5000

4500

4000

3500

3000

2500

2
3
3
6

SEA LEVEL ELEVATION, FEET
zg 3
° oO

500

-500

“1000

-1500

FIGURE 12. PRESSURES AND TEMPERATURES, ELK BASIN FIELD

2000
a) eee a
h+++++
[-] > GROUND LEVEL
% 1500 ‘a 7
© a
a 72.
LY g | vi
& 1000
peta.
#
uN = yy
=
3 500
\ = 2
Ws
\ 9 200 400 600
EMPERATURE GRADIENT [~ PRESSURE DRAW-DOWN, psi
ELK BASIN FIELO | PRODUCTIVITY OF WELLS 12,3,4,6,88
a ELK BASIN FIELD
\
IN
1000
\ TTTTTT {I
INITIAL PRESSURES
IN 4
TTENSLEEP RESERVOIRI-]
. ELK BASIN FIELD [4 s00
\
me °
x
“500
“1000
: he
-1500
70 90 no 130 150 1800 2000 2200 2400-2600
DEGREES, °F. PRESSURE, psio

SEA LEVEL ELEVATION, FEET


TABIE 3. - Hydrogen sulfide content of liberated gas, differential gas

liberation, oil sample from well 1

Pressure range in 1 Hydrogen sulfide
which gas increment liberated, | content of gas increment,
p.s.ia. | percent
1250 to 835 7.6
835 to 725 dal
725 to 625 59
625 to 535 6.8
535 to k50 6.1
450 to 365 10.2
365 to 300 13.0
300 to 225 16.0
225 to 165 194
165 to 120 26.7
120 to 85 28 6
85 to 60 35.0
60 to 45 38.0
U5 to 11 secessecosscccerececscsceresses 36.1

The great change in HoS content of the gas as it is released from solution
when the pressure is lowered 1s significant, It reflects the solubility of
HoS in crude ofl. These data and the "gas in solution" greph indicate that at
Elk Basin the HoS content of the gas from the oil in the upper mart of the
reservoir would increase as reservoir pressures decreased in accordance with
the graph, provided the gas;oil ratio during production would not exceed that
in solution in the oil. However, increasing gassoil ratios that normally
occur in solution gas drive reservoirs as the pressure declines will result in
decreasing the HoS content of the produced gas, because any volume of gas in
excess of that in solution in the oil will contain a smaller percentage of HoS
than the solution gas. Later in the life of the reservoir and declining
gassoil ratios the reverse can be expected.

Owing to the method by which the data were obtained, those data relating
to the specific gravity of the increments of gas liberated from the subsurface
oil samples and the hydrogen sulfide content of the gas samples are the least
accurate of all results reported. In fact, the data show changes in one or
two instances that are inconsistent with the data from other samples.

The writers are indebted to two of the operators for information on the
viscosity of the reservoir oil from well 2. Oil from well 2 is representative
of the oil in the upper 500 or more feet of closure. The viscosity, in
centipoises, of the ofl at a temperature of 123° F. and at several gage
pressures and the estimated viscosities of the oil near the edge of the
reservoir are given in table 4,

4o73 -T-


Well 1, 1,930; well 2, 1,265; well 3, 1,320; well 4, 1,440; and well 5, 1,150.
The curve for well } shows the effect of completion in a fractured zone in
the Tensleep formation. All curves show that a pressure determination made 1
hour after shutting in a well would yield a pressure almost as great as that
which would be obtained after a shut-down of several days.

Analyses of Produced Oil

Bureau of Mines routine analyses of the crude oils from six of the wells
in the Elk Basin field are given in tables 5, 6, and 7 on the following pages.
The samples of crude oil came from wells 1, 4, 6, 7, 8, and 9, and the
analyses show the slight variations in the characteristics of the produced oil
from wells at the top of the structure and increasingly farther down toward
the edge.

4073 -R-


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VARIABLE CHARACTERISTICS OF THE OIL IN THE TENSLEEP
SANDSTONE RESERVOIR, ELK BASIN FIELD,
WYOMING AND MONTANA

by
Ralph H. Espach2/ and Joseph Fry2/

CONTENTS

Introduction.
Acknowledgments......
Subsurface oil sampling...
Reservoir oil characteristics
DISCUSSION. .ceececeeveccccecccccecccccetsccssescccsees
Relationship of oil characteristics to reservoir structure.
Hypothesis for variable reservoir-oil characteristics.
APPCNGIX. cr ececcercncccrccrereccrecceeseeeseseesecsesecees
Reservoir data and producing characteristics of wells.
Analyses of produced OLl. cusses cccccccvcccccccccsecces

BEES oounon npg

TABLES

1. Data relative to wells sampled, Tensleep reservoir,
Elk Basin Ficld.. cece ee cece ccc c eee cecececeenes
2. General results of analyses of subsurface oil samples,
Tensleep reservoir, Elk Basin Pield.....ccccsecesees
« Hydrogen sulfide content of liberated gas, differen-
tial gas liberation, oil sample from well 1.........
Viscosity of reservoir O1l....c cece ecccccccecccvecces
Bureau of Mines routine analyses of crude oils from
wells Land bees c ccc cece cee ceceeececsceceseecs 13
. Bureau of Mines routine analyses of crude oils from
Wells 6 ANd Tecsececcececceceveceececscsecscescesess Li

or nn F

1/ Petroleum engineer.

Report of Investigations 4768


RESERVOIR OIL CHARACTERISTICS

The subsurface oil samples were analyzed by the differential gas-
liberation method outlined by Grandone and Cook .2/ The laboratory analyses
of the oil samples were performed at the Petroleum Experiment Station (now
Petroleum and 01l1-Shale Experiment Station), Bureau of Mines, Laramie, Wyo.,
during several months following sampling of the wells. The general results
are given in table 2. Individual gas-liberation data for the nine samples
are given in tables 8 to 16, inclusive (in the Appendix), and are show in
figures 2 to 10, inclusive.

In these figures the graphs of “expansion of sample“ are self-explanatory.
The values for the gas in the "gas-in-solution” graphs are figured at con-
ditions of 60° F. and 14.4 pounds per square inch absolute pressure. The
"relative oi1 volume” graphs show the volume relationship between the oil in
the reservoir under reservoir conditions and the residual oil at 60° F. The
"density of sample” graphs indicate the change in -density.as changes in _
pressure and release of solution gas take place. The "specific gravity of
gas in solution" graphs show the specific gravity of the gas that is in
solution in the oil at the different pressures and in volumes as shown by the
"eas in solution” graphs. The velues for the specific gravity of the gas in
solution were obtained by using a 220 ml. gas density balloon, weighing
samples of gas liberated between the pressure points as shown on the graph,
and calculating the composite to any pressure by using the volumes and
weights of the several increments of liberated gas.

The "hydrogen sulfide content of gas in solution” graphs show the per-
centage of hydrogen sulfide in the gas that is in solution in the oil at the
different pressures and in volume as shown by the "gas in solution" graphs.
The percentages of HpS were obtained by running both Tutwiler and Orsat
analyses of the increments of liberated gas during analyses of samples from
wells 5, 6, 7, and 9, and Orset analyses for acid gases of the increments of
liberated gas during analyses of samples from wells 1, 2, 3, 4, and 8.
Where Orsat values alone were determined, from 5 to 6-1/2 percent was sub-
tracted from them as a correction for ‘the carbon dioxide content of the gas.
The “hydrogen sulfide content of gas in solution” graphs represent values
obtained from the determined percentages of hydrogen sulfide in the several
increments of gas liberated between any two adjacent pressures, as indicated
by the points on the curves, the volumes of the several increments, and the
calculated composites of these to various pressures. Of interest in con=
nection with the “gas in solution” graphs are the data on the H58 contents:
of the several increments of gas liberated during the differential gas
liberation of a subsurface sample of o11. Such data from the sample of oil
from well 1 are given in table 3.

4073 25+,


of Gas in Solution,
percent

Hydrogen Sulfide Content

Gas in Solution,
air=1.0

Specific Gravity of

Density,

om/mi.

10

residual oil volume

Relative Oil Volume,
at 60°F.

rr

cu.ft./bbl. residual
oil at

Expansion of Sample,
percent

40

30

20

08

91

87

83

79)

Sj

at top of Tenslee:

Pressure,psia

FIGURE 5. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT 123°F.,

SUBSURFACE OIL SAMPLE FROM WELL 4, ELK BASIN FIELD.


z 40
to
88 30
si.
rs
= & 8 20
oes |
a
sé 10 =
gs P=
3
= ° +
t
16
Be it
>S
ss 4 K |
B20 q I
Sas eT t
ges 12 t
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B° 0 >
|
os
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ao :
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. 79 “+
ge T
3 §, 8 t
332 i
Sse 12 | rT
£33
332 Lt 4
Bos 1
2é am t
Lo at 60°F.
H
+
500]
83 uz 400) 1
323 4
223 °°
85 ° 200
1oo| Lt
° Tt Soturation pressure
¢ 8 I
2 FE Flowing pressure at sample point
5
8 6
3§ aT
§ Hy 4 T }Original reservoir pressure
2 T at top of Tenslee,
8 t
3 2 H
° I
° 500 1000 1500 2000 2500 3000 3500

Pressure, psia

FIGURE 8. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT 134°F,,
SUBSURFACE OIL SAMPLE FROM WELL 7, ELK BASIN FIELD.


Specific Gravity of Hydrogen Sulfide Content

Density,

s
i=
2
$
é
ry
2
s
3
é

Gas in Solution,
cu.ft./ bbl.residual

Expansion of Sample,

of Gas in Solution,

Gas in Solution,

gm./mi.

e
5
3

residual

percent

percent

air= 1.0

oil at 60°F.

40,

30

20

08

a

87

83)

79

500

300

200

100,

I

ii

[
oe

p=-tat 60°F.

awe

Saturation pressure

tt Flowing pressure at sample point

\ +L original reservoir pressure

H at top of Tensile

al

FIGURE 6. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT 129°F.,

500 1000 1500 2000 2600 3000 3500
Pressure, psia

SUBSURFACE OIL SAMPLE FROM WELL 5, ELK BASIN FIELD.



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Elk Basin, an interesting old oil field that has been producing oil from
the Frontier formation since 1915, is situated in a highly eroded basin, re-
sulting from the wearing away of the crest of an anticline and some of the
underlying softer shales. The field came back into national prominence during
1943, when it became known that it was the largest single reserve of new oil
found in the United States that year. The Tensleep sandstone was found to
contain oil on November 26, 1942, when a well in the NE 1/4, Ne 1/4, ne 1/4,
sec. 31, T. 58 N., R. 99 We, Park County, Wyo., drilled to a depth of 4,538
feet (4 feet into the Tensleep sandstone) , flowed oil at the rate of 2,500
barrels a day. By the end of 1945, 125 oil-producing wells and four dry
holes had been drilled, and 9,419,271 barrels of oil had been produced.

Three. additional wells were drilled in each of the years 1946 and 1947, and
the oil produced amounted to 5,605,833 barrels in 1946 and 5,869,123 barrels
in 1947. Approximately 6,000 acres may.be considered as. productive of oil.
in the Tensleep reservoir, and estimates.of the oi1 that will be produced
average 200 million barrels.

The Tensleep reservoir is of further interest because it has’ greater
closure than any 011 field in the Rocky.Mountain area; the closure of the
Elk Basin anticline is variously estimated to be 5,000 to 10,000 feet, and
the top 2,000 feet of the structure contained oil.

ACKNOWLEDGMENTS

The work covered in this report was conducted under a cooperative
agreement between the Bureau of Mines, United States Department of the Interior,
and the University of Wyoming.

The writers and those engaged in the work appreciate the assistance of
the operators in the Elk Basin field, who gave the engineers of the Bureau of
Mines access to their properties, supplicd the wells for testing and sampling
parposes, and supplied most of the information pertaining to the Tensleep
reservoir itself.

The report was prepared under the general supervision of R. A, Cattell,
chief, Petroleum and Natural Gas Branch, Bureau of Mines, Washington, D.C.,
and H. P. Rue, supervising engineer, Petroleum end O0il-Shale Experiment
Station, Bureau of Mines, Laramie, Wyo. The constructive criticisms of the
manuscript by C. B. Carpenter (deceased), A. B. Cook, H. C. Miller, and
E. M. Tignor, engineers of the Bureau of Mines, are gratefully acknowledged.
The work of three engineers formerly at -the Laramie experiment station -

H. Karl Schmidt and Warren J. Mason on the first group of oil samples and
Raymond A. Morgan on the second group of samples - in addition to that of the
writers resulted. in acquiring the data presented in this report. 0. C.
Heustis prepared the illustrations.

SUBSURFACE OIL SAMPLING

An electromagnetic-type sampler developed by the Bureau of Mines and
described by Grandone and Cook ,2/ was used in obtaining the subsurface oil
samples.

2/ Grandone, Peter, and Cook, Alton B., Collecting and Examining Subsurface

Samples of Petroleum: Bureau of Mines Tech. Paper 629, 1941, pp. 12-19

and 59-64,

4073 -2-


= 40
2
2.
85 30
g2
Sse
38 20
Cary
88" 10
Ss a
=
£ °
- 16
Se
Se
$ 2 val :
£52
SBS
228 MPR
33
BF io in
os
. 9 hat 60°F i
3¢ t
2 £ 87 4
&& :
83 T
T
79 +
13 T
12
ww —
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i
Ww
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500] }
$3, 400] t
228
B22
ea H
85° 200 H
3
to +
100 al
6 [tt Saturation pressure
T
é 8
¢ [tp Flowing pressure at sample point
ae 6 T
38
s§ 4 ii }_| Original reservoir pressure
2 ° at top of Tensleep
3
oO 2
i |
° 1 I
° 500 1000 1500 2000 2500 3000 3500

Pre

@,psia

FIGURE 10. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT I4I°F.,
SUBSURFACE OIL SAMPLE FROM WELL 9, ELK BASIN FIELD.




10.

ll.

12.

4073

ILLUSTRATIONS (Continued)

Results of differential gas-liberation analysis at
129° F., subsurface oil sample from well 5, Elk
Basin PLCld.. ccc ee cece cece cece ccceeecncccavees

Results of differential gas-liberation analysis at
133° F., subsurface oil sample from well 6, Elk
Basin field......... see eeeee

Results of differential gas-liberation analysis at
134° F, ,» subsurface oil sample from well 7, Elk
Basin field............ sae eeeeee

Results of differential gas- eration analysis at
140° F., subsurface oil sample from well 8, Elk
Basin field..... Dene e cere reser ee ceeeenenes

Results of different gas-liberation analysis at
141° F,, subsurface oil sample from well 9, Elk
Basin PLClLA. cece cece cece ccc cccenccerenvceccees

Relationship of the characteristics of the oil at
original conditions to the location of the oil
in the Tensleep reservoir, Elk Basin field.

Pressures and temperatures, Elk Basin field.......

Bottom-hole pressure build-up, wells 1, 2, 3, 4,
5, Elk Basin field.......

- ili -

Follows
page


10.

ll.

13.

14.

b.

16.

4073

TABLES (Continued)

Page
Bureau of Mines routine analyses of crude oils from
wells 8 and 9... eee seeeceess 15
Results of differential gas-liberation analysis at
121° F, » subsurface oil sample from well 1, Elk
Basin field.. we eeeees seeee 16

Results of differential gas-liberation analysis at

121° F., subsurface oil sample from well 2, Elk

Basin PICA. cece cece eee e cece recesses eseeesees 17
Results of differential gas-liberation analysis at

121° F., subsurface oil sample from well 3, Elk

Basin PICld. cece eee eee cece cere sececsseeeseseves 18
Results of differential gas-liberation analysis at

123° F., subsurface oil sample from well 4, Elk

Basin PiClA. cece cece cece cece eeccccrecenesssessses 19
Results of differential gas-liberation analysis at

129° F., subsurface oil sample from well 5, Elk

Basin PICLA... cece eee eee ceececcererccececesesess 20
Results of differential gas-liberation analysis at

133° F., subsurface oil sample from well 6, Elk

Basin PLClA. .. cece cece cece eee escsreseeeseseeses 21
Results of differential gas-liberation analysis at

134° F., subsurface oil sample from well 7, Elk

Basin PLCLA.. cece eee ee cece cccccceeseeessesess 22
Results of differential gas-liberation analysis at

140° F., subsurface oil sample from well 8, Elk

Basin PICld..... cece cece cece reece eteeeteesesesvees 23
Results of differential gas-liberation analysis at

141° F., subsurface oil sample from well 9, Elk

Basin ficld..e.csece cece s eee e eee eeeeceeeecesceeeses Ob

ILLUSTRATIONS
Follows
page
Elk Basin, Tensleep reservoir, MAD......eeseeeuees 2

Results of differential gas-liberation analysis at

121° F., subsurface oil sample from well 1, Elk

Basin Ficld. ce eee cece cece cece cece cence cecece 4
Results of differential gas-liberation analysis at

121° F ., subsurface oil sample from well 2, Elk

Basin PLClA... cece cece eee ccecee reese ceccsesencs 4
Results of differential gas-liberation analysis at

121° F., subsurface oil sample from well 3, Elk

Basin Picld. cc cee cece reece ec ee reese nceceesenes 4
Results of differential gas-liberation analysis at

123° F., subsurface oil sample from well 4, Elk

Basin field..

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FIGURE 4. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT 12I°F.,
SUBSURFACE OIL SAMPLE FROM WELL 3, ELK BASIN FIELD.


SUMMARY

In the spring of 1943, when it was evident that the Tensleep sandstone
in the Elk Basin field, Wyoming and Montana, held a large reserve of petrol-
eum, Bureau of Mines engineers obtained samples of oil from the bottom of
nine wells and analyzed them for such physical characteristics as the volumes
of gas in solution, saturation pressures or bubble points, shrinkage in
volume caused by the release of gas from solution; expansion of the oil with
decrease in pressure, and other related properties. The composition of the
gas in solution in the oil was studied. The pressures and temperatures exist-
ing in the reservoir and the productivity characteristics of the oil wells
were determined. :

The data obtained indicate that the oil in the Tensleep reservoir of the
Elk Basin field has unusually varying physical characteristics, such as a
saturation pressure of 1,250 pounds per square inch absolute and 490 cubic
feet of gas in solution in a barrel of oil at the crest of the structure,
and a saturation pressure of 530 pounds per square inch absolute and 134
cubic feet of gas in solution in a barrel of oil low on the flanks. The
hydrogen sulfide content of the gas in solution in the oil varies from 18
percent for oil on the crest to 5 percent for oil low on the flanks of the
structure. Of even greater significance is the fact that these and other
variable characteristics of the reservoir oil are related to the position of
oil in the structure. Many geologists and petroleum engineers have considered
all the oil in a petroleum reservoir to have uniform physical characteristics
and that equilibrium conditions prevailed in all underground accumulations of
oil and gas. That such is not always so is borne out by the results of the
study made by the writers.

INTRODUCTION

The Rocky Mountain area is one in which confirmation as well as invalida-
tion of accepted theories regarding oil and gas accumulation my be found. In
the area are striking examples of the unusual as is evident from the following
observations: The high helium content (7.6 percent) of the gas in thé Ouray-
Leadville limestone sequence in the Rattlesnake field, New Mexico and gases of
similar helium contents in other fields; 50 to 55° A.P.I. gravity distillate
in solution in carbon dioxide gas and recoverable through retrograde conden-
sation, in the North McCallum field, Colorado; the occurrence of gas and oil
in closely related structures contrary to the usual concepts of gravimetric
segregation; the accumulation of gas, oil, or both in structures closely
related to other structures that apparently are more favorable but do not
contain oil or gas accumilations; the high hydrogen sulfide content (as high as
ke percent) of the gas assdéciated with oil-in some fields in the Big Hom
Basin, Wyoming; and the wide range of fluid characteristics found in the Elk
Basin reservoir,

4073 -1-


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FIGURE 7. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT 133°F,,
SUBSURFACE OIL SAMPLE FROM WELL 6, ELK BASIN FIELD.


Bureau of Mines
Report of Investigations 4768

VARIABLE CHARACTERISTICS OF THE OIL
IN THE TENSLEEP SANDSTONE RESERVOIR,
ELK BASIN FIELD, WYOMING AND MONTANA

By RALPH H. ESPACH AND JOSEPH FRY

United States Department of the Interior— April 1951

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SEA LEVEL ELEVATIONS OF TOP OF TENSLEEP, FEET
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FIGURE II. RELATIONSHIP OF THE CHARACTERISTICS OF THE OIL AT ORIGINAL
CONDITIONS TO THE LOCATION OF THE OIL IN THE TENSLEEP RESERVOIR,
ELK BASIN FIELD.








wells shown in figuré 1 are identified by number along the abscissa of

figure 11 at. positions correspmding to the elevation of the top of the*
Toncleep in each well. Although the wells are plotted according to formatim
"tops," the oil from each of these wells came from same greater depth,
depending on the penetration (160 to 200 feet) of the formation in each well.

‘The canposite sample data for the oil from each of the wells sampled,
as derived from the individual analyses shown in figures 2-to 10, inclusive,
are shown in figure 11 to have a very definite relationship to the structure
of the reservoir from which the oil came, and to the writers! knowledge this
isthe first time that such a relatimship has been shown to exist,

A study of figure 11 would indicate that possibly the subsurface semple
of oil from well 7 was not truly representative of the reservoir oil at well
7, end that the semple had been conditioned somewhat. This was referred to
in the section on subsurface oi] sampling. ‘The data for the specific gravity
of the gas in solution show the least consistent relationship of all the data.
Attention has been called carlier in this report to the manner in which these
data were acquired.

The physical characteristics of the oil throughout the crestal 500 to
€00 feet of the reservoir are very much alike - all oil] samples fram the
crestal part of the reservoir showed a saturetion pressure of 1,235 to 1,250
pounds per square inch absolute end contained 460 to 490 cubic teet of gas
in solution based on a barrel of residual oil. As a consequence of: this gas
in solution, 1.22 barrels of oil in the reservoir are required to produce one
barrel of residual or stock tenk oil. At a depth of about 100 feet above sea
level, a decided change begins to occur in the character of the reservoir
oil - the saturation pressure becomes lower, and the volume of gas in solution
decreases, The other characteristics begin to change also, These changes
continue with depth until the oil from a well in which the top of the producing
formation is. at an elevation of 1,107 feet below sea level, contains only 134
cubic feet of gas in solution per barrel of oil (on & residual-oll basis), and
its saturation pressure is only 530 pounds per square inch absolute. Edge
water is present in the reservoir at an elevation of about 1,500 feet below sea
level. The writers have no information on the characteristics of the oil
coming from wells in which the top of the Tensleep sandstone is below a depth
of 1,107 feet below sea level, Characteristics may be inferred fran an
extenaion of the several curves to elevations of 1,200 or 1 790 feet below
sea level.

The hydrogen sulfide content of the gas in solution in the oil as compared
to the volume of gas in solutio in the different oi] samples is a rather :
interesting relationship, The gas in solution in the oil on top of the
structure contains 18 percmt hydrogen sulfide, that in solutim in the oil m
the edge of the structure contains 5 percent, and the variation is almost
directly proportional. to the volume of gas in the different samples, This
suggests a plausible hypothesis for the movement and characteristics of the
fluids-in the reservoir and further substantiates the supposition that .
equilibrium has not yet bee atteined in the whole mass of reservoir oil,

4073 “9°


Hypothesis for Variable Reservoir-0i1 Characteristics

The sedimentary rocks in the north end of the Big Horn Basin were folded
between the Beartooth Mountains. on the west and the Big Horn Mountains on the
east. In the early stages of folding, an accumlation of gas containing
possibly. 13 percent 4S may have been surrounded by a ring of oil with water
beneath. With the uplift of the mountain masses, an increasing hydrostatic
head would have been applied to the fluids. Under these conditions the oil
ring would have moved into the top of the Elk Basin anticline following the
compression of the gas above and solution of the gas in the oil. This action
would have continued until all the free gas disappeared into solution at a
eressure possibly of 1,500 pounds per square inch, and would have persisted
until some unknown pressure was reached.

At the time of discovery, the pressure was 1,853 pounds per square inch
absolute at the crest of the oil accumulation. As the oil ring moved higher
into the trap, it was confined in an increasingly smaller areal extent and
accordingly increased in thickness until, at the time the field was discovered,
oil filled more than 2,000 feet of closure. As the oil encroached into the
space occupied by Pree gas, the upper part or foremost edge of this oil con-
stantly mixed with and dissolved gas. The dissolved gas then passed from the
highly concentrated zone by diffusion to lower levels of the oil body, result-
ing in progressively. lower concentrations of solution gas with depth. The oil
at the advancing front dissolved HoS more rapidly than methane, because the
products of the partial pressure and solubility coefficient for HoS exceeded
the corresponding value for methane; the latter being by far the largest
single component in the gas phusc.e However, in the oil-gas solution the
methane moved down-dip in the reservoir more rapidly than the HDS,” inasmch as
this is a diffusion process, and among other things the rete of this jrocess
varies inversely as the square roots of the densities. The advancing upper
part of the oil, when pushed up into the top of the trap by the encroaching
water, assumed appreciable depth, which accounts for the fact that the ”
characterisitcs of the oil in the top 500 to 600 feet of the structure are
approximately uniform.

Had geologic time since this movement of the oil been of sufficient
duration, the volume and composition of the gas in solution in the oil at the
top of the structure would have been mch the same as that in the oil else-
where in the structure, and the state of equilibrium of the reservoir-oll
would have been similar to that found throughout other oil-bearing provinces
in the United States, where, according to the common concept, the whole mass
of ofl in a reservoir has rather uniform characteristics. In the Tensleep
reservoir at ilk Basin, the oil progressively farther dow structure below
the top 500 to 600 fcet was increasingly undersaturated with gas, and the gas
that was in solution contained progressively less HpS. The HS content of
less than 18 percent could be explained by the process of diffusion to the
lower levels of the oil body, and by assuming that equilibrium processes were
in action after accumlation but had not proceeded very far prior to the
discovery of the field. The low value of 5 percent HpS content might be
explained by assuming that the small volume of gos associated with the oil prior
to the orogenic movement contained 5 percent or less of HS.

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APPENDIX

When the first work of well testing and sampling by the writers was
being done in the Elk Basin field in the fall of 1943, less than 20 wells had
been campleted in the Tensleep reservoir, and only a limited amount of sub-
surface engineering data were available. Consequently, while conditioning the
wells for sampling, opportunities were available to obtain informatio rela-
tive to the formation and to the characteristics of the wells that would be of
value in the evaluation and interpretation of the subsurface oll-sample data.
Information and data on temperature gradients, original pressures, character-
istics of the producing formation, quality, and characteristics of the produced
oil are of appreciable import in 4 study of a reservoir. Consequently, such
information as was obtained in appended to this report,

Reservoir Data and Producing Characteristics of Wells

Figure 12 is a graph of the temperature gradient in the Elk Basin structire;
the data are essentially those obtained in wells 5 and 7, fran which little oil
had been produced before the tests were made. One well had been shut in for 4
days and the other 45 days at the time the temperatures were obtained. The
density of data points below soa level results fran the use of bottom-hole
temperature data obtained fran the other wells that were sampled. The tempera-
tures existing in any of the wells between ground level and 1,000 feet in depth
were not used. Figure 12 indicates that the temperature in the Elk Basin field
increases with depth at the rate of 1.640 F, per hundred feet.

The chart in the lower right corner of figure 12 has the same sea-level
elevation ordinate as the temperature gradient curve and shows the original
pressures that existed in the Tensleep reservoir at time of discovery. This
chart was developed by establishing the original pressure at sea-level
elevation and using the data from the curve of density at original pressure
shown in figure 11. The original pressure at sea level was determined by
plotting the pressures existing at successive cumulative productions and
extending the curve to zero production.

The graph in the upper right corner of figure 12 shows the productivity
or several of the wells. The graph shows the amount of drawdow in pressure
obtained below the tops of the Tensleep formation in several wells as a result
of flowing them at different rates. Well 4 wos campleted in a fractured zme
in the Tensleep formatim, which accounts for its high productivity as canpared
with other wells. The curves represent conditons in the early life of each
of the wells, or, with one exceptim, within 6 mmths of initial production.

Figure 13 shows sane curves of the pressure changes at sampling-point
elevations that tod: place following the shutting~in of several of the wells
that had been producing ut high rates. ‘The pressure points for the time
period up to 10 hours after the wells were shut in, were obtained by a sub-
surface pressure gage that had been lowered into the wells just prior to
shutting them in. The pressure points at 3 to 6 days were determined with a
subsurface pressure gege after the wells had been shut in prior to the time of
flowing the wells for sampling and productivity testing. The wells were pro-
ducing oil, before being shut in, at the following rates, in barrels, per day:

4073 -ll-


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FIGURE 2. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT !2I°F.,
SUBSURFACE OIL SAMPLE FROM WELL |, ELK BASIN FIELD.


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FIGURE 3. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT I2I°F.,
SUBSURFACE OIL SAMPLE FROM WELL 2, ELK BASIN FIELD.


VARIABLE CHARACTERISTICS OF THE OIL
IN THE TENSLEEP SANDSTONE RESERVOIR,
ELK BASIN FIELD, WYOMING AND MONTANA

BY RALPH H. ESPACH AND JOSEPH FRY

* * «* « «* «* « «* « Report of Investigations 4768

UNITED STATES DEPARTMENT OF THE INTERIOR
Oscar L. Chapman, Secretary
BUREAU OF MINES
James Boyd, Director

The Bureau of Mines will welcome reprinting

Work on manuscript completed March 1950.
“Reprinted from

of this paper, provided the following footnote acknowledgment is made:
Bureau of Mines Report of Investigations 4768.”

This report represents work done under a cooperative agreement between the Bureau of Mines,
United States Department of the Interior, and the University of Wyoming.

April 1951


TABLE 4, - Viscosity of reservoir oil

Pressure, Oil fran well 2, | O11 from edge wells,
psi. centipoise centipoise (estimated)
2500 sees 1.59- 16
2000 seaeeee 1.50 -
1500 Lhe 4A
1150 1.35 -
1000 1.40 -
GOO seeeeee 1.50- | 4D
600 1.65 | 4.0
HOO seseeee 1.82 4D
200 2.05 47
O eeeesee 450 8.0
Discussion

A review of the data shown in figures 2 to 10, inclusive, reveals a very
interesting conclusion that should be considered in all studies of petroleum
reservoirs, namely, that the oil in a reservoir may have rather widely varying
physical characteristics. That these characteristics may be related to the
location of the oil in the structure will be discussed in a subsequent section
of this report.

In general, many engineers and geologists have reasoed that, considering
the length of time involved in the formatio of an oil field, equilibrium must
have been attained between the fluids in the reservoir, and if any of the oil
was saturated with gas under existing pressures and temperatures, then all of
it was saturated, or very nearly so, or, conversely, if some of the oil was
undersaturated, then all of it would be undersaturated to about the same
degree. Some recorded observations in other fields seem to indicate that
equilibrium conditions may not exist in all fields, but incontrovertible
proof of that has been lacking. Estabrook and Rader3/ noted the presence of
a "zone of dead oil" in the Second Wall Creek sand in the Salt Creek field
and reported that "the distribution of dissolved gas in the oil seems to have
been fairly wmiform over the field except toward the edges."

The study of the subsurface oil samples fram Elk Basin definitely proves
that the fluids in a reservoir are not necessarily in equilibrium. It is con-
cluded that either extremely long periods of time are necessary to effect
equilibrium, or that accumulation can be quite young or is still taking place
in same petroleum reservoirs.

RELATIONSHIP OF OIL CHARACTERISTICS TO RESERVOIR STRUCTURE

The relationship of the characteristics of the oil to the location of
the oil in the Elk Basin, Tensleep reservoir, is shown in figure 11. In
figure, 11 values of the saturation pressure, gas in solution, relative oil
volume, density of the reservoir oil, specific gravity of the gas in solution,
and hydrogen sulfide content of the gas in solution for the nine subsurface
oil samples are plotted as ordinates against the common abscissa of sea-level
elevations of the top of the Tensleep formation over the structure. The nine

3/ Estabrook, E. L., and Rader, 0. M., History of Production of Salt Creek Oil
Field, Wyoming: Am. Inst. Min. and Met. Ing., Petroleum Development and
Technology in 1925, 1926, p. 216.

4073






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FIGURE 9. RESULTS OF DIFFERENTIAL GAS-LIBERATION ANALYSIS AT 40°F,
SUBSURFACE OIL SAMPLE FROM WELL 8, ELK BASIN FIELD.


As the wells were tubed nearly to bottom, the sampler was run as far
as possible in the tubing but never below the’ top perforations. Thus, oil
leaving the reservoir had to flow through or around the sampler on its way
to the well head, and pressures at the sampling point were the maximum
obtainable.

The following procedure in testing and sampling the wells was used: A
well was shut in for at least 3 days, after which shut-in subsurface pressures
and temperatures were measured. Generally, the well then was allowed to flow
at a 10- to 20-barrel per hour rate for approximately 1 day, when a flowing
temperature and pressure traverse was run. One day later, at the same
production rate, a subsurface sample was obtained and transferred to a storage
container. A second subsurface sample of oi] then was procured, and if, in
transferring, the pressure and volume date duplicated those of the first
sample, both were considered satisfactory for analyses. After sampling: was
completed, the Plow rate of the well was increased to two higher ‘rates, and
subsurface pressure traverses were obtained at each rate. A 12-hour
subsurface-pressure build-up was obtained when the well was shut in following
the highest rate of flow. A pressure gage recorded the tubing and casing
pressures at the well head during the tests, The oil produced by the well
during the tests was gaged in lease tanks at regular intervals, and the
separator gas was measured with a recording orifice meter. The essential
data pertaining to the wells are given in table 1.

Wells 7 and 9 were not producing when sampled. Well 7 had flowed most:
of the indicated production, but the flow gradually lessened, and the well
finally quit flowing. It was sampled several days later. well 9 was swabbed
unsuccessfully to cause it to flow; it had been standing for about 30 days
prior to sampling, and, as tubing had not been run to bottom, it was necessary
to sample the oil at a relatively high point in the well.

It will be noted that wells 2 and 3 have essentially the same structural
position, and for the purposes of this study it might seem that one well would
suffice. Well 3, however, produced from the upper 44 feet of the Tensleep
formation, and well 2 was completed so that. only the zone between 66 and 192
feet below the top of the Tensleep formation produced oil through the tubing.
Sampling both wells provided an excellent opportunity to study the possibility
of the existence of more than one reservoir in the Tensleep formtion, a
postulate that had merit because of the rather extensive dolomitic zones
extending throughout the sandstone body at depths greater than 70 to 80°feet
in the formation. The almost identical results obtained in the analyses of
the oil samples from both wells indicated that the oils were the same’ and that
the Tensleep formation contained only one reservoir.

Figure 1 is a structural map of the Elk Basin Tensleep reservoir, on
which the nine wells that were used in this study and the numbers corresponding
to the well designations herein referred to are shown. Wells 1, 2, 3, 4, and
8 were tested and sampled during October and November 1943, and ‘yells 5, 6, 7,
and 9 during June and July 19h.

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