1. Indiana Geological Survey
Reservoir Characteristics and Gas Production Potential of Woodford Shale in the Southern Midcontinent
John B. Comer
Indiana Geological Survey
Indiana University

2. Distribution of Devonian Black Shale
Modified from Juergen Schieber, 2004

3. Woodford Lithofacies in the Southern Midcontinent
Black shale
Siltstone
Dolostone
Chert
Sandstone
Mudstone
Bypassed settings
Proximal and basin centers
Distal to open ocean
Proximal and locally derived
Local

4. Log Characteristics Texas and New Mexico
Pan American
No. 1 Walker
Shell No. 1
Chriesman
Mobil No. 1918
Parks Unit 2
Upper Woodford is missing and middle Woodford is truncated in the Lea County, New Mexico, and Cochran County, Texas, wells indicating that there was a period of uplift and erosion before the Mississippian Limestone was deposited. In the Midland County, Texas, well, lower Woodford is missing due to onlap over the Central Basin Platform. A telescoped section of Woodford containing all three units is present in Glasscock County, Texas. Ellison (1950) suggested that telescoped sections indicate contemporaneous uplift during Woodford deposition. For more details see Comer (1991).
Shell No. 5
Pacific Royalty
Reference log from Ellison (1950) Comer (1991) Fig. 7

5. Fractured Woodford Shale Reservoirs
North Aylesworth Field Marshall County, Oklahoma Texaco 1-K Drummond, 11-6S-6E Comer (1992) location A33; Comer (1987) Fig. 5e
Southeast Joiner City Field Carter County, Oklahoma California No. 1 Mullen, 29-5S-2W Comer (1992) location A21; Comer (1987) Fig. 5a
Oil reservoirs in the North Aylesworth Field include Woodford Shale, the overlying Sycamore Limestone, and the underlying Misener Sandstone and Hunton Group. Oil reservoirs in the Southeast Joiner City Field include Woodford Shale and the underlying Bois D’Arc and Chimney Hill Formations in the Hunton Group.
3,065 ft % TOC
8,983 ft % TOC
1.0 mm
0.2 mm
Vertical fractures and stylolites filled with bitumen Tension gash filled with chert Recrystallized Radiolaria

6. Comer (1992) location A25; Comer (1987) Fig. 4a-d
Fractured Woodford, Anadarko Basin
Columbia Fuel #1 Rainy Mountain, Kiowa County, Oklahoma, 23-6N-15W, 760 ft
Comer (1992) location A25; Comer (1987) Fig. 4a-d
8.1 % TOC
0.61 % Ro
Bitumen
Calcite
Superposition of phases filling these fractures shows that calcite formed first and bitumen invaded the remaining porosity before the fractures were completely mineralized. Calcite precipitates from aqueous solution and represents an early period of time when the Woodford was water wet. Bitumen was generated from indigenous type II kerogen and represents the period of time when primary migration occurred during the early oil generation stage of thermal maturation. With oil generation the Woodford became oil wet and the pore system became oil saturated. Bitumen filled fractures are characteristic of the Woodford in southern Oklahoma and indicate that the bitumen invaded fractures shortly after they formed. Fractures mostly formed during tectonic adjustments associated with Late Paleozoic orogenic activity.
1 mm
Core diameter = 2 inches

7. Comer (1991) location C2, sample C2-5
Fractured Woodford, Permian Basin
Humble No. 43 Yarborough & Allen, Ward County, Texas, Section 66, E. J. Brady Survey
Comer (1991) location C2, sample C2-5
Calcite
7175 ft
Fractures in Woodford Shale from the Permian Basin formed early and are mostly calcite filled. Fractures are highly deformed and the calcite commonly is twinned due to compaction and shearing . Bitumen is rarely found in fractures and is restricted to the small residual cavities or vugs that formed when calcite crystals growing outward from opposite walls of a fracture failed to join.
10.1 % TOC
0.55 % Ro
Type II Kerogen
1 cm
0.2 mm

8. Interbedded Black Shale and Chert Lithofacies, Southern Oklahoma
Arbuckle Mountain Uplift outcrop, Murray County, Oklahoma, 1-2S-2E
Comer (1992) location OK26, Hwy miles north of Dougherty; Comer (1987) Fig. 7a
Chert
Black Shale
Chert
Black Shale
Chert
Black Shale
Interbedded organic-rich, biogenic chert and black shale are characteristic of Woodford in south-central and southeastern Oklahoma.
Chert
0.2 m

9. Black Shale and Chert Petrology, Southern Oklahoma
Comer (1992) location OK35 Arbuckle Mtn Uplift
25-2S-1E; Comer (1987) Fig. 7c
Comer (1992) location OK 55 Ouachita frontal zone
4-2N-15E; Comer (1987) Fig. 7b
Black Shale
Highly compacted
Flattened Tasmanites spores (T)
Amorphous Type II organic matter (AOM)
Little or no chert
Chert
Tasmanites spores (T) uncompacted or slightly flattened (early chert (CT) cementation)
Amorphous Type II Organic Matter (uncompacted)
Well indurated, brittle, and tightly sealed
Tasmanites are marine algal spores that are hollow and easily flattened during compaction unless cementation occurs very soon after deposition.
Up to 35 % TOC
Up to 6.4 % TOC
0.2 mm
0.2 mm

10. Chattanooga Shale, Ozark Uplift, Belle Vista, Benton County, Arkansas
Boone Formation
St. Joe Member
(Mississippian)
Chattanooga Shale
(Devonian)
2.1 % TOC
1.11 % Ro
Mixed marine/terrestrial kerogen
US 71
12-20N-31W
Comer (1992) location AR1

11. Black Shale – Sandstone Association, Ozark Uplift Sylamore Sandstone Type Area, 21-15N-11W, Stone County, Arkansas
Interbedded black shale and medium-grained supermature quartzarenite 8.5 ft thick; Quartz inherited from Middle Ordovician sandstones Comer (1992) location AR9
Comer (1987) Fig. 3a-c
Phosphate
Black Shale
Oil Residue
Black Shale
3.5 % TOC (avg)
0.83 % Ro (avg)
Black Shale consists of zones of
amorphous marine Type II kerogen
terrestrial Type III kerogen
mixed marine and terrestrial kerogen
Quartz
0.75 mm
0.2 mm

12. Black Shale Characteristics
Mobil No Parks Unit 2, Midland County, Texas,
Section 14, Block 40, C. F. O’Neal Survey
11,544 ft,
3.1 % TOC
Comer (1991)
location C1,
sample C1-5
Parallel laminae
Abundant pyrite
Fine grain size
Black color
High radioactivity
Abundant organic carbon
Amorphous (marine) type II kerogen
1 cm
11,555 ft
4.2 % TOC
Comer (1991) location C1,
sample C1-10, Fig. 4a
3 mm

13. Siltstone Characteristics
Shell A No. 1 Williamson, Gaines County, Texas,
Section 110, Block H, D&WRR Survey, 13,064 ft
Black Shale
Bioturbated Siltstone
Pyrite
1 cm
Comer (1991) location C11, sample C11-10

14. Siltstone Characteristics
Shell A No. 1 Williamson, Gaines County, Texas,
Section 110, Block H, D&W RR Survey, 13,064 ft
Comer (1991) location C11, sample 11-10, Fig. 5h
Siltstone Characteristics
Subequal random mixture of detrital dolomite (48%) and quartz (52%)
Median grain size is 0.05 mm (coarse silt) for both dolomite and quartz
Dolomite is angular and abraded with random orientations and uniform texture
Siltstone is dense and well indurated
Much of the dolomite in siltstones is resedimented from shallow-water evaporitic settings. See Comer (1991) for detailed discussion.
0.1 mm

15. Siltstone Depositional Processes
Silt was deposited by bottom flows
Ripple wavelength ~ 1.5 cm
C (Rippled)
B (Flat)
Bouma Sequence
A (Graded)
Scoured Base
Hybrid quartz/dolomite siltstones like the ones shown here were deposited from bottom flows that were mostly storm-generated . See Comer (1991).
Humble No. 1 A. E. State, Lea County, New Mexico,
16-15S-33E, 13,768 ft
Comer (1991) location C3, sample C3-5, Fig. 5e
3 mm

16. Lithologic Variations, Texas and New Mexico
Modified from Comer (1991) Fig. 9
Upper left panel: Siltstones are most abundant in two different types of locations (1) proximal settings near emergent source areas and (2) deep basin depocenters where bottom flows finally converge.
Upper right panel: Shallow-water sedimentary structures include burrows and desiccation cracks. They represent depositional settings that were topographically elevated during the Late Devonian.
Lower left panel: Dolomite is more abundant that quartz in shallow-water settings along the flank of the Eastern Shelf, on the Central Basin Platform, and on shoals in the far western part of the basin. The distribution suggests that these were the areas where most of the contemporaneous dolomitization was occurring.
Lower right panel: Biogenic chert (predominantly recrystallized Radiolaria) is found mostly along the Central Basin Platform and the eastern flank of the Midland Basin suggesting that these areas were bypassed by bottom flows and experienced intrabasinal upwelling.

17. Woodford Resource Potential
For TOC and thermal maturity data see Comer (1992).
Modified from Comer (2005)

18. Woodford Resource Potential
In-Place Estimates Based On Hydrogen Mass Balance
600 x 1012 ft3
60 x 109 bbl
4.4 x 1012 ft3
70 x 109 bbl
0.24 x 1012 ft3
For a detailed explanation of the calculations used to make these estimates, see Comer (2005).
TOTAL RESOURCE POTENTIAL
Total Estimated Oil-in-Place 130 x 109 bbl
Total Estimated Gas-In-Place 600 x 1012 ft3
Modified from Comer (2005)

19. Woodford Resource Potential
For TOC and thermal maturity data see Comer (1991).
Modified from Comer (2005)

20. Woodford Resource Potential
In-Place Estimates Based On Hydrogen Mass Balance
35 x 109 bbl
0.11 x 1012 ft3
84 x 109 bbl
9.0 x 1012 ft3
220 x 1012 ft3
For a detailed explanation of the calculations used to make these estimates, see Comer (2005).
TOTAL RESOURCE POTENTIAL
Total Estimated Oil-in-Place 120 x 109 bbl
Total Estimated Gas-In-Place 230 x 1012 ft3
Modified from Comer (2005)

21. Conclusions
Unconventional gas discoveries in Woodford Shale are likely in both the Anadarko and Permian Basins and adjacent provinces where
Woodford Shale is thermally mature
Fractures are common
Competent lithofacies (chert, siltstone, dolostone, sandstone, silty black shale) are abundant
Areas having greatest gas production potential and most prospective lithologies are the
Anadarko Basin in Oklahoma (siltstone and silty black shale)
Arkoma Basin in Oklahoma and Arkansas (silty black shale)
Frontal zone of Ouachita fold belt in Oklahoma (chert)
Delaware Basin in Texas and New Mexico (siltstone and silty black shale)
Val Verde and Midland Basins in Texas (siltstone and silty black shale)

22. Key References
Comer, J. B., 1991, Stratigraphic analysis of the Upper Devonian Woodford Formation, Permian Basin, West Texas and southeastern New Mexico: Austin, Texas, Bureau of Economic Geology, Report of Investigations 201, 63 p.
Comer, J. B., 1992, Organic geochemistry and paleogeography of Upper Devonian formations in Oklahoma and northwestern Arkansas, in K. S. Johnson, and B. J. Cardott, eds., Source Rocks in the Southern Midcontinent, 1990 Symposium: Norman, Oklahoma, Oklahoma Geological Survey, Circular 93, p
Comer, J. B., 2005, Facies distribution and hydrocarbon production potential of Woodford Shale in the southern Midcontinent, in B. J. Cardott, ed., Unconventional Energy Resources in the Southern Midcontinent, 2004 Symposium: Norman, Oklahoma, Oklahoma Geological Survey, Circular 110, p
Comer, J. B., and H. H. Hinch, 1987, Recognizing and quantifying expulsion of oil from the Woodford Formation and age-equivalent rocks in Oklahoma and Arkansas: American Association of Petroleum Geologists Bulletin, v. 71, p
Ellison, S. P., 1950, Subsurface Woodford black shale, west Texas and southeast New Mexico: Austin, Texas, Bureau of Economic Geology, Report of Investigations 7, 20 p.