GEOLOGICAL SURVEY OF ALABAMA
Geological Foundation for
Production of Natural Gas from
Diverse Shale Formations
Under subcontract agreement 07122-17
Project Duration: 7/31/2008-7/30/2011
To assist in the development of emerging gas shale plays in Alabama, the Geological Survey of Alabama has completed a three-year study on the geology and development potential of shale gas plays. The Black Warrior basin and Appalachian thrust belt of Alabama contain a diversity of emerging gas shale plays in Cambrian through Mississippian strata (fig. 1) with gas resources that may exceed 800 trillion cubic feet (original gas-in-place). Development of these reservoirs faces technical challenges, including uncertainty about best practices for exploration, drilling, and well completion. This uncertainty stems largely from inadequate characterization of the geologic framework of the targeted shale formations and is compounded by major differences of composition, thickness, geometry, and fracture architecture from proven gas shale reservoirs in other regions.
Figure 1.—Generalized map showing shale-gas development areas in Alabama
(modified from Pashin, 2008).
A critical need exists for a basic conceptual framework for the production of natural gas from shale formations with diverse characteristics. Accordingly, this study applied an interdisciplinary approach that synthesizes stratigraphy, sedimentology, geologic structure, hydrodynamics, geothermics, petrology, and geochemistry to characterize and quantify shale gas resources and reserves (fig. 2).This study was designed to increase knowledge of the mechanisms of gas storage and the sources of permeability in shale formations with diverse composition and geology. This integrated approach reduces risks associated with exploration and development and will provide for a reasonable assessment of resources and reserves. This study is assisting industry in the formulation of exploration and development strategies that are optimized for each gas shale play and has derived basic scientific concepts and models that can be applied to emerging and frontier shale plays throughout North America.
Figure 2.—Conceptual model showing geologic factors that influence the producibility of natural gas resources from shale.
The Middle Cambrian Conasauga Formation of the Appalachian thrust belt hosts the geologically oldest gas shale play in the world and one of the most structurally complex gas shale formations discovered to date (fig. 3). It can be characterized as a shoaling-upward succession in which shale passes vertically into a broad array of inner ramp carbonate facies. The shale was deposited on the outer ramp, and has been thickened tectonically into antiformal stacks. Complexly folded and faulted, the shale is in places thicker than 12,000 feet. In 2005, Dominion Exploration and Production, Inc. began the first commercial gas production from shale in Alabama in what is now Big Canoe Creek Field. Other antiformal stacks of intensely deformed Conasauga shale exist in the Alabama Appalachians, thus significant potential exists for expansion of the Conasauga play.
Figure 3.—Structural cross section showing tectonically thickened mass of gas shale in the Conasauga Formation (modified from Thomas and Bayona, 2005).
Shale of Devonian age has gas potential in both the Appalachian thrust belt and the Black Warrior basin (fig. 4). Natural gas has been produced from Devonian shale since the 19th Century, yet the potential for gas production from Devonian shale in Alabama is just beginning to be realized. To date, production has been established in the Chattanooga Shale along the Appalachian frontal structures, but potential also exists in Silurian-Devonian strata of the Appalachian Thrust Belt. The Chattanooga Shale is an extremely widespread, radioactive black shale unit that is in places jointed (fig. 5) and in others tectonically sheared. Thin compared to other established Devonian shale formations, development in the Chattanooga has been focused in shale between 30 and 50 feet thick. Early production efforts through vertical wells have been highly promising, and a series of horizontal wells have been drilled.
Figure 4.—Regional stratigraphic cross section of the Black Warrior basin showing the Chattanooga Shale and Neal (Floyd) Shale (modified from Pashin, 1994).
Figure 5. —Devonian Chattanooga Shale disconformably overlying Silurian Red Mountain Formation in northeastern Alabama. Note well-developed joint system in Chattanooga Shale.
The Upper Missisippian Floyd Shale is an equivalent of the prolific Barnett Shale of the Fort Worth Basin. Drillers have long recognized a resistive, organic-rich shale interval in the lower part of the Floyd Shale that is informally called the Neal shale (fig. 4). The Neal shale is an organic-rich, starved-basin deposit that is considered to be the principal source rock for conventional hydrocarbons in the Black Warrior basin (fig. 6). Several wells have been drilled in search of natural gas in the Neal shale of Alabama, but to date, these wells have achieved limited success. Thus far, only vertical wells have been drilled, and horizontal wells may help maximize resource recovery. Many of these wells are near fault zones, yet wells producing from the Barnett near faults have reduced production performance.
Figure 5. —Neal shale in a core from Pickens County, Alabama.
Core is 4 inches wide.
Jack Pashin, Principal Investigator (Stratigraphy, Structure)
Richard Carroll (Organic petrology)
Rashmi Grace (Geochemistry, outreach)
Denise Hills (Geophysics)
David Kopaska-Merkel (Petrology, Sedimentology)
Marcella McIntyre (Structure)
This project includes a vigorous technology transfer program that is designed to facilitate the development of gas shale resources. Results are being presented at technical meetings and workshops and are being published in technical journals and meeting proceedings. This website is central to technology transfer activities, and links to relevant reports and materials are given below:
Carroll, R. E., Pashin, J. C., and Kugler, R. L., 1995, Burial history and source-rock characteristics of Upper Devonian through Pennsylvanian strata, Black Warrior basin, Alabama: Alabama Geological Survey Circular 187, 29 p. (PDF 3.5 Mb)
Pashin, J. C., 2008, Gas shale potential of Alabama: Tuscaloosa, Alabama, University of Alabama, College of Continuing Studies, 2008 International Coalbed & Shale Gas Symposium Proceedings, paper 0808, 13 p. (PDF 2.8 Mb)
Pashin, J. C., 2008, Stacks, limbs, and hinges: Shale gas plays of the southern Appalachian thrust belt: Geological Society of America Abstracts with Programs, v. 40, no. 6, p. 233. (Link)
Pashin, J. C., 2009, Shale gas plays of the southern Appalachian thrust belt: Tuscaloosa, Alabama, University of Alabama, College of Continuing Studies, 2009 International Coalbed & Shale Gas Symposium Proceedings, paper 0907, 14 p. (PDF 1.5 Mb)
Pashin, J. C., Grace, R. L. B., and Kopaska-Merkel, D. C., 2010, Devonian shale plays in the Black Warrior basin and Appalachian thrust belt of Alabama: Tuscaloosa, Alabama, University of Alabama, College of Continuing Studies, 2010 International Coalbed & Shale Gas Symposium Proceedings, paper 1016, 20 p. (PDF 12 Mb)
Pashin, J. C., Kopaska-Merkel, D. C., Arnold, A. C., and McIntyre, M. R., 2011, Geological foundation for production of natural gas resources from diverse shale formations: Sugarland, Texas, Research Partnership to Secure Energy for America Final Report 07122.17.01; Geological Survey of Alabama Open-File Report 1110, 156 p. (PDF 30.9 Mb)
Carroll, R. E., Pashin, J. C., and Kugler, R. L., 1995, Burial history and source-rock characteristics of Upper Devonian through Pennsylvanian strata, Black Warrior basin, Alabama: Alabama Geological Survey Circular 187, 29 p.
Cleaves, A. W., and Broussard, M. C., 1980, Chester and Pottsville depositional systems, outcrop and subsurface, in the Black Warrior basin of Mississippi and Alabama: Gulf Coast Association of Geological Societies Transactions, v. 30, p. 49-60.
Cleaves, A. W., 1983, Carboniferous terrigenous clastic facies, hydrocarbon producing zones, and sandstone provenance, northern shelf of Black Warrior basin: Gulf Coast Association of Geological Societies Transactions, v. 33, p. 41-53.
Hill, R. J., and Jarvie, D. M., eds., 2007, Barnett Shale: American Association of Petroleum Geologists Bulletin, v. 91, p. 399-622.
Kidd, J. T., 1975, Pre-Mississippian stratigraphy of the Warrior Basin: Gulf Coast Association of Geological Societies Transactions, v. 25, p. 20-39.
Mars, J. C., and Thomas, W. A., 1999, Sequential filling of a late Paleozoic foreland basin: Journal of Sedimentary Research, v. 69, p. 1191-1208.
Pashin, J. C., 1993, Tectonics, paleoceanography, and paleoclimate of the Kaskaskia sequence in the Black Warrior basin of Alabama, in Pashin, J. C., ed., New Perspectives on the Mississippian System of Alabama: Alabama Geological Society 30th Annual Field Trip Guidebook, p. 1-28.
Pashin, J. C., 1994, Cycles and stacking patterns in Carboniferous rocks of the Black Warrior foreland basin: Gulf Coast Association of Geological Societies Transactions, v. 44, p. 555-563.
Pashin, J. C., and Kugler, R. L., 1992, Delta-destructive spit complex in Black Warrior basin: facies heterogeneity in Carter sandstone (Chesterian), North Blowhorn Creek oil unit, Lamar County, Alabama: Gulf Coast Association of Geological Societies Transactions, v. 42, p. 305-325.
Rheams, K. F., and Neathery, T. L., 1988, Characterization and geochemistry of Devonian oil shale, north Alabama, northwest Georgia, and south-central Tennessee (a resource evaluation): Alabama Geological Survey Bulletin 128, 214 p.
Thomas, W. A., 1988, The Black Warrior basin, in Sloss, L. L., ed., Sedimentary cover—North American craton: Geological Society of America, The Geology of North America, v. D-2, p. 471-492.
Thomas, W. A., and Bayona, G., 2005, The Appalachian thrust belt in Alabama and Georgia: thrust-belt structure, basement structure, and palinspastic reconstruction: Alabama Geological Survey Monograph 16, 48 p.