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Region: Northern Midcontinent
System: Ordovician
Viola Oil and Gas--Summary
This Viola Play information is from the
U. S. Geological Survey
1995 National Assessment of United States Oil and Gas Resources (available
on CD-ROM from the U.S.G.S. as Digital Data Series DDS-30, Release 2).
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Viola Oil and Gas Play
by Mitchell E. Henry and Timothy C. Hester
This play is composed of carbonate rocks of the Viola Limestone. The
play extends throughout the province except in the westernmost part and
that part included in the Deep Structural Gas Play. As with the
Simpson Group, early work by Cole (1975) showed that the Viola Limestone
was absent in the northern and western part of the play. Recent well
data, however, indicate that it is absent only in southeastern Colorado
and part of the Texas panhandle. These rocks range in depth from about
4,000 ft in the northern and western parts of the province to 13,000 ft
in the southern part (the upper depth limit of Deep Structural Gas Play).
Carbonate rock lithology is the primary defining feature of this
play.
Reservoirs
Reservoir rocks are limestone and dolomite. The entire formation ranges
in thickness from near zero in the northern part of the play to an
estimated 500 feet at the southern boundary (adjacent to
the Deep Structural Gas Play).
Only six major accumulations are known. According to Cole (1975) the
lithology is variable in the Viola Limestone and includes some
sandstones.
Source rocks, timing, and migration
The most likely source rocks for this play are Simpson Group shales. In
the northern part of the play, however, where the Silurian Hunton Group
is absent and the Woodford Shale directly overlies the Viola Limestone,
the Woodford Shale may also be a viable source rock. The upper
Ordovician Sylvan Shale may also contribute to Viola Limestone
production in some areas of the province. Burrus and Hatch (1989)
mention the Sylvan Shale as a potential source rock with a wide genetic
potential ranging from poor to good. They list TOC values (0 to 8.5
percent) not for the Sylvan Shale in particular, but for
undifferentiated Ordovician samples (Simpson Group and Sylvan Shale
combined). Thermal maturity levels for the Sylvan Shale and Simpson
Group are similar; 0.5 percent Ro (or greater) throughout the play.
Timing of trap development and hydrocarbon generation is favorable for
charging of Viola Limestone reservoirs.
Traps
Only one description of trap type was found for all the accumulations
assigned to this play. It was identified as a faulted anticline.
Because the areal distribution of major Viola Limestone reservoirs is
virtually identical to that of major Simpson Group reservoirs, this play
is also thought to be strongly influenced by structure. Reservoirs in
the Viola Limestone may be enhanced by fracturing or some subaerial or
near-surface stratigraphic modification possibly related to the major
structural uplifts along the eastern and northern province boundaries.
Probable seals for this play are the Viola Limestone or Sylvan Shale.
Exploration status
The Viola has been penetrated by about 6600 wells; about 150 of these
wells are productive. Drilling intensity of the Viola Limestone mirrors
that of the Simpson and Arbuckle Groups in that exploration is
concentrated along the eastern and northern province boundaries, where
virtually all known major accumulations are located. Away from these
productive areas, the Viola is uniformly but not intensely explored.
Gas is the primary product produced in four of the five major
accumulations. The largest accumulation is at the Union City field
which has an estimated ultimate recovery of 30 BCFG. Known reservoirs
are found at depths ranging from 5,600 to 10,000 ft.
Resource potential
It appears that reservoir enhancement and trap formation in this play
are strongly dependent on proximity to major structural features.
Although parts of the play are not well explored, they are not close to
major structural features and are thus, not highly prospective.
Therefore, this play is not expected to contain significant quantities
of undiscovered hydrocarbons. The outlook is based primarily on
historical drilling and reservoir discovery data.
Play Map
References
Burruss, R.C., and Hatch, J.R., 1989, Geochemistry of oils and
hydrocarbon source rocks, greater Anadarko basin--evidence for multiple
sources of oils and long-distance oil migration, in Johnson, K.S., ed.,
Anadarko Basin Symposium, 1988: Oklahoma Geological Survey Circular 90,
p. 53-64.
Cole, V.B., 1975, Subsurface Ordovician-Cambrian rocks in Kansas:
Kansas Geological Survey, Subsurface Geology Series 2, 18 p.
Kansas Geological Survey, Digital Petroleum Atlas
Updated May 28, 1998
Comments to webadmin@kgs.ku.edu
URL=http://www.kgs.ku.edu/DPA/frontEnd/Anadarko/Viola/provSum.html