Tertiary Coals in South Texas: Anomalous Cannel-Like Coals of Webb County (Claiborne Group, Eocene) and Lignites of Atascosa County (Jackson Group, Eocene) - Geologic Setting, Character, Source-rock and Coal-bed Methane Potential

Field Trip Guidebook

FIELD TRIP GUIDEBOOK

The 1999 AAPG Annual Convention Energy Minerals Division Field Trip # 15 April 14-15, 1999

Field Trip Leaders: Peter D. Warwick, U.S. Geological Survey, Reston, VA Robert W. Hook, Consultant, Austin, TX John R. SanFilipo, U.S. Geological Survey, Reston, VA

Edited by P.D. Warwick, C.E. Aubourg, and J.C. Willett

U.S. Geological Survey Open-File Report 99-301

Tertiary Coals in South Texas: Anomalous Cannel-Like Coals of Webb County (Claiborne Group, Eocene) and Lignites of Atascosa County (Jackson Group, Eocene) -- Geologic Setting, Character, Source-rock and Coal-bed Methane Potential

Field Trip Guidebook

The 1999 AAPG Annual Convention Energy Minerals Division Field Trip # 15 April 14-15, 1999

Leaders: Peter D. Warwick, U.S. Geological Survey, Reston, VA, Robert W. Hook, Consultant, Austin, TX, and John R. SanFilipo, U.S. Geological Survey, Reston, VA

Edited by P.D. Warwick, C.E. Aubourg, and J.C. Willett

U.S. Department of Interior U.S. Geological Survey Open-File Report 99-301

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S.G.S.

1999 Copies of this Field Guide are available from: USGS Information Services Box 25286 Denver Federal Center Denver CO 80225-0046 Tel: 303-202-4700; Fax 303-202-4188

or

http://energy.er.usgs.gov CONTENTS

Introduction Tertiary coals in South Texas: Anomalous cannel-like coals of Webb County (Claiborne Group, Eocene) and lignites of Atascosa County (Jackson Group, Eocene) Peter D. Warwick, Robert W. Hook, John R. SanFilipo...... 1 Itinerary...... 2 Other Information...... 4

Chapter 1 Regional geologic setting of South Texas and the Santo Tomas coal deposits Thomas E. Ewing...... 5 Introduction...... 5 General Overview of the Field Trip Route...... 5 Structural and Stratigraphic Setting of the Santo Tomas District...... 8 References Cited...... 11

Chapter 2 The San Pedro and Santo Tomas Coal Beds (Claiborne Group, Eocene) of Webb County, Texas Robert W. Hook and Peter D. Warwick...... 13 Introduction...... 13 Geologic Setting and Stratigraphy...... 13 Mining History...... 16 Depositional Setting...... 17 Coal Characteristics...... 18 Summary...... 20 Acknowledgments...... 21 References Cited...... 21

Chapter 3 The San Miguel lignite deposit, Jackson Group (Eocene), South Texas Peter D. Warwick, Sharon S. Crowley, Leslie F. Ruppert, and James Pontolillo...... 23 Introduction...... 23 Geologic Setting...... 23 Lignite Resources and Mining...... 24 Character of the San Miguel Lignite...... 26 Megascopic characteristics...... 26 Proximate analyses...... 28 Major, Minor, and Trace element data...... 29 Mineralogy of San Miguel Partings...... 31 Palynology and Organic Petrology...... 31

iii Relationship Between Ash yield, and Petrographic and Geochemical Characteristics of the San Miguel Lignite Deposit...... 33 Summary...... 34 Acknowledgments...... 34 References Cited...... 34

Chapter 4 Some speculations on coal rank anomalies of the South Texas Gulf Province and adjacent areas of Mexico and their impact on coal bed methane and source rock potential John R. SanFilipo...... 37 Introduction...... 37 Cretaceous bituminous coals...... 37 Tertiary bituminous and related coals...... 37 Extent of the coal rank anomaly and its possible impact on hydrocarbon generation...... 42 Conclusions...... 45 Acknowledgments...... 46 References Cited...... 46

FIGURES

Introduction

Figure 1. Map showing outcrop of coal-bearing formations in the U.S. Gulf of Mexico coastal area...... 2 Figure 2. Stratigraphy of coal-bearing intervals in the U.S. Gulf of Mexico Coastal Plain...... 3

Chapter 1

Figure 1. Route of the 1999 EMD field trip, with major tectonic features and outcrop belts shown...... 6 Figure 2. General northwest-southeast stratigraphic section of Mesozoic and Paleogene strata along the Rio Grande, from Eagle Pass to Freer...... 7 Figure 3. Subsurface structure, Top Middle Wilcox marker, Santo Tomas area...... 9 Figure 4. Electric log, Hughes & Hughes #1 Palafox Exploration “B”, showing the correlation from surface mines to subsurface stratigraphy...... 10

iv Chapter 2

Figure 1. Location of study area, undergroun mine entries and prospects...... 14 Figure 2. Representative corehole records from Vaughn (1900) that include both coal zones...... 15 Figure 3. Bed sections and sample intervals of the San Pedro, Santo Tomas, and Santo Tomas rider coal beds...... 16 Figure 4. Selected proximate and ultimate data...... 19 Figure 5. Schematic block diagram of depositional environments for the San Pedro and Santo Tomas coal zones in the middle part of the Claiborne Group...... 20

Chapter 3

Figure 1. Index map of the San Miguel Mine area showing towns, roads, and sample sites...... 24 Figure 2. Generalized stratigraphy and stratigraphic sections of the coal-bearing interval at the san Miguel Mine...... 25 Figure 3. Representative photographs and photomicrographs of outcrops and samples from the San Miguel lignite interval...... 26 Figure 4. Bar graphs shoing concentrations of ash yield, total sulfur, and caloric values...... 27 Figure 5. Bar graphs showing the distribution of trace element concentrations for twelve potentially environmentally sensitive elements identified by the 1990 U.S. Clean Air Act amendments...... 28-29 Figure 6. Bar graphs showing the distribution of trace element concentrations for those elements inferred to be derived from volcanic ash...... 30 Figure 7. Bar graphs of the major petrographic constituents for the channel sample sets...... 32

Chapter 4

Figure 1. Generalized columnar section for South Texas coal areas and adjacent areas of Mexico...... 38 Figure 2. Regional geology and location of bituminous coal deposits of South Texas and adjacent areas of Mexico...... 39 Figure 3. Standard analysis and elemental ratios for selected samples from the Santo Tomas coal field and Sandow Mine...... 41 Figure 4. Selected rank parameters for selected samples from the Santo Tomas coal field and Sandow Mine...... 43 Figure 5. Van Krevelen-type diagram for various coal types of South Texas and vicinity...... 45

v vi U.S. Geological Survey Open-File Report 99-301 Introduction

Tertiary coals in South Texas: Anomalous cannel-like coals of Webb County (Claiborne Group, Eocene) and lignites of Atascosa County (Jackson Group, Eocene) (AAPG - EMD Field Trip, April 14-15, 1999)

By Peter D. Warwick, Robert W. Hook*, and John R. SanFilipo

U.S. Geological Survey, 956 National Center, Reston, VA 20192 *Consultant, 1301 Constant Springs Drive, Austin, TX 78746

The coal-bearing Gulf of Mexico northwestern Louisiana. The cannel-like coal Coastal Plain of North America contains a deposits of south Texas are unique and are one variety of depositional settings and coal types. of the few mined cannel-like coal deposits in The coal-bearing region extends westward the world. This field guidebook contains from Alabama and Mississippi, across papers that focus on the regional geologic Louisiana to the northern part of the setting of south Texas (Chapter 1), and papers Mississippi Embayment, and then southward on the Santo Tomas and San Pedro coal to eastern Arkansas, Texas and northern deposits and San Miguel lignite deposits Mexico (fig. 1). Most of the coal currently (Chapters 2 and 3). Finally, there is an mined in Texas is lignite from the upper part of overview paper on the anomalous coal-rank the Wilcox Group (Paleocene-Eocene) and, in changes of south Texas, and a discussion of Louisiana, lignite is mined from the lower part the source-rock and coal-bed methane of the Wilcox (fig. 2). Gulf Coast coal is used potential for the south Texas coal fields primarily as fuel for mine-mouth electric (Chapter 4). plants. On this field trip we will visit the only The field trip will visit two active mines two non-Wilcox coal mining intervals in the in South Texas and will provide the Texas-Louisiana Coastal Plain; these include participants an opportunity to observe the the San Pedro - Santo Tomas bituminous mining operations and coal geology of the cannel-like coal zone of the Eocene Claiborne surface mines. The older Claiborne Group Group, and the San Miguel lignite coal zone of coals that occur near the U.S.-Mexico border the Eocene Jackson Group (fig. 2). Other (fig. 1), are long-known for their cannel-like coal-mining areas in northern Mexico are character and will be seen in workings of the currently producing bituminous coal from the Farco Mining Company. These unusual Cretaceous Olmos Formation of the Navaro nonbanded coals have high calorific values Group (fig. 2). (3800 to 6000 kcal/kg or 6800 to 10,800 btu, The Claiborne-Jackson coal-bearing dry basis) and substantial oil and gas yields intervals of south Texas are very different from upon distillation. Recent petrographic and the Wilcox coal-bearing intervals found in geochemical investigations have shown that central and northeastern Texas and these coals are bituminous in rank, hydrogen

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USA Mexico Geologic Units

Eocene Jackson Group Claiborne Group Mining areas Paleocene Wilcox Group Naheola Formation U. Cretaceous Olmos Formation 0 300 600 miles

Figure 1. Map showing outcrop of coal-bearing formations in the U.S. Gulf of Mexico coastal area. Active, recent and proposed mining areas are shown in red. rich, contain abundant green algae remains, depositional setting for these lignites, and and are potential sources of hydrocarbon. The regional South Texas/Northeastern Mexico field trip leaders will discuss the depositional coal geology will be discussed. The field trip setting, petrology, geochemistry, and regional will spend the first night in Laredo, a U.S.- geology of these coal deposits. Mexico border town, and will visit the two The lignites of the San Miguel deposit surface mines the next day. (Jackson Group, Eocene), differ greatly from the coals of the border area. Four principle ITINERARY lignite beds comprise the San Miguel deposit. The beds are separated by reworked volcanic This is a tentative schedule based upon ash partings (generally less than 0.25 m or ideal weather and traffic conditions. 0.75 ft thick). Drill records and in-mine studies indicate that these lignites were Wednesday, 14 April deposited in environments that were partially marine influenced. The geochemistry and 4:30 PM - depart H.B. Gonzalez Convention

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approximately 30 minute drive.

8:00 AM - arrive at Farco Mining's Palafox Mine Office. David Wadsack and Gustav Holm, hosts. Safety training in office. Jackson Group Stop 1: Trevino Mine. Observe San Pedro Coal (Bigford Formation) and overlying tidal

Claiborne Group deposits. Discuss unique mining methods. Eocene

Stop 2: Palafox Mine. Observe San Pedro Coal, overlying sandstone and mudstone Wilcox Group deposits, and thin coal seam.

Stop 3: Coal yard. Discuss coal preparation Midway Group and utilization.

Paleocene 10:30 AM - return to Farco office for Navarro Group convenience stop; depart for San Miguel Mine

at 10:45 AM.

Upper Cret.

Optional Stop A: FM 1472 roadcut east of Figure 2. Simplified stratigraphy of coal- Santo Tomas. Observe coastal-barrier facies bearing intervals in the U.S. Gulf of Mexico in El Pico Clay (10 minutes). Coastal Plain. Arrows indicate stratigraphic position of major coal-bearing intervals. 12:15 PM - lunch stop (30 minutes), Picnic Area south of Cotulla at Mile Marker 59 on I-35 North. This is a no-toilet rest area. Center in San Antonio for Laredo, a nonstop drive of approximately 2.5 hours. 1:00 PM - convenience and gas stop (15 minutes) at Mobil station in Cotulla at junction 7:00 PM - check-in at La Posada Hotel, of Business I-35 and Texas Route 97. downtown Laredo. 2:15 PM - arrive North American Coal's San 7:30 PM - short (less than 30 min.) meeting at Miguel Mine Office. Roger Fish, Engineering La Posada. We will provide an overview of Manager, host. Safety certification and mine South Texas coal geology and give overview at office. suggestions for dinner. Stop 4: E area final pit. Observe completed Thursday, 15 April highwall exposure of Jackson Group from spoil bank. Discuss lithofacies overlying coal, 7:00 AM - depart La Posada for Palafox, an ash disposal, and reclamation methods.

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Stop 5: E area, active pit. Observe coal seams and splits, discuss unique mining methods.

Optional Stop B: B area.

4:30 PM - depart San Miguel Mine for San Antonio.

OTHER INFORMATION

Both Farco Mining and North American Coal require hard hats, safety glasses, and steel-toe boots.

Exercise extreme caution at all times when on mine property. Do not approach highwall exposures without the consent of company personnel.

Coal sampling and photography are permitted by both operators. Later publication of coal-quality data or mine photographs requires company consent and acknowledgment.

We will stop at a mandatory U.S. Border Inspection station north of Laredo on Thursday morning. No illegal substances or firearms should be on your person or in your belongings. Non-U.S. citizens should have the appropriate travel papers available for presentation.

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CHAPTER 1

Regional geologic setting of South Texas and the Santo Thomas coal deposits

By Thomas E. Ewing

Venus Exploration, Inc., 1250 Loop 410, Suite 1000, San Antonio, TX 78209

INTRODUCTION of lesser subsidence than surrounding areas during Mesozoic and Cenozoic time. During This paper supplements the work that the Early Cretaceous, the area was repeatedly the U.S. Geological Survey has recently occupied by extensive carbonate platforms (the conducted in South Texas on the coal geology Sligo Limestone of Aptian age and the of the Eocene Claiborne Group of the Laredo Edwards Group carbonates of Albian age; fig. area (Warwick and Hook, 1995) and the 2). As regional sedimentation shifted from Jackson Group of the San Miguel lignite mine carbonates to siliciclastic material in the Late area (Warwick and others, 1996). I will first Cretaceous (Campanian - Maastrichtian), the present a general overview of the trip route in San Antonio area lay east of the major San relationship to the major structural and Miguel and Olmos wave-dominated deltas of stratigraphic features of South Texas. Then I South Texas, and mainly marine shales and will present subsurface well log data to show thin sandstones were deposited. In the the correlation of the coal zones into the Paleogene, Wilcox, Carrizo and younger subsurface, and place them in a regional sandstones were deposited in the San Antonio context. area, which lay in the transition between the largely strike-oriented strandplain, wave- GENERAL OVERVIEW OF THE dominated delta and bar - lagoon systems of FIELD TRIP ROUTE South Texas and the fluvial-deltaic deposits of East Texas. When we leave San Antonio, we will As we drive southwest toward Laredo, proceed southwest along Interstate 35 towards we cross the Rio Grande Embayment, a broad the city of Laredo. San Antonio is an oasis synclinal area between the San Marcos Arch of city, built around springs and deep wells Central Texas and the Laramide fold systems drawing water from the Lower Cretaceous of Mexico (fig. 1). The present Rio Grande Edwards Aquifer along the (Miocene-age) flows along the southwest side of this Balcones fault zone. The city is built on rocks embayment. The road slowly climbs the ranging in age from Albian (Edwards stratigraphic section, reaching the late middle Limestone) to Paleocene (lower Eocene ("Laredo Formation," Sparta and Wilcox Group), with younger rocks generally Cook Mountain equivalent; fig. 2) at Laredo. to the southeast. South of Cotulla (fig. 1), the road crosses the Tectonically, the San Antonio area lies buried Stuart City Reef margin of the Edwards on the southwest flank of the San Marcos Group carbonate platform. The reef margin is Arch (fig. 1), a broad southeast-trending area gas-productive, and also controls faulting and

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Figure 1. Route of the 1999 EMD field trip, with major tectonic features and outcrop belts shown. Open boxes represent locations of Upper Cretaceous coal deposits; “X” represents igneous rocks; Kig, Tig, QTig = Cretaceous, Tertiary, and Quaternary igneous rocks. shallower marine sandstone development in Monclova to San Antonio passed north of the the Olmos Formation. At Laredo, the position Laredo area, but the location of Laredo on the of the buried Sligo reefal shelf margin is shortest route from Monterrey to San Antonio, reached. This reef contains a major gas field coupled with the main rail links to both cities just across the river in the western part of and to the port of Corpus Christi, ensured Nuevo Laredo (Totonaca field) at depths of Laredo's economic growth. some 13,000 ft. Laredo, and the Santo Tomas district Jose de Escandon founded Laredo, the to its northwest, lie on the southwest flank of chief port of entry from Mexico into Texas, in the Rio Grande Embayment. Dips are the 1750s as the northernmost city of the new generally one degree or higher to the east in province of Nuevo Santander (the lower Rio outcrops near the river. Southwest into Grande valley). The early trade routes from Mexico, the dips continue until the base of the

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Figure 2. General northwest-southeast stratigraphic section of Mesozoic and Paleogene strata along the Rio Grande, from Eagle Pass to Freer (fig. 1). The stratigraphic location of significant coal and lignite deposits are shown.

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Tertiary outcrops about 12.5 mi into the with a strike-oriented sandstone maximum in interior. Several anticlinal axes, collectively eastern Webb County about 30 miles east of referred to as the "Salado arch" or the "Burro- Santo Tomas (Guevara and Garcia, 1972). Peyotes arch", trend southeastward, with Northwest of the sand maximum, the upper intermittent exposures of Austin limestone on part of the cycle contains a low percentage of anticlines. The east to northeast dip off the sandstone and much lagoonal shale; this has flanks of the arch towards the Rio Grande is been called the El Pico Clay by Eargle (1968). easily visible on satellite photography. The lower part of the cycle contains high Secondary folds are seen in some areas. percentages of sandstone (Bigford Formation After visiting the Santo Tomas area, as redefined by Eargle from Trowbridge, we will travel back to the north to Cotulla, 1923), and rests directly on the Carrizo turning east to Tilden, then north to the San Formation and upper Wilcox sandstones with Miguel lignite mine (fig. 1). The trip east from very little break. The downdip marine Reklaw Cotulla to Tilden proceeds over younger rocks formation passes westward into the Bigford than any other part of the trip. Sandy Yegua shoreline sandstones. Formation deposits are overlain by the lignite- The contact between Bigford bearing Jackson Group. The hills east of Formation and El Pico Clay is gradational. Tilden are underlain by the largely Sandstones do occur in the El Pico "Clay", and volcaniclastic Catahoula (or Gueydan) may become more abundant northward away Formation of Oligocene age. Zeolites have from the Rio Grande. The transitional beds been mined from the basal Jackson strata just between Bigford and El Pico are intimately west of Tilden, and used as kitty litter. North associated with the San Pedro and Santo of Tilden, we cross San Miguel Creek (with Tomas coal zones, as we will see. natural lignite exposures in its banks) and To better understand the location of proceed to the San Miguel mine. After this the Santo Tomas coal deposits within the classic Gulf Coast lignite deposit is viewed, we stratigraphic section, a pilot study was will travel back to San Antonio across Eocene undertaken of well logs in the area around and and Upper Cretaceous rocks. east of the mines. The area has been extensively drilled for gas reserves in the STRUCTURAL AND STRATIGRAPHIC Wilcox Group sandstones (Palafox field) and SETTING OF THE SANTO in the Olmos Formation (Las Tiendas, Booth TOMAS DISTRICT Ranch, and La Cruz fields); gas has also been found in reefal Edwards carbonates in Galan The coal beds of the Santo Tomas field, just south of the Rachal Mine (fig. 3). district lie within the Bigford Formation - El Many of these well logs have been released, Pico Clay succession of the Rio Grande and most were logged to within a few hundred Embayment. These two formations correlate feet of the surface. A regional shale which with the Queen City cycle of the Claiborne correlates to the Middle Wilcox can be Group, and are early Middle Eocene in age identified on all logs, and forms a good (fig. 2). In South Texas, the Queen City structural datum. The structure map (fig. 3) sandstones were deposited in a mixed wave- shows structural dip of 1.0-1.4 degrees to the dominated deltaic and barrier bar environment,

8 U.S. Geological Survey Open-File Report 99-301

Figure 3. Subsurface structure, Top Middle Wilcox marker, Santo Tomas area. The contoured data on the Santo Tomas and San Pedro coal beds are from Warwick and Hook (1995). Gas field names are also shown. east or east-northeast on the Middle Wilcox certainly, the San Pedro seam at the Trevino marker. This corresponds well to the 0.9-1.2 lease lies about 1600-1650 ft above the degrees of dip seen in the Trevino and Rachal marker, or 250-300 ft below the Santo Tomas. mine areas (data from Warwick and Hook, Elsewhere, however, the San Pedro is only 90 1995). From comparison of the Middle ft below the Santo Tomas. This suggests that Wilcox data and the mine elevations, the Santo the San Pedro beds are not Tomas bed at the Rachal Mine lies 1900±20 ft above the Middle Wilcox marker. Less

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Figure 4. Electric log, Hughes & Hughes #1 Palafox Exploration "B", showing the correlation from surface mines to subsurface stratigraphy. Location of well shown on figure 3. Vertical log scale in ft.

10 U.S. Geological Survey Open-File Report 99-301 wholly correlative, although there may be significant surface occurrence of barite in the other solutions. Pinto Creek drainage, just northeast of the There is slight thickening of some Rachal mine. The barite occurs in clays that stratigraphic units to the northeast, perhaps lie 80-250 ft above the Santo Tomas seam. indicating structural growth on the Salado folds during Eocene time. However, the total amount of growth is not more than 100-150 ft. REFERENCES CITED The coal beds in the Rachal area can be confidently projected into the Copano #A-1 Eargle, D.H., 1968, Nomenclature of Palafox Exploration well, an Edwards test formations of Claiborne Group, drilled in 1962 about 3 miles SSE of the Middle Eocene coastal plain of Texas: Rachal mine (fig. 3). The correlation indicates U.S. Geological Survey Bulletin 1251- that the coal beds correlate to a thin-bedded D, 25 p. interval lying above a 60-ft thick sandstone Guevara, E.H., and Garcia, R., 1972, and below a 100-ft thick shale unit. This shale Depositional systems and oil - gas ("Pink" Marker) can be observed on logs reservoirs in the Queen City Formation throughout the area mapped, and continues (Eocene), Texas: BEG Geological downdip at least to the Laredo airport (fig. 3). Circular 72-4, 22 p. The sequence can be correlated to the east into Kyle, J.R., 1994, The barite industry and the Hughes and Hughes #1-B Palafox resources of Texas: Bureau of Exploration well, where the overlying clay-rich Economic Geology, The University of sequence is also logged (fig. 4). The Pink Texas at Austin, Mineral Resources Marker and the overlying sandstone can also Circular 85, 86 p. be correlated to lithologic logs presented by Lonsdale, J.T., and Day, J.R., 1937, Geology Lonsdale and Day (1937) and Warwick and and ground-water resources of Webb Hook (1995). Although the individual coal County, Texas: U.S. Geological beds cannot be definitely located on the well Survey Water-Supply Paper 778, p.1- logs (the coal may have some SP response?), 104. the interval is reliably picked, and can be Trowbridge, A.C., 1923, A geologic carried throughout the area. Downdip past the reconnaissance in the Gulf Coastal old workings towards Laredo, the interval Plain of Texas, near the Rio Grande: becomes sand-rich and loses its thin-bedded, U.S. Geological Survey Professional resistive character. Paper 131-D, p. 85-107. Due to the density of drilling in the Las Tiendas area of northwest Webb County, it should be possible, with detailed study of the subsurface logs, to more closely define the depositional geometry of the algal coals of the Santo Tomas district and estimate their resource potential. As an added point of interest in the area, Kyle (1994, p. 41-46) has described a

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Warwick, P.D., and Hook, R.W., 1995, Petrography, geochemistry and depositional setting of the San Pedro and Santo Tomas coal zones: anomalous algae-rich coals in the middle part of the Claiborne Group (Eocene) of Webb County, Texas: International Journal of Coal Geology, v.28, p. 303-342. Warwick, P.D., Crowley, S.S., Ruppert, L.F., and Pontilillo, James, 1996, Petrography and geochemistry of the San Miguel lignite, Jackson Group (Eocene), South Texas: Organic Geochemistry, v. 24, no. 2 p. 197-217.

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CHAPTER 2

The San Pedro and Santo Tomas Coal Beds (Claiborne Group, Eocene) of Webb County, Texas

By Robert W. Hook and Peter D. Warwick*

Consultant, 1301 Constant Springs Drive, Austin, TX 78746 *U.S. Geological Survey, MS 956, 12201 Sunrise Valley Drive, Reston, VA 20192

INTRODUCTION not been determined. In adjacent parts of Mexico, numerous cannel-like beds occur The San Pedro and Santo Tomas coal within the Claiborne Group (SPP, 1989), but zones of Webb County, Texas, are unique no comparable deposits are known elsewhere within the realm of Gulf Coast coal geology. in Texas outside of Webb County. The Coal beds within these zones are distinguished question of rank also cannot be resolved at this by calorific yields and vitrinite-reflectance time. Previously, we attributed the high rank values that indicate a rank as high as of the San Pedro and Santo Tomas to high-volatile bituminous C, which is geothermal influences (Warwick and Hook, considerably greater than other Tertiary coals 1995). Although the suggestion of geothermal within the Gulf Coastal Province. In the past, influence is consistent with data that indicate the San Pedro and Santo Tomas were an elevated rank of Wilcox coal deposits in identified as cannel coal because of their adjacent counties, additional study is required blocky, nonbanded character, and conchoidal to confirm the origin and delineate the extent fracture (Vaughan, 1900; Ashley, 1919; of such alteration. Likewise, the significance Lonsdale and Day, 1937). Unlike true cannel of abundant green-algae fructifications in these coal known from the Upper Carboniferous of coals, identified only from petrographic study the Appalachians and Europe, the Webb of polished blocks, cannot be judged fully at County deposits are not particularly rich in this time because comparable polished block spores or cuticular material and are not petrographic data do not exist for the great restricted to local channel fills. Rather, the majority of other Tertiary coals. San Pedro and Santo Tomas coals contain abundant green-algae fructifications and occur GEOLOGIC SETTING AND as multiple tabular coal beds within zones that STRATIGRAPHY extend for tens of miles along strike (Warwick and Hook, 1995). The Farco mine complex of Webb Recent work has addressed the County lies within the outcrop of the depositional origin, petrography, and southwestern flank of the Rio Grande geochemistry of the San Pedro and Santo Embayment, which extends southwesterly Tomas coal beds, but several basic questions through Coahuila, Nuevo León, and remain. The full extent of these deposits has

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100 o 28 o Rio Grande FM1472

Cannel Coal Co. no. 6 Texas ST Channel 94 Rachal Mine Webb County Study Outline of area Rio Grande Embayment Trevino Lease Laredo

Approximate outcrop of San Pedro and Santo Tomas Palafox mine coal zones SP Channel 93 SP Channel 94 Texas Palafox Old mine prospect

Villa Rio Grande Hildago Mexico Minera Santo Tomas shaft drift mines

Cannel Coal Co. no. 2 Coahuila ' Colombia Darwin shaft Dolores shaft

Nuevo Leon 0 5 km

Tamaulipas 27 37'30'o 99 37'30"o Figure 1. Location of study area, underground mine entries and prospects, Cannel Coal Company coreholes 2 and 6, surface mines, and sample sites. Inset shows location of study area, Webb County, and Rio Grande Embayment in South Texas. From Warwick and Hook (1995).

Tamaulipas in Mexico (fig. 1). Within the Rio The San Pedro and Santo Tomas coal zones Grande Embayment, the lower to middle part occur within the fining-upward transition from of the Claiborne Group consists of marine sandstone-dominated rocks to mudstone-rich mudstones (Reklaw Formation) in the east and deposits. The coal beds dip approximately 2E northeast, and sandstones and mudstones to the northeast, toward the synclinal axis of (Bigford Formation) in the south and the embayment. southwest (fig. 2). The marine mudstones The San Pedro coal zone ranges up to coarsen upward into the fluvialdeltaic Queen 10 m (33 ft) in thickness and contains as many City Sand, which prograded gulfward across as five organic-rich beds that vary from eastern and central Texas. To the west and carbonaceous mudstone to nonbanded coal in southwest, the interval overlying the Bigford composition. The thinner Santo Tomas coal Formation becomes less sandy and claystones zone occurs approximately 25 to 35 m (82 to of the El Pico Clay Formation predominate.

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Cannel Coal Company no. 6 corehole

Cannel Coal Company no. 2 corehole Carbonaceous shale and lignite

Eastern and South Santo Tomas Santo Tomas central Texas Texas coal zone coal zone Unnamed Unnamed Jackson Group coal bed coal bed

Yegua Formation Fossil bed Cook Mountain Fm. Sparta Sand Laredo San Pedro Fm. Weches coal zone Formation El Pico Clay Queen City Sand San Pedro coal zone 15 m

Bigford

Eocene Group Claiborne Reklaw Fm. Formation 29 km Carrizo Sand

Wilcox Group

Midway Group

Claystone and Paleocene mudstone

Sandstone

Limestone

Figure 2. Representative corehole records from Vaughan (1900) that include both coal zones. Approximate locations shown on figure 1. Regional stratigraphy after Eargle (1968) and Guevara and Garcia (1972). From Warwick and Hook (1995).

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Rachal Mine Palafox Mine Sandstone, flat- bedded, 100 cm+

Coal, nonbanded, blocky,

20 cm (STR94-1) Mudstone coal Rider Sandstone, scoured Claystone, base, 100 cm+ burrowed, 10 cm Siltstone, rooted, becoming sandy upwards, 193 cm Coal, nonbanded, Coal, nonbanded, cleated, 42 cm cleated, 34 cm (SP93-A1) (SP94-2) Coal, nonbanded, less 50 cm blocky than below, Coal, massive, 7 cm Coal, impure, 36 cm (ST94-2) (SP93-A2) massive, 26 cm Coal, impure, 7 cm (SP94-1)

(SP93-A3) coal Pedro San San Pedro coal Pedro San Claystone, rooted Coal, nonbanded, cleated, Mudstone, rooted 33 cm (ST94-1)

Lat. 27 o 50'02"N Lat. 27 o 49'01"N Mudstone, rooted, coal Tomas Santo Long. 99 o 51'26"W Long. 99 o 50'19"W <4.8 m Coal, impure, <90 cm (core data, bench not exposed in mine) Lat. 27 o 56'32"N Long. 99 o 51'17"W

Figure 3. Bed sections and sample intervals of the San Pedro, Santo Tomas, and Santo Tomas rider coal beds. Sample numbers are in parentheses. Approximate locations are shown in Figure 1. From Warwick and Hook (1995).

115 ft) above the San Pedro zone, averages MINING HISTORY approximately 2.4 m (7.9 ft) in thickness, and consists of a lower main bed and an overlying The Santo Tomas coal zone of Webb rider, both of which may occur as nonbanded County was first mined commercially by the coal, impure coal, or carbonaceous mudstone Rio Grande Coal and Irrigation Company (fig. 3). Within the Farco property and leases, between 1881 and 1914 in modest drift coal beds of the San Pedro zone are mined at openings in the vicinity of Minera (fig. 1). By the Palafox and Trevino mines, and the Santo 1895, the Cannel Coal Company had Tomas zone is developed at the Rachal mine completed a shaft and had driven mains in both (fig. 1). the Santo Tomas and San Pedro zones at Darwin (fig. 1). The company, which employed several hundred miners, was directed by David Darwin Davis, a mining

16 U.S. Geological Survey Open-File Report 99-301 engineer from Wales (Hopson, 1975). A nonbanded upper bed and a thin, impure bed second shaft of the Cannel Coal Company at approximately 3 m (9.8 ft) below; additional Dolores also mined both coal zones. The thin coal beds and carbonaceous mudstones Santo Tomas Coal Company opened a shaft to occur in the lower part of the coal zone but are the Santo Tomas coal bed between 1910 and not mined (fig. 3). The upper coal bed is 1912 near the village of Santo Tomas. This underlain by a dark gray, root-penetrated, silty operation ceased in 1918 after a mine fire. claystone, and the uppermost portion of this The Darwin mines were abandoned in 1921, coal bed contains mud-filled invertebrate and the Dolores mines closed in 1939. burrows and trails. As seen in-mine, the In 1979, Farco Mining Company of contact between this coal and overlying Texas, Inc., initiated surface mining at Palafox mudstone or sandstone generally is sharp. (fig. 1). The Rachal and Trevino mines began The interval immediately above the San production in 1983 and 1996, respectively. Pedro coal zone consists mainly of sandstone, Cumulative production from 1979 through a slightly smaller proportion of mudstone, a 1997 totaled approximately 6 million short minor intraformational conglomerate, local tons. The coal is washed and sized at a fine-grained limestone nodules or bands, and cleaning plant at Palafox. A significant thin (< 0.45 m or 1.5 ft), discontinuous coal amount of the coal is exported to the Republic beds that range from nonbanded coal to clay of Ireland as lump coal for domestic use, and or silt-rich impure coal. The sandstones are other production is consumed by local cement calcareous and are composed chiefly of fine to plants. Kelmac, Inc., a subsidiary of Chevron very fine, subangular to rounded quartz grains; Corporation, recently acquired the Webb individual beds are massive, cross bedded, County mines of Farco Mining Company of streaked or interbedded with mudstone, Texas and operates currently under the name rippled, or rooted or otherwise biotrubated. Farco Mining, Inc. An en echelon pattern characterizes sandstone bodies above the San Pedro coal zone in the DEPOSITIONAL SETTING Palafox Mine. These sandstone bodies are flat botttomed, flat to ripple bedded, and exhibit The coal-bearing portion of the lower characteristics of mouth-bar to crevasse-splay to middle Claiborne Group of Webb County is sands deposited in shallow water. Between a fining-upward sequence situated above a the main coal bed and these sandstone thick, sandstone-dominated section that deposits, coarsening-upward, rhythmically contains poorly preserved marine interbedded mudstones and sandstones contain invertebrates, and below a mudstone-rich flaser bedding suggestive of tidal influences. A section that yields mainly nonmarine fossils. few zones of poorly preserved marine Corehole records and natural exposures show invertebrate fossils (pelecypods, gastropods) that the San Pedro coal zone overlies a barren occur above the San Pedro coal zone interval dominated by thick (7.6-10.7 m or 25- (Lonsdale and Day, 1937). 35 ft), predominantly flat-bedded, sandstone At the Rachal Mine (fig. 1), the Santo deposits. Tomas coal zone consists of a thick (0.69 m) At Palafox, the San Pedro coal zone nonbanded main seam, a clastic interval that consists of a thick (< 0.85 m or 2.9 ft), ranges from 0.91 to 1.98 m (3 to 6.5 ft) in

17 U.S. Geological Survey Open-File Report 99-301 thickness, and a thin (0.20 m or 0.6 ft) rider coal bed (fig. 3). Both beds are underlain by COAL CHARACTERISTICS paleosols, and the top of the main bed contains invertebrate burrows and trails. The interval The nonbanded coals that are mined in between these coal beds consists of Webb County are extremely hard, fracture flat-bedded siltstone and fine to very fine conchoidally, and have cleats developed at a grained, noncalcareous sandstone that fine scale similar to that of bituminous nonbanded laterally into coarsening-upward mudstone coals. The lower part of the San Pedro coal deposits. These mudstones lack the flaser- bed at Palafox is very massive and breaks into bedded sandstone interbeds that occur above blocks that average 0.5 m (1.6 ft) in width; to the San Pedro coal bed at Palafox. Channel- a lesser extent, the main Santo Tomas coal bed form deposits within this interval are small becomes more massive and less fractured (0.25 m or 0.8 ft thick, 1 m or 3.3 ft wide), downward. few in number and limited to the top of thick Physical and geochemical analyses (fig. sandstone deposits. Overall, the interval 4) reported by us previously (Warwick and between the main Santo Tomas coal bed and Hook, 1995) are comparable to earlier rider bed records the drowning of an initial analyses of these coal beds (Evans, 1974; mire, infilling of the resulting floodbasin by Mukhopadhyay, 1989). All the San Pedro and minor crevasse splays, and re-establishment of Santo Tomas samples have relatively high ash mire conditions. yields (9.5-18% dry basis) at their base. Total The presence of several thin coal and sulfur content (0.8-2.0%, dry basis) generally carbonaceous shale beds a short distance increases upward, and organic sulfur is above the Santo Tomas rider bed indicates that predominant. Calorific values increase upward clastic-rich mires persisted locally. Overlying within each coal bed, with a maximum of coarsening-upward sequences represent the 7,434 kcal/kg (13,371 Btu, dry basis) found in terminal flooding and clastic infilling of the the top part of the main San Pedro bed at floodbasin. Lonsdale and Day (1937) reported Palafox. Hydrogen content follows calorific thin sandstone beds in the area of Darwin that values, ranging from 5.4 to 6% (dry basis). contained poorly preserved nonmarine Additional physical and geochemical data, molluscs and abundant green algae including trace-element analyses, are reported fructifications. Although they interpreted in Warwick and Hook (1995). these fossil occurrences as a unique bed of Petrographic study of particle pellets considerable persistence, Gardner (1945), who and unetched polished blocks from San Pedro measured and collected many of the and Santo Tomas coal samples (Warwick and fossiliferous sections reported by Trowbridge Hook, 1995) indicate that a highly degraded (1923, 1932) and Lonsdale and Day (1937), groundmass composed of eugelinite is the stated clearly that several such beds occur. main organic component (approximately 71%, The presence of these locally rich beds of mineral-matter-free basis). An enriched nonmarine aquatic fossils above the Santo liptinite fraction (approximately 23%) Tomas coal zone indicates that lacustrine accounts for a portion of the high calorific settings may have been common within the floodbasin.

18 U.S. Geological Survey Open-File Report 99-301

Santo Tomas and

rider coal beds

10000 10000

14000 14000

6000 6000

2000 2000

0.2 0.2 0.4 0.6 0.8 1.2

16 16

12 12

0 0 4 8

0 0

0 0 1 2 4 6 0 0

ASH (db) S (db) BTU (db) H (db)

14000 14000

10000 10000

2000 2000 6000

0.4 0.4 0.6 0.8

0.2 0.2 1.2

12 12 16

0 0 1

0 0 4 8

0 0

0 0 2 4 6

ASH (db) S (db) BTU (db) H (db)

50 cm San Pedro 93

channel sample

10000 10000 14000

6000 6000 2000 2000

0.5 0.5

1.5 1.5

20 20

40 40

60 60

0 0

0 0

1 1 2 2 0 0

ASH (db) S (db) BTU (db)

San Pedro 94 10000 10000

channel sample 14000

6000 6000 2000 2000

0.5 0.5 1.5 2.5

10 10 15

20 20

0 0

2 2 6 6

4 4

0 0

0 0 0 0 5 1 2 2

ASH (db) S (db) BTU (db) H (db) Figure 4. Selected proximate and ultimate data. Sample descriptions and symbols are the same as those used in figure 3. All values as percentages except Btu. Sample locations are shown in Figure 1. Complete sample data are given in Warwick and Hook, 1995. From Warwick and Hook (1995).

19 U.S. Geological Survey Open-File Report 99-301

values that distinguish these coals. Most of reported by Mukhopadhyay (1989) for one this enrichment represents fusiform San Pedro sample. Compared to standard green-algae fructifications, which because of rank parameters, the San Pedro and San their size (up to 2 mm or 0.1 in. in length), Tomas coal deposits fall within the range of cannot be identified or quantified readily by subbituminous A to high-volatile bituminous C conventional coal petrography techniques or coal. palynologic preparations that involve acidification. There is a negligible proportion SUMMARY of inertinite macerals in these coals. Average maximum vitrinite reflectance The unusual coals of the San Pedro

(Romax) values for San Pedro and Santo Tomas and Santo Tomas coal zones of the Claiborne coal samples reported previously by us was Group in Webb County, Texas, are

0.53. This was comparable to the Romax distinguished by physical, petrographic, and

Alluvial Crevasse floodplain splay Santo Tomas rider bed Clastic swamp Algae-rich Distributary channel lake and crevasse splay

Algae-rich lake

Mouth bar sandstone Algae-rich lake

Santo Tomas coal bed Sandstone San Pedro Mudstone/claystone Coal coal bed Carbonaceous mudstone

Figure 5. Schematic block diagram of depositional environments for the San Pedro and Santo Tomas coal zones in the middle part of the Claiborne Group. No scale implied. The geometric distribution of lithologies on the panels is inferred from borehole and mine highwall data. From Warwick and Hook (1995). 20 U.S. Geological Survey Open-File Report 99-301 geochemical characteristics unlike those of REFERENCES CITED other Gulf Coast Tertiary coal deposits. Whereas the San Pedro deposits accumulated Ashley, G.H., 1919, The Santo Tomas Cannel in marine-influenced lower delta plain settings, Coal, Webb County, Texas: U.S. the overlying Santo Tomas interval represents Geological Survey Bulletin 691-I, p. a nonmarine, mudstone-dominated sequence 251-270. (fig. 5). Mineable coal beds within both coal Eargle, D.H., 1968. Nomenclature of zones contain an abundance of green algae Formations of Claiborne Group, fructifications, which account in part for the Middle Eocene Coastal Plain of Texas: anomalously high calorific value. Data from U.S. Geological Survey Bulletin1251- Mexico (SPP, 1989), however, record D, 25 p. Quaternary igneous intrusive rocks less than Evans, T.J., 1974, Bituminous Coal in Texas: 100 km (62 mi) west of the Rio Grande area, Bureau of Economic Geology, The and coal-quality data from coal beds in the University of Texas at Austin, underlying Wilcox Group of Texas counties Handbook 4, 65 p. bordering Webb County indicate increased Gardner, J.A., 1945, Mollusca of the Tertiary rank. Additional work is required to Formations of Northeastern Mexico: determine the postdepositional history of the Geological Society of America coal deposits of South Texas. Memoir 11, 332 p. Guevara, E.H., and Garcia, Roberto, 1972, ACKNOWLEDGMENTS Depositional systems and oil-gas reservoirs in the Queen City Formation This summary is based mainly upon a (Eocene), Texas: The University of 1995 paper published in the International Texas at Austin Bureau of Economic Journal of Coal Geology. We thank Elsevier Geology Geologic Circular 72-4, 22 p. Science B.V. for allowing us to reproduce Hopson, F.S., 1975, Resume of cannel coal portions of our previous work. Our original development, in, Triple Energy Field study was made possible through the Trip, Duval, Webb, and Zapata assistance of Farco Mining Company of Texas, Counties, Texas: Corpus Christi Inc., and the Railroad Commission of Texas. Geological Society Guide Book, 14 p. We also thank those at the U.S. Geological Lonsdale, J.T., and Day, J.R., 1937, Geology Survey and the U.S. National Museum who and Ground-Water Resources of Webb contributed to our earlier work. County, Texas: U.S. Geological Survey Water-Supply Paper 778, 104 p.

21 U.S. Geological Survey Open-File Report 99-301

Mukhopadhyay, P.K., 1989, Organic petrology and organic geochemistry of Texas Tertiary coals in relation to depositional environment and hydrocarbon generation: Bureau of Economic Geology, The University of Texas at Austin, Report of Investigations188, 118 p. SPP [Secretaría de Programación y Presupuesto], 1989, Carta Geologica [Monterrey]: Dirección General de Geografía del Territorio Nacional, San Antonio Abad, Mexico, scale 1:1,000,000. Trowbridge, A.C., 1923, A geologic reconnaissance in the Gulf Coastal Plain of Texas, near the Rio Grande: U.S. Geological Survey Professional Paper 131-D, p. 85-107. Trowbridge, A.C., 1932, Tertiary and Quaternary Geology of the Lower Rio Grande Region, Texas: U.S. Geological Survey Bulletin 837, 260 p. Vaughan, T.W., 1900, Reconnaissance in the Rio Grande Coal Fields of Texas: U.S. Geological Survey Bulletin 164, 100 p. Warwick, P.D., and Hook, R.W., 1995, Petrography, geochemistry, and depositional setting of the San Pedro and Santo Tomas coal zones: anomalous algae-rich coals in the middle part of the Claiborne Group (Eocene) of Webb County, Texas: International Journal of Coal Geology, v. 28, p. 303-342.

22 U.S. Geological Survey Open-File Report 99-301

CHAPTER 3

The San Miguel lignite deposit, Jackson Group (Eocene), South Texas

By Peter D. Warwick, Sharon S. Crowley+, Leslie F. Ruppert, and James Pontolillo*

U.S. Geological Survey, MS 956 (MS 910+, MS 432*), 12201 Sunrise Valley Drive, Reston, VA 20192

INTRODUCTION updip of a strike-elongated sand complex that has been interpreted as a barrier-bar/strand The San Miguel lignite deposit (late plain complex by Kaiser and others (1980) and Eocene, lower Jackson Group) of South Texas Gowan (1985). Gowan (1985) and Ayers is mined by the North American Coal (1986) suggest that the mud-dominated Corporation, at the San Miguel Mine near sediments the comprise the floor, partings, and Jourdanton, Texas (fig. 1). The deposit roof of the San Miguel lignite deposit consists of four or more thin (generally < 1 m originated in lagoonal mires landward (west) or 3.3 ft thick) lignite benches that are of the sandstone-dominated barrier island separated by claystone and mudstone partings complex. The lignite-bearing interval is (fig. 2). The partings are composed of altered laterally persistent along strike for more than volcanic air-fall ash that has been reworked by 40 kilometers (25 mi) (Ayers, 1986, 1989; tidal or channel processes associated with a Tewalt and others 1983). More detailed back-barrier depositional environment descriptions of the geological setting of the (Snedden and Kersey, 1981; Gowan, 1985; area can be found in McNulty (1978), Snedden Ayers, 1986; Senkayi and others, 1987; and (1979), Snedden and Kersey (1981), Tewalt Warwick and others, 1996). The field trip and others (1983), Gowan (1985), and Ayers stop at the San Miguel Mine will include a trip (1986, 1989). into the working opencast pit. We will be able The San Miguel lignite interval consists to observe mining methods, and the rocks of four (or more) thin (generally <1 m or 3.3 ft associated with the coal-bearing interval. This thick) lignite benches that are separated by summary of the geology of the San Miguel claystone and mudstone partings. The upper deposit is based largely on Warwick and others bed is designated as the A bed and is underlain (1996), which focuses on the petrology and by the B, C, and D beds (figs. 2, 3a) (Gowan, geochemistry of the lignite deposit. 1985). Tewalt and others (1983) referred to this interval as South Jackson seam no. 10. GEOLOGIC SETTING The rocks above the lignite beds are dominated by fine siltstone and claystone with The San Miguel lignite deposit is in the occasional thin layers of gastropod and bivalve undivided lower Jackson Group of South shells. Texas (fig. 2). In the mine area, the Jackson Kaiser (1982), Gowan (1985), and strata dip to the southeast about 17 m/km (90 Ayers (1986, 1989) suggested that the rock ft/mi) (Ayers, 1986). The mine is located partings within the San Miguel lignite deposit

23 U.S. Geological Survey Open-File Report 99-301

o o 98 35' 98 o25' 98 15'

To Jourdanton 791

140 28 45'o Approximate Campbellton 16 3387 boundary of San Miguel Mine Permit

281

Texas ALT 281 SM-1 791

Whitset SM-2

McMullen Co

Atascosa Co

Live Oak Co 99 0 5 km Crowther 99 28 35'o Figure 1. Index map of the San Miguel Mine area showing towns, roads, and sample sites. Mine outline from the San Miguel Lignite Mine Permit Application, on file at the Texas Railroad Commission in Austin, Texas. From Warwick and others (1996). are composed of volcanic air-fall ash that has the San Miguel Mine began in 1980 and the been reworked by possibly tidally influenced 1997 annual production was about 3.5 million channel processes. Senkayi and others (1987) tons (3.2 metric tons) (data from the U.S. examined the partings in detail and found that Department of Energy, Energy Information most contained altered kaolinite and mixed Agency and the Railroad Commission of detrital and volcanic quartz grains, thereby Texas). Since 1997, the San Miguel Mine has supporting the interpretation of a volcanic been operated by the North American Coal origin and subsequent detrital mixing of the Corporation; prior mining was carried out by sediment that forms these layers. These the Atascosa Mining Company. The findings are consistent with the conclusions of Keystone Coal Industry Manual (PRIMEDIA Warwick and others (1996) (see below). Intertec, 1999) reports the total reserves for the North American Coal Corporation in the LIGNITE RESOURCES AND MINING San Miguel Mine area to be about 42 million tons with 35 million tons recoverable (38.2 Jackson Group lignite resources for the and 32 million metric tons respectively). South Texas area have been estimated to be Average overburden is about 30 m (98 ft), about 6 billion short tons (5.4 metric tons) with a stripping ratio of 7:1. Overburden is with 750 million tons (680 metric tons) removed by dragline methods. The coal is occurring near surface and the remainder removed by Easi-Miners (a modified asphalt classified as deep-basin deposits (Kaiser and stripper) and front-end loaders. The lignite is others, 1980). Surface mining operations at

24 U.S. Geological Survey Open-File Report 99-301

SM-1 SM-2 * Mudstone roof Mudstone roof 0.15cm (RFT) Carbonaceous shale * Mudstone roof 0.15cm (RFB) N. American 20cm (A.1) Series European Carbonaceous shale 18cm stage Group (RF2) stage A Lignite 33cm (A.2) Lignite 22cm (A1) Mudstone 12cm Lignite 8 cm (A2) Mudstone 14cm Mudstone 13cm (P1.1) Priabonian Jackson Group Claystone 16cm Mudstone 15cm (P1.2) Jacksonian San Miguel Mine Claystone 8cm (P1.3) Lignite 23cm (B.1) Lignite 17cm (B1) Bartonian B Lignite 13cm (B.2) Lignite 15cm (B.3) Lignite 32cm (B2)

Claystone 23cm Claiborne Claystone 29cm (P2) Group Lignite 31cm (C.1)

Lignite 31cm (C1) Lutetian Eocene C Lignite 37cm (C.2)

Claibornian Lignite 35cm (C2) Mudstone 10cm Claystone 8cm Claystone 9cm (P3.1) Claystone 9cm (P3.2) * Claystone 21cm Claystone 9cm (P3.3) * Claystone 11cm (P3.4) Ypresian Wilcox Group Lignite 36cm (D1) D Lignite 60cm (D.1) Sabinian Lignite 33cm (D2) Selandian Claystone floor (GRAB) Lignite 40cm (D.2) Midway Group * Danian Paleocene Midwayan * Mudstone floor 33cm 1 m Green claystone 300 m Figure 2. Generalized stratigraphy and stratigraphic sections of the coal-bearing interval at the San Miguel Mine. Sample numbers are in parentheses, and channel sample sites are shown on figure 1. The approximate positions of the volcanic ash beds identified by Senkayi and others (1987) are indicated by an asterisk. Stratigraphy is after Breyer (1991). From Warwick and others (1996). moved by truck and used as steam coal in the meters in length and are less than 0.5 m (1.6 mine-mouth 409 megawatt San Miguel Power ft) wide. Minor faulting is usually associated Plant operated by the San Miguel Electric with these dikes. Ayers (1986) suggested that Cooperative (PRIMEDIA Intertec, 1999). emplacement of these dikes may be related to Some mining problems that are loading and dewatering processes. Another encountered in the San Miguel Mine include interesting feature that is often observable in floor heaves, mudstone dikes, and channels the San Miguel Mine are small interseam within the coal interval. Some of these channels that usually scour into lignite beds C features may be observed during our field trip. or B and are filled with inter-seam parting In some areas, a greenish mudstone located 1- material. The channels are usually no more 2 meters (3.3-6.6 ft) below the D bed (fig. 2) than a few meters (several ft) wide and are intrudes into the floor of the open pit causing confined to a single clastic parting zone within problems with pit traffic and general mining the A-D coal bed complex. Ayers (1996) operations (fig. 3a). Ayers (1986) described described these channels in detail and suggests clastic dike intrusions that originate primarily that were formed by marsh drainages that from the partings between beds C and D. The occupied the area between periods of peat intrusions are generally no more than a few accumulations.

25 U.S. Geological Survey Open-File Report 99-301

Floor heave

15 cm

A B C D A B

EG

EL C D

scale Figure 3. Representative photographs and photomicrographs of outcrops and samples from the San Miguel lignite interval. (a) View of a highwall with A (upper), B, C, and D beds and the partings. Note that the debris on the mine floor seen on the left of the photograph is due to floor heaves. (b) Clay-filled burrows are common in the San Miguel lignite. Scale = 15 cm. (c) Arrow points to a compressed log in the D bed. (d) Typical photomicrograph of an etched and unetched surface of a polished pellet of San Miguel lignite. EG = eugelinite, EL = etch line. Scale bar = 25µm. From Warwick and others (1996)

CHARACTER OF THE SAN MIGUEL by organisms that lived in the water that LIGNITE encroached over the top of the peat deposit. The burrows, usually filled with a mixture of Megascopic characteristics organic material and kaolinitic claystone, is As described by Gowan (1985), the one source of ash in the lignite beds (fig. 3b). San Miguel Lignite interval has been Individual lignite beds generally exhibit a dull, extensively reworked by burrowing somewhat massive texture near their base and organisms. This is apparent from the grade upward into attrital-rich lignite with lithologic descriptions of the channel sample increasing xylinitic bands (3-4 cm or 1.2-1.6 in sites described by Warwick and others (1996) thick) towards the top of the individual bed. (fig. 3b). Some of the burrows near the top of Xylinitic bands can compose up to 30% of the A lignite bed were identified as Gyrolithes individual lignite beds. Occasionally, by Gowan (1985) and probably were formed compressed logs are exposed in

26 U.S. Geological Survey Open-File Report 99-301

Ash Yield mass %

40 40

30 30

10 10 20 20

0 0

20 20 30 40 60 70

10 10 50 0 0

1 m SM-1 SM-2

Sulfur mass %

3.5 3.5

0.5 0.5 2.5

1.5 1.5

0 0

0.5 0.5 1.5 2.5 3.5

1 1 2 3

0 0 1 2 3

SM-1 SM-2

Btu/lb

CV 12000

10000 10000

2000 2000 4000 6000 8000

10000 10000

6000 6000

2000 2000 4000 8000

0 0

0 0

30 10

10

30 20

20 0 0 0 0 Mj/kg Mj/kg SM-1 SM-2

Figure 4. Bar graphs showing concentrations of ash yield, total sulfur, and calorific value (CV, dry basis) for the sample sets SM-1 and SM-2. Location of samples are shown on figure 1 and lithologic symbols are the same as used on figure 2. Distance between sample sites is about 300 m. From Warwick and others (1996).

27

U.S. Geological Survey Open-File Report 99-301

100

16 16 12 12

As 100 Cd Co

60 60 80

0 0 20 20 40

40 40 4 8

20 20 80

60 60

12 12

0 0 3

0 0

1 1 2

2 2

0 0 8

0 0

1 1 0 4

SM-2 SM-2 SM-2

SM-1 SM-1 SM-1

0.2 0.2

0.6 0.6 0.4 0.4 0.2 0.2 Be Cr 100

Hg 0.4

20 20

40 40

60 60

40 40

20 20 80

15 15

10 10 0 0

0 0

0 0

0 0

0 0

5 5

25 25

20 20 15 15

10 10

0 0 5 5

SM-2 SM-2 SM-1 SM-2 SM-1 SM-1 1 m

Figure 5. Bar graphs showing the distribution of trace element concentrations (dry USGS ash basis) for twelve potentially environmentally sensitive elements identified by the 1990 U.S. Clean Air Act Amendments. Location of samples are shown on figure 1 and lithologic symbols are the same as used on figure 2. All values are parts per million (ppm). From Warwick and others (1996). cross section on the exposed face of the lignite (Tewalt, 1986). Total sulfur contents (on a bench (fig. 3c). The middle lignite beds (B and dry basis) range from 1.5 to 3.1% (Tewalt, C) appear to contain the greatest percentage of 1986). Organic sulfur is the dominant sulfur xylinitic banding when compared to the upper type. Calorific values (on a moisture-free and lower beds (A and D). basis) for the San Miguel samples range from 5384 to 8690 Btu/lb (12.5-20.2 Mj/kg) Proximate analyses (Tewalt, 1986). Tewalt (1986) and Chemical and physical data obtained Mukhopadhyay (1989) report that the rank of from San Miguel lignite samples have been the Jackson Group and the San Miguel lignites reported on an individual bed basis (5 samples; is lignite A. Mukhopadhyay, 1989) and on a combined bed Warwick and others (1996) reported basis (23 samples; Tewalt, 1986). From these proximate analyses for 17 lignite samples data, the dry ash yields of samples range from collected from the San Miguel deposit (fig. 4). 25 to 55% (avg 38%; Tewalt, 1986) with the The data indicate that dry ash yields average A bed having the greatest average ash yield 30.16%; total dry sulfur content averages (39%, Mukhopadhyay, 1989). Equilibrium 2.76% (dominated by organic sulfur, 2.04%); moisture values range from 27 to 33%

28 U.S. Geological Survey Open-File Report 99-301

Mn Pb Se

100 100

300 300 500

500 500

700 700 300 300

100 100

100 100

40 40

20 20

0 0

20 20 15 15

10 10 40 40 80

20 20 60

5 5 0 0

0 0

0 0

5 5 0 0 0 0

SM-1 SM-2 SM-1 SM-2 SM-1 SM-2

NI Sb U

20 20

10 10

10 10

10 10 20 30

10 10 20

0 0

10 10

20 20

25 25

0 0

6 6 8 0

0 0 2 4

6 6

0 0 4 4 2 2 0 0

SM-1 SM-1 1 m SM-2 SM-1 SM-2 SM-2

Figure 5. Continued - Bar graphs showing the distribution of trace element concentrations (dry USGS ash basis) for twelve potentially environmentally sensitive elements identified by the 1990 U.S. Clean Air Act Amendments. Location of samples are shown on figure 1 and lithologic symbols are the same as used on figure 2. All values are parts per million (ppm). From Warwick and others (1996). and dry calorific values average 8611 Btu/lb other lignite deposits of Texas (range of (20 Mj/kg) (fig. 4). As-received moisture averages: 0.41-1.22%), and suggested that this content averages 33.4% for the SM-2 channel enrichment was either the result of sodium sample set. At both sample sites, ash yields sorption from saline waters during and after were greatest from the upper and lower lignite deposition, or of sodium cation exchange with benches. Total sulfur content is fairly uniform present-day ground water. Tewalt (1986) both within and between each sample site (fig. reported trace-element concentrations for 4). fifteen elements (As, B, Be, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, U, V, Zn) that were Major, Minor, and Trace element data obtained from two samples from the San Tewalt (1986) reported major oxide Miguel deposit. Based on the results from and trace-element data for the San Miguel other Jackson Group lignites of East Texas, lignite interval. She found that the San Miguel Tewalt suggested that the Jackson lignites lignite interval contains greater amounts of contain greater amounts of B, Be, Mn, and Zn

Na2O (avg 3.67%; N=66) as compared to

29 U.S. Geological Survey Open-File Report 99-301

Ce Ta Zr 1000

1000 1000

100 100 200

200 200 400 600 800

100 100 200 300

200 200 400 600 800

10 10

0 0 0

0 0 0 2 4 6 8

0 0

0 0 2 4 6

SM-2 SM-2 SM-2 SM-1 SM-1 SM-1

Nb

100 100 120 120

100 100 Th

60 60 80 60 80

20 20 40 10 20 30 40 50 60

10 10 20 30 40 50 60

20 20 40

0 0

0 0

0 0 0 0

SM-2 SM-2 SM-1 SM-1 1 m

Figure 6. Bar graphs showing the distribution of trace element concentrations (dry USGS ash basis) for those elements inferred to be derived from volcanic ash. Location of samples are shown on figure 1 and lithologic symbols are the same as used on figure 2. All values are parts per million (ppm). From Warwick and others (1996). than the older Wilcox lignites of eastern Texas. containing less than 50% ash (i.e. what is Warwick and others (1996) reported normally mined) showed that only Mn trace element data obtained from two channel has a strong negative (r <-0.5) relationship to samples taken from the San Miguel mine (figs. sample ash yield. On a whole-coal basis (for 1, 2). Of particular interest for this study was those samples containing less than 50% ash) a the distribution of 12 environmentally sensitive strong positive correlation (r >0.5) was found elements that were described by the 1990 U.S. between ash and Pb, Se, and Na2O. Clean Air Act Amendments. A summary of Warwick and others (1996) also the trace element data (on a dry, ash basis) for plotted bar graphs for both San Miguel rock both sample sites is plotted on figure 5. Visual and lignite samples to show the vertical analysis of the SM-2 bar graphs shows that distribution of elements that may be associated some of the elements are concentrated in the with volcanic ash partings (fig. 6). Mean organic-rich (or lignite) samples. Linear concentrations are: Ce, 117 ppm: Nb, 36 ppm; correlations between ash yield and the Ta, 2.4 ppm; Th, 29 ppm; and Zr, 477 ppm (all concentrations of the environmentally sensitive concentrations in coal on an ash basis) (fig. 6). elements on an ash basis for those samples Concentrations of these elements are generally

30 U.S. Geological Survey Open-File Report 99-301 enriched in the B and C lignite beds. Previous sample, but they are rare. Quartz, pyrite, and work has indicated that concentrations of these Ca-Al phosphate (crandalite group?) are elements in coal can be enriched by present in trace amounts. incorporation of volcanic ash in peat or by Examination of samples from the leaching from volcanic ash beds (Zielinski, claystone parting between the C and D beds 1985; Crowley and others, 1989). In coal (P3.1, P3.2, P3.3, and P3.4, fig. 2), which was samples from intervals adjacent to altered described as a major ash bed by Senkayi and volcanic ash partings in the Cretaceous C coal others (1987), shows that the matrices are bed of Utah, Crowley and others (1989) composed primarily of mixed-layer clays. reported mean concentrations (on an ash basis) Kaolinite is present as cell-fillings, vermicular of elements having a volcanic origin as clasts, and rarely as matrix material. follows: Zr (752 ppm), Nb (57 ppm), Th (60 Accessory grains are uncommon. The ppm), and Ce (246 ppm). Although the mean dominant accessory minerals are subhedral and concentrations of these elements in the San often etched alkali feldspars. Plagioclase is Miguel samples do not reach the reported also present but not as abundant as the averages for the Utah C coal bed, specific alkalies. Barite and pyrite, and rare samples from SM-1 and SM-2 exceed the subrounded quartz and euhedral zircons were Utah averages for Zr, Nb, and Ce. observed in each of the examined samples.

Mineralogy of San Miguel partings Palynology and Organic Petrography Warwick and others (1996) reported Gennett (1985) and Gennett and results from preliminary examination of San Raymond (1986) examined incremental bench Miguel rock parting samples based upon samples from a core of the San Miguel lignite scanning electron microscopy (SEM) and and found that the lower D bed had pollen energy-dispersive X-ray analyses (EDXA). derived from marsh ferns and palms which These analyses indicate that the upper part of suggested a brackish-water influence on the the mudstone parting below the A bed (SM-2 deposition of the D bed paleo-mire. Pollen P1.1, fig. 2) is composed primarily of kaolinite types found in the overlying beds showed with some mixed-layer clay present. fresh-water associations as indicated by a Vermicular kaolinite is present throughout the vertical transition from Nyssa (tupelo) to sample, as are small (<50µm) K-feldspars. Engelhardia (walnut family) and Fagaceous Isolated plagioclase, quartz, barite, framboidal (oak family) types. Mukhopadhyay (1989) pyrite, anatase, and a single Al-phosphate were reported that beds B and D contain marsh also observed. The mudstone is very organic pollen types and suggested that the San Miguel rich. Although individual organic stringers are lignites are dominated by rare, irregular pieces of coaly material are Engelhardia/Juglandaceae and herbaceous(?) found throughout the sample. monocots. The matrix of the claystone below the Mukhopadhyay (1986, 1989) B bed (sample SM-2 P2, fig. 2) is composed examined the petrographic characteristics of primarily of mixed-layer clay and abundant channel and lithotype samples from each of organic material. Both alkali and plagioclase the four major lignite beds exposed at the San feldspars were observed throughout the

31 U.S. Geological Survey Open-File Report 99-301

SM-2

SM-1 100

100 100

20 20 40 60

80 80 40 60

20 20

80 80

0 0 0

Major Huminites (%)

1 m

EG HM DG EG HM DG

10 10

10 10 30

20 20

15 15 25

5 5

0 0

0 0 5

Major Liptinites (%)

1 m

LPD ALG RSD LPD ALG RSD

0 0

1 1

0 0 1 2 3 4 5 6 2

Major Inertinites (%)

1 m MAC SCL FUS MAC SCL FUS

Figure 7. Bar graphs of the major petrographic constituents for the channel-sample sets SM-1 and SM-2. Location of samples are shown on figure 1 and lithologic symbols refer to figure 2. Abbreviations are as follows: EG = eugelinite, HM = humotelinite, DG = detrogelinite, LPD = liptodetrinite, ALG = alginite, RSD = resinite in attritus, MAC = macrinite, SCL = sclerotinite, FUS = fusinite. From Warwick and others (1996)

32 U.S. Geological Survey Open-File Report 99-301

Miguel mine. Mukhopadhyay's channel finding is in agreement with megascopic sample data indicate that bed A (fig. 2) descriptions of the coal in the field. contains greater amounts of liptinite macerals (32%, primarily sporinite and liptodetrinite) RELATIONSHIP BETWEEN ASH than the huminite-rich middle lignite beds. YIELD, AND PETROGRAPHIC AND Bed C contains the greatest amount of GEOCHEMICAL CHARACTERISTICS huminitic material (85%, primarily ulminite OF THE SAN MIGUEL LIGNITE and attrinite/densinite) when compared to the DEPOSIT other beds. Inertinite content for the beds range from 1 to 4% and is primarily in the Warwick and others (1996) examined form of sclerotinite and fusinite. The D bed the relationship between the ash yield and the contained a few marine dinoflagellates (algal petrographic and geochemical characteristics cysts). of the San Miguel lignite as mined. The major Warwick and others (1996) reported conclusions of their study are as follows: (1) that the San Miguel lignite deposit is The distribution of Mn is inversely related to dominated by the huminite maceral group the ash yield of the lignite samples. This (89%) followed by the liptinite (10%), and indicates an organic affinity, or an association inertinite (1%) maceral groups (figs. 3d and 7). with finely disseminated minerals in the lignite The three dominant huminite maceral that contain this element. (2) On a whole-coal subgroups in decreasing order of abundance basis, the concentration of the environmentally are: eugelinite (73%), humotelinite (11%), and sensitive element Pb is positively related to ash detrogelinite (3%). The liptinite maceral yield in lignite samples. This indicates an group is dominated by liptodetrinite (4%), inorganic affinity for Pb. (3) Average followed by alginite (2%) and resinite in whole-coal concentrations of As, Be, Sb, and attritus (1%). The inertinite maceral group is U in the San Miguel samples are greater than composed of macrinite, sclerotinite, and published averages for these elements in other fusinite. U.S. lignites. (4) The upper and lower lignite Vertical and lateral variations in benches of the San Miguel deposit are both maceral composition can be observed in the ash- and algal-rich, indicating that these petrographic data (fig. 7). Channel-sample set intervals were probably deposited in wetter SM-1 contains similar amounts of huminite conditions than those in which the middle macerals as found in SM-2 samples, but SM-1 intervals formed. (5) The dominance of the samples have markedly less liptinite and eugelinite maceral subgroup over the huminite inertinite components. Vertical trends in the subgroup indicates that the San Miguel lignites petrographic data include slightly increased were subjected to peat-forming conditions liptodetrinites in samples from the top and (either biogenic or chemical) that enabled bottom of the lignite interval and a sharp degradation of wood cellular material into increase in inertinite (primarily macrinite and matrix gels, or that the plants that formed fusinite, fig. 7) in the lignite samples directly these lignite benches were less woody and below the parting between the A and B beds. more prone to formation of matrix gels. (6) Humotelinite content increases, although An inertinite-rich layer (top of the B bed) slightly, in the upper part of each bed. This might have formed from widespread oxidation

33 U.S. Geological Survey Open-File Report 99-301 of the San Miguel peat as a result of a volcanic lignites (Warwick and others, 1996). ash fall that was subsequently reworked. ACKNOWLEDGMENTS SUMMARY This summary is based mainly upon a There have been numerous studies 1996 paper published in Organic conducted through the years detailing the Geochemistry. We thank Elsevier Science geology of the San Miguel lignite deposit. The Ltd. for allowing us to reproduce parts of our depositional setting of the lignite deposit is previous work. We wish to thank John Palfy, interpreted to have originated in back-barrier General Manager, and Roger C. Fish, lagoonal mires (Snedden and Kersey, 1981; Engineering Manager, and North American Kaiser and others,1980; Gowan,1985; Ayers, Coal Corporation for facilitating our visit to 1986). The deposit consists of four or more the San Miguel Mine. thin (generally < 1 m or 3.3 ft thick) lignite benches that are separated by claystone and REFERENCES CITED mudstone partings (fig. 2). The partings are composed of altered volcanic air-fall ash that Ayers, W. B., Jr., 1986, Geology of the San has been reworked by tidal or channel Miguel lignite mine, Jackson Group, processes associated with a back-barrier South Texas, in Finkelman R. B. and depositional environment (Snedden and Casagrande D. J., eds, Geology of Kersey, 1981; Gowan, 1985; Ayers, 1986; Gulf Coast Lignites: Geological . Senkayi and others, 1987; and Warwick and Society of America, Coal Geology others, 1996). Division Field Trip Guidebook, p. Individual lignite beds generally exhibit 54-67, Environmental and Coal a dull, somewhat massive texture near their Associates, Houston, Texas. base, which grades upwards into attrital-rich Ayers, W. B., Jr., 1989, Depositional setting lignite with increasing xylinitic bands (3-4 cm of the San Miguel Lignite Mine, or 1.2-1.6 in thick) towards the top of the Jackson Group (Eocene), South individual bed. Proximate data indicate that Texas, in Ayers W. B. Jr., Breyer J. A. dry ash yields from the San Miguel deposit and Finkelman R. B, eds., Depositional average 30.16%; total dry sulfur content settings of Texas Lignites, 28th averages 2.76% (dominated by organic sulfur, International Geological Congress: 2.04%); and dry calorific values average 8611 Field Trip Guidebook T173, p. 33-37. Btu/lb (20 Mj/kg) (Warwick and others 1996). Breyer J. A., 1991, Tertiary coals of the Gulf Petrographically, the San Miguel lignite Coast, in Gluskoter, H. J., Rice D. D. deposit is dominated by the huminite maceral and Taylor, R. B., eds., Economic group (89%) followed by the liptinite (10%), Geology U. S.: Geological Society of and inertinite (1%) maceral groups (Warwick America, The Geology of North and others, 1996). Average whole-coal America P-2, p. 573-582. concentrations of As, Be, Sb, and U in the San Miguel samples are greater than published averages for these elements in other U.S.

34 U.S. Geological Survey Open-File Report 99-301

Crowley, S. S., Stanton, R. W., and Ryer, T. McNulty, J. E., Jr.,1978, Geology of the San A., 1989, The effects of volcanic ash Miguel lignite deposit (Atascosa and on the maceral and chemical McMullen Counties, Texas, in Kaiser composition of the C coal bed, Emery W. R., ed., Proceedings, Gulf Coast Coal Field, Utah: Organic lignite conference: geology, utilization, Geochemistry, v. 14, p. 315-331. and environmental aspects: The Gennett, J. A., 1985, A core from the Eocene University of Texas at Austin, Bureau San Miguel lignites, South Texas: of Economic Geology Report of palynomorphs and depositional Investigations 90, p. 79-83. environments [abs.]: American Mukhopadhyay P. K., 1986, Petrography of Association Stratigraphic selected Wilcox and Jackson Group Palynolologists. Program and lignites from the Tertiary of Texas: in Abstracts, 18th Annual Meeting. p. 12. Finkelman R. B. and Casagrande D. J., Gennett, J. A. and Raymond, A. L., 1986, eds., Geology of Gulf Coast Lignites: Changes in floral composition with Geological Society of America Coal depositional environment in Texas Geology Division Field Trip Eocene Manning Formation lignites Guidebook, Environmental and Coal [abs.]: American Association of Associates, Houston, Texas, p. 126- Petroleum Geologists Bulletin, v. 70, 145. p. 1181-1182. Mukhopadhyay, P. K., 1989, Organic Gowan, S. W., 1985, Depositional petrology and organic geochemistry of environment of the San Miguel lignite Texas Tertiary coals in relation to deposit in Atascosa and McMullen depositional environment and Counties, Texas: College Station, hydrocarbon generation: The Texas A&M University, Ph.D. University of Texas at Austin, Bureau dissertation, 199 p. of Economic Geology, Report of Kaiser, W. R., 1982, Lignite depositional Investigations 188, 118 p. models, Texas Eocene: a regional PRIMEDIA Intertec, 1999, Keystone coal approach to coal geology, in Shobert industry manual:Chicago, Illinois, H. H., ed., Proceedings of the Basic PRIMEDIA Intertec, 807 p. Coal Science Workshop, Houston, Senkayi, A. L., Ming, D. W., Dixon, J. B., and Texas, 1981: U. S. Department of Hossner, L. R., 1987, Kaolinite, Energy, Grand Forks Energy opal-CT, and clinoptilolite in altered Technology Center, p. 10-67. tuffs interbedded with lignite in the Kaiser, W. R., Ayers, W. B., Jr., and La Brie, Jackson Group, Texas: Clays & Clay L. W., 1980, Lignite resources in Minerals, v. 35, p. 281-290. Texas: The University of Texas at Austin Bureau of Economic Geology Report of Investigations 104, 52 p.

35 U.S. Geological Survey Open-File Report 99-301

Snedden, J. W., 1979, Stratigraphy and environment of deposition of the San Miguel lignite deposit, northern McMullen and southeastern Atascosa counties, Texas: College Station, Texas A&M University, M.S. thesis, 161 p. Snedden, J. W., and Kersey, D. G., 1981, Origin of San Miguel lignite deposit and associated lithofacies, Jackson Group, South Texas: American Association of Petroleum Geologists Bulletin, v. 65, p. 1099-1109. Tewalt, S. J., 1986, Chemical characterization of Texas lignite: University of Texas at Austin, Bureau of Economic Geology Geological Circular 86-1, 54 p. Tewalt, S. J., Bauer, M. A., Mathew, D., Roberts, M. P., Ayers, W. B., Jr, Barnes, J. W., and Kaiser, W. R., 1983, Estimation of coal resources in Texas Gulf Coast, Ohio Northern Appalachian, and Wyoming Powder River Basins: a comparison of statistical approaches: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations 136, 137 p. Warwick, P.D., Crowley, S.S., Ruppert, L.F., and Pontilillo, James, 1996, Petrography and geochemistry of the San Miguel lignite, Jackson Group (Eocene), South Texas: Organic Geochemistry, v. 24, no. 2 pp. 197- 217. Zielinski, R. A., 1985, Element mobility during alteration of silicic ash to kaolinite--a study of tonstein: Sedimentology, v. 32, p. 567-579.

36 U.S. Geological Survey Open-File Report 99-301

CHAPTER 4

Some speculations on coal-rank anomalies of the South Texas Gulf Province and adjacent areas of Mexico and their impact on coal-bed methane and source rock potential

By John R. SanFilipo

U.S. Geological Survey, 956 National Center, Reston, VA 20192

INTRODUCTION Btu/lb on a moist, mineral matter-free (MMmF) basis. Samples from Carbon I and II There are two known occurrences of are reported to have an average (Rm?) vitrinite bituminous coal within the generally lignitic reflectance of 0.58% (Verdigo and Arciaga, Gulf of Mexico Coal Province. One occurs in 1991). These and other rank parameters for the Olmos Formation (Maastrictian) and the Sabinas Basin coals are somewhat higher than other occurs in the lower-middle part of the those for most shallow occurrences of Upper Claiborne Group (middle Eocene, fig. 1). Cretaceous coals of the southern Rocky Both occur within the Rio Grande vicinity of Mountain Province, for example the southern south Texas and Mexico, and each can be part of the San Juan Basin of New Mexico, but considered somewhat anomalous in rank in they are consistent with rank-elevated coals comparison with typical U.S. coals of similar towards basin interiors or near igneous age, which are generally less mature. provinces on basin margins. The rank elevation of the Olmos Formation coals can CRETACEOUS BITUMINOUS COALS probably be attributed to residual heat flow from slightly younger shallow intrusive activity The Olmos Formation coals, which will of the Balcones igneous province, or much not be visited on the field trip, occur in the younger igneous activity within Mexico (fig. Eagle Pass area of Maverick County, Texas, 2). where a permit to mine is pending (Dos Republicas permit, fig. 2). This deposit TERTIARY BITUMINOUS AND extends into the Sabinas Basin of Mexico, RELATED COALS where high-volatile C bituminous coal is being mined for electrical power generation by the The second documented occurrence of Minera Carbonifera Rio Escondido (MICARE) bituminous coals in the Gulf Province is the for the Comission Federal de Electicidad Santo Tomas coal field in Webb County, (CFE) Carbon I and II projects (fig. 2). Core Texas, which will be visited on the field trip. samples of these high-ash coals from the Eagle Here the Farco Mining Company (fig. 2) is Pass area are typically about 7500 Btu/lb on an mining high-volatile A bituminous coal from as-received (AR) basis and about 12,000 the lower-middle part of the Claiborne Group

37 U.S. Geological Survey Open-File Report 99-301

SOUTH TEXAS EAST-CENTRAL TEXAS (RIO GRAND EMABAYMENT) (HOUSTON EMBAYMENT)

Whitsett upper Tj Manning

lower Wellborn

UPPER GROUP San Miguel Mine JACKSON Caddell

Yegua Ty Yegua

Cook Mtn. Tcm

Laredo Tl

Sparta Ts MIDDLE

EOCENE Weches Tw

El Pico Tep

Santo Tomas Field GROUP CLAIBORNE Tqc (Farco Mines) Queen City Bigford Tb Reklaw Tr

Carrizo Tc Carrizo TERTIARY

Calvert Bluff LOWER

Indio Tin Simsboro Twi

Hooper WILCOX GROUP WILCOX Uvalde area UPPER

upper

Kincaid Tm

PALEOCENE Tk

LOWER GROUP MIDWAY lower

Escondido Kes Marlbrook Marl

Ki

(south Texas) (south MAASTRICHTIAN

NAVARRO GROUP NAVARRO Olmos Ko Eagle Pass / MICARE Anacacho

Limestone Kac CRETACEOUS San Miguel

Ksm

EXPLANATION

TAYLOR CAMPANIAN Wilcox and above modified from Ayers and Kaiser, 1987 Symbols (e.g. Ksm) where shown for formation STRAT COLUMN SYMBOLS names refer to geologic map (fig. 2) Fluvial Strandplain/barrier-bar Carbonate OTHER MAP UNITS Bituminous coal Ti Tertiary intrusive rocks Fluvial-deltaic Lagoonal Lignite

Qb Quaternary basalt sand Deltaic (fluvial - Marine/prodelta shale Upper Cretaceous undivided dominated) Ku (includes Olmos in places) mud

Delataic (wave- Intrusive rock of the Lower Cretaceous undivided dominated) Marl Ki Kl Balcones Province Figure 1. Generalized columnar section for south Texas coal areas and adjacent areas of Mexico.

38 U.S. Geological Survey Open-File Report 99-301

101 100 99

Balcones Igneous Province Kac Ki Ki Ki Kes Tm Uvalde Ki Ku Ki Ki U.S. Twi Kac Tc MEXICO Ksm Tr Tin 29 Kl Dos Tqc Republicas

Permit Ksm Tb Tw Ts MICARE Kol LIGNITE Tcm Mine Eagle Pass Tep

Blocks Kes Tl

CFE Tk Tc SUBBITUMINOUS Power BITUMINOUS Plant

Kol

Tin Ty

Tep

Tb

28

Ku Qb FARCO MINING CO. Tj Qb

Qb Laredo

Ku

N INDEX MAP

Tk

27

TX Ti Geology modified from published maps of the Texas Bureau of Economic Geology and the Govt. of Mexico.

Ti 30 mi Kl 0 50 km

Kl

Figure 2. Regional geology and location of bituminous coal deposits of south Texas and adjacent areas of Mexico. See figure 1 for map units. Also shown are speculative boundaries of coal-rank changes.

39 U.S. Geological Survey Open-File Report 99-301

for use in local cement plants and for export as Maxwell's table 30, which is lignite on an lump coal for use in the Republic of Ireland. MMmF basis, is clearly weathered, and some The coal occurs in two primary zones, the San other samples from his tables appear to have Pedro and Santo Tomas zones, near the lost moisture; normalizing to 15% moisture ill-defined contact of the Bigford and overlying consistent with table 32 results in an apparent El Pico Formations, about 3,000 ft rank of subbituminous B to high-volatile C stratigraphically higher than the Olmos bituminous. Proprietary analytical data Formation (fig. 1). Speculations on the cause recently made available to the USGS for the of the relatively high rank of these coals are National Coal Resource Assessment supports more problematic than those for the Olmos these determinations. Together they indicate coals. Because these coals occur higher in the that on the basis of heating value, the Wilcox section than Wilcox Group coals from the coals range from subbituminous B just east of same general area, which have traditionally Uvalde to high-volatile bituminous A towards been described as lignite (Maxwell, 1962, the west, although even the most mature of Breyer and McCabe, 1986), depth of burial these coals is generally described as lignite in would presumably not account for the rank company drilling records. An average elevation. Numerous lignite beds have also reflectance value of 0.37% (Rm; range been described as intercalated within the San 0.30-0.45%) reported by Breyer and McCabe Pedro – Santo Tomas canneloid coals (1986) from an area just east of Uvalde is themselves (Lonsdale and Day, 1937), which somewhat equivocal, but consistent with would also presumably rule out depth of burial subbituminous coals of the Powder River or local thermal gradients as the cause of rank Basin of Wyoming (R.W. Stanton, USGS, elevation. personal communication). These values can be Based on these considerations, it compared with some previously unpublished would be logical to assume that the highly rank parameters and standard analysis of the sapropelic nature and enriched liptinite content Santo Tomas and San Pedro coal zones and (reported to be 23% in Warwick and Hook, typical Wilcox lignites from the Sandow mine 1995) of these coals could account for their near Austin, approximately 250 mi northeast anomalously high heating value (~12,000 of the Santo Tomas field, in figures 3 and 4. Btu/lb AR, 14,000 MMmF, figs. 3 and 4), To the author's knowledge, no without an added thermal factor. A published analysis of the intercalated "lignites" re-examination of nearby Wilcox coals of the Bigford-El Pico interval exist. One suggests, however, that a much wider area and highly weathered sample of a banded, stratigraphic range of South Texas has been somewhat splintery, stray bed that occurs subjected to elevation in rank, probably by within the San Pedro interval about 40 ft local heat flow, than previously thought. above the main bed at the Farco Trevino Mine Recalculating the Wilcox coal quality has been analyzed by the USGS with equivocal parameters for the Uvalde area (fig. 2) from results (sample SP-UN-96-1, figs. 3 and 4). Maxwell (1962, tables 30, 31, and 32) gives a On a Btu basis, this coal would seem to be range of MMmF Btu/lb from 10,494 to classified as lignite; however, an average Rmax 11,893, (i.e. subbituminous A to high-volatile of 0.74% and extremely high C bituminous). Note that sample no. 1-A from

40 U.S. Geological Survey Open-File Report 99-301

USGS PROXIMATE ANALYSIS BTU % Sulfur ULTIMATE ANALYSIS ATOMIC RATIOS SAMPLE NO. (as-rec'd) (as-rec'd) (as-rec'd) (dry, ash-free) (dry basis)

ST-UN-96-1 NA (pollen sample)

%Moist %Ash %VM %FC %H %C %N %S %O H/C O/C C/N C/S

12,000 14,000

10.5 ft 10,000

2,000 4,000 6,000 8,000

100

100

0.5 1.0

1.5 2.0 2.5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

0 0

5

0

0 5

STR2-96-1 42.82 32.88 21.40 7739 0.90 6.58 77.98 12.34 1.00 0.12 63 126

2.90 1.45 1.65

4.7 ft 1.21 3.49

27.72 37.81

ST2-96-3 6.97 27.50 8622 2.28 6.32 72.03 16.95 1.04 0.18 69 55 1.22 48.42 35.18 12151 79.71 1.38 0.97 0.11 67 174 ST2-96-2 5.07 1.02 6.49 11.20

RACHAL MINE (Santo Tomas Zone) Tomas (Santo MINE RACHAL 11.33

ST2-96-1 10.38 51.73 22.50 15.39 4678 0.55 6.47 70.37 20.60 1.09 0.22 74 130 1.11 1.45 ~ 40 ft 2.18 3.35

SP-UN-96-1 26.56 32.62 26.51 14.31 3449 1.37 3.20 61.38 29.89 0.62 0.37 33 49 M. CLAIBORNE CLAIBORNE M.

~ 40 ft

1.64 1.02 2.22 SP-3-96-2 13.32 51.61 32.85 12730 0.86 6.69 80.36 10.29 0.99 0.10 57 210

SP-3-96-1 19.70 45.09 32.02 11499 0.99 6.66 80.14 10.13 0.99 0.09 52 167

3.19 1.28

1.79 TREVINO MINE (San Pedro Zone) Pedro (San MINE TREVINO 1 ft ~ 2,000 ft 0.5 0

? 1.43

AL-H2-6-1 34.35 14.38 27.12 24.15 6497 1.53 5.41 73.46 16.72 0.88 0.17 60 66 2.98

NA (pollen sample)

AL-H2-P

WILCOX

SANDOW MINE (lower Calvert Bluff Fm) Bluff Calvert (lower MINE SANDOW 2.03

AL-H2-8-1 33.87 9.90 29.60 26.63 7284 1.14 5.54 75.13 15.89 0.88 0.16 62 99 1.41

Canneloid Dirty coal Carb Lignite Siltstone Claystone coal (> 25% ash, shale d.b.) Figure 3. Standard analysis and elemental ratios for selected samples from the Santo Tomas coal field (middle Claiborne) and Sandow Mine (Wilcox).

41 U.S. Geological Survey Open-File Report 99-301 volatile-matter content imply a rank more proposed, as shown in figure 2. Any such similar to the canneloid coals. Other revisions are speculative, however, without maturation parameters (apparent specific analysis of fresh samples of the intercalated gravity, residual moisture, agglomerating stray beds in the El Pico – Bigford interval and character, etc.) are more consistent with additional reflectance values and analyses from Wilcox lignite, although the weathered nature the Wilcox, Jackson and Yegua intervals (figs. of the sample is readily apparent form the high 1 and 2). oxygen content, and none of these results can Coal-bed methane (CBM) shows have be considered definitive. The higher been recorded in some gas wells in Maverick reflectance value for the stray bed versus the and Dimmit counties, and a number of gas main San Pedro – Santo Tomas beds as shown fields occurring in the general area produce in figure 4 may be attributable to from Olmos and Wilcox sandstones (see weathering-related diminution of the vitrinite Ewing this volume). The extent of the suppression typical in canneloid coals. This coal-rank variation within South Texas can sample fluoresces when examined under blue therefore be considered particularly important light and shows algal cell structures similar to from the consideration of the CBM and the San Pedro and Santo Tomas canneloid conventional source-rock potential. The H/C coals (P.D. Warwick, USGS, personal and O/C ratios from figures 3 and 4 are plotted communication). on a Van Krevelen diagram in figure 5, along with data for Olmos Formation coals derived EXTENT OF THE COAL-RANK from various sources (Evans,1974, table 5; ANOMALY AND ITS POSSIBLE Miller and others, 1998; and unpublished IMPACT ON HYDROCARBON USGS files). A few examples of Jackson GENERATION Group coals, which are generally not discussed in this paper, but will be seen on the field trip, As previously stated, it now appears are also shown for completeness; these are that a much wider area of South Texas has from unpublished USGS sampling of the San been subjected to coal-rank elevation than has Miguel Mine. As to be expected, the Wilcox, previously been recognized. Early versions of Olmos, and Jackson coals plot within the type the "Coal Fields of the United States" map III kerogen path, and the less humic Santo (e.g. Campbell, 1908, through Trumball, 1960) Tomas and San Pedro coal zones plot closer to showed small pods of bituminous coal within the type II field. The Santo Tomas coals, the overall lignite trend, and no intervening which appear slightly more oil prone on the subbituminous halos. The area of diagram, yielded 52 gal/ton oil and 5672 SCF hypothesized rank elevation was slightly per ton gas from low-temperature distillation expanded for a preliminary revision to this map done by the U.S. Bureau of Mines (Ashley, (Tully, 1996), without the benefit of most of 1919). Rock-Eval pyrolysis for one Santo the new data or rank recalculations cited Tomas sample by Mukhopadhyay (1989, fig. herein. Based on the information currently 28a), which yielded 66% total organic carbon available, additional revisions to the map that and very high and low hydrogen and oxygen will show a much wider and more graduated indices respectively, placed these coals directly area of rank elevation in South Texas are being on the type II kerogen field.

42 U.S. Geological Survey Open-File Report 99-301

USGS % Volatile % Fixed Apparent Sp. Gr.* % Residual Reflectance, Free-swelling BTU (MMmF) SAMPLE NO. matter carbon moisture Avg. Rmax (oil) Index (as-rec'd)

ST-UN-96-1 NA (pollen sample)

10,000 16,000 12,000

10.5 ft 14,000

2000 4000 6000

8000

1.2 1.3 1.4 1.5 1.6 1.7 1.8

1.0 1.1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

50 52 54 56 58 60 64

62 66 33 35 39 41 43 45 47 49

37

0 0

1.13 1.0 STR2-96-1 14,446 60.57 39.43 1.54 1.38

4.7 ft

12,365 57.90 42.10 1.20

ST2-96-3 1.43 2.13 0.5

.30 .38

1.16 .46 .54 .62 .70 .78 .86 .22 1.0 ST2-96-2 13,876 57.91 42.09 1.24 2.14

RACHAL MINE (Santo Tomas Zone) Tomas (Santo MINE RACHAL 0.53**

1.12 0 ST2-96-1 10,611 59.38 40.62 1.68 2.06

~ 40 ft

SP-UN-96-1 0

5,281 64.93 35.07 1.63 1.77 9.26 0.74 M. CLAIBORNE CLAIBORNE M.

~40 ft

1.12 1.0 SP-3-96-2 14,901 61.11 38.89 1.22 1.39 0.51**

SP-3-96-1 14,645 58.47 41.53 1.11 1.26 1.32

TREVINO MINE (San Pedro Zone) Pedro (San MINE TREVINO 1 ft *calculated from as-rec'd values using **approx. interval, sampled in 1994 an assumed ash density of 3.0 g/cc 0.5 ~ 2,000 ft ***modified from Rm in similar interval, except as noted, all are dry, ash-free adjacent pit, Mukhopadhyay, 1989 0 basis

?

AL-H2-6-1 7,677 52.89 47.11 1.35 8.51 0.39*** 0

NA (pollen sample)

AL-H2-P

WILCOX SANDOW MINE (lower Calvert Bluff Fm) Bluff Calvert (lower MINE SANDOW

1.29 0 AL-H2-8-1

8,149 52.65 47.35 8.84 0.33*** 1.32

Dirty coal Canneloid coal Lignite (> 25% ash, Carb shale Siltstone Claystone d.b.) Figure 4. Selected rank parameters for selected samples from the Santo Tomas coal field (middle Claiborne) and Sandow Mine (Wilcox).

43 U.S. Geological Survey Open-File Report 99-301

The remarkable ease of ignition that enables select "typical" samples with relatively similar these coals to be exported in considerable ash values from the various data sets, with the quantities for domestic use is further evidence exception of the "old" samples, where the of their oil-generating potential. Most of the widest range is shown. The two San Miguel Olmos data shown on figure 5 approaches the samples selected for the diagram are probably lower limits of hydrocarbon generating slightly atypical, in that they are at the higher maturity. In addition to being somewhat end of the recorded heating values for USGS oil-prone in composition, these coals may lack samples from that mine. These and the the permeability to adsorb gas or produce Sandow samples shown on the diagram fall in CBM commercially. the relatively less mature end of the humic The Olmos Formation data shown on pathway as expected. All of the oxygen values figure 5 is generally consistent with that of a used for figure 5 are by difference rather than somewhat more mature humic coal than the direct measurement, with the possible but other samples. Available coal-quality data for unlikely exception of the "old Eagle Pass" the Olmos Formation is, however, scarce and samples. somewhat questionable. The data shown on No elemental data is currently available figure 5 from the MICARE mine samples to the USGS for Wilcox, Yegua, or Jackson (Miller and others, 1998) shows consistently coals from the higher rank areas shown in lower oxygen values than the other Olmos figure 2. Based on their location, at least some samples, and thus more apparent maturation, of the reported CBM shows in the area are placing them well beyond fluid hydrocarbon without question from Olmos Formation coals. generation thresholds. While this conceivably It appears likely that a considerable area of could reflect increasing rank towards local Wilcox coals may also have reached thermal- igneous activity, it seems more probable that gas-generating maturity in the South Texas this is an analytical bias. Other data from the area. Although highly speculative, based on MICARE mines (Verdigo and Arciaga, 1991, the extent of Tertiary volcanism in nearby table 2) show higher average oxygen values, areas of Mexico, it appears that Yegua and but the reporting basis is not clear. The Dos Jackson coals could have been similarly Republicas sample analysis appears to be elevated in rank as far south as Falcon reliable, but these and other recent analyses Reservoir (about 90 mi southeast of Laredo, from the Eagle Pass area show a very wide fig. 2). In a detailed study of the petrology range of ash, and corresponding elemental and organic geochemistry of Texas coals, values where available, that may be Mukhopadhyay (1989) suggested that many attributable to sampling techniques. The "old Texas coals are enriched in liptinite relative to Eagle Pass" samples on figure 5 came from a other coals of similar rank and may have variety of mine sources sampled before 1912 generated liquid hydrocarbons earlier on the (see Evans, 1974), including unwashed and maturation pathways than is normally washed samples, with some apparent drying, expected. Although very few of his samples and possibly weathered. The available data on are from South Texas, where the maturation heating value for the MICARE coals is also potential would seem highest, he suggests (p. inconclusive in this regard. The Olmos data 102) that some of the oil produced from the shown on figure 5 represent an attempt to

44 U.S. Geological Survey Open-File Report 99-301

Jackson (SanMiguel) Type I Kerogen Claiborne (Santo Tomas)

1.5 Wilcox (Sandow)

Type II Olmos (old Eagle Pass) Olmos (Dos Republicas) Olmos (MICARE) 1.0

Type III Atomic H/C Atomic

0.5

Arrows represent mean maturation paths

0 0.1 0.2 0.3 0.4 Atomic O/C Figure 5. Van Krevelen-type diagram for various coal types of South Texas and vicinity.

Wilcox and Claiborne intervals of South and coal-bearing facies of the Olmos Formation is East Texas may be sourced in the Wilcox. mapped with an eastward pinch-out into One potential constraint on commercial marine rocks a few miles southwest of Uvalde CBM viability in South Texas may be the (fig. 2), anecdotal evidence presented in thickness of the coal beds, which for individual Maxwell (1962, p. 90) suggests that beds is generally less than ten feet thick. Cretaceous coal-bearing rocks may be present There are multiple beds in each of the somewhat farther east and north. coal-bearing intervals, however, and in general coal thicknesses beyond shallow (i.e. mineable) CONCLUSIONS depths have not been thoroughly investigated. It should be noted that nothing in this paper Available information based on limited addresses the potential for early or late and poorly controlled sampling and analysis biogenic gas occurrences in Texas coals, which suggests that the coal rank anomalies should be considerable based on recent activity recognized in the Olmos Formation and middle in the Powder River Basin of Wyoming. It is Claiborne Group of South Texas and adjacent also worth noting that although the areas of Mexico may extend to a considerably

45 U.S. Geological Survey Open-File Report 99-301 wider area and stratigraphic range. Additional REFERENCES CITED sampling and analysis of the entire interval from the Upper Cretaceous Olmos Formation Ashley, G. H., 1919, The Santo Tomas cannel through the upper Eocene Jackson Group coal, Webb County, Texas: U.S. would be useful in refining the known extent Geological Survey Bulletin 659, p. of rank variation. Coal-bed desorption and 1-127. drill-stem or more sophisticated down-hole Ayers, W. B., Jr., and Kaiser, W.R., 1987, testing would help determine the methane- Regional depositional setting o, producing potential of these coals. Biomarker resources, and quality of lignite in the and stable isotope studies of hydrocarbons Wilcox Group of east Texas and the produced from more conventional reservoirs in Jackson Group of east and South the area may also provide useful information Texas, in Finkelman, R.B., on the source-rock potential of these coals. Casagrande, D.J., and Benson, S.A., eds., Gulf Coast lignite geology, ACKNOWLEDGMENTS Prepared for the fourteenth biennial lignite symposium on the technology The author wishes to thank the staff of and utilization of low-rank coals: the Farco Mining Company and the Alcoa Environmental and Coal Associates, Sandow Mine for their permission to visit and Reston, Va., p. 83 – 128. sample their mines, and the USGS staff who Breyer, J.A., and McCabe, P.J., 1986, Coals collectively participated with me in the associated with tidal sediments in the sampling used for this report: Peter Warwick, Wilcox Group (Paleogene), South Robert Hook, and Sharon Crowley. Texas: Journal of Sedimentary Petrology, v. 56, no. 4, p. 510-519. Cambell, M. R., 1908, Coal fields of the United States: U.S. Geological Survey, scale1:7,000,000. Evans, T.J., 1974, Bituminous Coal in Texas: Bureau of Economic Geology, University of Texas at Austin, Handbook 4, 65 p., 1 pl.. Lonsdale, J.T., and Day, J.R., 1937, Geology and groundwater of Webb County, Texas: U.S. Geological Survey Water Supply Paper 778, p. 1-104. Maxwell, R. A., 1962, Mineral resources of South Texas, region served through the Port of Corpus Christi: Bureau of Economic Geology, University of Texas at Austin, Report of Inv. No. 43, 140 p., 5 pl., 44 tables.

46 U.S. Geological Survey Open-File Report 99-301

Miller, J.D., Parga, J.R., Drelich, Jaroslaw, and Lin, C.L., [1998?], Coal cleaning

opportunities for SO2 emission reduction in the border region, Final Report to the U.S. Southwest Center . for Environmental Research and Policy: University of Utah, Salt Lake, and Institute of Technology, Saltillo, Mexico, 37 p. Mukhopadhyay, P.K., 1989, Organic petrology and organic geochemistry of Texas Tertiary coals in relation to depositional environment and hydrocarbon generation: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations 188, 118 p. Trumball, James, 1960, Coal fields of the United States: U.S. Geological Survey, sheet 1 of 2, scale1:5,000,000. Tully, John, digital compiler, 1996, Coal fields of the conterminous United States: U.S. Geological Survey Open-File Report 96-92, 1:5,000,000. Verdigo, F.D., and Arciaga, M., 1991, The Fuentes-Rio Escondido coal basin, Coahuila, in Salas, G. P., ed., Economic Geology, Mexico: Boulder, The Geological Society of America (Decade of North American Geology), Geology of North America, V.P-3, p. 107-128. Warwick, P.D., and Hook, R.W., 1995, Petrography, geochemistry, and depositional setting of the San Pedro and Santo Tomas coal zones: anomalous algae-rich coals in the middle part of the Claiborne Group (Eocene) of Webb County, Texas: International Journal of Coal Geology 28, p. 303-342.

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