a-c illustrates the proportion of coal removed by these restrictions and the 

 


remaining available resource for each of the zones. Figure 18, Tables 1 and 2 demonstrate how 

the restrictions influence the available coal resources in each zone. Overall, the original 

resources in the Green zone are depleted the most by these restrictions. 

The Seven Lakes and Hospah oil and gas pools have little effect on surface minable coal, 

but would impact underground mining, removing 56 million st from the resource. The railroad 

spur removes 2 million st from the original resource and none of this restricted resource is within 

the 10:1 stripping ratio category. 

Restrictions with potential for mitigation 

The restrictions applied to the remaining resource with potential for mitigation are 

buildings, and ponds. Buildings remove 5 million st from the remaining resource. If mining were 

to take place these buildings, which are mostly Native American hogans, would be moved and 

the owners compensated. The small ponds called Laguna Castillo and Orphan Annie Tank 

remove 6 million st of coal resource. There are minor washes in the study that are not Alluvial 

Valley Floors and lack flow except during rainy seasons, consequently mining would not be 

restricted in the wash areas. 

 
Red Zone (milions of st)

Available

63.9% (702)

Ponds

0.2% (2)

Buildings

0.1% (1)

Land-use: Chaco 

National Monument

0.4% (3)

Land-use: pipelines, 

powerlines, roads

0.5% (5)

Technological 

Restrictions

34.7% (392)

Oil And Gas: Seven 

Lakes and Hospah 

pools

0.2% (3)

Railroad: South 

Hospah and Lee 

Ranch spurs

0.1% (1)

Blue Zone (millions of st)

Railroad: South 

Hospah and Lee 

Ranch spurs

0.1% (2)

Oil And Gas: Seven 

Lakes and Hospah 

pools

2.2% (37)

Technological 

Restrictions

22.9% (380)

Land-use: pipelines, 

powerlines, roads

0.7% (11)

Land-use: Chaco 

National Monument

0.1% (1)

Ponds

0.3% (3) Buildings

0.1% (2)

Available

73.5% (1,119)

Green Zone (millions of st)

Available

57.8% (667)

Ponds

0.2% (2) Buildings

0.2% (2)

Land-use: Chaco 

National Monument

0.0% (0) Land-use: pipelines, 

powerlines, roads

0.4% (5)

Oil And Gas: Seven 

Lakes and Hospah 

pools

1.4% (17)

Railroad: South 

Hospah and Lee 

Ranch spurs

0.0% (1)

Technological 

Restrictions

40% (469)

Figure 17a-c. Original zone resources removed by land-use and 

technical restrictions and remaining available resources. 

 
0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

Millions of st

Original

resource

Original minus 

Technical

Remaining-landuse restrictions

=Available

Red Blue Green Overall

Coal Zones

Figure 18. Standing Rock area coal resources by zone. 

Comparison with other resource studies 

 The DOE Demonstrated Reserve Base study (Hoffman, 1996b) estimate of 0.84 billion st 

for the entire Standing Rock field is difficult to compare to the 2.5 billion st available resource 

for the four-quadrangle study area. The DRB did not include calculations for inferred reliability 

and for depths greater than 1000 ft. The inferred category increases the available resource by 

1.70 billion st and 0.31 billion st of coal are added for depths greater than 1000 ft for the study 

area. Removing these two categories leaves 0.49 billion st of available resources, about 58% of 

the DRB estimate for the entire Standing Rock field, although the study area is about 1⁄4 the total 

area of this field. New point source data was added during this study, which increased the 

accuracy of the resource estimate and filled in areas of no data 

 The DOE Demonstrated Reserve Base study (Hoffman, 1996b) estimate of 0.84 billion st 

for the entire Standing Rock field is difficult to compare to the 2.5 billion st available resource 

for the four-quadrangle study area. The DRB did not include calculations for inferred reliability 

and for depths greater than 1000 ft. The inferred category increases the available resource by 

1.70 billion st and 0.31 billion st of coal are added for depths greater than 1000 ft for the study 

area. Removing these two categories leaves 0.49 billion st of available resources, about 58% of 

the DRB estimate for the entire Standing Rock field, although the study area is about 1⁄4 the total 

area of this field. New point source data was added during this study, which increased the 

accuracy of the resource estimate and filled in areas of no data 

 
Summary 

The Cleary Coal Member of the Menefee Formation in the Standing Rock study area 

contains moderate-ash, moderate-sulfur, Subbituminous A-rank coals. This resource does not 

meet compliance coal standards of less than 0.6 lbs sulfur/MMBtu, but with blending or washing 

these coals potentially could meet this guideline. Within the Cleary Coal Member, three of the 

seven coal zones recognized were evaluated for original and available resources in the study 

area. These zones, Green, Red, and Blue contain relatively thin, variable coal seams with average 

seam thickness of 3-4 ft. 

Using Arc Info® facilitates calculating resources for areas with many restrictions and 

allows for faster recalculation of resources with new data. Original resources for this study area 

are 3.8 billion st and are greater than previous studies because the inferred category and depths 

greater than 1000 ft are included. Another factor influencing the resource total is the acquisition 

of data during the study. Technical restrictions removed about 1.2 billion st from the original 

resource, which is the largest restriction on the resource. The available resource is 2.5 billion st 

for the Standing Rock study area. A percentage breakdown by zone of this total is 27% in the 

Green, 45% in the Blue, and 28% in the Red. Of this available resource, 0.15 billion st is within 

the 10:1 stripping ratio category (Table 1). The Blue zone contains 75% (0.11 billion st) of the 

strippable (10:1) resource. 

 
 

References 

Anderson, O. J., Jones, G. E., Green, G. N., 1997, Geologic Map of New Mexico: U.S. 

Geological Survey, Open-file report 97-052, scale 1:50,000. 

Beaumont, E. C., 1998, Distribution of near-surface coal deposits in the San Juan Basin, New 

Mexico: New Mexico Bureau of Mines and Mineral Resources, Resource Map 19, scale 

1:250,000. 

Chaco Energy Company, 1981, South Hospah mining plan: Geology: Permit Application filed 

with the New Mexico Energy, Minerals and Natural Resources Department, vol. 2, 16 p., 

9 plates. 

Energy Information Administration, 1993, U.S. Coal reserves: An update by heat and sulfur 

content: Energy Information Administration DOE/EIA–0529(92), p. viii. 

Hoffman, G. K., 1996a, Coal resources of New Mexico: New Mexico Bureau of Mines and 

Mineral Resources, Resource Map 20, 22 p. text, 1 sheet, scale 1:1,000,000. 

Hoffman, G. K. 1996a, Demonstrated reserve base for coal in New Mexico, modified from final 

report for cooperative agreement DE-FC0193EI23974: New Mexico Bureau of Mines 

and Mineral Resources, Open-file report 428, 89 p. 

Wood, G. H., Kehn, T. M., Carter, M. D., and Culbertson, W. C., 1983, Coal resource 

classification system of the U.S. Geological Survey: U.S. Geological Survey, Circular 

891, 65 p.