Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1942-11-07
Production at Reagan Well Explained

This week we had occasion to refer to a
publication OF the United States Geological
Survey and our eyes fen upon a paragraph
which seemed 10 answer some questions re-
garding the oil in the Reagan horizon on
Twin Rivers Nose.
we have been wondering whether Twin
Rivers Nose is a freak: whether the oil
found so feet below the top of the Madison
lime is a condition that win not be found
elsewhere. Certainly those who found it had
reason to continue drilling below the Madi-
son contact. Why should they have violated
the rules and gone below the known produc-
ing horizons, especially on the lower TANKS
Of the SweetgraSs Arch where there is every
reason to expect water2
In the U.S.GS. publication we read:
THE distribution Of the oil may be due
in part to its source and method of migra-
ton. The small fisures noted In the Troy
SweetgraSs well may be parts of fracture
zones or even faults which reach down into
the Devonian formation and through which
the oil reported in that formation may have
collected at the base Of the Ellis forma-
ton. If this could be definitely proved to
be the source and method Of migration of
the oil, it would In part account for the I'M
equal distribution Of the oil, the larger wells
being located near the more open assures,
the small wells being farther laterally from
the assures, and the barren areas repre-
senting places far removed from assures or
fractures"
That was written in 1929 by Arthur j.
Collier for the q. s. Geological Survey pub
lished in Bulletin S12-B.
In those days, when Mr. Collier made his
survey Of the Sweetgrass Arch, it was be-
neved by many biologists that the oil was
found in the Ellis shales The bottom of the
Ellis contains about 80 IT, Of black shale that
drillers call "black lime" Biologists argued
that the oil at the contact had its origin In
these black shales and Travelled downward
into the porous contact material by capil-
larry attraction That was fine theory until
oil was found go feet below solid, massive
limestone. No Oil penetrated downward
against a strong hydrostatic table through
solid lime to come to rest at the Reagan
horizon OF that every oil man is today GER
tain. The Ellis-origin theory is definitely
"out"-just as in the case Of Turner Valley
where the oil occurs from 300 to 500 feet
below the top of the hard, massive lime.
We suddenly realized that Arthur j. Col
ler had a keen grasp of the accumulation
of oil at the top of the Madison lime. We
took time to read the whole chapter. It is
worth republication at this time:
ERRATIC OCCURRENCE OF THE OIL
"On in the basal sand" Of the Ellis is
very rregularly distributed, without any
system that can be detracted from study of
the surface Many of the large wells in the
field having initial yields above 100 barrels
a day. are offset by wells 440 feet away
that are dry or have only small produc-
ton For some of the wells this unfavorabe
result is known to be due to the release Of
the gas pressure in the first well drilled but
generally the wells appear to be wholly in
dependent of each other A group of wells
drilled around a section corner may include
one well yielding a small Oil production,
offset by three dry holes. Very little can be
predicted from proximity to a producing well
as to the probability of success in any par
ticular area and according to the popularly
expressed opinion ALL THE WELLS ARE
WILDCATS"
This uncertainty is probably due, in part
at least, to the irregular surface Of the
Madison limestone and the uncertain char
acter of the soil or other material making
up the basal sand" Of the Ellis at the con-
tact of the Ellis and Madison formations
In some places the wells may strike buried
channels in the limestone which are filled
with porous material; in other places chan
nels are absent, the Ellis and Madison lime
stones are almost in contact, or, if they are
separated, the soil and other material at
the contact may not be porous. An example
of a buried channel is probably presented
by the Big West Steele No well, in Sec
2-34N-1W, in which a large quantity of
more or less rounded gravel containing water
and Some oil was found. In the Handbook
Craig NO 2 well in Sec. 1334-2W, drilled in
1925 a similar occurrence is reported. In
places where the Madison limestone was
more resistant to erosion or was left at
slightly higher level for any cause, the basal
sand" of the Ellis is likely to be thin, but
such places can not be detected from the
surface, and the graphic well logs given in
Plate II do not indicate any large area OF
his kind. Such high places are presumably
he results of an unequal erosion ON a nearly
surface. The thinness or thick
less of the basal sand" of the Ellis is only
me of the factors controlling production. If
he Ellis formation above the basal sand"
S, as it appears to be in the southwestern
art of Twp. 35N, r. NV, composed Of very
,orous limestone, the high places in the
Madison limeStones may be the most pro-
uctive In a few wells the basal sand" OF