The future of natural gas exploration in the
Foothills of the Western Canadian Rocky
Mountains
ANDREW C. NEWSON, Moose Oils Ltd., Calgary, Alberta, Canada
T
he Foothills of the Western
Canadian Sedimentary Basin (WCSB)
cover 40 000 miles2 in the fold and
thrust belt of the Canadian Rocky
Mountains. The topmost northwestern point lies just north of the
Northwest Territories border at the
town of Fort Liard. The Foothills also
occupy part of the adjacent Yukon
Territory, run southeast through
British Columbia and Alberta, and
terminate near the U.S./Canadian
border (Figure 1).
The northwestern and southeastern limits are controlled by political
boundaries and the extent of the natural gas gathering system. The width
is defined on more geologic grounds.
The Triangle Zone (Figure 2) defines
the eastern side. This is a descriptive
term for the subsurface shape of the
rocks (in cross-section) that form the
effective edge of the fold and thrust
belt beyond which lie WCSB’s conventional exploration and development plays. The western edge is
defined generally by the topographic
high formed by the front ranges of the
Rocky Mountains. The extreme relief
of this topographic high limits access.
It is also frequently the eastern edge
of national or provincial parks,
another restriction to access.
The Foothills are part of the larger
fold and thrust belt of the Rocky
Mountains, where the sedimentary
rock has been deformed by horizontal compression. The rocks have been
effectively shortened by one of two
mechanisms. In some cases, reservoir
rocks faulted and stacked on top of
each other to form structures in
which the reservoir rock may be fault
repeated two or three times. In other
cases, horizontal compression created
tight folds in which the reservoir rock
may be broken or fractured. In areas
where the reservoir rock has been
fault repeated, fields may have multiple individual pools of hydrocarbon stacked on top of each other.
Where the reservoir rock has been
tightly folded, the resultant fractures
can greatly enhance the productive
capacity of a reservoir that would not
produce had it not been folded.
0000
THE LEADING EDGE
JANUARY 2001
Many Foothills fields have reservoir rock that has been fractured naturally. This fracturing causes a degree
of uncertainty in the calculation of
marketable gas reserves. This is
reflected by the unusually large difference between the marketable and
gas-in-place figures in certain Foothills
pools. A good example of underevaluating a reservoir is Moose Mountain,
a natural gas field that has a naturally
fractured reservoir. Between 1985 and
2000, the field produced steadily from
two pools. No additional wells were
tied in, nor was there any work on the
existing wells to access more reserves.
The original marketable reserves were
given as 130 billion ft3 with the inplace reserves of 250 billion ft3 (AEUB
1985). In 1999, total production
exceeded the original marketable
reserves and the field is still producing at 40 million ft3 per day.
Figure 1. The Foothills are enclosed
by the blue dots. Red = Rocky
Mountain Fold and Thrust Belt.
Green = Western Canadian
Sedimentary Basin. White line indicates the position of Figure 2.
The rock formations that produce
hydrocarbons in the Foothills are
spread throughout the stratigraphic
column (Figure 3). The youngest producing formation is the sandstone of
the Cretaceous Cardium formation.
The oldest producing formation is the
carbonates of the Devonian Beaverhill
Lake group. The bulk of the gas
reserves booked to date is in the
Mississippian aged rocks (26 trillion
ft3). The next largest reserves are found
in the Triassic and Devonian aged
rocks (6 trillion ft 3 apiece). The
Cretaceous has 2 trillion ft3.
Data published in 1999 by the
Alberta Energy and Utilities Board and
the British Columbia Ministry of
Economic Development estimated the
total gas-in-place reserves for the
Foothills at 40 trillion ft3 of which 19
trillion ft3 is considered marketable
and 13 trillion ft3 has been produced
to date.
Opportunities. Exploration and
development of hydrocarbon
reserves in this area are significantly
assisted by several factors, including:
• A large public domain data set that
is available for this area because of
technical mistakes made developing
the Turner Valley Field. When
exploitation of this field was just
beginning in the 1920s, the gas cap
of the yet undiscovered oil leg was
depleted for about five years—leaving a billion barrels of oil in the field
that are beyond recovery. As a result,
a joint government and industry regulatory agency, the Energy
Resources Conservation Board (a
precursor to AEUB), was formed.
Among the legacies of this agency is
the data set it compiled and made
public on all of Alberta’s wells, pools,
and fields. Similar action in British
Columbia, the Yukon, and the
Northwest Territories has also made
these data publicly available.
• The Geological Survey of Canada
began mapping the Rocky
Mountains in 1886 and has produced high-quality surface geologic
maps for much of the Foothills.
• Each year about 15 000 wells are
JANUARY 2001
THE LEADING EDGE
0000
added to the 333 000 wells already
drilled in the WCSB. At present,
300 wells are drilled in the Foothills
per year. As a result, a wide range
of equipment is readily available
for drilling, seismic acquisition, gas
processing, or laying pipelines.
• The Foothills have inspired much
advanced research over the years.
Articles in technical bulletins have
made a significant contribution to
the effectiveness of the exploration
and development in the area, and
many technical papers on the
Foothills have become landmark
papers for overthrust belts around
the world. Thus, technical sophistication about this area is high.
• Lastly, the Foothills belt is already
connected to the North American
gas gathering system. With naturalgas pipelines and plants in existence from Fort Liard in the north
to Waterton in the south, the area
is well served by infrastructure and
has good access to the natural-gas
markets of North America.
Foothills models. Foothills play
types have been broken up into five
categories based on three components: structural style, stratigraphic
framework, and history of discovery.
Structures in the Foothills may be
considered in the light of two end
members of structural style: fault
bend folds and detachment folds. The
fault bend fold (Figure 4) is a structure in which the fold shapes and size
are controlled by the relative position of the fault ramp in the hangingwall and the fault ramp in the footwall. It has nearly equal amounts of
displacement on both sides of the
structure. The detachment fold
(Figure 5) is a structure in which the
fold shape and size is controlled by
the amount of displacement and the
position of a flat fault in the core of
the structure. It has very marked difference in displacement on both sides
of the structure.
Five components are used to
define these two structural styles. In
a fault bend fold, (1) displacement
on the fault on both sides of the structure will be nearly equal; (2) beds will
be flat, unless a later-stage movement
deforms them; (3) bed thickness will
remain constant throughout the bulk
of the structures; (4) the angle
between the limbs will be high (110130°); and (5) the structure will have
low bedding dips on the fold limbs,
and it will be a low-amplitude structure.
In a detachment fold, (1) dis0000
THE LEADING EDGE
JANUARY 2001
Figure 2. Cross-section through the Alberta Foothills. Green = Cretaceous.
Blue = Mississippian. Purple = Devonian. Red = Cambrian. (After
Widdowson, 1995).
Figure 3. Generalized stratigraphic column for the Foothills showing the
main producing horizons.
placement on both sides of the structure will be very unequal; (2) beds
will have a syncline; (3) bed thickness
will vary greatly throughout the
structure due to disharmonic folding
and small-scale back thrusts and forethrusts; (4) the angle between the
limbs of the structure will be small
(40-90°); and (5) the structure will
exhibit high bedding dips on the fold
limbs, and it will be a high-amplitude structure.
The application of the stratigraphic model in classifying structural styles can be illustrated by the
Mississippian-aged Turner Valley
Formation of the south-central
Alberta Foothills, an area that contains 70% of Foothills reserves. This
part of the Foothills has several major
far-traveled thrusts that run northwest and southeast, parallel with the
edge of the Foothills belt. They generally lie in the center of this belt.
These thrusts are the Livingstone,
Moose, and Brazeau thrust faults. The
facies of the Turner Valley Formation
varies considerably across these
thrusts. To the northeast side of the
fault, this formation is dominantly a
grainstone; on the southwest side the
facies is dominated by packstone and
wackstone. This transition in facies of
the reservoir rock can occur in just 68 miles, as between the Jumping
Pound West and Morley gas fields. If
the reservoir rock were returned to its
prethrusting position, this distance
would be more than 66 miles. This
has a considerable impact on the
interpretation of the stratigraphic
model for these two fields. In other
Foothills formations, the relationship
is more complicated, especially if the
direction of movement on the fault or
fold is oblique to the facies belt.
The earliest significant Foothills
discovery was in 1913, the Cretaceous
and Mississippian gas condensate
and oil play at Turner Valley. This
JANUARY 2001
THE LEADING EDGE
0000
started 25 years of active development of this field. In 1957 came the
discovery of Waterton, the Foothills'
largest gas condensate field. This
Mississippian and Devonian play has
in-place reserves of 4 trillion ft3. Then
in 1968 came the discovery of the
Ricinus Cretaceous gas condensate
play. All three fields are dominated
by a fault bend fold structural style.
It was not until 1977, with the
Sukunka discovery in the Triassicaged rocks, that the industry began
to appreciate the importance of the
detachment fold. The last important
date is 1979, the first well in
Blackstone Field. This proved the
potential of the Devonian Beaver Hill
Lake group reefs.
Foothills play types. The Foothills
have been divided into five categories
based on structural style, stratigraphic framework, and history of
exploration (Figure 6).
First-Generation plays of the
Mississippian aged reservoir formation dominated Foothills exploration
from 1914 to 1960. These plays have
contributed 37% of in-place gas
reserves. They are in structures
formed by single thrust sheets and
generally follow a fault bend fold
structural style. Formed along the
outer edge of the Foothills belt, these
plays lie in the part of the Turner
Valley Formation where the environment of deposition was dominantly high energy, as is reflected in
the grainstones that form most of the
Turner Valley Formation. The outer
Foothills area has less rugged topography with softer Cretaceous surface
rocks that allow better seismic
imagery of the structures. Turner
Valley (Figure 7) is an example of this
type of play.
Second-Generation plays dominated exploration from 1945 to 1980.
These plays represent 27% of in-place
reserves. They contain both
Mississippian and Devonian aged
reservoir rocks and are formed in
structures composed of multiple
thrust sheets with a dominant fault
bend fold structural style. SecondGeneration plays lie in the inner
Foothills belt in the part of the Turner
Valley Formation formed in a lower
energy environment of deposition
that resulted in the rock matrix being
dominated by wackstones and packstones. These rocks are generally inferior to the grainstones of the outer
Foothills and need fractures to
enhance productivity. The inner
Foothills area has rugged topogra0000
THE LEADING EDGE
JANUARY 2001
Figure 4. Diagram illustrating constant displacement across a fault bend
fold and its key elements. (1) Nearly equal amounts of displacement on
both sides of the structure carried on one major thrust. (2) Flat beds in the
footwall. (3) Constant bedding thickness throughout the structure. (4)
Generally high interlimb angles (>120°). (5) Low dips on fold limbs/lowamplitude structure.
Figure 5. Diagram illustrating variable displacement across a detachment
fold and its key elements. (1) Unequal amounts of displacement on both
sides of structure due to folding. (2) Frontal syncline developed to the
equivalent level of the lower detachment. (3) Variation in bedding thickness throughout the structure due to disharmonic folding and small-scale
backthrust and forethrust. (4) Generally low interlimb angles (<90°). (5)
High dips on fold limbs/high-amplitude structure).
phy, often with high-velocity
Paleozoic Carbonates at the surface.
These factors contribute to the poor
quality seismic data recorded.
However, because some parts of the
structure may be exposed at surface,
geologic surface mapping becomes
an effective tool. Moose Mountain
(Figure 8) is a good example of this
play type.
Third-Generation play types have
become increasingly important in
Foothills exploration since 1970. To
date, these plays have contributed
about 20% to the in-place reserve
base. Third-Generation plays form
structures that are dominated by the
detachment fold structural style. This
style has the ability to fracture the
reservoir rock due to the way this
fold style develops. As a result, rocks
with moderate to poor matrix reservoir can become good producers.
However, the disadvantage is that
fields that are made up of good
matrix reservoir rock may have their
cumulate production negatively
affected by fractures. The structures
in this play type have steep dips and
disharmonic folding. This leads to
problems with seismic imaging,
although they can often be effectively
mapped using surface mapping techniques because of the amplitude of
the folds. Findley (Figure 9) is a good
Figure 6. The five Foothills play
types.
example.
Reef/stratigraphic plays have
had a relatively minor role in
Foothills exploration strategy from
1970 to 2000 and contributed only 5%
of in-place reserves. This is dominantly a stratigraphic play that
extends from the conventional part of
the WCSB and can occur in either the
regional or on thrust sheets in the
Foothills. In both cases, seismic imagJANUARY 2001
THE LEADING EDGE
0000
ing is hampered by the complex
geology of the shallower strata, large
variations in topography, and steep
dips of the reservoir rock itself or in
the overlying strata. On the positive
side, the reef plays to date have been
prolific producers from good matrix
reservoirs in the Devonian Swan Hills
group and moderately good from the
Devonian Palliser, Nisku, and Leduc.
It is a play that relies heavily on
advances in seismic technology for
the imaging necessary to provide successful drilling locations.
The Triangle Zone is actually a
complex interaction of the two structural styles; i.e., it has multiple thrust
sheets as well as a detachment folding component to enhance tight reservoir rock. There has been a
resurgence in exploration of this play
type since 1995. It has contributed
10% of in-place reserves. This play
type involves primarily Cretaceous
aged sandstone reservoir rocks.
Because of its position in the outer
Foothills and the velocity contrast
between the rocks, it is well imaged
on seismic data. Cabin Creek is a
good example (Figure 10).
Geographically, the Foothills can
be divided into two areas by using
the Alberta/British Columbia border.
The play types in the southern part
are dominated by the fault bend fold
structural style. The northern half
(British Columbia, Yukon, and
Northwest Territories) is dominated
by the detachment fold style,
although examples of both types of
structural style occur in either area.
Alberta’s First-Generation plays
are generally in the outer part of the
Foothills. Second-Generation plays
are in the inner part close to the front
ranges. Third-Generation plays are
not numerous but are more common
in the north near the Alberta/British
Columbia border. Triangle Zone
plays are concentrated on the outer
edge of the Alberta Foothills. Reef/
stratigraphic play types also are concentrated in the central part of the
Foothills.
As of the end of 1998, Alberta’s
Foothills fields were producing 500
billion ft3 of natural gas per year.
They have 29 trillion ft3 in place, of
which 16 trillion ft3 is marketable.
Production to date has been 10 trillion ft3.
To the north, the distribution of
plays is very different. The dominant
play is the Third-Generation play
type. First-Generation play types do
not form a significant part of these
fields, and there is no production
0000
THE LEADING EDGE
JANUARY 2001
Figure 7. Section through Turner Valley, a First-Generation play type (after
Gallup, 1951).
Figure 8. Section through Moose Mountain, a Second-Generation play
type.
Figure 9. Section through Findley, a Third-Generation play type.
JANUARY 2001
THE LEADING EDGE
0000
from Second-Generation, Triangle
Zone, or reef/stratigraphic play
types. As of the end of 1998, these
northern fields were producing 100
billion ft3 of natural gas per year.
They have 9 trillion ft3 in place, of
which 5 trillion ft3 is marketable. This
area has produced 3 trillion ft3.
The future. The real excitement in
the Foothills is the opportunities for
new play development—the ideas
that add new reserves and value to
some existing reserves. These ideas
tend to fall into the following categories:
New concepts—ideas that have the ability to make significant changes to the
exploration and development potential
of the Foothills plays.
There is an uncertainty in the
booked reserves of the Foothills
because of the fractured nature of
reservoir rocks. The published figures are 40 trillion ft3 in place, with
21 trillion ft3 being marketable. Even
allowing for the sour nature of the
gases, the difference between these
numbers is too high. Some of this 19
trillion ft3 represents an uncertainty
of booked reserves. As seen at Moose
Mountain, marketable reserves of
some Foothills play types are underevaluated. Better understanding the
generation of fractures and their contribution to production may add substantial marketable reserves to the
Foothills.
Also, as exploration groups deal
with more plays formed by the
smaller, poorly imaged detachment
folds, linkages within structure and
between structure become important.
Much existing interpretation assumes
constant displacement faults that are
“hard” linked. However, in detachment folding domains, many structures are not linked in this manner
and they are best described as “soft”
linked.
Lastly, there are numerous examples of wells missing the leading edge
of structure in the Foothills. Although
this is often attributed to poor seismic imaging, enough evidence suggests that the kinematic model for
fold and fault generation at the leading edge of thrust sheets is more variable than presently understood. This
is well illustrated by the F-25
Northcore et al. Liard well on the
Liard anticline (Figure 11). There is
enough evidence on the dipmeter
and sample description to interpret
a detachment between the Nahanni
Formation carbonates and the over0000
THE LEADING EDGE
JANUARY 2001
Figure 10. Section through Cabin Creek, a Triangle Zone play type.
Figure 11. Section through F-25 Northcore et al. Liard illustrating localized
detachment of the shale from the carbonate.
lying Besa River shale. This detachment appears very localized to the
leading edge of the structure.
New structural opportunities that
involve all play types. First-Generation
plays are still being successfully
developed and are frequently close to
established production. The northern extension to Wildcat Hills is a
good example.
Second-Generation plays are still
being sought as well. The 1998 well
Northstar et al. Highrock c-67-B 82G-15 west of the Crowsnest Pass in
southeastern British Columbia is a
good example. This well was
designed to test a large structure
found beneath the Lewis Thrust.
Third-Generation plays are being
very actively pursued. The well
Husky Benjamin 16-28-28-8 W5M
drilled by Husky in 1991, is a good
example of a very successful well
drilled into a detachment fold.
Triangle Zone plays are being
developed in several locations along
the Alberta Foothills (e.g., in the eastern edge of the Foothills between
townships 27 and 56). In addition,
potential exists for older abandoned
triangle zone faults to the west of the
traditional location of these play
types.
New stratigraphic opportunities based
on advanced technology. The Devonian
Reef play in the Central Alberta
Foothills is exciting. The technical
challenge of directly imaging it is one
that has yet to be overcome.
Other opportunity exists in
untested areas in the Foothills where
reef development may exist on either
JANUARY 2001
THE LEADING EDGE
0000
the thrust sheets or in region.
Lastly there is the opportunity to
develop stratigraphic plays on the
large far-traveled thrust sheets. In this
situation the formation may be gently dipping. This could allow direct
detection of hydrocarbons by seismic
attributes.
New formation opportunities. A large
percentage of existing reserves that
have been added are in the
Mississippian-aged rocks. Other
rocks in the stratigraphic column
offer potential as well. Many of the
criteria for a successful play are found
deeper in the Cambrian Formation.
Elsewhere, the Permo-Penn in the
Foothills appears to be a worthwhile
target. Shallow Cretaceous and
Triassic rocks also offer potential in
the correct structural position.
New area opportunities. Some areas of
the Foothills have less than one well
per township. With about 2000 key
wells scattered throughout the
Foothills, it is far from being “drilled
up.” To the north, beyond the present
end of Foothills development at Fort
Liard, is another 40 000 miles2 that
terminates at the Arctic Ocean. It has
many of the characteristics of the
Foothills. It is underexplored, with
only 200 wells drilled in to it. It is a
relatively narrow band of complex
geology in a remote area with difficult access. At the moment, it lacks
infrastructure to get any discovered
gas to market. However, for the exporationist willing to accept the challenge, it offers structural and
stratigraphic plays in abundance.
Suggested reading. Alberta’s Reserves
of Crude Oil, Oil Sands, Gas, Natural Gas
Liquids and Sulphur (AEUB, Calgary,
1999). “Structure, seismic data and orogenic evolution of southern Canada
Rocky Mountains” by Bally et al.
(Bulletin of Canadian Petroleum Geology,
1966). Facies Relationships at the
Mississippian Carbonate Margin, Western
Canada by Bamber et al. (GSC Open
File
Report
OF-674,
1980).
“Mississippian Stratigraphy and
Sedimentology, Canyon Creek (Moose
Mountain), Alberta” by Bamber et al.
(in Field Guides to Geology and Mineral
Deposits, 1981). “Sukunka-Bullmoose
gas fields: Model for a developing trend
in the southern Foothills of northeastern B.C.” by Barss and Montandon
(Bulletin of Canadian Petroleum Geology,
1981). Hydrocarbon and By-Product
Reserves in British Columbia (British
Columbia Ministry of Employment and
Investment, Energy and Minerals
0000
THE LEADING EDGE
JANUARY 2001
Division, 1996). “The analysis of fracture systems in subsurface thruststructures from the Foothills of the Canadian
Rockies” by Cooper (in Thrust Tectonics,
Chapman and Hall, 1992). “Balanced
cross sections” by Dahlstrom (Canadian
Journal of Earth Sciences, 1969). “Aspects
of three-dimensional structure of the
Alberta Foothills and Front Ranges” by
Fermor (GSA Bulletin, 1999). “Geology
of Turner Valley oil and gas field,
Alberta, Canada” by Gallup (AAPG
Bulletin, 1951). “The role of strain in
area-constant detachment folding” by
Groshong and Epard (Journal of
Structural Geology, 1994). “Folded faults
and sequence of thrusting in Alberta
Foothills” by Jones (AAPG Bulletin,
1971). Triangle Zones and Tectonic Wedges
(special issue of Canadian Petroleum
Geology, 1996). “Devonian reefs in
Canada and some adjacent areas” by
Moore (in Canada and Adjacent Areas,
Memoir 13, Canadian Society of
Petroleum Geologists, 1988).
Stratigraphy, Sedimentology, Structural
Geology and Exploration History of the
Mississippian at Moose Mountain,
Southwestern Alberta Foothills by Mundy
et al. (Field Trip Guide, 1995 CSPGCWLS First Joint Symposium).
Exploration Targets in the Canadian Rocky
Mountain Foothills by Newson and
Sanderson (Field Trip Guide, 1999
CSPG and Petroleum Society Joint
Convention). “Three-dimensional
geometry of the structural front
between Berland River and Smoky
River, central Alberta Foothills” by Liu
et al. (Bulletin of Canadian Petroleum
Geology, 1996). “The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains” by
Price (in Thrust and Nappe Tectonics,
Geological Society of London Special
Publication 9, 1981). “Geometry and
kinematics of fault bend folding” by
Suppe (American Journal of Science,
1983). “The triangle zone at Cabin
Creek, Alberta” by Teal (in Seismic
Expression of Structural Styles, AAPG
Studies in Geology Series 15, 1983).
“The nature and significance of large
‘blind’ thrusts within the northern
Rocky Mountains of Canada” by
Thompson (in Thrust and Nappe
Tectonics). “Structural cross-section
through Moose Mountain” by
Widdowson (in Stratigraphy,
Sedimentology, Structural Geology and
Exploration History of the Mississippian
at Moose Mountain, Southwestern Alberta
Foothills, Field Trip Guide, 1993 CSPG
Annual Convention). LE
HOUSE
AD
Corresponding author: A. C. Newson,
[email protected]
JANUARY 2001
THE LEADING EDGE
0000