Lookout Butte
Cedar Hills
800 MMBO
LEGEND
Bowman County
Little Beaver
Adams County
Wrench
Fault
Trend
Medicine Pole Hills
Cedar Creek
Horse Creek
ND
Harding Springs
6 MMBO
Repeat
1 MMBO
SD
Fracture
(dolomitized)
Horse Creek
Oil Field
Producing
Horizontal
Well
Buffalo
40 MMBO
Producing
Vertical
Well
Fee Acres
State Acres
MONTANA
Phoenix
GeoExplorati on
2009
www.phxgeo.com
SOUTH DAKOTA
Harding County
Perkins County
ACREAGE:
590,000 gross / 464,000 net acres (725 square miles net).
OUR PROJECT AREA:
Estimated reserves: 400 million to 1 billion barrels.
PROSPECT:
Horizontal drilling across hydrothermally-dolomitized fracture
zones in the Red River “B” has made the Cedar Hills area
into an 800 million barrel field. Previously, vertical wells in
the area were uneconomic. Plus Three Forks Potential.
RESERVOIR:
Reservoirs are dolomitized Red River “B”. Porosity averages
13%. Net pay is 15 feet. Oil is 31 degrees API. Vertical depth
is 8,500 ft. Horizontal legs average 5,000 ft. Average well is
230 MBO to 450 MBO. Average drilling cost $2 million.
A Low Risk, Continuous Drilling Project
The Cedar Hills Field extends over 125 square miles and has estimated recoverable
reserves of 150 million to 800 million barrels.
Vertical wells in the area were uneconomic.
Horizontal drilling is responsible for all of the reserves.
Our project area is even more prospective – vertical wells were economic.
We have 725 square miles on the “horsetail” of the Cedar Creek Anticline.
PLAN: Drill sparsely-spaced horizontal wells to prove up the play  ROI of 100 times.
Project Overview
100% working interest in the project area with an 80% NRI on all of the leases.
Immediately northwest of our Project Area, the Red River “B”, “D”, and “C” porosity intervals
host over 1 billion barrels of recoverable oil. Horizontal wells at the Cedar Hills Field average
0.46 to 1.2 MMBO per sq mi from the Red River “B” interval. Increased dolomitization above
areas of heavy basement fracturing in our project area and additional contributions from the “C”
and “D” intervals could yield even higher EUR than 0.46 to 1.2 MMBO per sq mi from our net
725 sq mi property, but those figures alone suggest 333 MMBO to 870 MMBO.
Geology
The primary target on this project is the 8’ to 18’ thick Ordovician Red River “B” porosity
interval at an average depth of 8,600 feet. The secondary target is the Red River “D” porosity
interval, which lies 120 feet below the “B” and which is even thicker and more productive than
the “B” interval, in the Horse Creek and Horse Creek South fields.
The Project Area lies within the Williston Basin, which is a cratonic basin which has undergone
little tectonic deformation through Phanerozoic time. More than 99% of the oil and gas in the
basin has been found in Paleozoic strata and 90% of that is found in carbonate reservoirs (Kerr,
1988). There are four structural features in the Williston Basin. The largest of these is the Cedar
Creek anticline which trends NNW along the western edge of the basin. Fields along the crest of
the Cedar Creek anticline produced more than 320 million barrels of oil by the end of 1986.
More recently, fields on the eastern flank of the Cedar Creek Anticline have added another 800
million barrels. A 1998 AAPG Meeting abstract immediately below shows that development
with horizontal drilling became economic in 1994, when oil prices were only $15 per barrel.
1998 Abstract: Red River “B” Zone - SW Williston Basin: Horizontal Drilling
Awakens a Sleeping Giant
MCCLELLAN, THOMAS S., Westport Oil and Gas Company; RICHARD E. GABER,
Consulting Geologist
Summary
This paper outlines the exploration and development history of the Ordovician Red River
“B” zone reservoir leading to the discovery of the “giant” Cedar Hills Field, Bowman
County, North Dakota. Production from vertical wells completed in this thin, oil
saturated dolomite yielded marginal economics at best. Detailed geologic evaluation of
the reservoir and drilling program revisions led to the successful economic exploitation
of the “B” zone through horizontal drilling. This application culminated in the discovery
and development of the Cedar Hills Field, a single reservoir covering more than 125
square miles with over 150 MMBO of recoverable reserves.
Introduction
The “B” zone dolomite is porous over a large area in the SW Williston Basin, averages
10 feet thick has high porosity but typically low permeability. This oil saturated
reservoir, at depths of 9,000 ft, had frustrated explorationists for many years in trying to
turn a profit. Burlington Resources first exploited the “B” zone in the mid-80's, drilling
vertical wells along the east flank of the Cedar Creek Anticline in Montana. Continued
development was halted due to marginal production rates and dropping oil prices.
Horizontal Drilling Application
In 1989, horizontal drilling was implemented along the flank of the Cedar Creek
Anticline in an attempt to economically develop the known “B” zone reserves. Although
higher production rates were achieved in an initial five-well program, it did not offset the
higher costs for horizontal drilling and continued low oil prices. In 1993, a second
horizontal program yielded solid economic returns. Geologic and engineering
reevaluation of the initial program led to a more defined stratigraphic target and
improved drilling technology which yielded lower costs, longer laterals, and higher
production rates.
In 1994, Burlington Resources expanded the play to the southeast into Bowman County,
North Dakota, where three wells indicated a large area with high oil saturation in the
“B” zone. A horizontal “B” zone wildcat was completed in October, 1994 flowing 250
BOPD, and produced 75 MBO in its first year. Following the discovery, more than 200
horizontal wells have been drilled in the play, with daily production exceeding 16,000
BO from horizontal wells.
Conclusion
The critical step leading to the current drilling activity was recognizing that the “B” zone
held significant oil in place over a large area, but was uneconomic to develop by
conventional methods. The combination of improved horizontal drilling technology,
interdisciplinary team work and perseverance were the key ingredients to unlocking the
potential of a “giant” oil field in the midst of a mature producing area.
Horizontal drilling has continued to the southeast through Medicine Pole Hills Field and Horse
Creek Field (both now undergoing air injection) and into Harding County, with its 83 horizontal
well permits since 2005. The 40 MMBO Buffalo Field is being re-developed with horizontal
wells in the Red River “B”, as are several other fields to the south, west, north, and northeast.
These fields and discoveries define an area of Red River “B” production that is about 30 miles
wide. There is additional production from vertical wells in the Lantry field, 100 miles to the
southeast in Dewey County, SD that suggest that the Red River “B” could be productive over a
very large area in the southern Williston Basin.
A series of three reports in the AAPG Explorer in 2004 documented re-awakened interest in
hydrothermal dolomitization in Paleozoic carbonates in the Eastern US. The two classic oil fields
of this type are the Lima-Indiana Field (500 MMBO) and the Albion-Scipio Field (250 MMBO).
The Albion-Scipio Field is obviously controlled by a basement fracture, where upwelling
hydrothermal fluids dolomitized the Ordovician Trenton-Black River to a distance of half a mile
on each side of the 40 mile long basement fracture. Until recently, relatively little exploration for
basement fractures or wrench faults has been carried out in the SW Williston Basin, although very
sudden lateral changes in porosity strongly suggest basement fracture control. Our leases extend
from the Buffalo Field to the southeast, where the basement is heavily fractured (Thomas, 1974),
and where we expect to see greater than average porosity due to hydrothermal dolomitization.
Reservoir
Primary Objective – Red River “B”
Red River strata consist of limestone, dolomite, anhydrite, and shale representing peritidal and
shallow-marine environments (Montgomery, 1997). The continuity of the Red River over much
of the Williston Basin suggests tectonic stability during deposition. The Red River is over 700
feet thick at the center of the basin and about 500 feet thick in Harding and Perkins County. The
underlying Winnipeg Group consists of siliclastics and shales of potential source rock quality.
Since the contact between the Winnipeg and the base of the Red River is conformable and sharp,
it provides a regional seismic and log marker. The Red River is divided into a lower portion of
non-porous bioturbated limestone, and an upper portion that is further subdivided into four
interpreted depositional cycles. From youngest to oldest these are the “A”, “B”, “C” and “D”
zones. The “B” and “C” zones each consist of: 1) an anhydrite at the top; 2) a middle “laminated
member” composed of porous, microsucrosic dolomite, non-fossiliferous cryptocrystalline
dolomite, or slightly porous, fossiliferous limestone; and 3) a lowermost burrowed member
dominated by non-reservoir, fossiliferous, bioturbated lime wackestone or mudstone.
There is regional porosity in the Red River “B” throughout much of western Bowman, southern
Slope, Harding and Perkins Counties. This contrasts with lateral changes in the “A”, “C” and
“D” zones. The “B” reservoir is divided into upper and lower zones, based on reservoir
properties. At Cedar Hills, the upper reservoir portion averages 3 to 8 feet of microsucrosic,
laminated dolomite with porosities from 12 to 25% and permeabilities ranging from 0.5 to 50
mD. The lower portion is also mainly dolomite, but texture is variable with poorlyinterconnected, vuggy porosity. Oil saturations in cores are consistently highest (up to 80%) in
the more permeable upper portion of the reservoir (Montgomery, 1997). At the Buffalo Field the
“B” zone is 15 to 18 feet thick. Porosities range from 16 to 29% and apparent air permeabilites
range from less than 1 mD to 36 mD.
Secondary Objectives – Red River “D” and “C”
The “D” zone contains several beds of black kerogenous dolomite up to 20 cm or more thick
with as much as 59% total organic carbon (Potter and St. Onge, 1991). The beds are continuous
across the basin and may have contributed to oil in the Red River (Kohm and Louden, 1988).
The “D” zone (equal to the “C” Burrowed Member) is comprised of burrowed dolomitized
carbonate mudstones and wackestones deposited in a shelf environment. The best reservoir
rocks occur where up to 25% porosity is present between completely dolomitized burrows
(Longman et al., 1992). Porosity is vuggy in finely crystalline dolomites. Permeability only
locally exceeds 30 mD. Sippel (1998) documents an average of 68 feet gross and 28 feet net
thickness for the “D”.
The “D” zone is the reservoir rock at Horse Creek and Horse Creek South Fields, whereas the
“B” zone is the main reservoir at Cedar Hills and Buffalo. The “B” and “C” zones are the
principal producers at Medicine Pole Hills.
Estimated Project Reserves:
From the Red River “B”
460,000 BO per section (Montgomery, 1997), but near basement fractures dolomitization
will be more extensive, resulting in perhaps triple that figure in fractured areas.
McClellan (1998) estimated 1.2 MMBO of recoverable reserves from the Red River “B”
alone. Thus from 725 net sections we could have 333 MMBO to 870 MMBO.
From the Red River “C” and “D”
We expect production similar to the “B” or better, but dolomitization may not be as
widespread as the “B”. Dolomitization may be confined near the basement fractures so
that the net sections may only be 20% of the “B”. Thus we may only have an additional
contribution of 20% more from the “C” and “D” or 67 to 174 MMBO.
Total Estimated Ultimate Recovery: 400 MMBO to 1 Billion barrels from net 725 sq mi.
Prove Up:
500+ MMBO in RED RIVER  ROI of 100 times
Possible:
THREE FORKS  Additional Potential
References:
Kerr, S. Duff, Jr., 1988, Overview: Williston Basin Carbonate Reservoirs, in S. M. Goolsby and
M.W. Longman, eds., Occurrence and petrophysical properties of carbonate reservoirs in the
Rocky Mountain Region: Rocky Mountain Association of Geologists, p. 251-274
Kohm, J. A., and R. O. Louden, 1988, Red River Reservoirs of western North Dakota and
eastern Montana, in S. M. Goolsby and M.W. Longman, eds., Occurrence and petrophysical
properties of carbonate reservoirs in the Rocky Mountain Region: Rocky Mountain Association
of Geologists, p. 275-290.
Longman, M. W., T.G. Fertal, and J.R. Stell, 1992, Reservoir performance in Ordovician Red
River Formation, Horse Creek and South Horse Creek fields, Bowman County, North Dakota:
AAPG Bulletin, v. 76, no. 4, p.449-467.
Montgomery, S.L., 1997, Horizontal Drilling in the Ordovician Red River “B” Williston Basin,
in Innovative Applications of Petroleum Technology Guidebook.
Montgomery, S.L., 1997, Ordovician Red River “B” horizontal oil play in the southern Williston
Basin, AAPG Bulletin, v 81, no. 4, p.519-532.
Potter, D., and A. St. Onge, 1991, Minton Pool, south-central Saskatchewan: a model for
basement induced structural and stratigraphic relationships, in J.E. Christopher and F. Haidl,
eds., Sixth International Williston Basin Symposium, Saskatchewan Geological Society Special
Publication 11, p.21-33.
Sippel, Mark A., 1988, Exploitation of Reservoir Compartments in the Red River Formation,
Southern Williston Basin: in R.M. Slatt, ed., Compartmentalized Reservoirs in Rocky Mountain
Basins, 1988 RMAG Symposium, p. 151-170.
Thomas, G. E., 1974, Lineament-block tectonics: Williston-Blood Creek Basin: AAPG Bulletin,
v. 58, p. 1305-1322.