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GEOLOGICAL ASPECTS OF ABNORMAL RESERVOIR PRESSURES IN THE GULF
COAST REGION OF LOUISIANA, U.S.A.
BY
GEORGE DICKINSON
S ynop'sis
High pressure zones frequently make drilling of wells most
difficult in a belt about 50 miles wide along the coastal plain
northwest of the Gulf of Mexico from the Rio Grande to the
Mississippi Delta. This study is an attempt to link geological
factors with occurrences of abnormal pressure in order to provide a
better understanding of their origin.
Abnormal pressure has been defined as any pres- sure which
exceeds the hydrostatic pressure of a column of water containing
80,000 parts per million total solids.
Dangerously abnormal pressures occur commonly in isolated porous
reservoir beds in thick shale sec- tions developed below the main
sand series. Their locations are controlled by the regional facies
change in the Gulf Coast Tertiary province, and they appear to be
independent of depth and geolog- ical age of the formation.
The high pressures are caused by compaction of the shales under
the weight of the overburden which is equivalent to approximately
one pound per square inch per foot depth. Difference in density
between gas and water causes abnormal pressure when hydrocarbon
accumulations occur above wa- ter, irrespective of whether the
water is at normal or abnormal pressures. The magnitude of this
press- ure depends upon the structural elevation above the source
of pressure in the water and may cause very high pressure gradients
in isolated sand bodies. However, the trend of pressures in the
Gulf Coast region indicates that maximum pressures will prob- ably
not exceed ninety per cent of the overburden pressure.
* Regional Production Department, Shell Oil Company, Houston,
Texas.
The abrupt increase in pressure above normal hydrostatic
pressure often occurs over a very short vertical interval which
makes control difficult. Suc- cessful drilling through abnormal
pressures involves cementing casing below the main sand series and
above the high pressure zones so that heavy mud may be used without
loss of circulation.
Rsum
Des zones de haute pression rendent frquem- ment trs difficile
le forage de puits dans une zone de 50 milles de largeur le long de
la plaine ctire au nord-ouest du Golfe du Mexique depuis le Rio
Grande jusqu'au delta du Mississi pi. Cette tude
ques et la prsence de pressions anormales afin de permettre une
meilleure comprhension de l'origine de celles-ci. Une pression a t
dfinie comme anormale quand elle dpasse la pression hydrostati- que
d'une colonne d'eau dont le contenu solide est de 80.000 parties
par million.
Les pressions dangereusement anormales se pr- sentent souvent
dans des couches-rservoir poreu- ses isoles, intercales dans d'
aisses sections de
principales. Leurs emplacements sont gouverns par les
changements rgionaux de facies dans la provin- ce tertiaire de la
Gulf Coast et paraissent tre ind- pendants de la profondeur et de
l'ge gologique de la formation.
ressions leves sont causes par-le tasse-
verture, lequel est quivalent environ une livre par pouce carr
par pied de profondeur. La dif- frence de densit entre le gaz et
l'eau cause des pressions anormales quand il y a des accumulations
d'hydrocarbures au-dessus de l'eau, que la pression de l'eau soit
normale ou anormale. Limportance de
essaie d'tablir des liens entre les F acteurs gologi-
shales dveloppes au-dessous c f es sries sableuses
Les ment B es shales sous le poids des terrains de cou-
Proceedings 3rd W.P.C., Section I 1
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2 PROCEEDINGS THIRD WORLD PETROLEUM CONGRESS-SECTION I
cette pression dpend de l'lvation structurale au- dessus du lieu
d'origine de la pression dans l'eau et peut causer des gradients de
pression trs mar- ques dans des amas de sable isols. Cependant,
l'allure des pressions dans la rgion de la Gulf Coast indique que
probablement les pressions maxima ne doivent pas dpasser 90% de la
pression des terrains de couverture.
L'augmentation abrupte de la pression au-dessus de la pression
hydrostatique normale a lieu souvent dans un intervalle vertical
extrmement court, ce qui rend le contrle difficile. La russite de
forages travers des pressions anormales implique la cimen- tation
du tubage au-dessous des sries sableuses principales et au-dessus
des zones de haute pression de manire pouvoir employer une boue
alourdie sans perte de circulation.
Introduction
Drilling operations in the coastal plain northwest of the Gulf
of Mexico frequently encounter high pressure zones which are most
difficult to control. These zones of excessive pressure are widely
dis- tributed in a belt 35 to 75 miles wide along the coast from
the Rio Grande in the southwest to the Vississippi Delta in the
east, a distance of approxi- mately 800 miles. This belt coincides
approx:mately with the arda of Pleistocene and Recent formations
shown on the index map, Figure 1.
There has been only limited success in drilling through high
pressure zones to rospectfve reser-
accumulation of oil and gas. An adequate under- standing of the
origin of pressure in reservoir for- mations becomes, therefore,
increasingly important as shallow objectives become fewer and as
attain- able drilling depths increase.
The present study of the geological aspects of the problem
attempts to link geological factors with oc- currences of abnormal
pressure.
The Gulf Coast region of Louisiana, as outlined on the index
map, Figure 1, was chosen for this purpose since it is part of a
relatively simple geolog- ical province favorable for analysis.
voir rocks thought to be favorab P y located for the
Stratigraphy
The general stratigraphic column of the Tertiary, shown in
Figure 5, is overlain by sediments of Re- cent and Pleistocene age,
whidh in some places ex- ceed 3,000 feet in thickness. In the
inland part of the area a few wells penetrated the Eocene, for
example in the Bear and Bannister districts shown in Figure 7.
A continental shelf environment similar to that prevailing at
the present time probably persisted throughout the Tertiary. The
distribution of the various geological units follows consistent
trends which nearly parallel the existing coast line ex- cept in
the area of the Mississippi Delta where the coast has been built
out into the Gulf of Mexico. Sedimentation was more or less
continuous and all the major stratigraphic units in the subsurface
thicken and become progressively more marine in character from the
outcrop toward the Gulf of Mexico. For. example, the Frio thickens
from about 1,700 feet in the Bannister wells to more than 4,200
feet in Iowa about thirty miles down dip. In gener- al the change
from mainly sandy sediments to ma- rine shales occurs at
progressively higher strati- graphic levels from the lower Frio
inland to high in the Miocene in the coastal zone as shown in Fig-
ures 7 and 8. However, the detailed facies studies of Lowman (27)"
have shown that while the gen- eral change of each zone is from
fresh water facies farthest shoreward through brackish water facies
and shallow marine facies to rogressively deeper marine facies, the
successive c ange is affected by rhythmic c cles caused by
transgressions and re-
R gressions o r I the sea.
Structure ' The regional structure of the Gulf Coast
consists
of an homocline dipping gently gulfwards. Surface dips are very
slight but increase with depth owing to the increasing thickness of
the sediments towards the Gulf. Regional faulting is typically
down-thrown towards the coast and is possibly connected with the
depositional environment and the increasing amount of compaction of
the more argillaceous sediments in that direction. The area is
typified by numerous salt domes in which the salt may be anywhere
from surface in piercement type domes to below the depth reached by
drilling at the present time. Some of these salt domes appear to be
connected with the regional faulting but in some cases there are
other faults dipping inland which appear to con- nect between
domes. The salt domes have charac- teristic fault patterns which
are caused by local up- lifting of the formations, but there is
little evidence of any other tectonic forces acting in the area
under review so that the effect of compaction of the sed- iments is
easily recognizable.
Normal Pressure Gradient
Throughout the Gulf Coast region the majority of * References
given at end of paper.
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G. DICKINSON-ABNORMAL RESERVOIR PRESSURES IN THE GULF COAST
REGION 3
wells encounter subsurface pressures which, when measured at the
oil/water or. gas/water interface, approximate very closely the
hydrostatic pressure of a column of water containing 80,000 parts
per million total solids, or a pressure gradient of 0.465 pounds
per square inch per foot depth. This grad- ient has been
established over a range from sur- face to about 16,000 feet in
Queen Bess Island as shown in Figure 2.
Occurrences of Abnormal Pressure
The available abnormal pressure measurements are plotted in
Figure 2, numbered to correspond with their locations as shown in
Figure 3. Actual measurements of abnormal pressures encountered in
a well are rare so that it is usually necessary to estimate the
bottom hole pressure from testing and production data or from the
mud weight in the hole at the time the abnormal pressure was
encount- ered, compared with that required to control the pressure.
Where actual bottom hole pressure meas- urements are available for
comparison, it appears that the former method is reasonably
reliable, al- though somewhat low pressures result; whereas the
latter method appears to give pressures which are about 10 per cent
too high as shown in Figure 4. All pressures estimated from mud
weights have, therefore, been reduced by this amount. However, this
correction factor is based upon very sparse data, and it is
possible that it may vary with hole size since the swabbing action
induced when pulling drill pipe necessitates an increasing pressure
differ- ential as the hole size is decreased *.
Many of the abnormal pressure occurrences which were reviewed,
flowed salt water with no oil, but it is probable that solution gas
was present (17) although it was not always reported. In the case
of most of the high pressure gas and oil ac- cumulations the depth
of the oil/water interface is not known, so that, depth for depth,
the abnormal pressures may be higher than if the zone contained
salt water only. However, a study of the pressure gradients given
in Figure 2 shows that the high- est pressures known have a
pressure gradient of about 0.87 pound per s uare inch per foot
depth,
tive of whether the reservoir contains salt water or gas and
oil.
or about 1.87 normal hy 1 rostatic pressure, irrespec-
* According to a verbal communication from J. M. Bug- bee, 800
to 1200 pounds per square inch overbalancing mud pressure is
required in a hole compared with only 200 to 500 pounds per square
inch in a 8-1/2 hole.
Abnormal pressures are encountered in forma- tions ranging in
age from the upper Miocene in the Mississippi Delta area, to the
base of the Oligocene in a strip extending from around Baton Rouge
to the Lake Charles area. Figure 3 shows the locations of all
abnormal pressure occurrences for which data were available, and
the geological zone in which the first abnormal pressure was
recorded. It is ap- parent from this map that these geological
zones follow trends which agree closely with the Bay Line of Lowman
(27) and with the established producing trends of the region
(Lowman, Figure 6). When plotted on a stratigraphic correlation
chart, Figure 5, the grouping of the occurrences of ab- normal
pressure is even more striking, so that some geological control
would appear to be indicated.
Cannon & Craze (12) and Cannon & Sullins (13) of the
Humble Oil & Refining Company, after reviewing a large number
of abnormal pressure oc- currences, concluded that depth alone
seemed to be the governing factor regardless of the age of the
formation. However, in the latter paper adjacent normal and
abnormal pressures in the same forma- tion were attributed to
depositional and faulting characteristics, but this geological
aspect was not further pursued.
The change from normal hydrostatic to abnormal pressure for some
wells is shown in Figure 6. A study of the available data appears
to lead to the conclusion that once the zone containing abnormal
pressure has been reached, the pressure will increase suddenly, as
in Iowa and Manilla Village, or some- what less rapidly, as in
Chalkley, South Roanoke and La Pice. No reliable examples of
gradual pres- sure increase over an appreciable depth range were
found. However, the use of progressively increasing mud weight in
many wells probably indicates that a gradual pressure increase does
occur.
In order to investigate possible geological con- trol of
abnormal pressures, a detailed study was made of the logs of all
the wells known to have encountered abnormal pressure and of many
neigh- boring wells with normal pressure. The results of this study
are illustrated by three diagrammatic stratigraphic sections,
Figures 7, 8, and 9, drawn through a series of typical wells across
the West, East and Delta areas of the region. These sections show
that abnormal pressure commonly occurs only below the base of the
main sand development in or below a major shaly series. Even though
most of the abnormal ressure occurrences reviewed conform to the
con 1; itions shown in the cross-sections, high pressure may also
be found in the main sand series where conditions are favorable for
isolation of sand
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4
lf!OOO
11000 O .- o C .- $- In .I 10000 a O .- c c
2 5 i?
9000
E e ... U : 8000 E .- c
o>
t
9. 7000 o 8 s
v> ln
D
u7 6000
5000
4000 4000
~
PROCEEDINGS THIRD WORI,D PETROLEUM CONGRESS-SECTION I
5000 6060 7000 8000 9000 10000 11000 i2000 Measured subsurface
pressure, in psig
Fig. 4. Relationship between subsurface pressures measured by
pressure bomb and estimated from minimum hydrostatic head of mud
required for control during drilling.
bodies by faulting or lensing out of the sand, for example, in
Darrow, Lirette and Venice.
The change in facies from mainly sand to mainly shale occurs at
the base of the Frio in the northern part of the area under review
but gradually climbs the stratigraphic section until it reaches
high in the hliocene in the Mississippi Delta area. These
Chang-
es of facies and of the accompanying fauna have been described
by Lowman (27). As a result of the present study, it appears that a
knowledge of the depth, at which the main facies change takes
place, is an important factor in forecasting the depth at which
abnormal pressures may be encountered in exploration wells.
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i p o
I
I I I I I I I I l l I l I I I l I l
T t + + I T t + t t + + + + 1 1
+ t t t t t t t + + + t t 4- t t t 4 4 4 -t 4 4
i
i I T
I
1-3 6 & I 8 9 10 & 16 II 14 18 5 19 & 20 21 22 23 i
24 24A 248 24C & D 27 28 & 50 29, 30, 42 & 49 31
33 & 34 36 & 45 31
32 f
38 I 39 I 40 43 & 44 46. 55-51 & 15 47 51 54 58 59 6a 61
82 63-66 75A 67, 76, 71, 80 & 84 68, 69, 18 & 81 10-14. 73,
86 6 87 85 8 & 89 90 91 92 96A 93A 93 94 95 96 97-108 108A 109
I10 111 114-118 119 h 122 IZI
Bay St. Elaine West Bay Four Isle Lirette Venice Dome De Large
East of Deer Island La Peyrouse Northeast of DeLargo Manilla
Village East of Lake Hermitage Southwest of Houma Deer Island
Gibson La Fitte Chacahoula Westwego Little Chenier North of Creole
Grand Lake East White Lake
Lake Borgne Area
Weeks Island Goodhope
Lake Pontchartrain
Near Hester West Lake Verret La Pice North Jeanerette North
Jeanerette Johnson's Bayou Little Chenier (see 27) Snake Lake West
Gueydan Mud Lake Abbeville Chalkley (South) Samstown Chalkley
(North) South Crowiey South Roanoke West Mermentau South Hayes
North of St. Gabriel West of Whitecastle East Hackberry Bon Air
Northeast of Black Bayou Nprtheast of Black Bayou North Crowley
Roanoke Bayou Choupiquc St. Gabriel East of Baton Rouge Box0
Roanoke TOPSY Iowa Bel Northeast of North Elton
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6 PROCEEDINGS THIRD WORLD PETROLEUM CONGRESS-SECTION 1
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G. DICKINSON-ABNORMAL RESERVOIR PRESSURES IN THE GULF COAST
REGION 7
Undoubtedly abnormal pressures have been encountered in wells
located between the occurren- ces shown in Figure 3, but the data
are not readily available. However, there are also other deep wells
which penetrated the same formations without encountering high
pressure reservoirs. It i s apparent, therefore, that other factors
must be present in ad- dition to the shaly facies with lenticular
sands.
Regardless of the origin of abnormal pressure, it is evident
that a reservoir containing high pressure must be effectively
isolated from any other porous formation which contains normal
hydrostatic pres- sure, otherwise the pressure would be dissipated.
This requires a suitable porous reservoir sealed in all directions
either by lensing or faulting. However, sand bodies in an
essentially shaly series are typical- ly lenticular and erratic so
that while faulting is not a pre-requisite for the preservation of
abnormal pressure, it is nearly always present in the wells
reviewed. Regionally, of course, the downdip seal of all reservoirs
can be the change to deepwater facies, but local pinchout may
produce more lim- ited reservoirs. These conditions are shown dia-
grammatically in Figures 10, a and b. The effect of the relation
between the position of the sand body in the shale series and the
throw of a fault on the preservation or dissipation of abnormal
pressure is shown in Figure 10c. It is obvious from these diagrams
that abnormal pressures can occur near the top of the shale series
only if the porous bed is isolated by pinchout or is faulted down
against the shale series as in Chalkley, whereas in the absence of
pinchout of bhe reservoir in upthrown blocks ab- normal pressures
can only be preserved deeper in the shale series by an amount
greater than the throw of the fault.
Geological conditions leading to the preservation or dissipation
of high pressures are well illustrated in the Chalkley Field.
Figure 11 is a north to south sketch section showing a series of
south dipping normal faults crossing a north-south trending domal
structure. The W sand in the upper part of a thick shale section
contains oil and gas under very high pressures in the south flank
but is under nor- mal pressure in the center and north of the
struc- ture. The downthrown block of the south flank is effectively
sealed updip by being faulted against the thick shale series,
whereas in the north flank the W sand is faulted against the main
sand series and is under normal hydrostatic pressure. In the south-
ernmost of the intermediate blocks sand develop- ment is poor and
no high pressure reservoirs were encountered. The W sand is present
in the other blocks, but it is clear from the section that it
is
faulted against other sands which have connection to the
normally pressured main sand series. How- ever, abnormal pressures
were encountered in the two northern blocks at greater depths where
the
orous zones are sealed by being faulted against figher parts of
the thick shale section.
Abnormal pressure occurrences in u thrown blocks similar to the
north flank of Chal I! ley are numerous in the Gulf Coast region.
The amount of uplift above regional is normally relatively small,
ranging from about 300 feet in Snake Lake to 1200 feet in South
Crowley and Grand Lake. Uplifts as large as 1600 feet for the north
flank of Chalkley and 3500 feet in East White Lake are
uncommon.
Abnormal pressures below an unsuspected fault are especially
difficult to control owing to the ab- rupt change in pressure
gradient, such as occurred in several wells in La Pice. Close
paleontological control may indicate such a fault and thus enable
the mud weight to be increased before a porous zone is penetrated.
In some cases, where abnormal pressure is encountered unexpectedly,
for example, as in Shell, Smith A-1, Weeks Island, the fauna may
show that the normal reservoir formations have been cut out by a
fault.
Little is known regarding the size of porous zones containing
abnormal pressures. Since most of them occur on faulted structures,
it is frequently assumed that they are only of limited extent. Most
of the abnormal pressure occurrences reviewed are in thin sands
containing salt water and occasionally some gas, although there are
also some very high pressure producing zones carrying oil and gas,
for example, the FV and FX sands in Iowa, the W sand in Chalkley,
the V sand in San Gabriel, and a Mio- cene sand in Manilla Village.
Rapid diminution in rate of flow of gas or salt waier, or rapid
drop in reservoir pressure indicates that soine of the high
pressure reservoirs are undoubtedly small in size or poorly
permeable, for example, the V sand in St. Gabriel appears to have
erratic development, and some wells were depleted in a few months.
On the other hand, some sand lenses must cover a con- siderable
area as indicated by the large volumes of fluid produced. For
example, the Bel crater in Allen Parish produced about seven
million barrels of wa- ter without apparent reduction in the rate
of flow (12). The rapid decline in reservoir pressure for the FV
sand in Shell, Fontenot No. 10 in Iowa seemed to indicate a limited
reservoir volume, but after 9 months the rate of decline decreased
considerably so that either the reservoir is larger than at first
supposed, or there has been failure of a fault seal
-
............ ....: ........ ........... ............. ..... :;::
.:.:: . >> ... .:. . . ......... .1 ........ ................
SMALL RESERVOIRS SEALED BY PINCHOUT.
a.
i\l ......... ................. ............ ;::: ......
...;:;.
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G. DICKINSON-ABNORMAL RESERVOIR PRESSURES IN THE G U L F COAST
REGION 11
ressure 9340 psi at 12500' ressure Gradient 0.757 psilft
I ~
Salt water sand
Pressure 10500 psi at 15000' Pressure Gradient 0.700 ps/ft.
i
Pressure 8175 psi at 10000' I
Pressure 9050 psi at 10000' Pressure Grodient 0.905 psiftt
/
Fig. 13. Effect of structure on pressure gradients in sands
containing fluids under abnormal pressure.
at points of contact of the sand grains. At similar depths,
about 10,OO feet, the sands and sandstones of the Miocene and
Oligocene apparently have not yet been similarly affected, so that
age, rather than depth of burial, appears to be the more important
factor in this exceedingly slow lithological change in the
character of a sand.
The effects of this process are twofold, firstly the solution of
silica from sand grains at points of con- tact results in
compaction with consequent decrease in porosity and expulsion of
water or rise in fluid pressure; and secondly the precipitation of
quartz around the sand grains and in the voids results in a further
decrease in porosity and expulsion of water or rise in fluid
pressure. The rate of volume reduc- tion from these causes is
probably so small com- pared with that of clays that its effect on
ihe fluid
region, whereas the alternative hypotheses discussed below are
not satisfactory in all respects.
P. E. Chaney (15) suggested that progressive degradation of oil
and gas in a closed reservoir could give rise to abnormal pressures
up to overburden pressure, and that higher pressures would be re-
leased by decompaction of fracturing. The disad-
pressure in a sand will be negligible during the greater part of
the compaction of the enveloping clays. However, in the later
stages of shale com- paction, its effect might cause fluid
pressures within isolated sand bodies to increase above the
residual abnormal pressure generated by the compaction of the
clays.
The foregoing hypothesis, that abnormal pressures are caused by
the weight of the overburden, appears to conform with known
conditions in the Gulf Coast
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12 PROCEEDINGS THIRD WORLD PETROLEUM CONGRESS-SECTION I
Shale-Density
I
0 -
O
O N -
O
O O
O O - o O Io
O
O 0
0 O O N
0
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13 G. DICKINSON-ABNORMAL RESERVOIR PRESSURES IN THE GULF COAST
REGION
Percentage of total Compaction and Porosity
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14 PROCEEDINGS THIRD WORLD PETROLEUM CONGRESS-SECTION I
vantage of this hypothesis is that many of the high pressure
zones in the Gulf Coast region contain salt water with solution gas
only. Illing (25) doubts whether changes in the composition of oil
and gas occur at so late a stage.
W. E. V. Abraham (1) thought that uplift of sand lenses from
great depths might account for abnor- mal pressures in Trinidad.
This hypothesis is unten- able for the Gulf Coast since the
geological history of the region does not allow postulation of
uplift of sufficient magnitude to account for even moderate- ly
high abnormal pressures. In addition, as Watts (35) has pointed
out, if the uplift is accompanied by the appropriate reduction in
temperature, contrac- tion of the confined fluids will decrease the
pres- sure rapidly and under some conditions sufficiently to
maintain normal hydrostatic pressure.
Tectonic forces undoubtedly may give rise to very high
subsurface pressures in some areas (25, 35) but such forces appear
to be absent in the Gulf Coast region, except perhaps locally
around salt domes.
Estimation of Overburden Pressure
A close approximation of overburden pressure based on the shale
density-depth relationship is given in Figure 16. It can be seen
from this curve that the commonly accepted pressure gradient of one
pound per square inch per foot depth is suf- ficiently accurate for
all practical purposes, al- though its use may lead to
underestimation of the overburden pressure at depths greater than
about 17,000 feet.
The effect of the great thicknesses of sand in the Gulf Coast
section is relatively small. According to Archie (2) Miocene,
loosely consolidated sandstone averages about thirty per cent
porosity, and Oligo- cene, consolidated sandstone, varies between
eight- een and thirty-five per cent porosity with an aver- age of
about twenty-five per cent. Assuming clean sand with a mineral
grain density of 2.65 (quartz) and salt water density of 1.08, the
bulk density of the sandstones will be about 2.18 and 2.26,
respec-
tively, see Figure 17. Although these densities are lower than
the equivalent shale densities at depths greater than about 3,000
to 4,000 feet, the effect on the overburden pressure is negligible.
For example,
if the upper 15,000 feet formation is assumed to be all sand
with thirty per cent porosity, the overburden pressure will be
about 14,300 pounds per square inch, compared with 14,900 pounds
per square inch for an all shale section. These ressures
represent
pressure may be some two to three per cent below that shown by
the curve in Figure 16 and will al- ways be less than one pound per
square inch per foot depth to drilling depths at present attainable
(say 20,000 feet).
The current maximum pressure gradients are 0.872 pounds per
square inch per foot de th for salt water, possibly with solution
gas, in JO nsons Bayou, and 0,876 pound per square inch per foot
depth for gas condensate in the FV sand of Iowa. Compared with the
maxima of about 0.865 of Can- non and Sullins (13) in 1946, 0.83 of
Denton (17) in 1943, and 0.765 of Cannon and Craze (12) in 1938,
these gradients appear to indicate that the upper limit of abnormal
pressure gradients is being ap- proached, and that it is unlikely
that it will exceed about 0.900 pound per s uare inch per foot
depth.
drilled through without excessive trouble using muds weighing 18
to 18.5 pounds per gallon. The main difficulty with such heavy mud
is loss of cir- culation. Where abnormal pressures have been
penetrated succesfully, for example in Iowa, St. Gabriel, and
Chalkley, casing was cemented in the top of the shale series before
drilling into the high pressure zones thus precluding the loss of
circula- tion into the main sand series.
the two extremes, so that, norma P ly, the overburden
Pressures approaching p1 t is gradient have been
Acknowledgements
The writer expresses appreciation to the manage- ment of the
Shell Oil Company for permission to publish this paper. Thanks are
due also to members of the staffs of the Exploration and Production
Departments in both the Regional office, Houston and the several
offices in the New Orleans Area who contributed suggestions and
assistance in as- sembling the information and in preparation of
the enclosures.
Manuscript received Nov. 23, 1950.
-
G. DICKINSON-ABNORMAL RESERVOIR PRESSURES IN THE GULF COAST
REGION
Porosity-Per cent
15
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16 PROCEEDINGS THIRD WORLD PETROLEUM CONGRESS-SECTION I
Bibliography (1) Abraham, W. E. V., Geological Aspects of Deep
Dril- (19) Gilbert, C. M., Cementation of Some California Ter-
ling Problems, Jour. Inst. of Petr., London, 1937, 378. tiary
Reservoir Sands, Jour. Geology, vol. LVII (1949),
pany, Production Dept. Report, February, 1949, Fig- (20)
Goldstein, Jr., A., Cementation of Dakota Sandstones ure 20. of the
Colorado Front Range, Jour. Sedimentary Pet-
(3) Athy, L. F., Compaction and Oil Migration, Am. As- rology,
vol. XVIII (1948), 108-125. soc* Bull., Vol. XIV (1930), 25-36.
(21) Goldstone, F., and Hafner, W., Geophysical Monthly
(4) Athy, L. F., Density, Porosity and Compaction of Report,
Shell Explor. Dept. Report, October, 1930. (22) Hedberg, H. D.,
Gravitational Compaction of Clays Sedimentary Rocks, ibid,
1-24.
and Shales, Am. Journ. Sci., Fifth Ser., vol. XXXI (5) Athy, L.
F., Compaction and Its Effect on Local . Structure, Problems of
Petroleum Geology, Am. Assoc. (1936), 279 (Includes a good list of
references).
(23) Heiland, C. A., Geophysical Exploration, Prentice Hall,
Petroleum Geologists, 1934, 814. New York 1940, 82-84, 278, and
280. (6) Barton, D. C., Belle Isle Torsion-Balance Survey, St.
(24) Hobson, G. D., Compaction and Some Oil Field Fea- Mary
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DISCUSSION Mr. B. P. BOOTS (N.V. De Bataafsche Petroleum
Maatschappij, The Hague, Netherlands) suggested that a clear
distinction be made between the two causes of abnormal pressures,
mentioned in Mr. Dickinsons paper, i.e.
1) difference in densities between hydrocarbon
2) compaction of formations. Dealing only with the second cause,
which is
characterized by an excess of pressure in the water
and water,
-
G. DICKINSON-ABNORMAL RESERVOIR PRESSURES IN THE GULF COAST
REGION 17
limb of the reservoir, the results of calculations were
presented showing the maximum posdble pressure which could occur in
the water, if formations in which originally hydrostatic pressures
prevailed, were buried to a greater depth. Such maximum pos- sible
pressure could only occur if the shales were entirely impermeable.
The calculations showed that maximum possible pressures approaching
over- burden pressure could only result from shale com- paction if
shallow formations with low bulk densities were buried deeply.
However, under such condi- tions the assumption that the shales
would be im- permeable cannot be expected to be tenable. The
importance of low bulk densities of shales at great depth as a
warning of possible excess pressure to be encountered was
emphasized. The results of Mr. Boots calculations throw some doubt
on the validity of the theory that the high excess pressures
encount- ered can be explained by compaction of shales only.
Mr. D. COMINS (Anglo-Iranian Oil) pointed out that in structures
with several thousand feet of ver- tical gas and oil column, as
occur in Iran, it was pos- sible for reservoir pressures at the
point of least cover to reach overburden pressure, without the
hydrostatic pressure of the edge water being abnor- mal. The
occurrence and magnitude of seepages were broadly related to the
ratio or reservoir pressure to overburden pressure at the point of
least cover. Where this ratio was under 0.6, seepages did not oc-
cur. Where the ratio approached 1.0, seepages were very heavy. The
practical significance is that if in a field with a competent
plastic cover and good or heavy seepages a discovery well shows a
low ratio, for example 0.4-0.5, it is probable that the structure
has been entered a long way down flank. Where the hydrostatic
pressure of the edge water itself is ab- normal, he could hardly
believe that compaction of shales was the only posshle cause. For
thick limestone reservoirs, such as occur in Iran, Iraq and
elsewhere, Mr. Comins preferred an explanation re- cently suggested
by Mr. Lees of the Anglo-Iranian Oil Company. This was that the
reservoir having no communication with the outcrop through lateral
change in permeability or nature of the productive rock, vertically
migrating gas from a deeper higher pressure source could pump up
the hydrostatic pressure to an abnormal figure. Should this hypo-
thesis be definitely substantiated it would, of course, enhance the
prospects of drilling for deeper horizons in fields where the
hydrostatic pressure is abnormal.
Mr. J. H. M. A. THOMEER (N.V. De Bataafsche Petroleum
Maatschappij, The Hague, Netherlands) did not deny the possible
effect of shale compaction on reservoir pressure, but proposed a
more simple explanation. In case the overburden were imper-
meable, the reservoir pressure should be equal to the weight of
the overburden. Owing, however,to the permeability of the overlying
rocks the amount by which the reservoir pressure exceeds the
hydrostatic pressure tends to be dissipated to the surface. I t
thus depends on sedimentation rate, permeability of sedi- ments and
time, whether or not hydrostatic equilib- rium will be found to
exist at a given moment.
Mr. G. M. LEES (Anglo-Iranian Co) comments, that if compaction
were responsible for high pressures in this way, we should find
high pressures much more frequently. In the main they are rather
abnormal. Mr. Comins has mentioned Persian conditions where high
pressures exist due to rather unusually high gas and oil columns in
oil reservoirs. The pressure at the top of the dome may actually be
referable to hydrostatic pressure in the water limb, but is
compounded of two factors: the height of the water on one side of
the U. tube, balanced against the very deep oil and gas column on
the other. How- ever there are some abnormal conditions observed,
much in excess of what could be explained by hydro- static pressure
balanced against oil and gas column.
The reservoir rock in this case consists of hard solid
limestone, not subject to compaction, and not immediately
associated with soft shales. This is an example where other factors
must have caused the abnormal high reservoir pressure.
Mr. Lees explanation is that in this case the reser- voir is
being pumped up by leakage of gas froin a deeper source below.
Mr. G. E. ARCHIE (Shell Oil Cy), representative of Mr.
DICKINSON, argued that the various comments touch on a point that
Mr. Dickinson did not stress, namely that subsurface conditions
related to high pressures are not in equilibrium. It should be
empha- sized that the pressures reported in this paper per- tain
only to the Gulf Coast of Louisiana, U.S.A. Mr. Archie is convinced
that Mr. Dickinson does not believe that only the compaction
contributes to hi h
and general area studied. Mr. Boots comments are well taken and
bear on
non-static conditions. It would seem, of course, that as shale
compresses, it {becomes more competent and will hold more
overburden pressure. But shales even at great depth will deform and
as this takes place pressure is transmitted to the fluid. The
Lhick- ness of shales in the area under discussion has some bearing
on Mr. Lees statement that man lenses are
resent in the geological section, but f ew contain figh
pressure. Lenses embodied in very thick shales would be more apt to
contain high pressure, for it would take longer for the pressure to
equalize under compaction.
pressures, but that it is most important for the we H 1s
Proceedings 3rd W.P.C., Section I 2