Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin, Las Animas County, Colorado

Home , Raton Basin, Raton Formation, Trinidad Sandstone, Vermejo Formation

Prepared in cooperation with the Colorado Water Conservation Board

Scientific Investigations Report 2006–5129

U.S. Department of the Interior U.S. Geological Survey Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin, Las Animas County, Colorado

By Kenneth R. Watts

Prepared in cooperation with the Colorado Water Conservation Board

Scientific Investigations Report 2006–5129

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

Posted online November 2006 This publication is available only online at http://pubs.usgs.gov/sir/2006/5129/ For more information on the USGS–the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation: Watts, K.R., 2006, Hydrostratigraphic framework of the Raton, Vermejo, and Trinidad aquifers in the Raton Basin, Las Animas County, Colorado: U.S. Geological Survey Scientific Investigations Report 2006–5129, 31 p. iii

Contents

Abstract...... 1 Introduction...... 1 Purpose and Scope...... 3 Hydrogeologic Setting...... 3 Data ...... 5 Water Wells...... 5 Coalbed-Methane Wells...... 5 Geologic Maps...... 7 Vertical Datum...... 7 Geostatistical Methods and Computer Contouring...... 8 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers...... 10 Raton Aquifer...... 11 Vermejo Aquifer...... 11 Trinidad Aquifer...... 12 Structure and Apparent Thickness of the Raton, Vermejo, and Trinidad Aquifers...... 12 Descriptions of Geostatistical Models and Cross-Validation Statistics...... 12 Calculated Surfaces...... 14 Structural Surfaces...... 14 Depth to Bottoms and Tops...... 19 Apparent Thicknesses...... 19 Regional Water Table...... 19 Limitations and Other Uses of Hydrostratigraphic Maps...... 27 Summary...... 30 References Cited...... 30

Figures

1. (A) Generalized geologic map of the Raton Basin, Colorado and New Mexico and (B) location of the study area and the Raton Basin in Colorado...... 2 2. Generalized geologic map and locations of oil and gas wells in the Raton Basin, Las Animas County, Colorado, 2004...... 6 3. Bar graph showing annual ground-water and gas production by CBM wells in the study area, 1999–2004...... 8 4. Theoretical variograms of (A) exponential, (B) Gaussian, (C) linear, and (D) spherical functions...... 9 5–17. Maps showing: 5. Generalized configuration of the bottom of the Trinidad Sandstone in Las Animas County, Colorado...... 15 6. Generalized configuration of the top of the Trinidad Sandstone in Las Animas County, Colorado...... 16 7. Generalized configuration of the top of the Vermejo Formation in Las Animas County, Colorado...... 17 iv

8. Generalized configuration of the top of the Raton Formation in Las Animas County, Colorado...... 18 9. Generalized depth to the bottom of the Trinidad Sandstone in Las Animas County, Colorado...... 20 10. Generalized depth to the top of the Trinidad Sandstone in Las Animas County, Colorado...... 21 11. Generalized depth to the top of the Vermejo Formation in Las Animas County, Colorado...... 22 12. Generalized depth to the top of the Raton Formation in Las Animas County, Colorado...... 23 13. Generalized apparent thickness of the Trinidad Sandstone in Las Animas County, Colorado...... 24 14. Generalized apparent thickness of the Vermejo Formation in Las Animas County, Colorado...... 25 15. Generalized apparent thickness of the Raton Formation in Las Animas County, Colorado...... 26 16. Generalized configuration of the long-term regional water table in the northern part of the study area...... 28 17. Estimated depth to water below land surface in the northern part of the study area...... 29

Tables

1. Spatial data for the Raton, Vermejo, and Trinidad aquifers in Las Animas County, Colorado...... 3 2. Hydrostratigraphic section of the Raton Basin...... 4 3. Geostatistical models, model fitting parameters, and cross-validation statistics...... 13

Conversion Factors

Inch/Pound to International System Multiply By To obtain Length foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area acre 4,047 square meter (m2) acre 0.4047 hectare (ha) acre 0.004047 square kilometer (km2) section (640 acres or 1 square mile) 259.0 square hectometer (hm2) square mile (mi2) 259.0 hectare (ha) square mile (mi2) 2.590 square kilometer (km2) Volume barrel (bbl), (petroleum, 0.1590 cubic meter (m3) 1 barrel=42 gal) cubic foot (ft3) 0.02832 cubic meter (m3) gallon (gal) 0.003785 cubic meter (m3) million gallons (Mgal) 3,785 cubic meter (m3) Energy (natural gas)1 cubic foot (ft3) 1,027 British thermal unit (Btu) cubic foot (ft3) 0.01 therm (approximate) thousand cubic feet (Mcf) 1 dekatherm (10 therms) 1American Gas Association (2006).

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum. Concentrations of chemical constituents in water are given in milligrams per liter (mg/L). Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin, Las Animas County, Colorado

By Kenneth R. Watts

Abstract Introduction

Exploration for and production of coalbed methane has Exploration for and production of coalbed methane increased substantially in the Rocky Mountain region of the (CBM) has increased substantially in the Rocky Mountain United States since the 1990s. During 1999–2004, annual pro- region of the United States since the 1990s. Where coalbeds duction of natural gas (coalbed methane) from the Raton Basin also are aquifers, the reduction in hydrostatic pressure needed to in Las Animas County, Colorado, increased from 28,129,515 produce methane may have unintended consequences, includ- to 80,224,130 thousand cubic feet, and the annual volume of ing reduction of water levels in nearby water-supply wells ground water coproduced by coalbed methane wells increased and mining of ground water. CBM production from the Raton from about 949 million gallons to about 2,879 million gallons. Basin in Huerfano and Las Animas Counties, Colorado (fig. 1), Better definition of the hydrostratigraphic framework of the has increased substantially since 1999. Production of natural Raton, Vermejo, and Trinidad aquifers in the Raton Basin of gas (coalbed methane) from coals in the Vermejo and Raton southern Colorado is needed to evaluate the long-term effects Formations in Las Animas County increased almost threefold of coalbed methane development on the availability and sus- during 1999–2004, from 28,129,515 thousand cubic feet (Mcf ) tainability of ground-water resources. during 1999 to 80,224,130 Mcf during 2004 (Colorado Oil and In 2001, the U.S. Geological Survey, in cooperation Gas Conservation Commission, variously dated). The volume with the Colorado Water Conservation Board, began a study of ground water coproduced by CBM wells in Las Animas to evaluate the hydrogeology of the Raton Basin in Huerfano County during 1999–2004 also increased about threefold, from and Las Animas Counties, Colorado. Geostatistical methods 22,597,972 barrels (about 949 million gallons) during 1999 to were used to map the altitude of and depths to the bottoms and 68,557,832 barrels (about 2,879 million gallons) during 2004. tops (structure) and the apparent thicknesses of the Trinidad Because domestic and livestock water-supply wells are Sandstone, the Vermejo Formation, and the Raton Formation completed in coal beds and associated sandstones in the Raton in Las Animas County, based on completion reports and Basin, water-well owners and resource managers are con- drillers’ logs from about 1,400 coalbed methane wells in the cerned that CBM production locally may affect ground-water Raton Basin. There was not enough subsurface control to map availability and sustainability. As a result of these concerns, the structural surfaces and apparent thicknesses of the aqui- the U.S. Geological Survey, in cooperation with the Colorado fers in Huerfano County. Geostatistical methods also were Water Conservation Board, began a study in 2001 to evaluate used to map the regional water table in the northern part of the hydrogeology in the Raton Basin of Colorado. A better Las Animas County, based on reported depth to water from understanding of the hydrostratigraphic framework, including completion reports of water-supply wells. Although these the external geometry (structure and thicknesses), the internal maps were developed to better define the hydrostratigraphic geometry (heterogeneity), discharge and recharge conditions, framework, they also can be used to determine the contribut- and ground-water levels and fluid pressures in the principal ing aquifer(s) of existing water wells and to estimate drilling hydrostratigraphic units, is needed to evaluate the potential depths of proposed water wells. These maps of the hydro- effects of CBM production on ground-water availability and stratigraphic framework could be improved with the addition sustainability. of measured sections and mapping of geologic contacts at Previous studies of the hydrostratigraphic framework of outcrops along the eastern and western margins of the Raton the Raton Basin (Abbott and others, 1983; Geldon, 1989) were Basin. limited because of the paucity of subsurface data. Subsurface Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

(Geology from Flores and Bader, 1999, their figure SR–1)

B 108° 106° 104° 102°

40°

Denver

COLORADOCOLORADO

38° HUERFANOHUERFANO CO CO RatonRaton Basin Basin Walsenberg Study area LAS ANIMAS CO Trinidad

Base from U.S. Geological Survey County Map Series, 1979, 1:50,000 Albers Equal-Area Conic projection Standard parallels 37º30'N and 40º30'N, central meridian105º30'W

Figure 1. (A) Generalized geologic map of the Raton Basin, Colorado and New Mexico and (B) location of the study area and the Raton Basin in Colorado. Introduction data, available from logs and completion reports of CBM wells Hydrogeologic Setting that have been drilled since the 1990s, permit subsurface mapping of the Raton-Vermejo-Trinidad aquifer (Geldon, 1989) as The Raton Basin (fig. 1A) is an asymmetrical structural three distinct hydrostratigraphic units: the Raton, Vermejo, and trough in southeastern Colorado and northeastern New Mexico Trinidad aquifers. that contains 10,000 to 25,000 ft of sedimentary rocks that range in age from Pennsylvanian to Eocene (Geldon, 1989). As much as 10,000 ft of clastic sedimentary rocks of Late Purpose and Scope Cretaceous and Tertiary age overlies the Late Cretaceous-age Pierre Shale in the Raton Basin in Colorado (table 2). The The purpose of this report is to provide an improved Raton Basin lies between the Sangre de Cristo Mountains on definition of the hydrostratigraphic framework of the Raton, the west and the Apishapa, Las Animas, and Sierra Grande Vermejo, and Trinidad aquifers in Las Animas County, arches on the east (fig. 1A). The eastern side of the study Colorado. The study area (fig. 1) was restricted to the por- area is defined by an escarpment formed by resistant ledges tion of the Raton Basin in Las Animas County because not of the Trinidad Sandstone. The western side of the basin is enough data were available to map the hydrostratigraphic defined by uplifted sedimentary rocks, which are warped framework with confidence in Huerfano County. Contour vertically. Igneous rocks (dikes, sills, and stocks) intruded maps are presented for the Trinidad Sandstone, the Vermejo the sedimentary rocks. Spanish Peaks, prominent igne- Formation, and the Raton Formation that show (1) the gen- ous intrusions (stocks), dominate the landscape along the eralized configuration and altitude of their bottom and top Huerfano–Las Animas County line (fig. 2). Dikes, which are surfaces; (2) the generalized depth from land surface to their tabular igneous intrusions that cut across the bedding of the bottom and top surfaces; and (3) their generalized apparent sedimentary rocks, form vertical ridges because the igne- thicknesses. Maps also are presented that show the general- ous rocks are more resistant to erosion than the surrounding ized configuration and altitude of the regional water table sedimentary rocks. In some places, dike swarms that radiate and the generalized depth from land surface to the water from the Spanish Peaks intersect a set of subparallel dikes table in part of Las Animas County. The contours shown that trend easterly across the study area (fig. 2). Sills, tabular on the maps in this report and the data points on which they igneous bodies parallel to bedding, intruded the middle part of were based are available electronically as spatial data at the Raton Formation and are exposed in the Purgatoire River URL http://water.usgs.gov/lookup/getgislist. Table 1 lists valley about 10–20 mi west of Trinidad and about 10–15 mi file names and an abbreviated description of the spatial data northwest of Trinidad (fig. 2). South-southeast of Trinidad on prepared for this study. The file names are a combination of the Raton Mesa, basalt flows overlie the Tertiary-age Poison the report number (SIR-06-5129) and a data descriptor. For Canyon and Raton Formations (fig. 2). Relatively narrow and example, the name of the spatial data set of contours of the thin alluviums of Pleistocene and Holocene age (not shown in generalized altitude of the top of the Trinidad Sandstone is fig. 2) fill stream valleys cut into the sedimentary rocks and SIR-06-5129_alt_top_trinidad_con. cap pediments (Geldon, 1989, his fig. 26).

Table 1. Spatial data for the Raton, Vermejo, and Trinidad aquifers in Las Animas County, Colorado.

[Spatial data available at URL http://water.usgs.gov/lookup/getgislist]

Spatial data file name Data type Data description sir-06-5129_seo_wells_point Points Location and selected attributes of water wells (Colorado Division of Water Resources (2003). sir-06-5129_alt_bottom_trinidad_con Points Locations and selected attributes of oil and gas wells. sir-06-5129_cogcc_production_point Points Annual gas and water production by oil and gas wells during 1999–2004. sir-06-5129_formation_tops_point Points Depth to tops of selected hydrostratigraphic units. sir-06-5129_alt_bottom_trinidad_con Contours Generalized altitude of the bottom of the Trinidad Sandstone. sir-06-5129_alt_top_trinidad_con Contours Generalized altitude of the top of the Trinidad Sandstone. sir-06-5129_alt_top_vermejo_con Contours Generalized altitude of the top of the Vermejo Formation. sir-06-5129_alt_top_raton_con Contours Generalized altitude of the top of the Raton Formation. sir-06-5129_depth_bottom_trinidad_con Contours Generalized depth below land surface to the bottom of the Trinidad Sandstone. sir-06-5129_depth_top_trinidad_con Contours Generalized depth below land surface to the top of the Trinidad Sandstone. sir-06-5129_depth_top_vermejo_con Contours Generalized depth below land surface to the top of the Vermejo Formation. sir-06-5129_depth_top_raton_con Contours Generalized depth below land surface to the top of the Raton Formation. sir-06-5129_thickness_trinidad_con Contours Generalized thickness of the Trinidad Sandstone. sir-06-5129_thickness_vermejo_con Contours Generalized thickness of the Vermejo Formation. sir-06-5129_thickness_raton_con Contours Generalized thickness of the Raton Formation. sir-06-5129_alt_water_table_con Contours Generalized altitude of the water table. sir-06-5129_depth_water_table_con Contours Generalized depth below land surface to the water table. Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

Table 2. Hydrostratigraphic section of the Raton Basin (modified from Geldon, 1989).

[--, not defined]

Maximum Formation Hydrostratigraphic System Series thickness1 Description and member unit (feet) Sedimentary rocks Clay to boulder-sized Holocene Stream alluvium 10–70 detritus. Quaternary Alluvial aquifers. Clay to boulder-sized Pleistocene Pediment alluvium 40–50 detritus. Variegated shale and Huerfano Forma- 2,000 sandstone with basal Huerfano aquifer (?). tion conglomerate. Eocene Pink and white sandstone Cuchara Formation 5,000 with shale layers. Cuchara–Poison Canyon aquifer Tan, gray, and (Geldon, 1989). Poison Canyon Tertiary 2,500 olive sandstone, Formation conglomerate, and shale. Gray, green, and tan Raton Formation, Paleocene 100–300 siltstone, sandstone, Upper2 and shale. Dark-gray, carbonaceous, Raton Formation, Raton aquifer. 800–1,000 and silty shale with coal Raton-Vermejo- Middle2 and sandstone lenses. Trinidad Tertiary and Paleocene and Raton Formation, Tan to gray sandstone and aquifer 1–250 Cretaceous Upper Cretaceous Lower2 conglomerate. (Geldon, Dark-gray, carbonaceous, 1989) Cretaceous Upper Cretaceous Vermejo Formation 65–590 and silty shale with coal Vermejo aquifer. and sandstone lenses. Tan to gray sandstone Trinidad Sandstone 40–340 Trinidad aquifer. with thin silty shales. Dark-gray shale with Pierre Shale 1,800–2,300 limestone and Pierre confining unit. sandstone layers. Igneous rocks -- Basalt and associated Potentially an aquifer but generally Basalt flows Pliocene–Miocene tuff, breccia, and unsaturated. (extrusive rocks) conglomerate. -- Syenodiorite, granodio- Potentially an aquifer Tertiary Intrusive rocks, rite, granite, and syenite where fractured. stocks Miocene porphyry. Intrusive rocks, -- Granite to gabbro in Flow barriers. dikes and sills composition. 1All formations present in the area are locally removed by erosion. Thickness or range in thickness given represents measured thickness revealed in outcrops and drill holes (Geldon, 1989). 2On the basis of completion reports for coalbed methane wells in Las Animas County, the reported thickness of Raton Formation, the combined thicknesses of the Upper, Middle, and Lower members, ranges from 1,100 to 1,900 feet.

The Trinidad Sandstone and the Vermejo, Raton, Poison Shale. An estimated 1,500 water-supply wells in the study Canyon, and Cuchara Formations are the primary water- area are completed in the coal, sandstone, and conglomerate in bearing rocks that overlie the Pierre Shale in the study area. these rocks or in alluvium that is hydraulically connected with Geldon (1989) defined two principal aquifers in the study area, them. Coal beds in the Vermejo and Raton Formations also are the Cuchara–Poison Canyon and the Raton-Vermejo-Trinidad the sources of methane for CBM wells and the ground water aquifers. Thin alluvial deposits along the Apishapa and which must be pumped to produce the methane. Purgatoire Rivers and their tributaries also are water bearing Geldon (1989) provides an analysis of the ground-water but are limited in extent. Ground water in the study area that hydrology of the Raton Basin in Colorado and New Mexico. has less than 10,000 milligrams per liter (mg/L) dissolved The aquifers in the study area are recharged by infiltra- solids occurs in the sedimentary rocks that overlie the Pierre tion of precipitation on outcrops in upland areas. Regional Introduction ground-water flow generally is from west to east, except where database (Colorado Oil and Gas Conservation Commission, intercepted by valleys cut into the rock. Most ground-water variously dated). Locations, geologic, construction, and flow occurs in the permeable rocks (coal, conglomerate, and production data, and scanned documents for oil and gas sandstone) but also occurs in fractures in other, less permeable wells can be accessed at the COGCC Web site at URL rocks. Ground water is stored in both the permeable rocks and http://www.oil-gas.state.co.us/. less permeable rocks (siltstone and shale). Ground-water flow The digital geologic map of Colorado (Green, 1992) generally is lateral and parallel with bedding but also flows was used to define the surface geology at wells. The digital downward where fractures connect permeable rocks. Local geologic map of Colorado can be accessed, in a spatial data ground-water flow from stream divides to valleys intercepts format, at URL http://pubs.usgs.gov/of/1992/ofr-92-0507/ much of the regional ground-water circulation. colorado.htm. The geologic map of the Trinidad quadrangle The volume of ground water pumped by about 1,500 (Johnson, 1969) was used to estimate the locations and water-supply wells in the study area is unknown; however, altitudes of geologic contacts at outcrop. The geologic map estimated annual ground-water use for all of Las Animas of the Trinidad quadrangle can be accessed, as an image, at County during 2000 was 1,055 Mgal (U.S. Geological Survey, URL http://ngmdb.usgs.gov/ImageLibrary/. 2004a; Hutson and others, 2004). During 1999–2004, the cumulative volume of ground water coproduced by CBM wells from the Raton and Vermejo Formations in Las Animas Water Wells County was about 299 million barrels (about 12,558 Mgal) Well data were acquired in 2003, and the locations of (Colorado Oil and Gas Conservation Commission, vari- water wells in the SEO database were reported in Universal ously dated). During 1999–2004, the volume of ground water Transverse Mercator (UTM) coordinates (zone 13, NAD 27, coproduced by CBM wells in Las Animas County also meters). The UTM coordinates are calculated from the loca- increased about threefold, from 22,597,972 barrels (about tion reported by the well driller or owner. Most of the study 949 million gallons) during 1999 to 68,557,832 barrels (about area south of the Purgatoire River is the Maxwell Grant 2,879 million gallons) during 2004 (fig. 3). Average annual (fig. 2), which is a land grant that does not have official town- ground-water production by CBM wells in the study area dur- ship, range, and section descriptions. Consequently, water ing 1999–2004 was about 2,093 Mgal, which is about twice wells located in the Maxwell Grant generally do not have the 1,055 Mgal of estimated ground-water withdrawals for all UTM coordinates in the SEO database. Minimum accuracy of other uses in Las Animas County during 2000. water-well locations is the nearest quarter-quarter section, the The response of fluid pressure in hydrostratigraphic nominal 40-acre tract in which the well is located; however, units to changes in recharge and discharge rates primarily is many well locations also are reported as distances in feet from controlled by the three-dimensional (spatial) distribution of the north or south and from the east or west section lines. lithology. Lithology (rock type) is a primary control of poros- Accuracy of distances of water wells from section lines is not ity. The hydraulic and storage properties of rocks depend on reported. Assuming that a well is located in the center of the the amount, size, and interconnection of pores. Understanding 40-acre tract, it is 660 ft from the sides of the tract and, thus, the external geometry (hydrostratigraphic framework) of the location error for the well could be as much as about 933 ft Trinidad Sandstone and Vermejo and Raton Formations is an (√(2*660 ft*660 ft) ≈ 933 ft). In addition to well locations, the initial step in improving understanding of ground-water flow SEO database includes well-permit and ownership information in the Raton Basin. Other hydrogeologic factors that affect and, generally, well depth, depths to the tops and bottoms of ground-water flow in the study area and that also need better perforated or screened intervals, reported depth to water, and definition, but which were not evaluated during this study, reported well yield. Generally, the altitude of the land surface include the internal geometry of the hydrostratigraphic units at the well and the aquifer in which the well is completed are and the effects of faults, fractures, and igneous intrusions not reported in the SEO database. (dikes and sills) on ground-water flow. Coalbed-Methane Wells Data Since January 2003, locations of oil and gas wells Data used for this study were retrieved from two agen- are reported to a precision of five (5) decimals of a degree cies of the Colorado Department of Natural Resources, the (about 3 ft) using NAD 83, as required by COGCC rule Division of Water Resources, and the Colorado Oil and Gas 308A (Colorado Oil and Gas Conservation Commission, Conservation Commission (COGCC). Water-well data for 2003). However, wells installed before adoption of rule 308A this study were obtained from the database and files of the may have less accurate locations. Currently (2006), accu- State Engineer’s Office (SEO) of the Colorado Division of racy and vertical datum of land-surface altitudes of oil Water Resources (Colorado Division of Water Resources, and gas wells are not specified in COGCC rules. 2003). Selected data for water wells can be accessed at the The primary source of subsurface geologic data for this SEO Web site at URL http://water.state.co.us/. Data for study consisted of depths to formation tops and bottoms listed oil and gas (CBM) wells were retrieved from the COGCC on “scout cards” for CBM wells. A scout card is a summary

Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

105°00' 104°45' 104°30' 104°15'

25 T. East Spanish Aguilar 30 Peak 160 350 S. West Spanishpani HUERFANO CO Peak NIMAS CO LAS AANIMAS CO 85 12 Peak

87 T. 31 S.

T. 37°15' 32 S.

160

T. Trinidad Cokedale 33 S.

Starkville

T. MAXWELL GRANT 34 S. Raton Mesa

T. Raton Mesa 35 25 COLORADO 37°00' S. NEW MEXICO I R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. 0 5 10 15 20 MILES Standard parallels 37°30' N and 40°30', central meridian 105°30' W

0 5 10 15 20 25 30 KILOMETERS

Figure 2. Generalized geologic map and locations of oil and gas wells in the Raton Basin, Las Animas County, Colorado, 2004. Introduction

EXPLANATION Geologic Maps QUATERNARY The digital geologic map of Colorado (Green, 1992) was Gravel and alluvium used to identify the surface geology at water wells and CBM wells. Because the subsurface data used in this study primar- Glacial drift ily are data for CBM wells and most CBM wells are located several miles from the basin boundaries, there is little subsur- TERTIARY face data to define the altitudes of formation tops and bottoms Basalt flows near outcrops. Therefore, altitudes of selected points along mapped geologic contacts between selected stratigraphic units Intrusive rocks, stocks were estimated from the 1:250,000-scale geologic map of the Trinidad quadrangle (Johnson, 1969). Intrusive rocks, dikes and sills

Huerfano Formation Vertical Datum Cuchara Formation The land-surface altitude at water wells and at CBM wells Poison Canyon Formation that were used for this study were estimated, on the basis of reported location, using geographic information (GIS) software TERTIARY—UPPER CRETACEOUS and the National Elevation Dataset (NED) at a resolution of Raton Formation 1/3 Arc Second (U.S. Geological Survey, 2004b). The NED is a raster (geospatial grid) of land-surface altitudes, which can be UPPER CRETACEOUS accessed at URL http://seamless.usgs.gov/. At a resolution of Vermejo Formation and Trinidad Sandstone 1/3 Arc Second each raster value represents the altitude of an area of about 100 m2 (about 929 ft2). Horizontal datum for the Pierre Shale and Niobrara, Carlile, Greenhorn, NED is the North American Datum of 1983 (NAD 83) and the and Graneros Formations vertical datum is the North American Vertical Datum of 1988 LOWER CRETACEOUS—JURASSIC (NAVD 88). The altitudes (raster values) are given in meters (m) but were converted to altitudes in feet (ft) above NAVD 88 Dakota, Purgatoire, Morrison, Ralston Creek, for this study. Vertical accuracy of the NED is approximately and Entrada Formations one-half the contour interval of the original source data, which PERMIAN—PENNSYLVANIAN in the study area were 7.5-minute topographic quadrangles with contour intervals of 20 or 40 ft. Because the land surface Sangre de Cristo, Minturn, and Belden Formations in the study area is rugged with local relief of several hundred feet, inaccurate geographic coordinates for a well could result PRECAMBRIAN in large error in the estimated land-surface altitude. The dif- Igneous and metamorphic rocks ference between NED-estimated altitude and reported altitude for 1,305 CBM wells in the study area ranged from 0.01 to Fault—Bar and ball on downthrown side about 929 ft with a median difference of about 15 ft; 25 percent Low-angle thrust fault—Sawteeth on upper plate of the differences were less than 7.3 ft, and 25 percent of the Syncline differences exceeded 29 ft. The causes of these differences in reported and estimated altitude at oil and gas wells include Oil or gas well—Shows location of oil or gas well error in the altitude estimated from the NED (±10–20 ft), errors in reported altitudes of wells, vertical-datum correction of Figure 2.—Continued. reported altitude (3.27–4.45 ft), and errors caused by inaccurate horizontal coordinates of wells. of information for an oil or gas well that includes location, Vertical datum corrections are needed to convert altitude geologic, well-completion, and other data for the well. Depths that is referenced to National Geodetic Vertical Datum of 1929 to selected stratigraphic horizons from drilling and comple- (NGVD 29) to altitude referenced to NAVD 88. The vertical- tion reports of CBM wells, which are referred to as “log top” datum correction increases from northeast to southwest across and “log bottom” on the scout cards, were entered into a the study area, from about 3.27 to 4.45 ft. Because altitudes relational database for computation of altitudes of the bottom at water wells and CBM wells were estimated from the NED, and top and thicknesses of the Trinidad Sandstone and the which is referenced to NAVD 88, they did not need datum tops of the Vermejo and the Raton Formations. The reported corrections. Altitudes of geologic contacts at outcrop were depths to formation tops and bottoms were screened to ensure estimated from the geologic map (Johnson, 1969) but a datum that reported depths of formations were in stratigraphic order correction was not applied because the error resulting from (increasing geologic age with depth) and that calculated thick- the estimation method likely was substantially larger than the nesses of the formations were within reasonable limits. datum correction. Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

90 Kriging is a geostatistical method that can be used to determine the optimal weighting of measurements at measured or sampled locations for prediction at unmeasured or unsam- 80 Water Gas pled locations and for estimating the uncertainty associated with those predictions (Bossong and others, 1999). A rigorous 70 discussion of the theoretical aspects of kriging is beyond the scope of this report, and the interested reader is referred to Davis (1986) and to Bossong and others (1999). 60 Kriging is based on the concept of regionalized variables. A regionalized variable has properties intermediate between a

50 truly random variable and a completely deterministic variable and consists of two components, commonly referred to as the “drift” (trend) and the “residual” (Davis, 1986). The 40 drift consists of the average or expected value of the variable within a neighborhood and is the slowly varying (nonstation-

WATER PRODUCTION, IN MILLION BARRELS 30 ary) part of the variable. The residual is the difference between the expected value (drift) and the actual (measured) value.

GAS PRODUCTION, IN MILLION THOUSAND CUBIC FEET When there is a known or suspected trend in the variable, for 20 example, the altitude of a sloping water table or the surface 1999 2000 2001 2002 2003 2004 of a geologic formation that has been folded, the trend is first Figure 3. Annual ground-water and gas production by CBM removed from the point values and the detrended data (residu- wells in the study area, 1999–2004. als) are fit to a mathematical function. Mathematical functions are used in kriging to describe the variability of the data as a function of distance and, if Geostatistical Methods and Computer needed, direction. Figure 4 shows four mathematical func- Contouring tions commonly used in kriging—the exponential, Gaussian, linear, and spherical functions. In figure 4, the semivariance (γ ) is one-half the sample variance between measured values The two methods commonly used for interpolation h (mapping) of geologic surfaces are deterministic-interpolation or , if drift is present, the residual; h is the distance between measurement points for which γ is calculated; the nugget (g) and geostatistical methods. Deterministic-interpolation h is the value of γ for data near h = 0 when nearby values are methods predict values at unmeasured locations from mea- h not identical; the sill (s) is the value of γ which approaches sured values at known locations, based on either the extent of h the sampled population’s variance and at which the data are similarity of values or the degree of smoothing. Geostatistical not spatially correlated; and, the range (r) is the distance, methods predict values at unmeasured locations on the basis h, at which γ approaches the sample variance and beyond of the statistical properties of the measured points and the h which data values are not spatially correlated. Equations for spatial configuration of the sample points around the predic- these mathematical functions are given in Bossong and others tion location. Both deterministic and geostatistical methods (1999). The shape of the graphs of the functions shown in can be implemented in GIS and either can produce similar figure 4 are all similar in that they have a sill and a range but interpretations. differ in shape near h = 0. Deterministic methods work well if data are regularly Development of geostatistical models, for this spaced and there is little local-scale variability in the data; study, consisted of the following general steps: (1) trend however, deterministic methods may work poorly if data are analysis to determine if the spatial mean of the data was sparse, locations are clustered, or local-scale variability is nonstationary (had trends) or stationary (did not have trends); large. Geostatistical methods have the following advantages: (2) removal of the drift (trends) from the data, if the spa- the variance between data points is a function of distance and tial mean was nonstationary; (3) variogram analysis to fit a direction between points; local-scale variability can be rela- theoretical mathematical function to the sample variogram; tively large; and errors of prediction are estimated. Geostatisti- (4) evaluation of anisotropy to compute directional sample cal methods may not provide a unique solution and, because variograms, if needed; and (5) evaluation of cross-validation geostatistical methods are basically linear-regression methods, statistics. tend to smooth (underestimate) variability. Geostatistical Trend removal is done when the spatial mean value is methods, rather than deterministic-interpolation methods, were nonstationary, using either ordinary or universal kriging. The used in this study to map the hydrostratigraphic framework type of trend may be selected based on knowledge of the shape because the subsurface data were relatively variable at local of the surface. A constant trend subtracts the mean value from scales. the data. A first-order trend is simply a sloping plane, which Introduction

A B Exponential Gaussian s s 0.95 s 2/3 s Gamma [ γ (h)] Gamma [ γ (h)] g { g { r/3 Lag (h) r r/2 r Lag (h)

C D Linear Spherical s s Gamma [ γ (h)] Gamma [ γ (h)] g { g { r = h 2r/3 r Lag (h) max Lag (h)

Figure 4. Theoretical variograms of (A) exponential, (B) Gaussian, (C) linear, and (D) spherical functions. is defined by a linear equation, a second-order trend is defined Cross-validation statistics are computed from the mea- by a quadratic equation, and a third-order trend is defined by a sured values and the predicted values from kriging. Prediction cubic equation. error for a measurement equals the measured value minus

In kriging, the semivariance (γh) for the sample data is the predicted value. Mean prediction error is the mean of the calculated and a sample variogram, a graph of the semivari- prediction errors and should be near 0. Because the mean pre- ance (γh) plotted against lag (h), is fit to one of the mathemati- diction error depends on the scale of the data, the prediction cal functions. Lag (h) is used in the variograms rather than standardized errors also are calculated. The prediction errors h because, typically, there are not enough data pairs at each divided by the prediction standard errors equals the prediction finite distance to calculate a valid γh; therefore, γh is estimated standardized errors. Root-mean-squared error is the square based on a finite range (lag) of distances. The objective of root of the mean of the prediction errors squared and should be fitting a variogram is to find a mathematical function that minimized. The average standardized error is the mean of the characterizes the spatial continuity of a regionalized variable prediction errors divided by the prediction standard errors and as a function of distance. The mathematical function that best should be near zero (0). The root-mean-squared-standardized fits the sample data is determined by trial and error. Because error should be close to 1 if the prediction standard errors are geological data from gas, oil, and water wells generally are valid. If the root-mean-squared-standardized error is greater widely spaced, on the order of 1,000 ft or more, there gener- than 1, the variability of the predictions is underestimated; if ally are not enough closely spaced data to define the shape of the root-mean-squared-standardized error is less than 1, the the variogram near zero lag (h = 0). Consequently, the sample variability of the predictions is overestimated. variograms were not particularly sensitive to which mathemat- The general procedure used to prepare the structure and ical function was used. apparent thickness maps consisted of the following steps: Simple kriging may be adequate to predict variables, 1. Point values of the altitudes of the formation top were which do not have a substantial trend in the spatial mean calculated as land-surface altitude (estimated from the value of the variable. Ordinary kriging can be used for NED) at the CBM well minus the reported depth to the data that seem to have a trend. However, there is no way to formation top or were altitudes of geologic contacts that decide, based on the data alone, whether the observed pat- were estimated from the geologic map of the Trinidad tern is the result of autocorrelation among the residuals or quadrangle (Johnson, 1969). Point values of formation a trend. A polynomial regression with the spatial coordi- thickness were calculated from the reported depths to the nates as the explanatory variables is used to predict the drift tops of successive stratigraphic units. in universal kriging. As with ordinary kriging, there is no way to decide whether the observed pattern is the result of 2. Spatial trends in the variable (tops or thicknesses) were autocorrelation among the residuals or a trend, on the basis evaluated to determine whether the variable was station- of the data alone. ary (no trend) or nonstationary (trend). Because the trend 10 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

analyses indicated that variables were nonstationary, a Hydrostratigraphic Framework of the geostatistical method that incorporated trends was used to model the residuals, the difference between the trend Raton, Vermejo, and Trinidad Aquifers (drift) and the point values. Better definition of the hydrostratigraphic frame- 3. Because trends were indicated by the trend analyses work (external geometry) of the Trinidad Sandstone and of the variables, ordinary kriging, as implemented in ArcMap 9.1 (Environmental Systems Research, Inc., the Vermejo and Raton Formations are needed to increase 1999–2005), was used to develop variogram models to understanding of ground-water flow in the Raton Basin. Other predict values of the altitude of the tops or the thicknesses hydrogeologic factors that also need better definition include of the formations. Several types of theoretical mathemati- the spatial variability within these formations (their internal cal functions (spherical, exponential, and Gaussian) and geometry) and the effects of faults, fractures, and igneous parameter sets (lag distances, anisotropy, and ratio of intrusions (dikes and sills) on ground-water flow. Because global to local drift) were evaluated by trial and error, lithology is a primary control on the porosity and permeability to find an acceptable fit of the sample variogram to a theo- of sedimentary rocks, understanding the effects of changes retical mathematical function. Anisotropy refers to spatial in fluid pressure requires knowledge of internal and external variability that is dependent on the direction of measure- geometries of the hydrostratigraphic units. ment. Drift (trend) in spatial variability can be global, A hydrostratigraphic unit is a “formation, group of local, or a combination of global and local. The ratio of formations, or part of a formation, which by virtue of its global to local drift determines the size of the neighbor- porosity or hydraulic properties has a distinct influence on hood in which a polynomial equation is fit to detrend the the storage and movement of ground water” (American data. If the ratio of global to local drift is 1:0, the neigh- Nuclear Society, 1980). A permeable hydrostratigraphic unit borhood is global and the data are detrended by removing is an aquifer, and an impermeable or relatively imperme- regional trends; if the ratio of global to local drift is 0:1, able hydrostratigraphic unit is a confining unit. An aquifer the neighborhood is local and the data are detrended by is “a formation, group of formations, or part of a formation removing local trends. The software generates grids of that contains sufficient saturated permeable material to yield predicted values and standard errors for the variables and significant quantities of water to wells and springs” (Lohman output grid spacing ranged from about 780 to 970 ft. The and others, 1972). The definition of a gas or oil reservoir is differences in grid spacing result from differences in the similar to that of an aquifer, except that the fluids of interest number and distribution of data points for the variables. are gas and oil. A confining unit is “a body of impermeable The selected mathematical function and parameter sets material stratigraphically adjacent to one or more aquifers... were those that resulted in cross-validation statistics its hydraulic conductivity may range from nearly zero to some with a mean error (of prediction) near zero, the smallest value distinctly lower than that of the aquifer” (Lohman and root-mean-squared error, an average standardized error others, 1972). approximately equal to the root-mean-squared error, and Table 2 (modified from Geldon, 1989) summarizes the a root-mean-squared-standardized error of about 1. hydrostratigraphic section for the Tertiary- and Cretaceous- 4. Contour lines were computer generated from the output age rocks that overlie the Cretaceous-age Pierre Shale in grid values to show lines of equal altitude of a surface or Colorado and New Mexico. The Pierre Shale is considered thickness of the hydrostratigraphic unit. The computer- a regional confining unit because it substantially restricts generated contours were smoothed, using an ArcMap gen- the flow of ground water and, for the purposes of this study, eralization tool, to smooth the contour lines. The smoothed is considered the base of the hydrostratigraphic framework. contours were edited, using ArcEdit, an extension of Arc The two principal aquifers in the Raton Basin that overlie the (Environmental Systems Research, Inc., 1982–2000) to Pierre confining unit are the Cuchara–Poison Canyon and remove small closed contours, which likely were artifacts the Raton-Vermejo-Trinidad aquifers (Geldon, 1989). The of the computer-generated contouring. Cuchara–Poison Canyon aquifer, with a maximum combined 5. Grid values were generated from the smoothed and edited thickness of about 7,500 ft in Huerfano County (table 2), was contour lines using TOPOGRID, an ARC™ 8.0.2 com- considered to be a single aquifer because of compositional mand. Derivative maps were prepared from mathematical similarity of the Poison Canyon and Cuchara Formations combinations of the grids. For example, the depth to the (Geldon, 1989). The Raton-Vermejo-Trinidad aquifer, with top of the Trinidad Sandstone was calculated by sub- a maximum combined thickness of about 2,480 ft (table 2), tracting the grid of the altitude of the top of the Trinidad also was defined on the basis of compositional similarity of Sandstone from the NED (a grid of the land-surface alti- the Vermejo and Raton Formations and of similarity between tude). The values of the resultant grid were then contoured sandstone layers in these formations and those in the Trinidad and the contours were smoothed and edited. Sandstone (Geldon, 1989). Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers 11

By definition, the porosity or hydraulic properties of a Formation was deposited on an upper alluvial plain, charac- hydrostratigraphic unit (aquifer or confining unit) have a dis- terized by various modes of aggradation and erosion (Flores, tinct influence on the storage and movement of ground water 1984). Deposition on the alluvial plain was interrupted by sev- (American Nuclear Society, 1980). Because the Cuchara– eral types of fluvial sedimentation. Rapid uplift of the source Poison Canyon and Raton-Vermejo-Trinidad aquifers, as area to the west caused widespread erosion of the highland defined by Geldon (1989), consist of thick sequences of and concurrent deposition of a basal conglomerate. The basal interbedded and discontinuous sandstones, shales, and coals, conglomerate, which consists of medium- to coarse-grained in which hydraulic and storage properties vary spatially, the channel sandstones with lenses and stringers of conglomer- aquifers, as defined, are likely so heterogeneous and aniso- ate, probably was deposited in basin-margin braided streams tropic that the spatial distribution of their properties could merging basinward into meandering streams. After deposition only be defined in a probabilistic manner. Subdivision of the of the basal conglomerate, sandstones were deposited mainly Cuchara–Poison Canyon and Raton-Vermejo-Trinidad aqui- in meandering streams at the same time that interbedded fers, on the basis of differences in their lithologic composi- thin coal, carbonaceous shale, mudstone, and sandstone beds tion, allows definition of more distinctive (less heterogeneous) accumulated in flood plains and back swamps. Crevasse splays hydrostratigraphic units. periodically interrupted and filled low-lying areas during Subsurface data are available only to refine the definition floods. Though some thick coal beds were deposited locally of the Raton-Vermejo-Trinidad aquifer in Las Animas County. in the lower coal zone, most coal swamps that formed in the Fewer CBM wells have been drilled in Huerfano County flood basins were well drained, small, and shallow, which and where the Huerfano and Cuchara Formations overlie the limited peat accumulation and resulted in thin, lenticular coal Poison Canyon Formation in Las Animas County (fig. 2), beds. thus, it is not possible to map subsurface configurations of the Geldon (1989) reported that the thickness of the Raton Huerfano, Cuchara, and Poison Canyon Formations. Because Formation ranged from about 1,000 to 1,600 ft. However the Vermejo and Raton Formations are the primary targets for on the basis of completion reports from 126 CBM wells CBM exploration and production in the Raton Basin and more in Las Animas County, reported thickness of the Raton than a 1,400 CBM wells have been drilled in the study area, Formation ranges from about 1,100 to 1,900 ft, where it is not refinement of the hydrostratigraphic framework was limited to eroded (is overlain by the Poison Canyon Formation). Net coal Las Animas County. Also, there were enough subsurface data thickness is about 100 ft, with as many as 40 coal beds, which to subdivide the Raton-Vermejo-Trinidad aquifer into three are characteristically lenticular, 1 to 10 ft thick, and with distinct aquifers: the Raton, Vermejo, and Trinidad. limited lateral continuity of about 500 to 1,000 ft (Clarke and The sedimentary rocks, which make up the Raton- Turner, 2002). Vermejo-Trinidad aquifer, the Trinidad Sandstone, the The Raton Formation includes upper, middle, and lower Vermejo Formation, and the Raton Formation, were formed members (Wood and others, 1951). The three members of the from clastic sediments and organic matter that were deposited Raton Formation are not distinguishable, except by strati- by different processes in distinct sedimentary environments. graphic position, and recognition of them was based primarily These included barrier-island, delta-front, and coastal depos- on the lateral persistence of sandstone zones at outcrop. The its in the Trinidad Sandstone; fluvial-dominated-delta-plain lower member differs from the middle and upper members, deposits in the Vermejo Formation, and alluvial-plain deposits because it contains a basal conglomerate, which thins east- in the Raton Formation (Clarke and Turner, 2002). The rocks ward and becomes finer grained (Wood and others, 1956). that formed from these sediments were subsequently uplifted, Generally, the lower contact of the Raton Formation with the warped, fractured, and intruded and altered by igneous rocks. Vermejo Formation, as indicated by the basal conglomerate, Subdivision of the Raton-Vermejo-Trinidad aquifer, on is erosional, but locally, the contact may be conformable. the basis of stratigraphy (indirectly the sedimentary depo- Coal, conglomerate, and sandstone in the Raton Formation are sitional environment), results in hydrostratigraphic units in capable of transmitting substantial quantities of water to wells. which rock types, the primary control on the hydraulic and Because of the interbedded and lenticular nature of its strata, storage properties, are internally more consistent. Detailed the Raton aquifer is a highly heterogeneous and anisotropic analysis and correlation of geophysical logs and cores could aquifer. Definition of the internal geometry of the Raton provide a better understanding of the spatial distribution of Formation likely will require detailed analysis of core and hydraulic and storage properties within these aquifers but is geophysical logs and a probabilistic approach to approximate beyond the scope of this study. aquifer heterogeneity.

Raton Aquifer Vermejo Aquifer

The Raton aquifer consists of the Raton Formation. The The Vermejo aquifer consists of the Vermejo Formation. Raton Formation (table 2) consists of interbedded sandstone, The Vermejo Formation consists of gray to dark-gray, carbona- siltstone, silty shale, and lenticular beds of coal. The Raton ceous, and sandy shale with lenses of sandstone and coal. The 1 2 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin sediments and coals of the Vermejo Formation were deposited the north across the study area, with the maximum reported in contemporaneous fluvial-deltaic and back-barrier coastal thickness in Huerfano County. In comparison with the Raton plains fronted by barrier-bar and delta-front sandstones of the and Vermejo Formations, the Trinidad Sandstone is a relatively Trinidad Sandstone (Flores and Tur, 1982). Lower alluvial homogeneous aquifer. plains dissected by meandering streams separated by flood basins characterized the upper part of the Vermejo Formation. Sand-filled stream channels and fine-grained sequences of silt Structure and Apparent Thickness of the Raton, and mud were laid down in flood plains associated with cre- Vermejo, and Trinidad Aquifers vasse splay and minor crevasse-channel sandstone. Coal beds in the lower part of the formation formed in poorly drained The configurations of the top and bottom surfaces (the back barrier coastal swamps and in swamps adjacent to distrib- external geometry) of the Trinidad Sandstone and the Vermejo utary channels of delta plains. The thinner sandstone beds are and Raton Formations were mapped, using a combination of stratified and laminated, whereas the thicker beds are lenticu- geostatistical models and grid calculations. Reported strati- lar and cross-stratified (Wood and others, 1956). Thickness of graphic picks (depths to tops of the formations from drilling the Vermejo Formation in the study area, as indicated by com- and completion reports) for about 1,400 CBM wells, esti- pletion reports from 888 CBM wells, ranges from 65 to 590 ft. mated land-surface altitude at the wells, and the estimated The Vermejo Formation contains as many as 15 coal beds that altitudes of selected geologic contacts from the geologic map generally are 3 to 6 ft thick, with a total coal thickness of 5 of the Trinidad quadrangle (Johnson, 1969) were used with to 45 ft (Clarke and Turner, 2002). Coal beds in the Vermejo geostatistical models to estimate the altitudes of the tops of Formation generally are thicker, more regular in thickness, and the Trinidad Sandstone and the Raton Formation and the more laterally persistent (1,000 to 3,000 ft) than those in the apparent thicknesses of the Trinidad Sandstone and Vermejo Raton Formation. The contact of the Vermejo Formation with Formations. The altitudes of the bottom of the Trinidad the underlying Trinidad Sandstone is conformable (Wood and Sandstone and top of the Vermejo Formation were calculated others, 1956). A conformable contact indicates that the strata from the altitudes of the top of the Trinidad Sandstone and do not show physical evidence of erosion or significant breaks the apparent thicknesses of the Trinidad Sandstone and the in deposition (Bates and Jackson, 1980). The Vermejo aquifer Vermejo Formation, respectively. The thickness of the Raton is a heterogeneous and anisotropic aquifer but, on the basis of Formation was calculated as the difference between altitudes lateral persistence and thickness of coal beds, it likely is less of the tops of the Raton and Vermejo Formations. heterogeneous than the Raton aquifer. Detailed analysis of core and geophysical logs and a probabilistic approach likely Descriptions of Geostatistical Models and Cross- would be needed to approximate the heterogeneity of the Vermejo aquifer. Validation Statistics Descriptions of the geostatistical models and the cross- Trinidad Aquifer validation statistics are listed in table 3. Table 3 includes the method of kriging, the type or order of drift (trend), the The Trinidad aquifer consists of the Trinidad Sandstone. ratio of global to local drift, the theoretical mathematical The Trinidad Sandstone is tan to gray sandstone that contains functions that were used to fit the sample variograms, var- occasional thin beds of silty shale. The bedding is usually iogram fitting parameters (lag, range, sill, and nugget), and tabular but locally is lenticular and varies from medium to cross-validation statistics for the selected geostatistical mod- massive. The Trinidad Sandstone grades into and intertongues els. The geostatistical models listed in table 3 were the models with the underlying Pierre Shale, and their contact is conform- that had mean errors of prediction nearest to zero, the smallest able (Wood and others, 1956). The Trinidad was deposited root-mean-squared errors, average standardized errors nearest in contemporaneous delta-front and interdeltaic-barrier-bar the root-mean-squared errors, mean standardized errors near environments, as the sea regressed eastward. The delta-front zero, and root-mean-squared-standardized errors nearest to deposits include distributary-mouth-bar and distributary- one. channel sandstones (Flores and Tur, 1982). The barrier-bar All the geostatistical models (table 3) used ordinary krig- deposits consist of middle shoreface, river-estuarine-inlet, ing and assumed first-order trends to compensate for global and beach sandstones that are overlain by the fluvial-deltaic (regional) drift (trend) in the variables. All but one of the and back-barrier deposits of the Vermejo Formation. Clarke models used the spherical function (fig. 4) to model the semi- and Turner (2002) report that stacked channel sands in the variance of the variable as a function of distance; the model upper part of the Trinidad Sandstone indicate reworking of of thickness of the Vermejo Formation used the Gaussian distributary-mouth bars. Thickness of the Trinidad Sandstone, function (fig. 4). All of the geostatistical models, except as indicated by completion reports from 214 CBM wells, that for the water table, assume a first-order global trend to ranges from 40 to 340 ft. The Trinidad Sandstone thickens to compensate for regional drifts (trends) in the variables and lag Table 3. Geostatistical models, model fitting parameters, and cross-validation statistics.

Model fitting parameters1 Cross-validation statistics Drift Number Math- Root-mean- Kriging Trend ratio Average Mean

Sill Nugget Mean Root-mean- squared Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers Variable of data ematical Lag Range standardized standardized method order (global: (square (square error squared error standardized points function (meter) (meter) error error local) feet) feet) (feet) (feet) error (feet) (feet) (feet) Trinidad 822 Ordinary 1 1:0 Spherical 3,750 44,450 242,620 0.0 8.19 212.0 80.9 –0.015 1.00 Sandstone, top Trinidad 213 Ordinary 1 1:0 Spherical 3,600 40,866 0 1,241.2 –1.35 35.9 36.2 –0.037 0.99 Sandstone, thickness Vermejo 904 Ordinary 1 1:0 Gaussian 3,600 42,104 6128.5 3,121.2 –0.51 64.5 57.6 –0.010 1.10 Formation, thickness Raton 689 Ordinary 1 1:0 Spherical 3,600 42,672 249,800 8,450.5 1.04 148.0 134.0 0.002 1.00 Formation, top Water table 646 Ordinary 1 1:1 Spherical 2,700 23,346 92,316 5,389.0 –2.92 146.5 107.4 –0.01 1.06 1The length unit of the geographic coordinate system for spatial data is meter (m), but the length unit of the vertical datum is foot (ft).

1 3 14 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin distances of 3,600 or 3,750 m. The geostatistical model of the of the Trinidad Sandstone. Apparent thickness of the Raton water table also assumed a first-order trend that was equally Formation was calculated as the difference between the top of divided between global and local drift and a lag distance of the Raton Formation and the top of the Vermejo Formation. 2,700 m. [The length unit of the geographic coordinate system The altitude of the bottom of the Trinidad Sandstone, top for the spatial data is meter (m) but the length unit of the verti- of the Vermejo Formation and the apparent thickness of the cal datum is foot (ft).] The ranges (distances beyond which Raton Formation were calculated, rather than modeled, for semivariance is not a function of distance) for geostatistical several reasons. There were only 215 data points available to models of the formation tops, apparent thicknesses, and water model the altitude of the bottom of the Trinidad Sandstone. table are large (table 3), indicating spatial correlation of the Trend analysis of the 215 values for thickness and bottom residuals (measurement minus drift) over large distances. The altitude of the Trinidad Sandstone indicated that thickness sills (the semivariance beyond which the data are not spatially is relatively uniform and the altitude of the bottom of the correlated) for the geostatistical models of formation tops and Trinidad Sandstone is more variable because it is folded into a of the water table are relatively large and those for the appar- complex shape with large variance. Therefore, thickness of the ent thicknesses are relatively small, partly a result of the rela- Trinidad Sandstone could be modeled with smaller error than tive differences in scale of their values. The larger the nugget the altitude of its bottom surface. Calculation of the altitude of is compared to the sill, the more local-scale variability there the bottom surface of the Trinidad Sandstone by subtraction of is in the data, and the less important the correlation between its thickness from the altitude of its top also ensured that the neighboring locations becomes (Bossong and others, 1999). altitude of the bottom would not be greater than the altitude of The nuggets for the geostatistical models of altitudes of forma- the top. tion tops and the water table are small in comparison to their A comparison of preliminary cross-validation statistics respective sills, indicating that variability is primarily global for geostatistical models of the thickness of the Vermejo (regional) in scale. The nuggets for the geostatistical mod- Formation and the altitude of its top indicated that the model els of apparent thicknesses are large in comparison to their of thickness had better cross-validation statistics. The geo- sills, indicating that more of the variability is local scale. The statistical model of thickness had a smaller mean error, –0.51 geostatistical model of the apparent thickness of the Trinidad compared to –5.18; a smaller root-mean-squared error, 64.5 Sandstone has a sill of zero and a large nugget, indicating that, compared to 209; a smaller average standardized error, 57.6 after regional drift (trend) is removed, all the variability in the compared to 87, which also more closely approximated residuals is local scale. the root-mean-squared error; and a root-mean-squared- The cross-validation statistics for the models (table 3) standardized error nearer one (1), 1.1 versus 1.8. Additionally, indicate that the geostatistical models predict the selected vari- calculation of the altitude of the top of the Vermejo Formation ables reasonably well. However, the altitude of the top of the by addition of thickness of the Vermejo Formation and altitude Trinidad Sandstone is overestimated, as indicated by its mean of the top of the Trinidad Sandstone ensured that the surfaces prediction error of 8.19 ft. In addition, its average standardized would not cross. error is much smaller than its root-mean-squared error, indicat- Because the top of the Raton Formation is exposed at out- ing that the variability of the predictions is underestimated. crop in a large part of the study area (fig. 2) and is an erosional However, other combinations of types of drift (constant and surface, the thickness of the Raton Formation, in areas of out- second-order trends), ratios of global to local drift, mathemati- crop, varies not as a function of the original formation thick- cal functions (exponential and Gaussian), and lag distances, ness but ranges from values near zero, at the outcrop of its did not improve fit of the geostatistical model of the top of the contact with the Vermejo Formation, to its original thickness, Trinidad Sandstone. at outcrop of its contact with the Poison Canyon Formation. Because kriging is basically a regression technique and the preponderance of thickness values for the Raton Formation Calculated Surfaces are in areas of outcrop, the trend in thickness values that are The altitude of the bottom of the Trinidad Sandstone affected by erosion would overwhelm the thickness values (top of the Pierre Shale) and the top of the Vermejo Formation where it had not been eroded and result in a poor prediction (bottom of the Raton Formation) were calculated from the of thickness of the Raton Formation in the subsurface. final grids of the altitude of the top of the Trinidad Sandstone and of the apparent thicknesses of the Trinidad Sandstone and Structural Surfaces Vermejo Formation, respectively. The altitude of the bottom of the Trinidad Sandstone was calculated by subtracting the The external geometries of the Raton, Vermejo, and apparent thickness of the Trinidad Sandstone from the altitude Trinidad aquifers are defined by the structural surfaces on of the top of the Trinidad Sandstone. Similarly, the top of the the geologic contacts between the hydrostratigraphic units. Vermejo Formation was calculated by adding the apparent Figures 5–8 show the generalized configuration and altitude of thickness of the Vermejo Formation to the altitude of the top the bottom and top of the Trinidad Sandstone and the tops of 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. EastEa Spanishanish AgAguilarar 30 Peak HUERFANO CO 160 Vermejo Formation and 350 S. West Spanish Trinidad Sandstone Peak Subcrop of Trinidad Sandstone LAS ANIMAS CO 85 12 5000 Structure contour—Shows altitude 87 of bottom of Trinidad Sandstone. T. Dashed where approximately 31 located. Contour interval 200 feet. S. Datum is NAVD of 1988 5000 Control point—Shows location of 6000 gas well with thickness of Trinidad Sandstone Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers

T. 37°15' 32 S.

160

T. Cokedale Trinidad 33 S.

7000

Starkville

6000 MAXWELL GRANT T. 34 S.

6000 Raton Mesa

T. 8000 8000 35 COLORADO 25 Raton Mesa 37°00' 7000 S. NEW MEXICO

R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS

Figure 5. Generalized configuration of the bottom of the Trinidad Sandstone in Las Animas County, Colorado. 1 5 1 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish AguiAguilar 30 Vermejo Formation and 350 S. West Spanish Peak HUERFANO CO 160 Trinidad Sandstone Peak Subcrop of Trinidad Sandstone 85 12 LAS ANIMAS CO 5000 Structure contour—Shows altitude of top of Trinidad Sandstone. T. 87 Dashed where approximately located. 31 Contour interval 200 feet. Datum is S. NAVD of 1988 Control point—Shows location of gas well with depth to top of Trinidad

6000 Sandstone Outcrop point—Used for structural control T. 37°15' 32 5000 S.

160

T. Cokedale Trinidadrinidad 33 S.

Starkville 7000

6000 MAXWELL GRANT T. 34 Raton Mesa S. 7000 6000 COSTILLA CO 7000 T. 8000 8000 Raton Mesa 35 COLORADO 25 37°00' S. 9000 NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. 0 5 10 15 20 MILES Standard parallels 37°30' N and 40°30', central meridian 105°30' W

0 5 10 15 20 25 30 KILOMETERS

Figure 6. Generalized configuration of the top of the Trinidad Sandstone in Las Animas County, Colorado. 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish Aguilar 30 Vermejo Formation and S. Peak 160 350 West Spanish HUERFANO CO Trinidad Sandstone Peak LAS ANIMAS CO 85 Subcrop of Vermejo Formation 12 5000 Structure contour—Shows altitude 87 T. 5800 of top of Vermejo Formation. 5400 5600 31 Dashed where approximately located. S. Contour interval 200 feet. Datum is

6600 NAVD of 1988

Control point—Shows location of

6200 gas well with depth to top of Vermejo Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers 5200 Formation T. 37°15' 32 S.

7200

6600 160 6400

8000 T. Trinidad Cokedale 33 6400 S. 7600

7000 Starkville 7800

6000

6200 MAXWELL GRANT T. 6400 7600 34 6800 S. 6600 Raton Mesa 8800 7400

8400 6600 6800 T. 7800 7200 8200 35 COLORADO 7000 25 Raton Mesa 37°00' 7400 S. 8600 7800 NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS

Figure 7. Generalized configuration of the top of the Vermejo Formation in Las Animas County, Colorado. 1 7 1 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish 7000 AAguilar 30 Raton Formation S. West Spanish Peak HUERFANO CO 6800 350

Peak 160 Subcrop of Raton Formation ANIMAS LAS ANIMAS CO 6600 85 6800 12 6800 Vermejo Formation and Trinidad Sandstone T. 6800 6800 87 6000 Structure contour—Shows altitude 31 of top of Raton Formation. Dashed S. 6800 where approximately located. Contour interval 200 feet. Datum is NAVD of 1988 70007000 6800 Control point—Shows location of gas well with depth to top of Raton 6800 Formation T. 7200 Outcrop point—Used for structural 37°15' 7000 32 control S.

7400 8000 7200 160

7600 7000

8200

T. Cokedalek Trinidad 33 7800 6400 S.

6600 6400 8400 Starkville 66006600 64006400 7600 8800

8200 7800 T. MAXWELL GRANT 7400 34 8000 6800 S. 8800

8200 8000 8000 8400 8200 8600 Raton Mesa 7400 7200

8400 7400 9200 9000 8600 Raton MesaMesa T. 8800 8600 35 8200 8000

9400 9400 COLORADO 25 8400 37°00' 9200 7800 7400 8200 S. NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. 0 5 10 15 20 MILES Standard parallels 37°30' N and 40°30', central meridian 105°30' W

0 5 10 15 20 25 30 KILOMETERS

Figure 8. Generalized configuration of the top of the Raton Formation in Las Animas County, Colorado. Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers 19 the Vermejo and Raton Formations, respectively. The surfaces shown as dashed lines (figs. 13–15) because they are gener- are well defined, where there is adequate subsurface control alized from the difference between two estimated surfaces. from CBM wells, but are poorly defined in the northern part of Detailed measured (geologic) sections at outcrops along the the study area, near Spanish Peaks, and along the eastern and eastern and western sides of the study area could improve the western margins of the Raton Basin. The contours (figs. 5–8) maps of formation thickness and provide a better understand- are dashed where there is little subsurface control. The ing of the spatial variability of lithology (internal geometry) configuration of the top of the Trinidad Sandstone (fig. 6) is of these aquifers in the subsurface. similar to that previously mapped (Geldon, 1989, his fig. 43). The apparent thicknesses of the Trinidad Sandstone Measurements of the altitudes of formation contacts at outcrop (fig. 13), the Vermejo Formation (fig. 14), and the Raton and additional subsurface control likely would improve the Formation (fig. 15) equal the altitude of the top of the definition of the structural surfaces along the margins of the respective formation minus the altitude of the top of the Raton Basin. underlying formation. Apparent thicknesses of the selected formations were calculated using grids of the edited contours Depth to Bottoms and Tops shown in figures 5–8. For example, the apparent thickness of the Trinidad Sandstone equals the altitude of the top of the The generalized depths to the bottom and top of the Trinidad Sandstone (fig. 6) minus the altitude of the bottom Trinidad Sandstone and to the tops of the Vermejo and of the Trinidad Sandstone (fig. 5). Raton Formations (figs. 9–12, respectively) were calculated, The thicknesses are referred to as “apparent thicknesses” using the spatial analyst tools in ESRI ® ArcMap™ 9.1 because the formations dip from the horizontal and the (Environmental Systems Research, Inc., 1999–2005), as the formations’ thicknesses, which are calculated from reported difference between the estimated altitude of the land surface, formation tops and bottoms or from figs. 5–8, are not mea- from the NED, and the altitude of the respective surface sured perpendicular to the bedding planes of the formations. (figs. 5–8). Computer-generated lines of equal depth to the top Correction of apparent thickness is larger than 5 percent of stratigraphic units were smoothed and edited. Because the when the dip angle is larger than about 18º from horizontal. surfaces represent contacts between successive stratigraphic For example, near the western side of the study area, where units, the top of one stratigraphic unit is equivalent to the bot- bedding dips steeply, approaching 90º, formation thicknesses tom of the overlying stratigraphic unit, unless the overlying are overestimated. The thickness of a dipping formation, the unit has been removed by erosion. The depths to the bottom thickness perpendicular to the bedding plane, is equal to the and top of the Trinidad Sandstone and to the tops of the product of apparent thickness of the formation and the cosine Vermejo Formation (figs. 9–11) are equivalent to the thickness of the angle of dip. For example, if the apparent thickness is of the rocks overlying those surfaces. Except where overlain 100 ft and the angle of dip is 60º, the true formation thickness by basalt flows, the thickness of the rock overlying the top of is 50 ft (100 ft * cosine (60º) = 100 ft * 0.50 = 50 ft). When the Raton Formation (fig. 12) is equivalent to the combined estimating the transmissive and storage properties of a dipping thickness of the Poison Canyon, Cuchara, and Huerfano aquifer from apparent thickness and average material values, Formations. The contours of depth to the bottom of the the calculated values will overestimate the true values, unless Trinidad Sandstone and to the tops of the Trinidad Sandstone apparent thickness is corrected for the angle of dip. and Vermejo and Raton Formations from land surface are shown as dashed lines (figs. 9–12) because the depths are generalized from the difference between two estimated surfaces. Regional Water Table A potentiometric surface is defined as a surface that Apparent Thicknesses represents a static head, a water level that is not affected by pumping (Lohman and others, 1972). The water table is the Maps of the generalized thickness of the Trinidad potentiometric surface in an unconfined water body at which Sandstone (fig. 13), the Vermejo Formation (fig 14), and the pressure is atmospheric (Lohman and others, 1972). Where the Raton Formation (fig. 15), though previously estimated the water level is confined or varies appreciably with depth in using geostatistical models, also were calculated, using the an unconfined aquifer, the potentiometric surface is meaning- spatial analyst tools in ESRI ® ArcMap™ 9.1 (Environmental ful only if it describes the static head along a particular speci- Systems Research, Inc., 1999–2005), as a grid of the edited fied surface or stratum in that aquifer (Lohman and others, altitude of the top of a stratigraphic unit minus a grid of the 1972). edited altitude of the top of the underlying stratigraphic unit. Most water-supply wells in the Raton Basin probably are Lines of equal thickness of the stratigraphic units were com- drilled just deep enough to obtain an adequate supply of water. puter generated from the resultant grids and were smoothed Few recent measurements of water levels are available for the and edited. The contours of equal apparent thicknesses of the study area, in State and Federal databases. However, the SEO Trinidad Sandstone and Vermejo and Raton Formations are database includes the reported depth to water, at the time the 2 0

Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. AAguilar 30 East Spanish Vermejo Formation and HUERFANO CO 160 S. Peak Trinidad Sandstone West Spanish 350 2500 Peak 2500 500 Subcrop of Trinidad Sandstone 85 12 LAS ANIMAS CO 2500 2500 Line of equal depth to bottom of Trinidad Sandstone—Dashed T. 87 where approximately located. 31 500 Contour interval 500 feet. Datum S. is land surface Control point—Shows location of gas well with depth to bottom of 500 Trinidad Sandstone

500 35003500 T. 37°15' 3000 500 500 32 S.

25002500 500500 1000 160

500 2000 500 T. Trinidad Cokedale 33 500 500 500 S.

1500 10001000 500500 2000 1500 500 Starkville

1500 MAXWELL GRANT 1000 T. 2500 500 34 500500

500500 20002000 20002000 S. 500 2500 1500 10001000 2500 Raton Mesa

1000 500 1000 500 1000 1000 T. 15001500 500500 1500 Raton Mesa 2000 1500 25 35 1500 1000 1000 1000 COLORADO 1000 COLORADO 37°00' 500500 2500 1500 S. NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS

Figure 9. Generalized depth to the bottom of the Trinidad Sandstone in Las Animas County, Colorado. 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish AguilarAg 30 15001500 Vermejo Formation and Peak 160 350 S. West Spanish HUERFANO CO 2000 Trinidad Sandstone 2500 Peak 2500 Subcrop of Trinidad Sandstone 500 2500 12 LAS ANIMAS CO 85 2500 Line of equal depth to Trinidad Sandstone—Dashed where T. 87 approximately located. Contour interval 500 feet. Datum is land 31 500 surface S. Control point—Shows location of gas well with depth to top of Trinidad

3000 Sandstone Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers 35003500

T. 37°15' 500 32 S.

2500 160 500500 2000

500 500 Cokedale Trinidad 33 S. 1000 500

500 2000 Starkville

10001000 1500 1500 500 MAXWELL GRANT 500 T. 2500 2500 500 34 1000 S. Raton Mesa 500 20002000 500 500 500500 15001500

15001500 500 2000 T. 1000 1000 20002000

500 Raton Mesa 500 35 1500 COLORADO 25 1000 2500 1500 1000 S.

1500 37°00' 1500 1000 NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS

Figure 10. Generalized depth to the top of the Trinidad Sandstone in Las Animas County, Colorado. 1 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish AguilarAg 30 Peak S. West Spanish HUERFANO CO 160 Vermejo Formation and 350 2000 Peak Trinidad Sandstone 85 LAS ANIMAS CO Subcrop of Vermejo Formation 12 LAS ANIMAS CO 2000 Line of equal depth to Vermejo 2500 T. 87 Formation— Dashed where 31 approximately located. Interval S. 500 feet. Datum is land surface Control point—Shows location of gas well with depth to top of Vermejo Formation

2500 30003000 2500 T. 37°15' 32 S. 20002000

500500 160

T. 1500 Trinidad Cokedale 33 S.

1500 1000 1500 1000 500 Starkville

10001000 MAXWELL GRANT 1000 MAXWELL GRANT T. 34

20002000 Raton Mesa 500 S. 500 500500 1500

2000 500 500 10001000

1000 T.

1000 1000

500 1000 1000 500 Raton Mesa 500 COLORADO 25 35 37°00' 1500 20002000 500500 S. NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS

Figure 11. Generalized depth to the top of the Vermejo Formation in Las Animas County, Colorado. 105°00' 104°45' 104°30' 104°15'

R. 69 W. R. 68 W. R. 67 W. 25 Intrusive rocks, stocks T. Aguilar 30 East Spanish Raton Formation S. West Spanish Peak HUERFANO CO 160 350 Peak 500 Subcrop of Raton Formation 85 12 LAS ANIMAS CO1000 Vermejo Formation and Trinidad Sandstone 87 T. 1000 Line of equal depth to Raton 31 Formation—Dashed where S. approximately located. Interval 500 feet. Datum is land surface

10001000 Control point—Shows location of Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers 1500 gas well with depth to top of Raton Formation in subcrop T. Outcrop point—Used for structural 32 37°15' control S.

160

T. Cokedale Trinidad 33 S.

Starkville

500500 T. MAXWELL GRANT 15001500 500500 34 S. 500 Raton Mesa

10 0000 500 T. 35 COLORADO 25 500 Raton Mesa 37°00' 500 S. NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS Figure 12. Generalized depth to the top of the Raton Formation in Las Animas County, Colorado. Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish Aguilar 30 Vermejo Formation and Peak HUERFANO CO 350 S. West Spanish 160 Trinidad Sandstone Peak Subcrop of Trinidad Sandstone LAS ANIMAS CO 85 12 150 Line of equal thickness of Trinidad T. 87 Sandstone—Dashed where approximately 31 150 located. Contour interval 30 feet S. Control point—Shows location of gas well with thickness of Trinidad Sandstone

T. 37°15' 32 S.

160

Trinidad T. Cokedale 33 S.

Starkville 120

120 T. MAXWELL GRANT 34 S. Raton Mesa 120120 120 120 120 T. 35 COLORADO Raton Mesa 120 120 S. 37°00' 120 120120 25 NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS

Figure 13. Generalized apparent thickness of the Trinidad Sandstone in Las Animas County, Colorado. 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish Aguilar 30 S. West Spanish Peak HUERFANO CO 160 Vermejo Formation and 350 Peak Trinidad Sandstone 85 Subcrop of Vermejo Formation LAS ANIMAS CO 12 200 150 Line of equal thickness of Vermejo T. 87 Formation—Dashed where approximately 31 located. Contour interval 50 feet S. 150 Control point—Shows location of gas well with thickness of Vermejo Formation Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers

T. 37°15' 250 32 S.

160

T. 350 250 Cokedale Trinidad 33 S.

300 Starkville

200 250 150

250 MAXWELL GRANT 200 T. 34 S. 300 Raton Mesa 100 250

150 50 T. Raton Mesa 35 200 250 COLORADO 25 37°00' S. NEW MEXICO

0 5 10 15 20 25 30 KILOMETERS Figure 14. Generalized apparent thickness of the Vermejo Formation in Las Animas County, Colorado. Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin 105°00' 104°45' 104°30' 104°15'

25 Intrusive rocks, stocks T. East Spanish AguilarAg 30 S. West Spanish Peak HUERFANO CO 160 Vermejo Formation and 350 Peak 400400 Trinidad Sandstone 85 Raton Formation 12 LAS ANIMAS CO

Subcrop of Raton Formation T. 400 87 31 400 Line of equal thickness of S. Raton Formation—Dashed where approximately located. 1200 400 Contour interval 400 feet Control point—Shows location of gas well with thickness of Raton Formation in areas of subcrop T. 37°15' 32 1200 S.

800

400 160

400

400 T. 1600 Trinidad Cokedale 33 400 S.

800 400 1600 Starkville 400 1200

400400

400 T. RA MAXWELL GRANT 34 1600 S.

400400 800 400

400 Raton Mesa 400 1600 400 2000 T.

800 1200 400 800 Raton Mesa 35 25 1200 800 400 COLORADO 800 S. 37°00' 800 NEW MEXICO R. 69 W. R. 68 W. R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. Base from U.S. Geological Survey digital data, 1983. Geology modified from Green (1992) Albers Equal-Area Conic Projection. Standard parallels 37°30' N and 40°30', central meridian 105°30' W 0 5 10 15 20 MILES

0 5 10 15 20 25 30 KILOMETERS

Figure 15. Generalized apparent thickness of the Raton Formation in Las Animas County, Colorado. Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers wells were completed, in wells completed at various depths types of surfaces. The water table is a smooth surface and the within the Huerfano Formation and the alluvial, Cuchara– land surface is an erosional surface, with relatively large local Poison Canyon, Raton, Vermejo, and Trinidad aquifers. These relief. Although it generally is assumed that the water table reported water levels may represent water levels other than the mimics the topography of the land surface, in areas where water table that were measured over a period of decades. streams have cut deeply into the rock, the streams act as drains The configuration of the regional water table in the north- and the slope of the water table is more subdued than the slope ern part of the study area, primarily north of the Purgatoire of the land surface. Consequently, ground-water levels under River, was estimated from reported depth to water in water drainage divides likely are affected more by the relative height wells. In the study area, the depths of water wells that had of nearby drains than by the height of the land surface on the reported depth-to-water values ranged from 8 to 710 ft below divide. land surface. However, only depth-to-water values from wells that are less than 250 ft deep and that were estimated on the basis of reported well depth, estimated land-surface altitude, Limitations and Other Uses and figs. 5–8 to be completed in the uppermost aquifer at the of Hydrostratigraphic Maps well location were used to map the configuration of the long- term regional water table. The aquifer in which a water well The intended use of the hydrostratigraphic maps was completed was estimated on the basis of the altitude of (figs. 5–15), the long-term regional water-table map (fig. 16), the bottom of the water well (estimated land-surface altitude and depth-to-water map (fig. 17) is to provide a framework minus well depth) relative to the altitudes of the bottom and for evaluation of the availability and sustainability of ground- top of the Trinidad Sandstone and tops of the Vermejo and water resources in the Raton Basin in Las Animas County, Raton Formations (figs. 5–8). Only reported depth-to-water Colorado. A better understanding also is needed of the internal values from water wells that are less than or equal to 250 ft geometry (spatial variability of hydraulic and storage proper- deep and that are completed in the uppermost geologic unit, as ties) of the Raton, Vermejo, and Trinidad aquifers; of both the shown on the digital geologic map of Colorado (Green, 1992), internal and external geometry of the Cuchara–Poison Canyon were used in developing the map of the long-term regional aquifer; and of the three-dimensional fluid-pressure regime water table (fig. 16). Shallow water wells near the Purgatoire of the Raton Basin aquifers to evaluate the ground-water or Apishapa Rivers and their tributaries likely are completed resources in the Raton Basin. The hydrostratigraphic maps in alluvial aquifers but were considered to be completed in the of the Raton, Vermejo, and Trinidad aquifers are reasonably underlying formation. accurate in areas where there is adequate subsurface control Geostatistical methods, similar to those used to model from CBM wells; however, where there is little subsurface the structural surfaces, were used to model the altitude of the control along the eastern and western sides of the study area, regional water table (fig. 16). The altitude of the water table accurate surveying of geologic contacts and measurements of at wells was calculated as the estimated altitude of the land formation thickness (measured sections) could substantially surface at the well minus the reported depth to water. Because improve accuracy in the estimated surfaces and thicknesses. the range of dates of the reported water-level data in the SEO Detailed analyses of geophysical logs, cores, and measured database is many decades, the regional water table (fig. 16) sections and probabilistic methods of data analyses likely is assumed to represent average long-term conditions. A map would be needed to approximate the spatial distribution of of the potentiometric surface of the Raton-Vermejo-Trinidad lithology (internal geometry) within the hydrostratigraphic aquifer during April–July 1981 (Geldon, 1989) has a configu- units. ration similar to the long-term regional water table shown in The hydrostratigraphic maps (figs. 5–15) that were figure 16. developed to better define the hydrostratigraphy of the Raton, The estimated depth to the water table (fig. 17) is the Vermejo, and Trinidad aquifers also can be used for other depth at which one can expect to measure water levels in purposes. For example, if the contributing aquifer or aquifers wells drilled just into the water table. The generalized depth- in which a water-supply well is completed is not identified to-water surface (fig. 17) equals the estimated land-surface in the SEO database, it can be estimated using figures 5–8 or altitude minus the long-term regional water-table altitude. The 9–12. If the location, land-surface altitude, and depths of the estimated depth to water is greater than 600 ft on the flanks of completed interval of a water well are known, then figures 5–8 the Spanish Peaks and, locally, in an area about 6 mi north- can be used to estimate the aquifer or aquifers in which a well northwest of Trinidad (fig. 17). Estimates of depth to water is completed; or if only the location and depths are known, greater than 250 ft, the maximum depth of wells used to model then figures 9–12 can be used to estimate the aquifer or the water table (fig. 16), occur because the depth to water was aquifers in which a water well is completed. Figures 9–12 also calculated as the difference between the land-surface alti- can be used to estimate the depths to the Raton, Vermejo, and tude and the water-table altitude. The apparent discrepancy Trinidad aquifers for new wells. Figures 16–17 can be used to between the estimates of depth to water that are greater than estimate the minimum depth that a new well likely would have the reported depths to water result from differences in the to be drilled to reach the regional water table. Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin 105°00' 104°45' 104°30' 104°15'

. 7000 25 Basalt flows T. East Spanish HUERFANO CO 6600 30 AguilarAg ar Intrusive rocks, stocks Peak 160 350 S. West Spanish er iv Huerfano Formation Peak R

a 85 8200 9800 8200 ap 12 LAS ANIMAS CO h Cuchara Formation is 6600 Ap T. 87 Poison Canyon Formation 9600 31 8400 7800 10000 8000 S. 9800 8600 Raton Formation 8800 92009200 Vermejo Formation and 9400 6400 Trinidad Sandstone 9000 7000 Water-table contour—Shows altitude of the multidecadal 7000 6800 T. 37°15' water table. Contour interval 32 200 feet. Datum is NAVD S. of 1988 Control point—Shows location 7400 of water well with reported 160 depth to water less than 7600 250 feet below land surface 7200 6400 T. Cokedale Trinidad 33 S.

Purgatoire River Trinidad Lake 6400 6600 Starkville 6600 6600

T. MAXWELL GRANT 6800 34 S. Raton Mesa

T. 25 Raton MesaMes 35 37°00' COLCOLORADOORADO S. NEW MEXICO

0 5 10 15 20 25 30 KILOMETERS

Figure 16. Generalized configuration of the long-term regional water table in the northern part of the study area. 105°00' 104°45' 104°30' 104°15'

100 250 25 Intrusive rocks, stocks 250 T. AguiAguilarar 30 EaEast Spanishanish DEPTH TO WATER 2 50 350 S. West Spanish PeakP 400 160 (FEET) Peak 100 250 00 Less than 100

6 250 250 250 85 0 0 12 250 40 100 400 400 40 400 100−250 600 250 100 400 87 T. r 100 e 31 v 250−400 i S. CO 100 R 100 pa 50 sha 2 100 250 i 400−600 Ap 100 100 100 400 HUERFANO COCO 100 100 250 More than 600 100 250

250 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers LAS ANIMAS CO 100 400 400 Boundary of Raton Basin 250 100 100 250 00 T. 250 6 Line of equal depth to water— 37°15' 100 100 32 100 40 Contour interval variable. Datum S. 250 100 0 400 is land surface 250 10 0 400 0 250 10 160 50 2 100 0 0 100 250

25 25 250 COSTILLA CO COSTILLA CO T. 250 0 Trinidad 33 400 100 25 Cokedale 0 0 S. 1 Purgatoire River 100 Starkvilleville 100 250 4 100 00 400 250 T. MAXWELL GRANT 34 S. Raton Mesa

T. 35 COLORADO 25 Raton Mesa 37°00' S. NEW MEXICO

0 5 10 15 20 25 30 KILOMETERS

Figure 17. Estimated depth to water below land surface in the northern part of the study area. 30 Hydrostratigraphic Framework of the Raton, Vermejo, and Trinidad Aquifers in the Raton Basin

Summary American Nuclear Society, 1980, American national standard for evaluation of radionuclide transport in ground water for nuclear power sites: La Grange Park, Ill., American Nuclear Exploration for and production of coalbed methane has Society, ANSI/ANS–2.17–1980. increased substantially in the Rocky Mountain region of the United States since the late 1990s. Annual production of Bates, R.L., and Jackson, J.A., eds., 1980, Glossary of geology natural gas in Las Animas County, Colorado, increased about (2d ed.): Falls Church, Va., American Geological Institute, threefold from 28,129,515 in 1999 to 80,224,130 thousand 749 p. cubic feet in 2004. The annual volume of ground water coproduced by gas wells in Las Animas County increased Bossong, C.R., Karlinger, M.R., Troutman, B.M., and Vecchia, from about 949 million gallons in 1999 to about 2,879 million A.V., 1999, Overview and technical and practical aspects for gallons in 2004. Because coal beds and associated sandstones use of geostatistics in hazardous-, toxic-, and radioactive- also are aquifers in the Raton Basin, water-well owners waste-site investigations: U.S. Geological Survey Water- and resource managers are concerned that coalbed methane Resources Investigations Report 98–4145, 70 p. production may affect the availability and sustainability of Clarke, Paul, and Turner, Peter, 2002, Sedimentology and local ground-water supplies. In 2001, the U.S. Geological coalbed methane resources of the Raton Basin, USA: Survey, in cooperation with the Colorado Water Conservation accessed September 21, 2005, at URL http://www.gees. Board, began a study to compile and evaluate the hydrogeol- bham.ac.uk/downloads/COALBED_%20METHANE.pdf ogy in the Raton Basin of Colorado. A better understanding of the hydrostratigraphic framework of the Raton Basin Colorado Division of Water Resources, 2003, Database: is needed for quantitative evaluation of the ground-water Denver, Colo., accessed September 26, 2003, at URL resources. http://water.state.co.us/. As a result of much additional subsurface data from logs Colorado Oil and Gas Conservation Commission, 2003, and completion reports of CBM wells, the Raton-Vermejo- Rules and regulations: accessed November 8, 2005, at URL Trinidad aquifer could be mapped as three distinct hydrostrati- http://www.oil-gas.state.co.us/. graphic units (aquifers). Geostatistical methods were used to prepare the maps showing the hydrostratigraphic framework Colorado Oil and Gas Conservation Commission, variously of the Raton, Vermejo, and Trinidad aquifers and the gener- dated, Database: Denver, Colo., Colorado Oil and Gas Con- alized configuration of the water table in the Raton Basin, servation Commission, accessed various dates 2003–2005 at Las Animas County. URL http://www.oil-gas.state.co.us/. The maps, though primarily prepared for defining the hydrostratigraphic framework, also may be useful in estimat- Davis, J.C., 1986, Statistics and data analysis in geology, ing depths of proposed water wells and in assigning aqui- 2d ed.: New York, Wiley, 646 p. fer designations to existing water wells. The structure and Environmental Systems Research, Inc., 1982–2000, Arc, ver- thickness maps of the Trinidad Sandstone and the Vermejo sion 8.0.2: Redlands, Calif. and Raton Formations could be improved with surveying of geologic contacts and measurements of geologic sections at Environmental Systems Research, Inc., 1999–2005, ArcMap, outcrops along the eastern and western margins of the Raton version 9.1: Redlands, Calif. Basin. Additional study is needed to define spatial variability Flores, R.M., 1984, Comparative analysis of coal accumula- of hydraulic and storage properties and to determine fluid tion in Cretaceous alluvial deposits, southern United States pressure within the Raton, Vermejo, and Trinidad aquifers. Rocky Mountain basins, in Stott, D.F., and Glass, D., eds., The Mesozoic of Middle North America: Canadian Society of Petroleum Geologists Memoir 9, p. 373–385.

References Cited Flores, R.M., and Bader, L.R., 1999, A summary of Tertiary coal resources of the Raton Basin, Colorado and New Abbott, P.O., Geldon, A.L., Cain, Doug, Hall, A.P., and Mexico, chapter SR of Fort Union Coal Assessment Team, Edelmann, Patrick, 1983, Hydrology of Area 61, northern 1999 Resource Assessment of selected Tertiary coal beds Great Plains and Rocky Mountain coal provinces, Colo- and zones in the Northern Rocky Mountain and Great Plains rado and New Mexico: U.S. Geological Survey Open-File region: U.S. Geological Survey Professional Paper 1625–A, Report 83–132, 99 p. 34 p.

American Gas Association, 2006, How to measure natural gas: Flores, R.M., and Tur, S.M., 1982, Characteristics of deltaic accessed March 15, 2006, at URL http://www.aga.org/ deposits in the Cretaceous Pierre Shale, Trinidad Sandstone, Content/NavigationMenu/About_Natural_Gas/ How_to_ and Vermejo Formation, Raton Basin, Colorado: The Moun- Measure_Natural_Gas/How_to_Measure_Natural_Gas.htm tain Geologist, v. 19, p. 25–40. References Cited 1

Geldon, A.L., 1989, Ground-water hydrology of the central U.S. Geological Survey, 2004a, Estimated use of water in the Raton Basin, Colorado and New Mexico: U.S. Geological United States, county-level data for 2000: [Available online Survey Water-Supply Paper 2288, 81 p. at URL http://water.usgs.gov/watuse/data/2000/index.html] Green, G.N., 1992, The digital geologic map of Colorado in ARC/INFO format: U.S. Geological Survey Open-File U.S. Geological Survey, 2004b, USGS National Elevation Report 92–0507. Dataset: [Available online at URL http://ned.usgs.gov]

Hutson, S.S., Barber, N.L., Kenny, J.F., Linsey, K.S., Lumia, Wood, G.H., Jr., Johnson, R.B., and Dixon, G.H., 1956, D.S., and Maupin, M.A., 2004, Estimated use of water Geology and coal resources of the Gulnare, Cuchara Pass, in the United States in 2000: U.S. Geological Survey and Stonewall area, Huerfano and Las Animas Counties, Circular 1268, 46 p. Colorado: U.S. Geological Survey Coal Investigations Johnson, R.B., 1969, Geologic map of the Trinidad quad- Map C–26, scale: 1:31, 680, 2 sheets. rangle, south-central Colorado: U.S. Geological Sur- vey Miscellaneous Geologic Investigations Map I–558, Wood, G.H., Jr., Johnson, R.B., Eargle, D.H., Duggner, R.T., scale 1:250,000. and Major, Harold, 1951, Geology and coal resources of Lohman, S.W., and others, 1972, Definitions of selected the Stonewall-Tercio area, Las Animas County, Colorado: ground-water terms—Revision and conceptual refinements: U.S. Geological Survey Coal Investigations Map C–4, U.S. Geological Survey Water-Supply Paper 1988, 21 p. scale: 1:31, 680, 2 sheets.