Chapter 4 Petroleum Assessment of the Pottsville Coal Total Petroleum System, Black Warrior Basin, Alabama and Mississippi
By Joseph R. Hatch and Mark J. Pawlewicz Volume Title Page
Chapter 4 of Geologic Assessment of Undiscovered Oil and Gas Resources of the Black Warrior Basin Province, Alabama and Mississippi
Compiled by Joseph R. Hatch and Mark J. Pawlewicz
U.S. Geological Survey Digital Data Series DDS–69–I
U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Myers, Director
U.S. Geological Survey, Reston, Virginia: 2007
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Suggested citation: Hatch, Joseph, R., and Pawlewicz, Mark J., 2007, Petroleum assessment of the Pottsville Coal Total Petroleum System, Black War- rior Basin, Alabama and Mississippi, in Hatch, Joseph R., and Pawlewicz, Mark J., compilers, Geologic assessment of undiscovered oil and gas resources of the Black Warrior Basin Province, Alabama and Mississippi: U.S. Geological Survey Digital Data Series DDS–69–I, chap. 4, 28 p.
ISBN 1-411-31775-0 iii
Contents
Overview...... 1 Geologic Setting...... 2 Stratigraphic and Sedimentological Framework...... 2 Structural Framework...... 5 Appalachian Folds and Thrust Faults...... 5 Pottsville Formation Fracture Systems...... 8 Hydrocarbon Source-Rock Characterization...... 12 Organic Carbon Contents—Organic Facies...... 12 Organic Matter Thermal Maturity...... 12 Thermal Maturity Modeling...... 12 Produced Water Chemistry...... 14 Natural Gas Chemistry...... 15 Black Warrior Basin Assessment Unit...... 18 Gas and Water Production ...... 18 Historical Production ...... 18 Drilling Practices...... 20 Cumulative Gas and Water Production ...... 20 Well Production Controls...... 21 Assessment Results...... 24 References Cited...... 24
Figures
1. Map showing location of the Pottsville Coal Total Petroleum System and the Black Warrior Basin Assessment Unit...... 1 2. Petroleum system events chart...... 2 3. Generalized elevation contour map of the unconformity separating the Pennsylvanian Pottsville Formation and Upper Cretaceous strata in the southeastern part of the Black Warrior Basin...... 3 4. Stratigraphy of the upper part of the Pennsylvanian Pottsville Formation...... 4 5. Cross section showing idealized depositional cycle in the Pennsylvanian Pottsville Formation, Black Warrior Basin… ………………………………………………………… 5 6. Generalized isopach map of the Ream through Gwin coal zones, Pennsylvanian Pottsville Formation...... 7 7. Generalized map showing net coal isoliths in the upper part of the Pennsylvanian Pottsville Formation in the southeastern part of Black Warrior Basin...... 7 8. Generalized structural contour map of the top of the Pratt coal zone...... 8 iv
9. Photograph of a small normal fault and associated fracturing...... 9 10. Photograph of jointed shale above the Gwin coal zone...... 9 11. Photograph of a bedding plane exposure of a cleat system in Alabama coal...... 10 12. Generalized map showing joint (A), and cleat (B) systems in the southeastern part of the Black Warrior Basin...... 11 13. Stacked bar histogram showing distribution of hydrogen indices for 16 shale samples and 23 coal samples from the Pennsylvanian Pottsville Formation...... 13
14. Generalized map showing vitrinite reflectance (Ro) and rank of coals in the Mary Lee coal zone, Pennsylvanian Pottsville Formation...... 13 15. Plot showing relation between burial history and thermal gas generation in the Pennsylvanian Pottsville Formation in the Shell Holiman #13-16 well in Pickens County, Alabama...... 14 16. Stiff diagrams showing composition of formation water produced from the Pennsylvanian Pottsville Formation in the southeastern part of the Black Warrior Basin...... 15 17. Generalized map showing relation of freshwater plumes in the Mary Lee coal zone, Pennsylvanian Pottsville Formation, to geologic structure and Upper Cretaceous strata, along the southeast margin of the Black Warrior Basin...... 16 18. Generalized structural cross section showing meteoric recharge areas and subsurface flow patterns in the upper part of the Pennsylvanian Pottsville Formation...... 16 19. Generalized map showing distribution of gas wetness for samples of coal gas from the Pennsylvanian Pottsville Formation in the southeastern part of the Black Warrior Basin...... 17 20. Generalized map showing locations and status of coal degasification fields in the southeastern part of the Black Warrior Basin...... 19 21. Histograms of coal-gas and water production (A) and new well permits and producing wells (B) in the southeastern part of the Black Warrior Basin...... 19 22. Generalized map showing cumulative gas production from vertical wells in the southeastern part of the Black Warrior Basin...... 20 23. Generalized map showing cumulative water production from vertical wells in the southeastern part of the Black Warrior Basin...... 21 24. Generalized map showing alignment of exceptionally productive gas wells along a synclinal fold hinge in the Oak Grove coal degasification field in the southeastern part of the Black Warrior Basin...... 22 25. Generalized map showing relation of gas (A) and water (B) production to extensional structure in the Deerlick Creek coal degasification field in the southeastern part of the Black Warrior Basin...... 23 26. Graph showing distribution of estimated ultimate recoveries (EURs) for gas wells within the Black Warrior Basin Assessment Unit, Black Warrior Basin...... 25 27. Graph showing distribution of estimated ultimate recoveries (EURs) by thirds for gas wells within the Black Warrior Basin Assessment Unit, Black Warrior Basin...... 25
Plate
1. Stratigraphic cross sections for: (A) Black Creek and Ream coal zones; (B) Mary Lee coal zone; (C) Gillespy, Curry and Pratt coal zones; and (D) Cobb and Gwin coal zones, Pennsylvanian Pottsville Formation, in the southeastern part of the Black Warrior Basin, Alabama ...... 6
Tables
1. Ranges and median values of organic carbon contents, and hydrogen indices for coal and shale samples from the Pennsylvanian Pottsville Formation in the southeastern part of the Black Warrior Basin...... 12 13 2. Ranges and median values for gas wetness, N2, and CO2 contents, and d C1 for gases from coals in the Pennsylvanian Pottsville Formation, southeastern part of the Black Warrior Basin...... 17 3. Cumulative gas and water production data for the ten most productive coal degasification fields in the Black Warrior Basin...... 18 Petroleum Assessment of the Pottsville Coal Total Petroleum System, Black Warrior Basin, Alabama and Mississippi
By Joseph R. Hatch and Mark J. Pawlewicz
Overview unit, the Black Warrior Basin Assessment Unit (AU), which has the same boundaries. The Pottsville Coal Total Petroleum System (TPS) in Historical production data and the relation of this the Black Warrior Basin of Alabama and Mississippi (fig. production to the basin’s geologic framework were used to 1) produces natural gas from coals in the Lower to Middle estimate volumes of gas to be discovered in undrilled areas. Pennsylvanian Pottsville Formation. The areal extent of the Geologic framework factors, considered in the estimation Pottsville Coal TPS (about 7,100 mi2, fig. 1) is defined by of undiscovered hydrocarbon resources of the Pottsville the occurrence of coal in the upper part of the formation. The Coal TPS and the Black Warrior Basin AU, are shown in the Pottsville Coal TPS includes one continuous gas assessment petroleum system events chart in figure 2.
² ² ² ² ² ²
TENNESSEE ²
MISSISSIPPI ALABAMA Prentiss Colbert Tishomingo Union Lawrence Franklin Morgan Panola Lafayette Lee
Pontotoc Itawamba Quitman Marion Winston Cullman ² Yalobusha Tallahatchie Calhoun Chickasaw Monroe Grenada Lamar Walker Fayette Clay Montgomery Webster Jefferson Leflore Carroll Oktibbeha Lowndes Choctaw Pickens Tuscaloosa
Noxubee ² Attala Winston 4%..%33%% Greene ",!#+ Kemper Hale 7!22)/2 POTTSVILLE COAL TPS "!3). AND Sumter BLACK WARRIOR BASIN AU -)33)33)00) !,!"!-!
-),%3
² +),/-%4%23
Figure 1. Location of the Pottsville Coal Total Petroleum System (TPS) and the Black Warrior Basin Assessment Unit (AU) in the Black Warrior Basin Province of northwestern Alabama and northeastern Mississippi. Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
100
500
200
400
300
150
250
350
550
450
50 Geologic Time
Pennsylvanian
Mississippian Mississippian
Cretaceous
Ordovician
Proterozoic Paleogene Scale
Devonian
Neogene
Cambrian
Jurassic
Archean
Permian
Silurian Silurian
Triassic 2,500 (MYBP)
145
251
200
488
444
416
542
318
299
359
65
23 Petroleum
M & U Systems
Paleo.
Mio. Olig. Plio.
Eoc.
M
M
M
M
U
U
U
M
U U
U
L U
L U
U
L L L
L
L L L L Events
POTTSVILLE FM. DEPOSITION SOURCE ROCK RESERVOIR ROCK SEAL ROCK ? ? OVERBURDEN ROCK Water block TRAP FORMATION Biogenic gas ? Biogenic gas HYDROCARBON GENERATION Thermogenic gas HYDROCARBON MIGRATION HYDROCARBON ACCUMULATION HYDROCARBON PRESERVATION CRITICAL MOMENT
Figure 2. Petroleum system events chart showing interpreted timing of elements and processes related to hydrocarbon generation and accumulation for the Pottsville Coal Total Petroleum System and the Black Warrior Basin Assessment Unit, Black Warrior Basin, Alabama and Mississippi. Modified from Magoon and Dow (1994). MYBP, million years before present; Paleo., Paleocene; Eoc., Eocene; Olig., Oligocene; Mio., Miocene; Plio., Pliocene.
Source rocks for the gas accumulations in the Pottsville the AU to be 4.61 TCFG at the 95-percent certainty level, 6.85 Coal TPS and the Black Warrior Basin AU are the Pottsville TCFG at the 50-percent certainty level, 10.18 TCFG at the 5- Formation coals, which also form the principal reservoirs. percent certainty level, and a mean resource estimate of 7.06 Coal gas was generated primarily through thermal maturation TCFG. of coal during the late Paleozoic. Coal gases are characterized by having low gas wetness (median = 0.01 percent), low CO2 and N2 contents (medians = 0.08 percent and <0.01 percent, 13 respectively), and relatively negative δ C1 (median = –47.4 parts per thousand PDB), the latter indicating that some of the Geologic Setting gas formed from a late-stage microbial process. Volumes of gas and water produced are dependent on well design and completion practices, well spacing, coal Stratigraphic and Sedimentological Framework thickness, gas content (thermal maturity and coal chemical composition), and reservoir quality, which are determined by the abundance and openness of natural fractures, including The Lower to Middle Pennsylvanian Pottsville Formation cleats, joints, and fault-related shear fractures. Virtually all is composed principally of shale, sandstone, and coal; coal-gas production from the Pottsville Formation is from thicknesses of the formation locally exceed 6,000 ft (Thomas, high-volatile A bituminous or higher rank coals, which occur 1988). The formation is exposed at the surface in the eastern in areas of Tuscaloosa, Jefferson, and Walker Counties, one-third of the Black Warrior Basin and is overlain with Alabama, in the southeastern part of the Black Warrior Basin angular unconformity by poorly consolidated Mesozoic and AU. Cenozoic strata of the Gulf Coastal Plain and Mississippi Through the first quarter of 2000, more than 3,500 gas Embayment in the western two-thirds of the area (fig. 3) wells had been completed within the Black Warrior Basin (Mellen, 1947). Economically important coals, primarily in the AU, of which about 3,200 were still producing. Cumulative upper part of the formation, are assigned to several widespread production through January 2003 was about 1.42 trillion “coal zones” (McCalley, 1900). In ascending order, these are cubic feet of gas (TCFG). Our resource analysis resulted in the Black Creek, Ream, Mary Lee, Gillespy, Curry, Pratt, estimated volumes of potential additions to reserves within Cobb, Gwin, Utley, and Brookwood “coal zones” (fig. 4). Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province
7!,+%2
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!LABAMA 453#!,//3! %XPLANATION 5PPER #RETACEOUS 0OTTSVILLE &ORMATION
#ONTOUR INTERVAL FT '2%%.% MI KM (!,%
Figure 3. Generalized elevation contour map of the unconformity separating the Pennsylvanian Pottsville Formation and Upper Cretaceous strata in the southeastern part of the Black Warrior Basin, Alabama. Modified from Kidd (1976) and Pashin and others (2004, their fig. 25).
Butts (1926) was the first to recognize the evidence for contains burrows and scattered shells. Sandstone varies from repeated marine transgressions and regressions during Pottsville litharenite containing low-grade metamorphic rock fragments deposition. Wanless (1976) made passing mention of cyclicity to quartzarenite (Mack and others, 1983). The litharenite is in the Pottsville Formation, but it was not until the intensive dark and impermeable, whereas the lighter colored, quartzose exploration for conventional hydrocarbons and coal-bed gas in sandstone is permeable and of reservoir quality (Pashin and the 1980s that basinwide depositional cycles were confirmed others, 1991). In places, the litharenite is more than 150 ft thick (Sestak, 1984; Thomas, 1988; Pashin and others, 1991; Pashin, and includes progradational foresets and a variety of channel 1994a, b). The first cyclostratigraphic subdivision of the upper fills. Each cycle is capped by a lithologically heterogeneous part of the formation was made by Pashin and others (1991), coal zone containing one or more coals of variable geometry who defined a series of basinwide coarsening-upward and and thickness that are intercalated with mudstone and sandstone coaling-upward cycles. Pashin (1994a, 1998) and Pashin and (Pashin and others, 2004). others (2004) suggested that high-frequency glacial eustasy in The individual cycles are interpreted as flooding-surface- the Milankovitch long eccentricity band (about 0.4 Ma) was the bounded parasequences, in the terminology of Vail (1987). dominant causal mechanism of the depositional cyclicity. The basal surfaces are transgressive surfaces of erosion, or Pottsville Formation cycles are internally heterogeneous, ravinements, and the associated fossil concentrations are in consisting primarily of mudstones and sandstones (Pashin, condensed sections that rest directly on the ravinement surfaces 1994b, 1998) (fig. 5). The basal surface of each cycle is (Liu and Gastaldo, 1992; Pashin, 1998) (fig. 5). The coarsening- typically sharp and locally truncates strata of the underlying upward mudstone intervals above the fossil concentrations cycle; marine fossils are concentrated above this surface and litharenite foreset beds record deltaic progradation during (fig. 5). The mudstone above the fossil concentrations is highstand (for example, see Gastaldo and others, 1993), and typically between 30 and 300 ft thick, coarsens upward, and each mudstone unit apparently rests on a maximum flooding Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
,ITHOLOGY 'AMMA RAY $ENSITY ,ITHOLOGY $EPTH !0) GCC SS SH COAL $EPTH SS FT F M C CGL SH COAL FT F M C CGL &3 'AMMA RAY $ENSITY !0) GCC 0RATT COAL 'UIDE COAL #ORONA COAL "ROOKWOOD "ROOKWOOD COAL -ILLDALE COAL 0RATT .ICKEL 0LATE COAL ZONE #ARTER COAL COAL ZONE *OHNSON COAL COAL &3 !MERICAN COAL
&3 #URRY COAL #URRY COAL ZONE 5TLEY &3 COAL ZONE 'ILLESPY COAL
'ILLESPY COAL ZONE &3 &3 'WIN COAL .EW #ASTLE COAL 'WIN 4HOMPSON COAL ZONE -ILL COAL -ARY ,EE COAL -ARY ,EE "LUE #REEK COAL COAL ZONE *AGGER COAL
&3
#OBB COAL 4HOMAS COAL &3 #OBB 2EAM COAL ZONE COAL ZONE &3
,ICK #REEK COAL
&3 *EFFERSON COAL %80,!.!4)/. "LACK #REEK "LACK #REEK 2OCK TYPES 3TRUCTURES COAL ZONE COAL -UDSTONE -ARINE FOSSILS )NTERBEDDED MUDSTONE "URROWS AND SANDSTONE 0LANT FOSSILS ROOT TRACES 3ANDSTONE LITHARENITE 0EBBLES &3 -ARINE FLOODING SURFACE #OAL
Figure 4. Stratigraphy of the upper part of the Pennsylvanian Pottsville Formation, as shown in core and geophysical well logs in the Curry 12–10–370 well, Tuscaloosa County, Alabama. Modified from Pashin and Hinkle (1997, their fig. 3) and Pashin and others (2004, their fig. 5); coal zone and coal names are from Pashin and Hinkle (1997, their fig. 3). g/cc, grams/cubic centimeter; sh, shale; ss, sand- stone; f, fine; m, medium; c, coarse; cgl, conglomerate. Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province
Southeast TSE
Alluvial plain
Ravinement
Northwest Fluvial system Estuarine Constructive deltaic system Delta front deltaic system MFS TSE CS TSE TST Tidal sand banks, beaches Ravinement Shell bed LSE HST Prodelta CS Condensed section CS EXPLANATION MFS Maximum flooding surface TSE TSE Transgressive surface Gray shale Shells of erosion Porous sandstone LSE Lowstand surface Burrows of erosion 150 Tight sandstone Plants feet Approximate scale HST Highstand systems tract Coal TST Transgressive systems tract 10 miles Figure 5. Illustration showing an idealized depositional cycle in the Pennsylvanian Pottsville Formation, Black Warrior Basin, Alabama. Modified from Pashin (1994b, 1998) and Pashin and others (2004, their fig. 6). surface (fig. 5). Pottsville coal zones accumulated in diverse thickness exceeds 40 ft in much of the Moundville–Cedar coastal-plain settings from highstand into the early stages of Cove depocenter (see fig. 6) in northern Hale and southern marine regression (Pashin, 1998). The associated sandstone Tuscaloosa Counties, Alabama. In some localized areas in units include a broad range of fluvial and tidal-flat deposits, northern Hale County, net coal thickness exceeds 70 ft (Pashin and the mudstone units represent flood-basin and mudflat and others, 2004). environments (Rheams and Benson, 1982; Pashin and others, 1991; Demko and Gastaldo, 1996). Coals are the products of peat swamps, and those in the Pottsville Formation represent Structural Framework a spectrum of domed and low-lying peat swamp deposits that spanned the coastal plain (Eble, 1990; Pashin, 1994a, c; Pashin Strata associated with the Pottsville Formation coal and others, 2004). intervals lack matrix permeability to water; thus, virtually all The four cross sections on plate 1 illustrate the flow is through natural fractures (Pashin and others, 1991), stratigraphic architecture of the eight lower coal zones (Black which include joints, cleats, and fault-related shear fractures Creek through Gwin coal zones) in the Warrior coal field. (Ward and others, 1984; Pashin and others, 1991, 1999a; These cross sections were modified from Pashin and others Pashin, 1998). Pashin and others (1995) and Pashin and (2004; their cross sections D–D’ on pls. 2, 4, 6, and 8). Cycle- Groshong (1998) suggested that large-scale folds and faults bounding flooding surfaces are present at the base of the thick determine the abundance and openness of natural fractures mudstone units and are overlain by mudstone with slightly and thereby influence gas production from coals (Pashin and higher radioactivity than adjacent shale units. All cycles others, 2004). thicken toward the southeast (pl. 1), and the upper part of the Pottsville Formation is thickest in the Moundville–Cedar Cove depocenter in northern Hale and southern Tuscaloosa Appalachian Folds and Thrust Faults Counties, Alabama (Pashin and others, 2004). This southeastward increase in thickness is shown by the isopach The structural framework of the Black Warrior Basin map of the interval between the Ream and the Gwin coal can be characterized in general terms as a homocline that dips zones in figure 6 (Pashin and others, 2004). southwest toward the Ouachita orogenic belt and contains The total thickness of coal, in beds thicker than 1 ft, in numerous superimposed folds and faults (Thomas, 1988) (fig. 8). the upper part of the Pottsville Formation is shown in figure Folds of the Appalachian orogenic belt are superimposed 7. Net coal thickness in the Black Creek through Brookwood on the southeast margin of this homocline and include the coal zones increases toward the southeast, and net coal Sequatchie anticline, the Coalburg syncline, and the Blue Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province ) Cobb and Gwin coal D CHAPTER 4, PLATE 1 CHAPTER 4, PLATE 10 mi 15 km Pleasant Grove Pleasant 10 5 Field boundary Field Stratigraphic cross section cross Stratigraphic 5 Short Creek D' 0 DIGITAL DATA SERIES DDS–69–I DATA DIGITAL 0 D 5 5 Oak Grove Oak D' Brookwood Cycle-bounding flooding surface Minor flooding surface Well permit number Well Primary resource bed Primary resource Secondary bed resource markerThin bed sandstone Aggradational parasequence) by (color-coded Gamma-density logs Interbedded mudstone and mudstone Interbedded by (color-coded sandstone parasequence)
White Oak Creek Oak White
Blue Creek Holt
Creek Peterson
Cedar Cove Cedar Deerlick Creek EXPLANATION Big Sandy D Density Creek Little Sandy 6424 Moundville Coal ray Gamma Creek Thornton Bend Robinson's Buck Little Creek Creek Taylor Cobb Gwin Pratt Ream Curry Curry coal zone coal Gillespy coal zone coal coal zone coal coal zone coal Black Creek Mary Lee coal zone coal coal zone coal coal zone coal coal zone coal ) Gillespy, Curry and Pratt coal zones; ( ) Gillespy, 7390 7390 7390 C 7390 8019 8019 8019 8019 South T T T T 9344 9344 6911 6911 field field field Cedar Cove Cedar field 9812 9812 9812 9812 Cedar Cove Cedar Cedar Cove Cedar Cedar Cove Cedar 8449 8449 8449 8449 9692 9692 9692 9692 Peterson field Peterson Peterson field Peterson Peterson field Peterson Peterson field Peterson 8915 8915 8915 8915 ) Mary Lee coal zone; ( 9918 9918 B 9918 9918 Holt field Holt field Holt field Holt field 4166 4166 4166 4166 4003 4003 4006 4006 4308 4308 4308 4308 6195 6195 6195 6195 6365 6365 5903 Deerlick field Creek Deerlick field Creek Deerlick field Creek Deerlick field Creek T T T T 7499 7499 7402 7402 7399 7399 7399 7400 7979 7979 7979 7744 7957 7957 7957 7957 8314 8314 8314 8314 ) Black Creek and Ream coal zones; ( 8215 8215 8215 A 8215 9745 9745 9745 9745 10 km 10 km 10 km 10 km 6 mi 6 mi 6 mi 6 mi 5 5 5 5 8 8 8 8 9170 9170 9170 9170 Blue Creek field Blue Creek Blue Creek field Blue Creek Blue Creek field Blue Creek Blue Creek field Blue Creek 4 4 4 4 6 6 6 6 3 3 3 3 4 4 4 4 8874 8874 9117 8874 2 2 2 2 2 2 2 2 9527 9527 9527 9527 1 1 1 1 0 0 0 0 0 0 0 0 9446 9446 9446 9446 North U.S. DEPARTMENT OF THE INTERIOR U.S. DEPARTMENT U.S. GEOLOGICAL SURVEY 0 0 0 0 50 50
50
50 50 50 50 50 100 200 300 150 250
500 900 450 800 400 700 100 350 600 200 100 400 300 150 250 100 350 150 200 100 400 300 200 150 250 100 350 150 200 250 300
200 150 250 1000 Vertcal scale (ft) scale Vertcal Vertcal scale (ft) scale Vertcal Vertcal scale (ft) scale Vertcal Vertcal scale (ft) scale Vertcal A B D C Stratigraphic cross sections for: (
Plate 1. Basin, Alabama. Modified from Pashin and others (2004, their cross sections D–D’, on Warrior zones, Pennsylvanian Pottsville Formation, in the southeastern part of Black pls. 2, 4, 6, and 8). Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province
WALKER JEFFERSON FAYETTE
TUSCALOOSA CHAPTER 4, PLATE 1 CHAPTER 4, PLATE 10 mi 15 km Pleasant Grove Pleasant 10 5 Field boundary Field Stratigraphic cross section cross Stratigraphic 5 Short Creek D' 0 DIGITAL DATA SERIES DDS–69–I DATA DIGITAL 0 D 5 5 Oak Grove Oak
D' Brookwood Cycle-bounding flooding surface Minor flooding surface Well permit number Well Primary resource bed Primary resource Secondary bed resource markerThin bed sandstone Aggradational parasequence) by (color-coded Gamma-density logs Interbedded mudstone and mudstone Interbedded by (color-coded sandstone parasequence) PICKENS ).$%8 -!0
White Oak Creek Oak White
Blue Creek Holt
Creek Peterson Cedar Cove Cedar Deerlick Creek EXPLANATION Big Sandy D Density Creek Little Sandy 6424 Moundville Coal ray Gamma Creek
Thornton %XPLANATION Bend !LABAMA Robinson's Buck Little Creek n FT Creek Taylor n FT n FT Cobb Gwin Pratt Ream Curry Curry coal zone coal FT Gillespy coal zone coal coal zone coal coal zone coal Black Creek Mary Lee coal zone coal coal zone coal coal zone coal coal zone coal 7390 7390 7390 7390 #ONTOUR INTERVAL FT 8019 8019 MI 8019
8019 GREENE South -OUNDVILLEn#EDAR #OVE DEPOCENTER KM T T T T 9344 9344 6911 6911 HALE field field field Cedar Cove Cedar field 9812 9812 9812 9812 Cedar Cove Cedar Cedar Cove Cedar Cedar Cove Cedar 8449 8449 Figure 6. Generalized isopach map of the Ream through Gwin coal zones, Pennsylvanian Pottsville Formation, in southeastern part of 8449 8449 9692 9692 9692
9692 the Black Warrior Basin, Alabama. Modified from Pashin and others (2004, their fig. 8). Peterson field Peterson Peterson field Peterson Peterson field Peterson Peterson field Peterson 8915 8915 8915 8915 9918 9918 9918 9918 Holt field Holt field Holt field Holt field 4166 4166 4166 4166 WALKER 4003 4003 4006 4006 JEFFERSON 4308 4308 4308 FAYETTE 4308 6195 6195 6195 6195 6365 6365 5903 Deerlick field Creek Deerlick field Creek Deerlick field Creek Deerlick field Creek T T T T 7499 7499 7402 7402 7399 7399 7399 7400 7979 7979 7979 7744 7957 7957 7957 7957 TUSCALOOSA 8314 8314 8314 8314 8215 8215 8215 8215 ).$%8 -!0 9745 9745 9745 9745 10 km 10 km 10 km 10 km 6 mi 6 mi 6 mi 6 mi PICKENS 5 5 5 5 8 8 8 8 9170 9170 9170 9170 Blue Creek field Blue Creek Blue Creek field Blue Creek Blue Creek field Blue Creek Blue Creek field Blue Creek 4 4 4 4 6 6 6 6 3 3 3 3 4 4 4 4 8874 8874 9117 8874 2 2 2 2 %XPLANATION 2 2 2 2 9527 9527 9527 !LABAMA 9527 1 1 1 1 0 0 0 0 FT n FT 0 0 0 0 9446 9446 9446 9446 North U.S. DEPARTMENT OF THE INTERIOR U.S. DEPARTMENT U.S. GEOLOGICAL SURVEY n FT n FT 0 0 0 0 50 50
50 50 50 50 50 50 100 200 300 150 250
500 900 450 800 400 700 100 350 600 200 100 400 300 150 250 100 350 150 200 100 400 300 200 150 250 100 350 150 200 250 300 200 150 250 1000 Vertcal scale (ft) scale Vertcal Vertcal scale (ft) scale Vertcal
Vertcal scale (ft) scale Vertcal Vertcal scale (ft) scale Vertcal n FT n FT n FT A
B FT D C GREENE #ONTOUR INTERVAL FT MI
HALE KM
Figure 7. Generalized map showing net coal isoliths in the upper part of the Pennsylvanian Pottsville Formation in the southeastern part of Black Warrior Basin, Alabama. Modified from Pashin and others (2004, their fig. 9). Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
WALKER FAYETTE JEFFERSON Sequatchie anticline Coalburg syncline
-500
TUSCALOOSA Anticlinorium
-1,000 -1,500 Blue Creek anticline INDEX MAP PICKENS -1,000
0 -1,500 -2,000 -500 Birmingham
Explanation Alabama -2,500 Syncline -1,000 Anticline Normal fault; -3,000 -2,500 -2,000 -1,500 bar on downthrown side
Contour interval = 500 ft GREENE 5 0 5 10 mi -3,500 HALE 5 0 5 10 15 km
Figure 8. Generalized structural contour map of the top of the Pratt coal zone, Pennsylvanian Pottsville Formation (fig. 4), southeastern part of the Black Warrior Basin. Known normal faults are located. Modified from Pashin and others (2004, their fig. 21).
Creek anticline. The Sequatchie anticline, a prominent Mary Lee coal zone is exposed on both flanks (Semmes, 1929; frontal structure of the Appalachian orogen in Alabama and Pashin and others, 1991; Pashin and others, 2004) (fig. 8). The Tennessee, plunges southwestward, terminating in west-central forelimb of the structure locally dips more than 50° N.W. and Jefferson County, Alabama (fig. 8). The anticline is detached is thought to be a major recharge zone that fed freshwater deep along a slip plane in Cambrian shale, above crystalline into the Pottsville Formation through the exposed coal beds basement, and these relations provided the basis for the first and fractures (Pashin and others, 1991; Ellard and others, 1992; thin-skinned structural models of the Appalachian orogen Pashin and Hinkle, 1997). (Rodgers, 1950; Pashin, 1994c; Pashin and Groshong, 1998; Throughout the eastern part of the Black Warrior Basin, Pashin and others, 2004). the southwest-dipping homocline is broken by normal faults Structures along the southeast margin of the Black that generally strike northwest (fig. 8). Trace lengths of faults Warrior Basin include a series of northeast-striking folds mappable in the subsurface range from about 1 to 8 miles. Fault associated with the forelimb of the Birmingham anticlinorium strikes range from N. 7° W. to N. 54° W., averaging about N. (fig. 8), a broad anticlinal structure in which Cambrian- 30° W., and dip is generally between 50° and 70°. The faults Ordovician carbonate strata are exposed along a thrust fault form a horst-and-graben system in which about 60 percent of with basal detachment in Cambrian shale (Osborne and others, those mapped dip southwest, and the rest to the northeast. The 1989; Thomas, 2001). The Coalburg syncline is a flat-bottomed faults tend to be planar or are composed of planar segments structure that shares its northwest limb with the Sequatchie with sharp bends (fig. 8). Vertical separation is typically less anticline (Semmes, 1929) and has an axial trace that follows than 250 ft but approaches 1,000 ft along a fault in Hale County, the southeast margin of the basin (fig. 8). The southeast limb Alabama (Pashin and others, 2004). of the Coalburg syncline is nearly vertical and was overthrust in part by Cambrian-Ordovician carbonate strata of the Birmingham anticlinorium (Butts, 1910, 1927; Osborne and Pottsville Formation Fracture Systems others, 1989). The Blue Creek anticline is an arcuate structure that strikes northeast and is thought to have formed above a Fracture systems include joints, cleats, and fault-related blind detachment developed at least partly within Mississippian shear fractures (figs. 9, 10, 11). Vertical joints are widespread siliciclastic rocks (Thomas, 1985; Cates and Groshong, 1999). in shale and sandstone and are typically spaced between 0.5 Pottsville strata can be traced across the anticline, and the and 10 m apart. Closely spaced joints (about 0.5 to 2.5 cm) in Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province
4USCALOOSA 'ROUP
0OTTSVILLE &ORMATION 3ANDSTONE
#LEATS IN COAL 3HALE .ORMAL FAULT
3YNTHETIC SHEAR *OINT !NTITHETIC SHEAR 3ANDSTONE
3HALE
FT
Figure 9. Small normal fault and associated fracturing in an abandoned mine highwall, Brookwood coal zone, Pennsylvanian Pottsville Formation, Tuscaloosa County, Alabama. Modified from Pashin and others (2004, their fig. 26).
FT
Figure 10. Jointed shale above the Gwin coal zone, Pennsylvanian Pottsville Formation, Jefferson County, Alabama. Modified from Pashin and others (1991, 2004, their fig. 27). 10 Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
Bedding plane exposure FC BC
FC = Face cleat (systematic joint) BC = Butt cleat (cross joint) Figure 11. Bedding plane exposure of a cleat system in Alabama coal. Modified from Pashin and others (2004, their fig. 28). coals are called cleats and are a primary control on aquifer and shorter than systematic joints, tend to strike perpendicular to reservoir quality in the Pottsville coal interval. Joint systems systematic joints, and commonly terminate at intersections tend to be strata-bound (Pashin and others, 2003); that is, the with systematic joints. In coal, systematic joints are referred to upper and lower tips of the fractures tend to be at or near the as face cleats, and cross joints are called butt cleats (fig. 11). upper and lower contacts of the host shale, sandstone, or coal Systematic joints and cleats have distinctive orientations in the (figs. 9, 10, 11). Pashin and others (2003) observed that fracture eastern Black Warrior Basin (Ward and others, 1984) spacing in shale and sandstone increases logarithmically (fig. 12). Joints in sandstone and shale can be subdivided into a with bed thickness, and McFall and others (1986) and Pashin regional joint system and a localized cross-joint system that is and others (1999a) observed that cleat spacing decreases restricted to the area containing Appalachian folds on the east markedly as coal rank increases. Therefore, fracture height in basin margin (fig. 12A). Systematic joints of the regional joint the Pottsville Formation generally equals bed thickness, and system strike with a vector mean azimuth of N. 47° E., whereas fracturing of adjacent beds took place with a high degree of systematic joints of the fold-related system strike with a vector mechanical and thermal independence. Joint systems constitute mean azimuth of N. 64° W. Similarly, cleat systems can be orthogonal sets of vertical fractures composed of systematic subdivided into a regional cleat system and a localized system joints and cross joints (figs. 9, 10, 11) (Ward and others, 1984; that is restricted to the southeast basin margin (fig. 12B) (Pashin Pashin and others, 1999a, 2004). and others, 2004). Face cleats of the regional cleat system strike Systematic joints are planar fractures that can have with a vector mean azimuth of N. 62° E., which is 15° east of surface-trace lengths on the order of 100 m. Cross joints are the regional systematic joint system. Face cleats in the localized Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province 11 fracture system along the southeast margin of the basin Dipping fractures and fractures with slickensides strike with a vector mean azimuth of N. 36° W. are abundant within 10 m of normal faults and form Joints and cleats in the Pottsville Formation are crisscrossing networks that have been subdivided into commonly mineralized (Pashin and others, 1999b; synthetic and antithetic shear fractures by Pashin and Pitman and others, 2003), with calcite the dominant others (1991) (fig. 9). Synthetic shears dip parallel to the fracture-filling mineral in shale and sandstone, and associated fault, whereas antithetic shears dip opposite pyrite and calcite more common in coal. Fracture-filling to the associated fault. Faults and the associated shear minerals generally have a patchy distribution and seldom fractures cut across bedding and thus are possible fill fractures completely, so these minerals are generally avenues for hydrologic communication between the considered not to be major obstacles to fluid flow and, surface and the reservoir coals. In a study of gas seeps in places, may even help prop fractures open that would in the Black Warrior Basin, Clayton and others (1994) otherwise be closed (Pashin and others, 2004). investigated a number of exposed normal faults and
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" #LEAT SYSTEMS
Figure 12. Generalized maps showing joint (A), and cleat (B) systems in the southeastern part of the Black Warrior Basin, Alabama. Modified from Pashin and others (1991; 2004, their fig. 29). 1 Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province reported a significant gas seep along one fault in Oak Grove Table 1. Ranges and median values of organic carbon con- field. However, Pitman and others (2003) observed pervasive tents, and hydrogen indices for coal and shale samples from the cementation of coal cleats within 10 m of normal faults in Pennsylvanian Pottsville Formation in the southeastern part of the the Black Warrior Basin. This cementation appears to inhibit Black Warrior Basin, Alabama. flow in coal along large segments of many faults (Pashin and [wt %, weight percent; mg, milligram; HC, hydrocarbons; g, gram; TOC, total others, 2004). organic carbon; n, number of observations; for shale samples, Rock-Eval® Tmax ≤ 450 ºC; data from http://energy.cr.usgs.gov/prov/og/data2.htm]
Total Organic Carbon Hydrogen Index Hydrocarbon Source-Rock Lithology Statistic (wt %) (mg HC/g TOC) Characterization Median 71.2 126 Coal range 60.7–77.0 91–198 The potential for sedimentary rocks to generate (n) (23) (23) hydrocarbons, and the nature of the hydrocarbons generated, Median 1.3 74 depend on the amount, composition, and thermal maturity Shale range 0.5–27.8 39–246 of organic matter in the rock (Tissot and Welte, 1984). Total (n) (16) (16) organic carbon (TOC) content is measured in terms of weight percent (wt %). The relative hydrogen richness of the organic matter, and an indication of the types of hydrocarbons that Organic Matter Thermal Maturity will be generated, are measured by hydrogen index (HI, ® Thermal maturation (rank) of bituminous coals can Rock-Eval , S2/TOC, mg/g). Relative thermal maturity of organic matter is measured by Tmax (Rock-Eval®, °C), be determined through calculations made from proximate analyses of coal (moisture, ash, volatile-matter, and fixed- vitrinite reflectance (Ro), and calculations from proximate analyses and calorific value determinations (BTU/lb) of coal carbon contents), sulfur contents, and calorific values (Btu/lb), and (or) R measurements. Thermal maturation parameters (for example, see American Society for Testing and Materials o have been mapped for each Pottsville coal zone by Winston D–388, 2001). Source rocks for the gas accumulations in the (1990a, b), and the most recent thermal maturity mapping Pottsville Coal TPS and the Black Warrior Basin AU are the has relied on interpretation of proximate analyses and R Pottsville Formation coals and coaly shales, which also form o measurements (Telle and others, 1987; Winston, 1990a, b; the principal reservoirs (fig. 2). Levine and Telle, 1991; Pashin and others, 1999a). Rank of the Pottsville Formation coals ranges from high- Organic Carbon Contents—Organic Facies volatile C bituminous to low-volatile bituminous (Semmes, 1929; Winston, 1990a, b). For the Mary Lee coal zone (fig. 14), the coals are of high-volatile C bituminous coal rank in Ranges for TOC and HI for 39 coal and shale samples northern Walker County, western Fayette County, northeastern from the Pottsville Formation are summarized in table 1. The Pickens County, and eastern Lamar County, Alabama. Coals of distribution of HI and organic facies (designated A–D, see medium-volatile and low-volatile bituminous rank are centered below) for the samples is shown in figure 13. in an elliptical “bulls-eye” area in eastern Tuscaloosa County Jones (1987) defined a series of organic facies (A–D) for and western Jefferson County, Alabama, near the southeast rocks worldwide on the basis of microscopic and chemical margin of the Black Warrior Basin (fig. 14) (Winston, 1990a; characteristics (HI, H/C ratios) of organic matter that is Pashin and Hinkle, 1997). The southeast side of this elliptical marginally mature with respect to petroleum generation (Ro area is marked by the upturned northwest limb of the Blue about 0.50). The volume and type of generated hydrocarbons Creek anticline. The coal rank pattern in west-central Alabama vary widely with organic facies. Organic facies A, AB, does not reflect thickening of the Pottsville Formation coal and B are sources primarily of oil. Organic facies BC has interval into the Moundville–Cedar Cove depocenter (see fig. 6). capacity to generate both oil and gas. Hydrocarbons generated from organic facies C are nearly always condensate and gas. Organic facies CD has a moderate capacity for dry Thermal Maturity Modeling gas generation, whereas organic facies D is essentially nongenerative (Jones, 1987). Present burial depths of the upper part of the Pottsville As shown in figure 13, organic-matter compositions in Formation in coal-bed gas fields of the Black Warrior Basin the Pottsville Formation primarily represent organic facies range from 0 to about 4,600 ft. During maximum burial in the CD (median HI = 130, with the range in compositions from Early Permian, the formation was buried to depths of 6,500 organic facies C to D). Where thermally mature, the organic to 10,000 ft (Telle and others, 1987; Levine and Telle, 1991; matter in facies CD should primarily generate dry gas. Carroll and others, 1995). Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province 13
25 S
N Coal O
I 20
T Shale A V R
E 15 S B O
F 10 O
R E B 5 M U N
0 <50 50–125 125–250 250–400 400–650 650–850 >850 D CD C BC B AB A
HYDROGEN INDEX AND ORGANIC FACIES Figure 13. Stacked bar histogram showing distribution of hydrogen indices for 16 shale samples and 23 coal samples from the Penn- sylvanian Pottsville Formation, southeastern part of the Black Warrior Basin, Alabama. For shale samples, Rock-Eval® Tmax ≤ 450 ºC and total organic carbon ≥ 0.5 percent; organic facies boundaries (A through D) are from Jones (1987).
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Figure 14. Generalized map showing vitrinite reflectance (Ro) and rank of coals in the Mary Lee coal zone, Pennsylvanian Pottsville Formation, in the Black Warrior Basin, Alabama. Coal rank is based on calculations from proximate analyses, calorific values, and vitrinite reflectance measurements. Modified from Winston (1990a), Pashin and Hinkle (1997, their fig. 23), and Pitman and others (2003, their fig. 5). 1 Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
Lopatin modeling in Pickens County, Alabama (fig. 15) others, 1987; Winston, 1990a, b; Levine and Telle, 1991). provides evidence for a complex burial and thermal history, Accordingly, the paleogeothermal gradient is thought to have enabling some generalizations about burial and hydrocarbon been lowest in the area containing less thermally mature coal generation (Telle and others, 1987; Hines, 1988; Carroll and of high-volatile C and B bituminous rank and highest in the others, 1995). Major thermal maturation apparently took place area containing medium-volatile and low-volatile bituminous during rapid burial associated with Appalachian-Ouachita coal. The origin of the elevated geothermal gradient is unclear; orogenesis, and coal in the Pottsville Formation attained a the best hypothesis advanced to date is tectonic expulsion of rank of high-volatile A bituminous coal near maximum burial, hot fluids from the Appalachian orogen (Winston, 1990a, b; which occurred near the end of the Paleozoic (about 265 Ma). Goldhaber and others, 2003; Pashin and others, 2004). Importantly, major thermogenic gas generation is thought to begin as coal reaches that rank (Jüntgen and Karweil, 1966). In the Pottsville Formation, virtually all coal-gas production is from coal of high-volatile A bituminous or higher rank Produced Water Chemistry (Ro ≥ 0.8) (Pashin and others, 2004). According to the Lopatin models, minor thermogenic gas generation may have Stiff diagrams (fig. 16) demonstrate that subsurface continued during postorogenic unroofing but was effectively waters in the eastern part of the Black Warrior Basin contain completed by the Late Cretaceous, when the Gulf Coastal variable concentrations of sodium bicarbonate and sodium Plain and Mississippi Embayment formed. chloride (Pashin and others, 1991). Water containing about Although burial associated with foreland basin 30 equivalents per million of sodium bicarbonate is typical subsidence may have been an important mechanism of subsurface water at depths less than 1,500 ft (for example, influencing thermal maturation, rank anomalies in the Oak Grove and Pleasant Grove fields, fig. 16). However, water Pottsville Formation are not directly related to the regional containing significant concentrations of sodium chloride has isopach and structural contour patterns (figs. 6, 7), indicating been identified at depths near 1,000 ft (for example, see Oak that burial maturation was overprinted strongly by spatial and Grove field, fig. 16). Bicarbonate concentration is essentially temporal changes in the paleogeothermal gradient (Telle and constant regardless of depth, and increasing concentrations
0 70 Onset of thermal 2,000 gas generation 90 5 110 4,000 Onset of oil 3 generation 130 6,000 2 End of 150 1 oil generation 8,000 170 TEMPERATURE (ºF) TEMPERATURE 1- Base of Chattanooga Shale 10,000 2- Base Pottsville Formation 190 3- Preserved Pottsville Formation 4- Eroded section (Pennsylvanian + Permian ?) 5- Base of Cretaceous Tuscaloosa Group DEPTH BELOW SURFACE, IN FEET DEPTH BELOW SURFACE, 12,000 6- Cretaceous - Holocene unconformity 210 7- Eroded section
–350 –300 –250 –200 –150 –100 –50 0 TIME (MYBP)
Figure 15. Plot showing relation between burial history and thermal gas generation in the Pennsylvanian Pottsville Formation in the Shell Holiman #13-16 well in Pickens County, Alabama. After Carroll and others (1995); modified from Pashin and Hinkle (1997, their fig. 26). MYBP, million years before present; ºF, degrees Fahrenheit. Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province 15
#ATIONS %QUIVALENTS PER MILLION !NIONS
.A + (#/ 7ELL WATER FT DEEP
.A + #L /AK 'ROVE FIELD FT
.A + (#/ /AK 'ROVE FIELD FT
.A + (#/ 0LEASANT 'ROVE FIELD FT
.A + #L "LUE #REEK FIELD FT
.A + #L "ROOKWOOD FIELD FT .A + #L $EERLICK #REEK FIELD FT .A + #L #EDAR #OVE FIELD FT
Figure 16. Stiff diagrams showing the composition of formation water produced from the Pennsylvanian Pottsville Formation in the southeastern part of the Black Warrior Basin, Alabama. Modified from Pashin and others (1991; 2004, their fig. 43).
of sodium chloride dominate the water chemistry at depths The freshwater plumes terminate at the edge of Cretaceous greater than 1,500 ft (Raymond, 1991; Ortiz and others, 1993; outcrops, suggesting that the poorly consolidated strata of the Pashin and Hinkle, 1997; Pashin and others, 2004). Upper Cretaceous Tuscaloosa Group intercept the freshwater At depths less than 1,500 ft in the Pottsville Formation recharge in those areas (Pashin and others, 2004). (fig. 16), bicarbonate water apparently formed by meteoric recharge of the phreatic zone. Similar amounts of bicarbonate in deeper, higher salinity water (such as in the Brookwood field, fig. 16) suggest a genetic linkage with the shallower Natural Gas Chemistry water, but increased chlorinity indicates that the deeper water is sufficiently old to have mixed with brine from strata below Natural gases produced from coals in the Black Warrior the Pottsville coals. The presence of contrasting bicarbonate Basin are very dry, with median gas wetness only about 0.1 and chloride waters between 1,000 and 1,500 ft indicates mixing within the productive reservoir zone (fig. 16). percent; median CO2 content is 0.08 percent, and median Dissolved-solids content is less than 10,000 milligrams per N2 content less than 0.01 percent (table 2). There is no liter (mg/L) in most areas of coal-gas production (Pashin and measurable H2S. Geographic distributions of sampled gases others, 1991; Ellard and others, 1992, 1997; Sparks and others, and gas wetnesses (Oak Grove, Brookwood, and Deerlick 1993; Pashin and others, 2004). Creek degasification fields) are shown in figure 19. Rice Along the southeast margin of the basin, water in the (1993) interpreted the coal gases to be mainly of thermogenic Mary Lee coal zone consistently has dissolved-solids content origin. However, based primarily on the relatively negative less than 3,000 mg/L, and in places, dissolved-solids content δ13C compositions of the coal-bed gases, he (Rice, 1993) is less than 1,000 mg/L (Pashin and others, 1991) (figs. 17, 1 suggested that the gases are a mixture of thermogenic gas 18). Upturned and fractured strata in the forelimb of the and gas produced during a late-stage methanogenesis. This Birmingham anticlinorium form a recharge zone in which the coals are at or near the surface. Accordingly, water with methanogenesis was initiated by freshwater incursions into dissolved-solids content less than 3,000 mg/L has been the shallower coals on the east side of the basin (see figs. 17, interpreted to define a series of active freshwater plumes that 18) that may have begun during the late stages of Mesozoic extend from the forelimb of the Birmingham anticlinorium unroofing. These incursions are apparently controlled by deep into the Black Warrior Basin (Pashin and others, 1991). northwest-trending faults and fractures (Pashin and others, 1991). 1 Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
WALKER
FAYETTE JEFFERSON
TUSCALOOSA -!2').
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%XPLANATION !LABAMA 3YNCLINE !NTICLINE .ORMAL FAULT BAR ON DOWNTHROWN SIDE 5PPER #RETACEOUS 7ATER WITH LESS THAN MG, DISSOLVED SOLIDS IN -ARY ,EE COAL GROUP GREENE HALE 3TRUCTURE CONTOURS ON TOP OF 0RATT COAL ZONE CONTOUR INTERVAL FEET MI KM Figure 17. Generalized map showing relation of freshwater plumes in the Mary Lee coal zone, Pennsylvanian Pottsville Formation, to geologic structure and Upper Cretaceous strata, along the southeast margin of the Black Warrior Basin, Alabama. Modified from Pashin and McIntyre (2003) and Pashin and others (2004, their fig. 45). mg/L, milligrams per liter. A A' Northwest Southeast Major recharge area Blue Creek Recharge area for shallow aquifers anticline Utley Gwin Gwin Sea Cobb Vertical percolation Cobb level Pratt Pratt Mary Lee Freshwater plumes Black Creek (bed-parallel flow) Mary Lee Upturned basin margin Black Creek Variable reservoir pressure Normal reservoir pressure
5 0 5 10 mi Vertical exaggeration = 6x 5 0 5 10 15 km INDEX MAP
WALKER FAYETTE A Bicarbonate water (less than 3,000 mg/L JEFFERSON dissolved solids) Chloride water (greater than 3,000 mg/L A' TUSCALOOSA
dissolved solids) PICKENS General flow direction
GREENE HALE
Figure 18. Generalized structural cross section showing meteoric recharge areas and subsurface flow patterns in the upper part of the Pennsylvanian Pottsville Formation, along the southeast margin of the Black Warrior Basin, Alabama. Modified from Pashin and others (2004, their fig. 44). mg/L, milligrams per liter. Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province 17
Table 2. Ranges and median values for gas wetness, N2, and CO2 contents, 13 and d C1 for gases from coals in the Pennsylvanian Pottsville Formation, southeastern part of the Black Warrior Basin, Alabama.
[Gas wetness % = 100 x (1–[mole % C1/Σ mole % C1–C5]); n, number of observations; %, percent; ‰, part per thousand; PDB, relative to Pee Dee belemnite standard; < , less than.
C1 to C5, N2 and CO2 analyses are from Moore and Sigler (1987) and Hamak and Sigler 13 (1991); d C1 analyses are from Pashin and Hinkle (1997, their table 3)]
CO N δ13C Statistic Gas wetness (%) 2 2 1 (mole %) (mole %) (‰ PDB)
Median 0.01 0.08 <0.01 –47.4 Range <0.01 to 4.0 0.03 to 0.20 <0.01 to 1.2 –51.0 to –41.9 (n) (19) (19) (19) (17)
WALKER FAYETTE
Oak Grove
Deerlick Brookwood JEFFERSON TUSCALOOSA Creek
INDEX MAP PICKENS
Explanation !LABAMA Gas wetness percent 0 – 0.9 0.9 – 2.6 2.6 – 5.2 GREENE 5.2 – 9.2 HALE 9.2 – 14.5 14.5 – 28.5
Figure 19. Generalized map showing the distribution of gas wetness for samples of coal gas from the Pennsylvanian Pottsville Forma- tion in the southeastern part of the Black Warrior Basin, Alabama. Coal degasification field outlines (dashed blue lines) are from Pashin and others (2001a; 2001b; 2004); fields are identified in figure 20; gas wetness percent = 100 x (1–[C1mole %/ΣC1–C5 mol %]. 1 Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
Black Warrior Basin Assessment Unit
the 10 largest of these fields are listed in table 3. Cumulative Gas and Water Production production through January 2003 was about 1,415 billion cubic feet of gas (BCFG), with most of the production Estimates of volumes of undiscovered gas from undrilled concentrated in the southeastern part of the Black Warrior areas in the Black Warrior Basin are based on historical Basin AU in Tuscaloosa and Jefferson Counties, Alabama production data, well characteristics, distributions of gas and (Alabama State Oil and Gas Board at http://www.gsa.state. water production, and the relation of gas production to the al.us/ogb/db_main.html). Volumes of produced coal gas and geologic framework. water and numbers of well permits and producing gas wells for the years 1980 through 1998 are shown in figure 21. Wells in seven coal degasification fields (Little Buck Historical Production Creek, Taylor Creek, Little Sandy Creek, Carrolls Creek, Wolf Creek, Boone Creek, and Pleasant Grove) (fig. 20) have The first coal-gas production in the Black Warrior Basin been plugged and abandoned. Most wells in the Moundville was associated with degasification of underground coal mines. field are listed by the State Oil and Gas Board of Alabama as Production began in both the Oak Grove and Brookwood temporarily abandoned. Active exploration is now focused in coal degasification fields in 1981, and 20 coal-bed gas fields Blue Creek, White Oak Creek, and Short Creek fields in the have since been identified (fig. 20). Production data from northern part of the basin (Pashin and others, 2003).
Table 3. Cumulative gas and water production data for the ten most productive coal degasification fields in the Black Warrior Basin, Alabama. [BCFG, billion cubic feet of gas; MMbbl = million barrels of water. Data from the Alabama State Oil and Gas Board accessed at: http://www.gsa.state.al.us/ogb/db_main.html accessed May 2004; data summarized through December 31, 2003] Cumulative production Field County Discovery year Gas Water (BCFG) (MMbbl) Big SandyCreek Tuscaloosa 1990 9.0 25.8 Blue Creek Tuscaloosa 1988 256.3 28.4 Brookwood Tuscaloosa, Jefferson 1981 327.9 93.9
Cedar Cove Tuscaloosa 1983 279.1 302.3 Deerlick Creek Tuscaloosa 1983 120.5 50.0 Holt Tuscaloosa 1982 35.8 11.6 Oak Grove Jefferson, Tuscaloosa 1981 221.7 200.5 Peterson Tuscaloosa 1984 44.2 23.3 Robinson’s Bend Tuscaloosa 1990 103.9 250.5
White Oak Creek Walker, Jefferson, Tuscaloosa 1989 127.2 31.9 Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province 19
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0)#+%.3 #REEK $EERLICK "ROOKWOOD #REEK (OLT 2OBINSONgS 0ETERSON "END ,ITTLE 3ANDY #EDAR #OVE %XPLANATION #REEK !LABAMA &IELD BOUNDARY 4HORNTON 4AYLOR #REEK 0RODUCING #REEK -OUNDVILLE "IG 3ANDY #REEK 4EMPORARILY ABANDONED ,ITTLE "UCK #REEK 0LUGGED AND ABANDONED MI '2%%.% (!,% KM Figure 20. Generalized map showing locations and status of coal degasification fields in the southeastern part of the Black Warrior Basin, Alabama (modified from Pashin and others (2001a; 2001b; 2004, their fig. 69).
! 'AS PRODUCTION "#&' 7ATER PRODUCTION --BBL
02/$5#4)/.
" 9%!2 0ERMITS ISSUED 7ELLS PRODUCING
.5-"%2 /& 7%,,3
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Figure 21. Histograms of coal-gas and water production (A) and new well permits and producing wells (B) in the southeastern part of the Black Warrior Basin, Alabama, for the years 1980 through 1998. Histograms are from the Alabama Geological Survey Web site accessed at http://www.gsa.state.al.us, May/2004. BCFG, billion cubic feet of gas; MMbbl, million barrels. 20 Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
Drilling Practices wells are cased and perforated in multiple coals (Graves and others, 1983). Stimulation practices have also varied—early Three types of coal-gas wells are drilled in the Black wells were stimulated with water and sand, nitrogen foam, or Warrior Basin—vertical wells, gob wells, and horizontal wells. cross-linked gel, whereas the most recent vertical wells were Gob wells and horizontal wells are drilled in association with stimulated with nitrogen foam (Pashin and others, 2004). underground mining operations, whereas vertical wells are Essentially all vertical wells produce at least some water similar to conventional petroleum wells, have no dependence and gas, and the volumes of fluid produced and the production on mining operations, and are drilled in all coal-gas fields decline characteristics of these wells can be difficult to predict; (Pashin and others, 2004). however, some generalizations can be made. Gas and water Coal-gas development in the Black Warrior Basin production tend to peak during the first 2 years that wells are began in 1980, and most wells have produced gas for 9 to on line, although it is not uncommon for some wells to take 15 years. The earliest wells, which were underground mine more than 4 years to reach peak flow rates (Pashin and Hinkle, degasification wells drilled between 1975 and 1980, are 1997). Once peak gas production is reached, it tends to decline in closely spaced patterns (about 15 acres) in eastern Oak exponentially (a constant rate of decline), whereas water Grove field and in Pleasant Grove field. Subsequent field production tends to decline hyperbolically (a decreasing rate development has primarily utilized 40- and 80-acre well of decline) (Pashin and others, 2004). spacing. Most wells were drilled between 1988 and 1991 prior to expiration of the Section 29 tax credit, and nearly all are Cumulative Gas and Water Production still producing (Pashin and others, 2004). Completion practices in vertical wells vary considerably The cumulative gas production map (fig. 22) shows a (for examples, see Lambert and others, 1980; Holditch and general arcuate trend to the area in which most wells have others, 1989; Pashin and Hinkle, 1997). Many early wells were produced more than 300 million cubic feet of gas (MMCFG). open holes completed in single coals, but virtually all modern Within this trend, which extends from the Cedar Cove field
WALKER FAYETTE
JEFFERSON
TUSCALOOSA
).$%8 -!0 PICKENS
%XPLANATION !LABAMA
--#&' n --#&' 3YNCLINE &IELD n --#&' BOUNDARY !NTICLINE n --#&' .ORMAL FAULT n --#&' BAR ON DOWNTHROWN SIDE n --#&' MI GREENE n --#&' KM HALE --#&'
Figure 22. Generalized map showing cumulative gas production from vertical wells in the southeastern part of the Black Warrior Basin, Alabama. Modified from Pashin and others (2004, their fig. 77). Coal degasification field outlines (dashed blue lines) are from Pashin and others (2001a; 2001b; 2004). Fields are identified in figure 20. MMCFG, million cubic feet of gas. Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province 1 northward into the White Oak Creek field (fields identified high gas production (figs. 22, 23). However, farther north, in fig. 20), many wells have had cumulative production from the northern part of the Deerlick Creek field into the exceeding 1 BCFG, and some wells in the Cedar Cove field White Oak Creek field, less than 100,000 bbl of water have have produced more than 1.5 BCFG. Outside this arcuate been produced from the wells with high gas production trend, cumulative production exceeding 300 MMCFG of (Pashin and others, 2004). gas has a patchy distribution; in the Little Sandy Creek, Moundsville, Little Buck Creek, and Taylor Creek fields, for example, only one well has produced more than 100 MMCFG Well Production Controls (Pashin and others, 2004). Cumulative water production has a more heterogeneous Many factors influence the productivity of coal-gas distribution than cumulative gas production (fig. 23). Wells wells in the Black Warrior Basin, including well design have produced less than 100,000 barrels (bbl) over large parts and completion practices, well spacing, coal thickness, gas of the Black Warrior Basin, but some wells with cumulative content, and the abundance and openness of natural fractures production exceeding 1,000,000 bbl exist in most fields. (Pashin and others, 1991; Pashin and Hinkle, 1997). Within The largest areas, where cumulative water production has the basin, Pashin and others (1991) observed an extremely exceeded 100,000 bbl per well, are in the Robinson’s Bend poor correlation between net completed coal thickness and and Cedar Cove fields and the western part of the Oak Grove peak production of gas and water, concluding that although field. From the northeastern part of the Cedar Cove field to coal thickness and gas content were key determinants of the the southeastern part of the Deerlick Creek field, high water resource base, they were not the primary determinants of well production generally corresponds with the arcuate trend of performance (Pashin and others, 2004).
FAYETTE WALKER
JEFFERSON TUSCALOOSA
).$%8 -!0 PICKENS
!LABAMA %XPLANATION BBL n BBL &IELD 3YNCLINE BOUNDARY n BBL !NTICLINE n BBL .ORMAL FAULT n BBL BAR ON DOWNTHROWN SIDE GREENE n BBL n BBL MI HALE BBL KM
Figure 23. Generalized map showing cumulative water production from vertical wells in the southeastern part of the Black Warrior Basin, Alabama. Modified from Pashin and others (2004, their fig. 78). Coal degasification field outlines (blue dashed lines) are from Pashin and others (2001a, 2001b; 2004). Fields are identified in figure 20; bbl, barrels. Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
Detailed studies in some fields have identifed structural a series of linear to arcuate horst and graben blocks, which controls on well performance (Pashin and others, 1991, 1995; tend to compartmentalize production. In addition to the major Pashin and Groshong, 1998). In the Oak Grove field, for normal fault system, two distinct sets of cleats are developed example, exceptionally productive gas wells are aligned along in the Pottsville coals. One dominant set, which strikes to the the axial trace of a small syncline that defines a structural northeast, has been reported throughout the Alabama portion hinge separating the crestal uplift of the Sequatchie anticline of the basin (see fig. 12). The other set is developed locally from the more gently dipping northwest limb of the Coalburg on the southeast basin margin in Jefferson and Tuscaloosa syncline (Pashin and others, 1991) (fig. 24). Counties, where the face cleats strike to the northwest (Pashin Reservoir quality is structurally controlled. For example, and others, 1991, 2004). strata in anticlines (such as the Blue Creek anticline) adjacent Control of gas and water production by normal faults to the thrust faults of the Appalachian orogen are more is not obvious from regional maps. However, such control intensely fractured than are anticlines in other parts of the is indicated on detailed maps of the southeastern part of the basin. This increased fracturing enhances permeability and gas Deerlick Creek field (fig. 25), where peak production of both productivity. Normal faults, which generally trend northwest- gas (more than 300,000 cubic feet per day) and water (more southeast, are abundant in the Black Warrior Basin (fig. 8). than 750 barrels per day) is concentrated in a pair of half These faults are related to extensional tectonics and form grabens. There is little, if any, production from the intervening
/AK 'ROVE MINE
0EAK GAS PRODUCTION GREATER THAN -#&$ /AK 'ROVE FIELD .ORMAL FAULT BAR ON DOWNTHROWN SIDE
'AS WELL
MI )NDEX MAP KM
Figure 24. Generalized map showing alignment of exceptionally productive gas wells along a synclinal fold hinge in the Oak Grove coal degasification field in the southeastern part of the Black Warrior Basin, Alabama. Structure contours are drawn on the top of the Mary Lee coal. Modified from Pashin and others (1991; 2004, their fig. 83). MCFD, thousand cubic feet per day. Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province
&RANKLIN (ILL HALF GRABEN
3EXTON 3PRING HORST (OLT ,AKE HALF GRABEN
3TRIP -INE GRABEN MI
KM
0EAK GAS PRODUCTION GREATER THAN .ORMAL FAULT BAR ON DOWNTHROWN SIDE -#&$ #ONTOUR INTERVAL FEET 0EAK GAS PRODUCTION n -#&$
! 'AS PRODUCTION AND STRUCTURE
&RANKLIN (ILL HALF GRABEN
3EXTON 3PRING HORST (OLT ,AKE HALF GRABEN
3TRIP -INE GRABEN
MI
KM
0EAK WATER PRODUCTION GREATER THAN .ORMAL FAULT BAR ON DOWNTHROWN SIDE "0$ #ONTOUR INTERVAL FEET 0EAK WATER PRODUCTION n "0$
" 7ATER PRODUCTION AND STRUCTURE
Figure 25. Generalized map showing relation of gas (A) and water (B) production to extensional structure in the Deerlick Creek coal degasification field in the southeastern part of the Black Warrior Basin, Alabama. Modified from Pashin and others (1995; 2004, their fig. 84). MCFD, thousands cubic feet per day; BPD, barrels per day. Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province half graben and full graben (Pashin and others, 1995; Pashin additional gas production will focus primarily on infill drilling, and Groshong, 1998; Pashin and others, 2004). resulting in an increase in production within existing fields. The highly productive wells within the Strip Mine graben Our estimated median percentage of untested area with the penetrate the bounding fault and are completed within the Holt potential for additions to reserves in the next 30 years is about Lake half graben. Well performance within the half grabens is 27 percent (about 1,920 mi2). This value includes additions to extremely variable. Pashin (1998) and Pashin and Groshong reserves from infill drilling coupled with expansion of fields (1998) attributed this variability to variable structural into currently untested areas. conditions between wells that influence the abundance Our analysis resulted in estimated volumes of potential and openness of natural fractures, thereby determining the additions to reserves within the Black Warrior Basin AU of 4.61 effectiveness with which wells can dewater and depressurize TCGF at the 95-percent certainty level, 6.85 TCFG at the 50- the reservoir. percent certainty level, 10.18 TCFG at the 5-percent certainty level, and a mean of 7.06 TCFG (U.S. Geological Survey, 2003, their table 1). In comparison, for the four Black Warrior Basin Assessment Results coal-gas plays characterized by Rice and Fenn (1995) for the 1995 USGS National Assessment Oil and Gas Assessment, Through the first quarter of 2000, more than 3,500 gas Charpentier and others (1996) listed an estimated 2.2 TCFG wells had been completed within the Black Warrior Basin AU, at the 50-percent certainty level of potential additions to gas and some 3,200 of these wells were still producing (Pashin and reserves. others, 2004). A graph showing the estimated ultimate recovery (EUR) for gas wells in all fields, based on production from 3,200 wells, is shown in figure 26. Only wells with minimum recoveries of more than 10 MMCFG were used to calculate the References Cited EUR distributions shown in figure 26. The estimated minimum total recovery of 10 MMCFG was based on considerations of American Society for Testing and Materials, 2001, Standard the minimum recovery that might be required for an untested classification of coals by rank (ASTM designation D 388– cell to be economically viable within similar geologic and 99): 2001 Annual Book of ASTM Standards, p. 190–195. production constraints on producing gas wells in the basin. EUR distributions by “thirds,” shown in figure 27, are Butts, Charles, 1910, Description of the Birmingham quad- based on dates of initial production for the first (early), second rangle, Alabama: U.S. Geological Survey Geologic Atlas, (middle), and third (late) fractions of the 3,200 producing Folio 175, 24 p. gas wells; each third includes about 1,070 wells regardless of Butts, Charles, 1926, The Paleozoic rocks, in Adams, G.I., the number of years involved. Figure 27 shows two trends of Butts, Charles, Stephenson, L.W., and Cooke, C.W., eds., interest to resource assessment: (1) earlier wells have overall Geology of Alabama: Alabama Geological Survey Special lower maximum EUR projections, suggesting that more recent Report 14, p. 41–230. production has improved in terms of minimum recovery (likely the result of improved recovery technology); and (2) EUR Butts, Charles, 1927, Bessemer-Vandiver folio: U.S. Geologi- projections of later wells have decreased from those of the cal Survey Geologic Atlas, Folio 221, 22 p. middle wells, suggesting that the best wells (on average) have been drilled. Carroll, R.E., Pashin, J.C., and Kugler, R.L., 1995, Burial The EUR distributions were the primary basis for history and source-rock characteristics of Upper Devonian estimating minimum, median, and maximum ultimate through Pennsylvanian strata, Black Warrior basin, Ala- recoveries for untested cells in the assessment unit. Our bama: Alabama Geological Survey Circular 187, 29 p. estimated median total recovery of gas per well of 400 Cates, L.M., and Groshong, R.H., Jr., 1999, Confirmation of MMCFG was based on the EURs estimated for 50 percent regional thin-skinned extension in the eastern Black War- of the most recent one-third of the producing wells in the rior basin, Alabama, in Pashin J.C., and Carroll, R.E., eds., assessment unit (fig. 27). In contrast, our estimated maximum 1999, Geology of the Cahaba coal field: Alabama Geologi- total recovery of 20,000 MMCFG (fig. 27 at 100 percent) cal Society 36th Annual Field Trip Guidebook, p. 49–57. per untested cells reflects the possibility that other wells with this much production might exist. Within the Black Warrior Charpentier, R.R., Klett, T.R., Obuch, R.C., and Brewton, J.D., Basin AU, a general southeast to northwest decrease in well 1996, Tabular data, text, and graphical images in support of productivity is predicted, a likely result of the decreasing coal the 1995 National Assessment of United States Oil and Gas thermal maturity, decreasing number of coal beds, and resultant Resources: U.S. Geological Survey Digital Data Series 36, decrease in EUR per well. Our estimate for the minimum one CD–ROM. percentage of the untested area within the AU that has the potential for additions to reserves during the next 30 years is about 9 percent (about 640 mi2). This value assumes that Assessment of Pottsville Coal Total Petroleum Sytem—Black Warrior Basin Province
100,000
10,000
1,000
100 ESTIMATED ULTIMATE RECOVERY ULTIMATE ESTIMATED (MMCFG)
10 0 10 20 30 40 50 60 70 80 90 100 PERCENTAGE OF SAMPLED WELLS Figure 26. Distribution of estimated ultimate recoveries (EURs) for gas wells within the Black Warrior Basin Assessment Unit, Black Warrior Basin, Alabama. Only wells with minimum EURs exceeding 10 million cubic feet of gas (MMCFG) are shown.
100,000
%ARLY AVERAGE -IDDLE AVERAGE ,ATE AVERAGE 10,000
1,000
100
ESTIMATED ULTIMATE RECOVERY ULTIMATE ESTIMATED (MMCFG)
10 0 10 20 30 40 50 60 70 80 90 100 PERCENTAGE OF SAMPLED WELLS Figure 27. Distribution of estimated ultimate recoveries (EURs) by thirds for gas wells within the Black Warrior Basin Assessment Unit, Black Warrior Basin, Alabama. Only wells with minimum EURs exceeding 10 million cubic feet of gas (MMCFG) are shown. Assessment of Undiscovered Oil and Gas, Black Warrior Basin Province
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