Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1933-12-09
THE RFEECT OF SHORELTNTNC on THE
ARTICULATION OF OIL AND GAS IN
NOSES AND TERRACES ALONG THE
FLANKS OF THE SWEETGRASS
ARCH MONTANA
GRANVILLE GATES HOWARD
Consulting Geological Engineer

In the first few years OF oil and
gas IN Montana it was held to Be a
fallaey for any reputable geOlogist
70 recommend a test anywhere in the
state where the structure was not
known to be actual structural closure
or fault closure. In other words there
had to be actual structural closure be-
before any reputable geologiat would
recommend the drilling Of a we'll. In
view OF the conditions prevailing ia
the Sunburst and other fields this
opinion was extremely well sustained.
although, IN the case OF the Sunbursf
field, with tremendous closure, It was
not certain that wells drilled In seem
ingiy favorable spots would be pro-
ductive due TO the variability Of the
producing horizon.
While the drilling Of wells in the
Sweetgrass Hills and on the TANKS Of
the Arch showed that different sand
conditions existed on the fIanks than
on the crest Of the Arch, It was not
generally recognized by the geological
fraternity that shortening OF sands
along the TANKS Of the Sweet Grass
Arch existed. When the Cut Bank
Field was brought in on a structure
closed purely by shortening then the
geologist began to sit up and take
notice OF facts hitherto strongly fndI-
Gated but now proven deginiteIv.
namely that there is a wide belt
along the crest Of the Arch which was
apparently out Of the water during
several geological periods and that
while thds belt was exposed to erosion
Of the atmosphere during these periods,
sands and thicker formations were
being laid down along the water
covered flanks during the periods that
the Arch crest was exposed as an
island or peninsula
The period of socal1ed Great Uncon-
formity which occurred between Pale
ozoic and Jurassie times left several
great formations absent on the Crest
OF the Arch. These are the Penn
sylvania, Permian, Triassic, and part
OF the Jurassic. The thickness of
these formations, If present in aver
age normal thickness above the Madi-
son, the uppermost representative of
the Paleozoic present in most of the
Arch, would mean many thousands OF
feet OF intervening formation be
tween the Jurassic OF the Arch and the
Madison, with the probable presence
of a great many sands which would
be productive in favorable structures
the Jurassic (Ellis) and the Cretaceous
did not cover such a great period OF
geologic time but was Of such dura-
ton that important sands found In
the Lower Cretaceous and Upper
Jurassic of the Rocky Mountain Region
are here missing. That minor uncon-
foFmfties within the Cretaceous and
Jurassic formations themselves took
place Is OF greater Importance TO the
geologist and oil and gas operator than
the unconformities discussed above.
It Is these intra rather than Interior
national divergences with which we
have to deal In our search for new
oil and gas fields along the TANKS
of the Sweetgrass Arch.
In the Sweetgrass Hills area and
on the Eastern Flank of the Arch
the thickness OF the Ellis or Jurassic
is about 500 feet as against 235-
$50 feet on the crest. This allows for
a true sand at the base OF the Ellis
which is practically nonexistent on the
Arch crest. This true sand means
much more consistent drilling of pro-
ducing wells and much less chance for
dry holes In the structures such as
Flat Coulee, Bears Den, and Reserve,
where it has been proven to carry
commercial oil or gas, than in the
Sunburst Field, where it is nom
existent.
In the above named fields. as well
as in the Border and Cut Bank Oil
fields, there is, at the top of the
Ellis or base of the Kootenai (Cre-
taceous), another sand Of great i'm
portance as an oil and gas sand It
is variously called in the different
fields, the Darling. the Vanalta, or the
Baskoo On the crest Of the Arch
this sand is found only in isolated
areas where 1t is called the stray"
1t is probably the most important sand
OF North Montana region at present as
it IS the sand which yields BY far the
greatest amount OF oil and gas in the
Cut Bank Field and Is the principal
productive horizon in the Hills area.
On the crest OF the Arch the Kootenai
averages from S25 to 400 IT. in thick
ness. On the TANKS the average is
from 435 to 500 It. It is at the
bottom Of the formation that the sand
is found.
The KootenaI, which lies directly