EMD Gas Shales Committee Chair
note: Bustin is a professor in the Department of Geological Sciences
with the University of British Columbia.
are currently one of the hottest plays in the United States as a
result of high gas prices, the remarkable success in the Barnett
Shale in the Fort Worth Basin, technological advancements in drilling
and completions, and predicted near-term shortages of natural gas.
gas shales occur behind pipe in many developed basins where conventional
production is declining, an underutilized gathering infrastructure
exists and markets are accessible.
over 35,000 producing gas shale wells in the United States, with
a current cumulative production of about 600 bcf/year. Total gas
shale resources in the United States have been estimated in the
range of 500-600 tcf.
the United States gas shales have received little attention. Gas
shales resources in Canada are estimated to be over 1,000 tcf, and
many operators are assessing potential gas shales either as potential
incremental production or as stand alone, Barnett-type prospects.
are fine-grained rocks in which a significant component of gas storage
is by adsorption. Gas shales are unconventional, continuous-type
natural gas reservoirs (Schmoker, 1996) where accumulations are
shales, like coalbed methane, the shale is the source, reservoir
and trap for primarily methane and minor to significant amounts
of other gases, including ethane, carbon dioxide and nitrogen.
In as much
as the volume of adsorbed gas is strongly correlated with surface
area and the source of high surface area is in the microporous organic
fraction, it follows that gas shales are organic-rich and generally
fine grained. Rocks included in this definition of gas shales are
organic rich, true shales, mudrocks (non-fissile rocks comprised
mainly of clays), siltstone and very fine-grained sandstones.
producing gas shales additional gas storage capacity exists in intergranular
porosity and/or fractures; some gas shales, such as the Lewis Shale
in the San Juan Basin, grade into tight sands. Many gas shales have
substantial gas stored in the free state, and many tight sands have
gas stored in the sorbed state.
elements for successful development of gas shales, like coalbed
methane, are the presence of adequate gas in place and either the
existence of adequate permeability or a rock of suitable mechanical
properties for efficient completion and fracture stimulation.
of gas in place requires adequate gas generative organic matter
to generate either biogenic or thermogenic gas, and to retain significant
gas. The minimum amount of organic matter needed is unknown. Rocks
classified as gas shales have as little as 1.5 percent total organic
carbon (TOC) content to over 20 percent TOC.
are invariably fine-grained rocks, and as such have low matrix permeability
(<< 1 md). Past conventional wisdom has been that natural
fracturing is essential (i.e., Antrim Shale, Michigan Basin) or,
alternatively, more permeable interbeds of siltstone or sandstone
are required (i.e., Lewis Shale, San Juan Basin).
work on the Barnett Shale has brought the importance of pre-existing
natural fractures in question; some recent studies suggest the existence
of natural fractures in the Barnett impedes fracture stimulation,
whereas other studies suggest that wells with high pre-existing
fractures are the best producers.
from Devonian shales occurred as early as 1821 near Fredonia, N.Y.,
and by the late 1880s significant production was recorded from the
eastern United States' Appalachian Devonian shales, which were exploited
principally because of their proximity to a gas market.
a long period of limited activity and higher gas prices in the late
1970s, the introduction of the Section 29 Tax Credit in 1980 for
development of unconventional resources stimulated gas shale exploration
throughout the United States. Presently significant commercial gas
shale production occurs in the Barnett Shale in the Fort Worth Basin,
Lewis Shale in the San Juan Basin, Antrim Shale in the Michigan
Basin, Ohio Shale (and equivalents) in the Appalachian Basin and
New Albany Shale in the Illinois Basin (table 1, figure 1).
the biggest gas shale success story is the Mississippian-aged Barnett
Shale of the Fort Worth Basin. The undiscovered natural gas resources
in the Barnett Shale have been estimated at 26.2 tcf (Pollastro
et al. 2004), and currently there are over 3,500 producing wells.
(The history of the Barnett Shale and developments to 2002 were
summarized in the July
developed by Mitchell Energy, the Barnett play has continued to
grow outside the initial "core area" as a result of improved completion
and fracturing practices, horizontal drilling and successful refracing
of existing wells.
the biggest operator, has drilled over 800 wells since taking over
Mitchell Energy in 2001 and now operates over 1,700 Barnett wells.
In August 2004, Devon announced the drilling of its 100th horizontal
hole with rates up to 4 mmcfgd.
last several years Devon has been joined in the Barnett play by
other gas players, including Burlington, Chief Oil and Gas, EOG,
Quicksilver and EnCana.
are invited to view a presentation, bibliography and Web links on
gas shales in the members-only area (log in required) of the EMD Web site (emd.aapg.org).
Faraj, B., H. Willims, G. Addison, and B. McKinstry, 2004, Gas potential of selected shale formations in the Western Canadian Sedimentary Basin: Houston, Hart Publications, Gas TIPS, v. 10, no. 1, p. 21-25.
National Petroleum Council, 2003, Balancing natural gas policy, Volume II, Integrated report: National Petroleum Council Committee on Natural Gas.
Pollastro, R.M., R.J. Hill, T.A. Ahlbrandt, R.R. Charpentier, T.A. Cook, T.R. Klett, M.E. Henry, and C.J. Schenk, 2004, Assessment of undiscovered oil and gas resources of the Bend Arch-Fort Worth Basin Province of North-Central Texas and Southwestern Oklahoma, 2003: U.S. Geological Survey Fact Sheet 2004-3022, 2 p., available online at: http://pubs.usgs.gov/fs/2004/3022/
Schmoker, J. W., 1996, Method for assessing continous-type (unconventional) hydrocarbon accumulations, in D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds., 1995 National assessment of United States oil and gas resources – Results, methodology, and supporting data: U.S. Geological Survey Digital Data Series DDS-30, Release 2, [CD-ROM].
Shirley, K, 2002, Barnett Shale living up to potential: AAPG Explorer, v. 23, no. 7, p. 18-19, 27.