Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1941-02-22

amount. but only in very rare instances
is 1t greater The temperature, If we fig
ure a surface temperature of so degrees
Fahrenheit. and an increase of one de-
gree Fahrenheit for every so feet OF
depth would be about 226 degrees. This
temperature would vaporize light oil at
atmospheric pressure, but would be much
too low to vaporize even pure gasoline
under a pressure of more than 1000
pounds. We may therefore conclude that
so far as the temperature and pressure
are concerned oil could exist at a depth
of 10000 feet, ALTHOUGH BOTH
THESE FORCES WOULD TEND TO
MAKE THE OIL MIGRATE SURFACE
WARD THROUGH ANY FTSSURE OR
OTHER AVENUE OF ESCAPE THAT
MIGHT PRESENT ITSELF, AND FOR
THIS REASON If FOR NO OTHER IT
is UNREASONABLE TO EXPECT AS
MUCH OIL IN GREAT DEPTH AS WILL
BE FOUND NEARER THE SURFACE.
IF we conclude that so far as the eff
feet of heat and pressure on the oil it.
self is concerned the oil can persist at
depth greater than any we are likely
to reach We may inquire into the effect
OF these factors on the reservoir Sand and
on the cap rock Could rocks retain their
porosity under the pressures to which
they would be subjected at such depths,
with the heat tending to make them
more plastic? The experimental work of
Frank D. Adams and others shows that
openings in rocks such as sandstones
and limestones will not be mashed shut
by such pressure as would result from a
column of rock 10000 feet high, so
that so far as temperature and pressure
are concerned the rocks will retain their
porosity. When we consider porosity
however we must also take account of
cementation-the filling of the openings
in the rocks by some such substance as
calcite, gypsum, anhydrite, or silica.
Here is a very important factor that pre-
sumably will reduce the pore space in
deeply buried rocks more than it reduc-
es that space near the surface However,
it is perfectly safe to say that the mere
burial of a stratum to a depth of 10000
feet will not destroy all the oil in it,
NOR WILL 1T REDUCE THE POROS-
ITY TO SUCH AN EXTENT THAT THE
OIL WILL EITHER BE HELD IN
THREAD LIKE OPENINGS FROM
WHICH IT CANNOT ESCAPE OR ELSE
FORCED TO MIGRATE NEARER THE
Theory, in this case, is a
bundantly borne out by observation. We
can visit places where the rocks now
exposed by fitting and erosion have I'M
questionably been under more than TO-
900 feet of cover. We know this is so be-
cause we have actually measured the
it has not destroyed the oil-yielding pos
sibilities because in some instances oil
is found today in such beds that were
once deeply buried
Thus the theory by Mr.. Head that
the heat and pressure may force oil
from lower horizons up through very thin
cracks and creYices in the formation
arising as a very volatile gas. to collect
in whatever soft materials it may en-
counter on its upward migration.
The questions are, how light can the
oil get before it's place is taken almost
entirely by gas, and at what depth will
we reach this critical gravity of the
oily" he asked. He then cited fields
Of California where deeper drilling has
shown lighter oil with depth, ranging
from an increase of about one degree Be-,
for every SO feet to as much as 125
feet. "In applying this rule it must not
be forgotten that the depth figure re-
lates to a given zone, and NOT to depth
below the surface of the ground.
Facts not now available may modify
-hese conclusions. Very possibly 1t will
be found that the rate of increase is
much less when great depths are reach
I'd, so that instead of increasing one de-
gree for every 125 feet, IT will only