Montana oil and mining journal., 1942-06-06
FOR PR0SPECT0Rs TO THINK ABOUT

structuraIIy high-15O feet higher than Bsb
Engleking'S oil wells a few miles eastward
toward Kevin-and had a good flow of gas
perhaps 100000 cubic feet) at the base
Of the Colorado. Then the bit passed into
formation which could not be identified
as Madison lime, his well was too low struc-
turaIy to produce. However, he found oil
IN an S-inch @break" in the lime. It was alt
gravity OiE-the highest gravity oil ever
found IN that area. Below the oil was a
layer of water. He tried vainIy for several
months to shut off the water by squeeze"
cementing. If he could have shut off the
water, he had small producer But 1t could
not be done because the lime seemed 70 be
broken up so badly as to make it impossible
to exclude the water.
The U.S.G.S. was unable to explain the
situation. So the Erdmann- Davis map u9I9)
OF the West Kevin-Cut Bank area, issued
by U.S.G.S., has the notation, @StructuraIIy
lowest Madison well" beside the well Cassidy
drilled.
The combination Of the evidence of fault
ing on the rimrOck, coupled with the very
definite evidence of fauIting in the Cassidy
well would have been enough TO have con-
vinced operators In the HTidcOntinent, but
not in Montana. Recently another wen was
drilled, southwest Of the aCssidy we'll It
had the entire KO0tenai section present.
with q large flow Of gas in the Cut Bank
sand horizon, as compared with NO KOOt-
enai at the Cassidy. This new well, too,
checked high structuraIIy until 1t reached
the Ellis. Again the Madison lime was found
too low for commercial oil production Again
it carried a showing of oil which did not
respond to acfdizatlon
This "RimrOck fault" Is something for
good petroleum engineers to work out II Is
certain that the fault runs from northeast
to southwest. 1t Is certain that the south
east side is the "high" side and the north
west side Is the "down" side. If we were
good engineers we would perhaps know from
the logs of the wells that have penetrated
1t whether the fault Is vertical or whether
its throw is to the southeast or whether Il
reclines to the northwest. Those are import
ant determinations for guidance of driIIers
in the future.
The experience of other oil states Is that
wells must be very carefully spotted with
regard to faults. If too close they penetrate
the fault, as did these wells, and get water
If too far away, they do not get the ad-
vantage of the lime breakage and resultant
porous formation. We have heard many old
time operators say just how far to get back
from fault to get production. But since
then we have learned that the position of
the fault determines the location of the
weL rather than any rule-Of-thumb.
As we have said many times before. we
would like to see a geOphysicaI crew at work
along this RimrOck fault line We believe
that with scientific location of wells some
big producers may be found along this line
which, If not true fault" Is doubtless the
granddaddy Of the big lime cracks A geo
physical crew might explain why the High
Gravity producers, located about a mile back
from the fault, have a s70-barreI well among
them.
In view of the fact that this area west
Of Kelvin yields oil Of 36 to 40 gravity, rating
Cut Bank prices and In view Of the further
fact that it is only 2SO0-fOOt drilling, IL
seems probable that more drills win investi-
gate the nature Of the indicated fault
Anyone interested in further study Of the
Rimrock fault may have copy Of cur pub
ication in which we illustrate the theories
md show the relative location of the wells
IrilIed. We are particularly anxious to inter
st prospectOrs In the area.