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.
Target
Shale Formations
Conasauga Formation
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).
Devonian
Shale
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.
Neal
(Floyd) 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.
Project
Team
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)
Technology
Transfer
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)
Final
Report
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)
Some
Useful References
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.
