Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1940-12-28
member A2f feet.
Total thickness Of entire Devon
fan section. 100 feet.

A0AA1 thickness OF GMtArE Devon
ian section. 100 feet
or the above section Deiss OBJ
served THE most striking charac-
teristics OF the Lone Butte lime
stone are its drab. dark brown
color, its vitreous lnster upon fresh
/ fracture faces and its extremely
petroliferous content-porous in the
upper part"
PRAIRIE REEF (entire Sec
tiOn) OF DEVONIAN
Measured In Sec. 33, n. 22N.
r. II w.
Limestone. sandstone. shale
and dOlOmite; mixed beds of
buff-weathering caleareous,
thin-bedded sandstOnes and
limestones I'm basal 83 feet,
overlain by SO feet OF rcd, buff
and green-gray shale and ar
gillaCeous limestone. i'm turn
over-Iain by buff and gray.
thick and ehin-bcdded petrOli-
ferrous limestOnes. 1877 feet in
thickness, and at top 600 or
more feet of drab-brOwn ex-
tremeIy petroliferOus and mas
sive doIomitic limestone, total
thickness 100 feet.
OF the Prairie Reef cotton Delss
wrote: lone OF the most remark
able facts brought out by this sec
ton is the great thickness of the
lone Butte type locality. top
member Devonian) brown drab
sandy petroliferous limestones"
BIG GEORGE GUI,CH SECTION
OF DEVONIAN
Approximately ten miles due east
of the Prairie Reef section. and
similar to the Prairie Reef section
as described by Deiss the Devon
ian section. examined by the writer
Is well exposed between the upper
Cambrian and Madison formations
1t lies in W1hW1s Sec 81. T. %
NJ. r. 9 W. on the extreme west
side OF the mouth OF Big George
Gulch about two miles along the
trail above Gibson Dam Most Of
the petroliferous beds can be seen
above the Sun River trail over the
cliffs The Madison-Devonian con
tact lies in the small gulch cut in
the soft upper Devonian buff and
drab weathering beds. Like the
Prairie Reef section the upper
several hundred feet of the Devonian
is composed OF drab brown. sandy.
extremely petroliferOus dolomftic
limestone Samples taken through
the section range from massive,
brown, spongy appearing and porous
dOlomite extremely petroliferous
through the uppermost $50 feet or
more, through thinner bedded pet
roliferous compact I5mestones with
beds downward. so feet
or more in thickness, OF porous,
extremely petroliferous dolomite.
The rocks have a strong smell OF
petroleum when heated or struck.
CONCLUSIONS
The Devonian dolomitic lime
stones are a prolific producing
horizon Of oil and gaD when tested
on favorable structure, notably the
fields in the Illinois Eastward fold.
The westerly side of the broad
nOrthwesterly plunging fold of the
Sweet Grass Arch dipping into the
trough from which the
mountain upHift took place, forms
a Eastward gradient ideal for
localized accumulation of oil and
gas in the porous horizons of the |
Madison and Devonian formations
Such areas of accumulation will
likely be in domes or folds. prob
IDLY effected by faulting, interme
lately situated on the flank of the |
eastward fold. Such locations |
,annot be ascertained by surface |
geology alone, due to the great un-
at the Ellis-Madison and |
he Madison Devonian contacts. |
ieophysical methods must be adopt
d in the prairie region and of these
he seismographic survey is most
Elective.
The Devonian formations. as
ound and examined over a wide
rea near the mountain front un-
extend across the basin
yard flank of the Sweet Grass Arch
plift. They can be penetrated in
lost places In the oil counties of
he Northwestern Montana plains at
epths ranging from $500 to 1000
aet.
Several hundred feet OF the upper
levOnian strata in the nearby
is found to be petrolifer-
us and extremely petroliferous
trough the top 200 feet or more.
hese uppermOst extremely petroH-
srous beds are also found TO be
leal reservoir rock for oil and gas.
cling composed of porous dolomitic
} sandy limestone.