Abnormal pressures in Azerbaijan: A brief critical review and recommendations

Journal of Petroleum Science and Engineering 13 ( 1995) 125-135

Abnormal pressures in Azerbaijan: A brief critical review and recommendations Alexander E. Gurevich ‘9a, George V. Chilingar 2,b aAllied Oil and Tool Co., Inc., 1323 N. Harvard Blvd., Ste. 4, Los Angeles, CA 90027, USA ’ Department

of Civil Engineering,

University of Southern California, Los Angeles, CA 90089-2531, USA

Received 15 September 1994; accepted 30 November 1994

Abstract The authors exhaustively reviewed Soviet publications on abnormal pressures in Azerbaijan. According to these data, pore pressures in shales are determined in Azerbaijan by a calculation using well-logging data; pore pressures in shales, even in thin beds, significantly exceed pore pressure in sand reservoirs. On the basis of their review, the authors recommend the following additional research work to achieve a higher efficiency of drilling and production operations in Azerbaijan. ( 1) Differentiate causes of heaving and sloughing of different shales, in addition to that of the pressure abnormality, and identify cases when special formulations of drilling mud can be used instead of increasing mud density.

(2) Thoroughly evaluate the precision of pressure calculation using well-logging data for different shales and environments. (3) Distinguish cases where pressure abnormality is due to other causes than undercompaction. (4) Check the validity of indirect pressure measurements in thin shale beds. (5) Analyze local and regional hydrodynamics of formations for better prediction of the difference between pore pressures in shales and sand reservoirs.

1. Introduction The American petroleum industry has a great interest in Azerbaijan and joint Azerbaijan-American production from several fields is now being negotiated. Many oil and gas fields in Azerbaijan have abnormal formation pressures and serious problems are often encounwhile drilling through tered in wells abnormally-high-pressure formations and undercompacted shales. Abnormal pressures and associatedproblems in the oil and gas fields of Azerbaijan have been described in many Russian-language papers and a recent book ( Abasov, 199 1) . ’ Tel. (213) 4644405. ’ Tel. (213) 932-8369. 0920-4105/95/$09.50

0 1995 Elsevier Science B.V. All rights reserved

SSDJ0920-4105(95)00007-O

This paper is a critical review of Russian-language publications on abnormal pressures in Azerbaijan’s subsurface. First, it summarizes current information theories and empirical data - on the subject. Second, future studies of the abnormal pressures are recommended, to achieve more efficient drilling and production operations.

2. Geologic environment subsurface

of the Azerbaijan

The South Caspian Basin was formed mainly during Early and Middle The total of the sedimentary rocks reaches 22,000 m (72,13 1 ft) in the

126

A. E. Gurevich, G. V. Chilingar / Journal of Petroleum Science and Engineering 13 (1995) 125-135

Fig. 1. Distribution of mud volcanoes in Azerbaijan. (Modified after Melik-Pashaev et al., 1983. Editors of the map: A.A. Ali-Zadeh, E.M. Shekinskiy and A.A. Yakubov.) 1: faults separating large structural elements; 2: faults important for mud volcanism; 3: smaller faults; 4: large, periodically erupting mud volcanoes; 5: buried mud volcanoes not active for 100 years and more; 6: all other mud volcanoes; and 7: oil and gas fields.

deepest part of the basement. Major oil, condensate and gas reserves are present in the shale-sand sequences (“Productive Formation”) of Middle Miocene. The thickness of this formation is up to 4000 - 4500 m (13,115 - 14,754 ft) (Ali-Zadeh et al., 1985). Basic anticlinal structural elements developed during the Middle Pliocene when intense tectonic activity took place. A younger period of tectonic activity, during the second half of the Quaternary, completed the development of regional and local tectonic structures, forming most faults, and strongly enhancing mud volcano activity. The tectonically most active areas were the Apsheron Peninsula, the Apsheron Archipelago and the Sub-Kura region. The Mesozoic-Miocene tectonic stage has produced mostly sub-latitudinally oriented structures, whereas structures of the Oligocene-Quaternary stage are sublongitudinally oriented.

Mesozoic to Oligocene-Miocene deposits are represented mostly by flysch and finer marine molasses. For Pliocene to Quaternary rocks, coarser, mostly continental, molasses are typical. Marine formations constitute W-90% of the whole sedimentary section of the region. Owing to the very rapid Oligocene-Quaternary sedimentation (that began in Pontian time) and thick shales with low thermal conductivity, the geothermal gradient in the Azerbaijan part of the South Caspian Basin is as low as l6-18”C/m, which helps to preserve the high porosity of shales and their sealing properties. For example, formation temperatures in the Baku Archipelago fields are 110-l 15°C with a high content of montmorillonite at depths of about 6000 m. The least compacted shales are encountered in the Middle Pliocene Productive Formation in the Lower Kura Depression and Baku Archipelago (Kheirov et al., 1990). The

A.E. Gurevich, G.V. Chilingar/Journal

Sea levei

92

_‘.~_.

_,__”

41

.

.

Wells 79

63

.

:

==

=-

75

127

,_(17;246 R)

----

5-

of Petroleum Science and Engineering I3 (1995) 125-135

-5

-5

Fig. 2. Bulla Island geologic section. 1: mud volcano breccia, 2: faults. (Modified after Mel&Pashaev et al., 1983.)

of shales in a vertical geologic section vary appreciably due to variations in lithology: it is actually impossible to find lithologically identical rocks even within the same horizon (Kheirov et al., 1990). Although the density of rocks increases with depth, poorly consolidated, highly permeable sands can be encountered at depths of 5 to 6 km and more. It is believed that, despite some consolidation, shales retain their plasticity and good sealing property even at depths of more than 5-6 km (Mekhtiev et al., 1988) owing to their mineral composition. The montmorillonite content varies widely laterally and in vertical section, but mostly remains between 20 and 60% (Bunyatov and Imanov, 1989; Asadov et al., 1988; Kheirov et al., 1990; Buryakovskiy and Djevanshir, 1985). Xray analysis and electron micrographs (SEM) of shales show definitely that the rather stable content of montmorillonite in the Baku Archipelago fields is caused by the dominance of the formation of secondary montmorillonite over its destruction down to depths of at porosities

least 6200 m (20,328 ft) (Buryakovskiy et al., 1986). Very intensive Quaternary tectonic stresses and movements greatly affected the physical properties of the shale: many shales, strongly deformed by tectonic and diapir movements, are not cohesive enough and, thus, are very unstable in the boreholes, i.e., large fragments of shales fall down from the borehole walls (Melik-Pashaev et al., 1983). Instability of some shales is due to their lithology. For example, Lower Pliocene (Pontian) shales have gypsum inclusions. They easily soak in water and separate into small laminae that fall down from the borehole walls. Faults are numerous in the area. In the Apsheron zone nearly all anticlinal structures are cut by faults. Longitudinal faults with amplitudes of several hundred to 2000 m (6557 ft), are especially important. Such faults contribute strongly to the formation of mud volcano channels. Yakubov et al. (1971) showed that longitudinal faults in the southwestern Caucasus cut not only the Pliocene formations but also Miocene-

128

A.E. Gurevich, G. V. Chilingar/Journal

of Petroleum Science and Engineering 13 (1995) 125-135

Paleogene and Mesozoic strata. This provides a possibility for mud volcanoes to have deep roots into the Mesozoic formations. Active and buried mud volcanoes are widespread over the territory of the South Caspian Basin. Active mud volcanoes are well known in Azerbaijan in the Apsheron, Kobystan and Kuraregions, in the Apsheron and Baku Archipelagos (Fig. 1) . Mud volcanoes are situated along the axes of anticlinal structures (Fig. 1) , but not necessarily along the anticlinal crests (Melik-Pashaev et al., 1983). Yakubov et al. ( 1971) described more than 220 mud volcanoes and their numerous gryphons and salses in the Azerbaijan. In the SW Kobystan alone, there are more than 650 active gryphons and salses that emanate an average of 500 m3 of gas per day each. Many mud volcanoes are buried and their fluids cannot reach the Earth’s surface. The activity of mud volcanoes clearly shows the scale of the vertical upward fluid migration (gas and water mostly). Faults constitute another important avenue for vertical fluid migration (Fig. 2). Oil and gas generation and vertical upward migration, tectonic movements forming faults, and deformation of plastic shales forming diapirs and mud volcanoes have been considered to be part of an integral process as early as 1934 by I.M. Gubkin (in: Melik-Pashaev et al., 1983). This process still continues now. Tectonic movements cause strong earthquakes in the region. Mud volcanoes erupt periodically, transferring great volumes of water and gas to the surface and atmosphere, and also to the subsurface strata. Melik-Pashaev et al. (1983), for instance, believe that abnormal pressure in the Bakhar oil field formations is partially caused by mud volcano activity: a big subsea mud volcano nearby erupts fiercely at time intervals of l-2 years to 16-17 years. One of the evidences of the upward fluid migration can be seen in the Oil Stones field. The oil composition in this field is more or less uniform with a slight tendency of increasing density toward the oil-water contact. But, at the pool’s edge, a light oil appears which is quite alien for this group of fields (Samedov, 1959). The distance of vertical migration can be judged by the lithology and age of rocks brought by mud volcanoes to the surface. Large fragments of Cretaceous limestones and marls are present in the deposits of many mud volcanoes such as Lokbatan, Otman-Bozdag, etc. (Melik-Pashaev et al., 1983).

Sediments in the South Caspian Basin have been accumulated at a very high rate of 1.3 km per million years (Djevanshir, 1987). With shales being predominant in the geologic section, compaction makes a major contribution to the distribution of formation pressures. The presence of thick, highly permeable sand formations, exposed by subsequent erosion in some places, and the vertical migration of fluids, discharged from deeper formations to the surface or to the overlying formations allow rapid redistribution of pressure. This may be the cause of a specific distribution of pressures, e.g., pore pressures in thick shales noticeably exceed those in the permeable formations.

3. Methods used to determine abnormal pressures Authors who studied abnormal pressures in Azerbaijan distinguished three different pore pressures: ( 1) Abnormal pore pressures in permeable formations ( APPF) ; Abnormal pore pressures in shales (APPS) ; and (2) Abnormal pore pressures in thin permeable sand (3) lenses in shales (APTL). In each case, different measurement methods were used. APPF were mostly measured by wellbore pressure gauges. APPC were determined by calculation using well-logging data. APTL were assumed to be equal to pressures in the surrounding shales. Calculated values of pressures in shales were compared with pressures exerted by the weight of the drilling mud column having density necessary to maintain wellbore stability or that corresponding to the beginning of gas penetration into the mud. Judging from the texts of the reviewed papers, the static pressure of the drilling mud column (depth times specific weight) was used. No estimates of the pressure evaluation precision were presented. Deformations of wellbore walls in wells intersecting shales are considered, by almost all authors, to be a result of abnormal pressure influence.

4. Distribution of abnormal pressures Pressure abnormality in the region continuously increases from NE to SW, from Apsheron Peninsula and Apsheron Archipelago toward the Kura Depression

A.E. Gurevich, G.V. Chilingar/Journal

Pressure

gradient,

(psi/ft)

0.015

0.010

5,000 (16,393)

MPa/m

ofpetroleum

2

13

Fig. 3. Average pressure gradients at the ( 1) Southern [ Yuzhnaya], (2) Sangachaly-Duvannyy-Sea-Bulla Island fields and (3) SubKura Depression. (Modified atIer Buryakovskiy et al., 1986.)

Table 1 Statistics of abnormally-high pore pressures Apsheron offshore zone (after Buryakovskiy

in shales (APPS) et al., 1986)

Oil and gas fields

Zhiloy Island, Oil Stones, Apsheron Bank Peschanyy-Sea Southern, Southern-2 Bakhar Sangachaly-Sea - Duvannyy-Sea - Bulla island Khamamdag-Sea, Garasu, Sangi-Mugan, Petsiyanin Bank Sangachaly-Sea - Duvannyi-Sea - Bulla island, Khamamdag-Sea, Persiyanin Bank Bulla-Sea

Science and Engineering I3 (1995) 125-135

(Buryakovskiy et al., 1986). Fig. 3 shows the changes in vertical abnormality distribution in this direction. Table 1 illustrates the abnormality distribution in the oil and gas fields of the Apsheron and Baku Archipelagos and the South Apsheron offshore zone. According to Durmishian ( 1972), deeply-buried Miocene-Paleogene rocks exhibit considerable overpressures everywhere except near their outcrops. The upper part of the Productive Formation of Middle Pliocene age, devoid of oil and gas, does not show any or only very mild pressure abnormalities. The middle part has widely distributed but mostly moderate abnormalities. The lower part of the Productive Formation has high abnormal pressures all over the area. Laterally, as all authors indicate, pressure abnormality increases with an increase in depth and the shale content of rocks. The Apsheron petroleum zone has locally some noticeable abnormalities (Oil Stones, Mud Cone, Zhiloy Island, Makarov Bank and other oil fields in the Apsheron Archipelago and Lokbatan-Karadag in the southwestern part of the Apsheron Peninsula). Abnormalities in the upper and middle parts of the Productive Formation in this zone exist mostly within the boundaries of oil and gas pools. Beyond these boundaries pressure is mostly hydrostatic. Shales with abnormal (calculated) pressure in the Apsheron Archipelago are first encountered as shallow as 480 m ( 1574 ft) in the Peschanyy field in the lower part of the Apsheron Formation (Yusuf-Zadeh et al., 1979). The pore pressure gradient reaches 0.0158 MPalm in the Akchagyl Formation and changes stepwise with depth. For example, pressure gradients in the

determination

Number of determinations

for the fields of Apsheron

Pressure gradient (MPa/m)

and Baku Archipelagos

Standard of deviation,

Average

Median

Modal

600 391

0.0136 0.0132 0.0156 0.0166 0.0171 0.0176

0.0135 0.0132 0.0153 0.0165 0.0176 0.0181

0.0135 0.0135 0.0145 0.0165 0.0185 0.0185

0.00168 0.00155 0.00150 0.00156 0.00202 0.00223

991

0.0173

0.0178

0.0185

0.00210

646

0.0182

0.0184

0.0185

0.00111

353 35 74 232

129

and South-

(T (MPa/m)

130

A. E. Gurevich, G. V. Chilingar / Journal of Petroleum Science and Engineering 13 (I 995) 125-l 35

Pore pressures, MPa (psi) 50 (712)

loo (1,423)

1

I

I

gradients, MPa/m @si/ft)

Pressure

0.0’10

(0.043)

0.015

(0.065)

l

1

0.020

(0.087)

Fig.4. Porepressuresin ( 1) shales and (2) sand reservoirs in oil aad gas fields along the Khamamdag-Sea-Karasu-Sangi-Mugan-Persiyanin Bank trend. (Modified after Buryakovskiy et al., 1986.)

Lower Surakhan shales are normal. Reservoir pressures in the Apsheron Archipelago are about normal. Abnormal pressures in the Baku Archipelago and especially in the Kura region are much higher and are widespread. Abnormality at shallower depths (about less than 1500 m, i.e., above the Productive Formation) in the Baku Archipelago is mostly owing to vertical migration of fluids from the deeper formations through deep faults and active and buried mud volcanoes. The idea of a deep source of abnormality is strongly supported by the fact that abnormal pressures in the Bulla-sea field were encountered only in wells situated near longitudinal deep faults (Khalilov et al., 1988). At the same time, in the much deeper sediments of the Productive Formation, pressure abnormality decreases near such

faults (Yusuf-Zadeh et al., 1979). A mud volcano in this field is buried below the Akchagyl Formation. Roots of mud volcanoes in the Baku Archipelago reach Paleogene-Miocene sediments. Pressures are highest in the Kura region. Pressure gradients reach 0.0226 MPa/m in the northeastern slope of the Kyurovdag anticline (Kasumov et al., 1976). Difference between the measured pressures in sand reservoirs and calculated pressures in shales reaches up to 40 MPa in both the Kura region and the Baku Archipelago (Fig. 4). Calculated abnormalities are rather high even in thin shale beds (l-2 m) (Buryakovskiy et al., 1986) and increase with shale bed thickness (Fig. 5). The abnor-

A. E. Gurevich, G. V. Chilingar / Journal of Petroleum Science and Engineering 13 (I 995) 125-135

Table 2 Effect of length of time after the beginning

of production

Oil and gas fields

Sangachaly-Sea

- Duvannyy-Sea

on calculated*

Well #

-Bulla island

Bulla-Sea

pressures in shales (after Buryakovskiy

Well-logging

43

12/09/1965

62

310211961

135

g/01/1968

141 191

9/03/1970 4/05/1973

197

3/07/1974

531

10/08/1975

18

9

23

6/11/1973

10/26/1974

7/10/1974

date

131

et al., 1986)

Formation depth

Thickness

(m)

(m)

4570 4610 4620 4440 4480 4100 4110 4125 4045 4555 4580 4595 4525 4550 4620 4160 4620 4650 4670 4713 4115 4730 4740 4800 4145 4167 4770 4175 4795 4810 4840 4850 4660 4670 4685 4690 4715 4720 4735

3 1.5 1 1 1 1 3 1 I 2 1 2 1 2 4 6 2 2 1.5 1 3 1 4 3 1.5 1.5 1 1.5 1 2 4 2 1.5 2.5 1 2 1

Pressure gradient (MPa/m) 0.0190 0.0188 0.0186 0.0189 0.0190 0.0174 0.0175 0.0173 0.0166 0.0169 0.0170 0.0164 0.0169 0.0164 0.0167 0.0180 0.0185 0.0177 0.0178 0.0176 0.0176 0.0171 0.0173 0.0183 0.0 176 0.0 112 0.0173 0.0170 0.0172 0.0168 0.0170 0.0173 0.0174 0.0169 0.0169 0.0170 0.0174 0.0174 0.0163

*On the basis of well logging data.

mality level in such beds decreases production (Table 2).

with time upon

5. Analysis and recommendations The reviewed data were analyzed by the authors. Their goal was to determine: ( 1) whether or not current

theories of pressure abnormality distribution in Azerbaijan are complete enough, and (2) whether or not the pressure measurement methods are fully reliable. Although up to now a considerable amount of research work has been done on abnormal pressures in Azerbaijan, not all aspects of this problem were exhaustively studied. The analysis by the authors revealed some new areas and directions of possible research work that can

A.E. Gurevich, G.V. Chilingar/Journal

132

of Petroleum Science and Engineering I3 (1995) 125-135

0.020 (0.087)

3 2 T 4/ 0.010 1 (0.043)

I

t

8 (26) 4 (13) Thickness of shale beds, m (ft)

Fig. 5. Pore pressure gradients in shales versus thickness of shale beds. 1: Zhiloy Island, 2: Oil Stones, 3: Bakhar, 4: Sangachaly-SeaDuvannyy-Sea-Bulla Island, 5: Bulla-Sea, and 6: KhamamdagSea- Karasu-Sangi-Mugan-Persiyanin-Bank. (Modified after Buryakovskiy et al., 1986.)

be done by American companies to increase drilling and production efficiency in Azerbaijan. ( 1) Deformation of wellbore walls is considered to be mostly the result of shale plasticity combined with the impact of pressure abnormality. This may not be completely true. In the reviewed publications, there were no special analysis and experimental confirmation of the plasticity of shales at deep horizons. The conclusions on plasticity seem to have been made indirectly on the basis of observed deformations of walls in the wellbore. It is not apparent that shales at temperatures above 100°C can be plastic, because at such temperatures mineral grain surfaces have no or almost no bound water, which could reduce dramatically the grain-to-grain friction. Deformational behavior of shales in the geologic section of the Azerbaijan formations with abnormal pressures should be studied thoroughly. Possibly, one can stabilize these shales by means of a special formulation of drilling muds and, thus, reduce the required mud density. Also, the possibility of using direct electric current (DC), in conjunction with various chemicals, for electrochemical stabilization of heaving and

sloughing shales should be thoroughly investigated (see Chilingarian, 199 1, p. 293). It is always better to drill at the lowest possible wellbore pressure to achieve higher penetration rate. For geologic sections where shales with higher pressures are interbedded with reservoir sands with significantly lower pressures this may be crucial. Therefore. it is necessary to separately evaluate contributions of shale plasticity and pressure abnormality to the deformation of wellbore walls. In the case of high shale plasticity, special composition of drilling mud may be used instead of higher mud density. This can provide a significant increase in the penetration rate and, therefore, reduce costs. In a well drilled through shales, with clays still retaining colloidal properties and maintaining thixotropy, there is a possibility that vibrations of the drilling tools and tubing may also contribute to fluidization of shales in the wellbore walls. In some wells, although drilling mud weight was less than that calculated to be necessary to balance pore pressure in shales, no problems were encountered. In the reviewed papers, there were no analysis of the difference between shales with and without problems. The properties of shales and the drilling procedures in both cases should be thoroughly investigated. The emphasis should be on the relative roles of ( 1) pressure abnormality, that causes or increases plasticity or other deformational properties of shale; and (2) shale plasticity and other properties indigenous to the shale itself. It is important to determine the relative contributions to deformation of shales made by (a) abnormal water and gas pressure in shale and (b) plasticity of shale itself with different mineral compositions and at different temperatures. The effect of temperature on the amount of bound water and, thus, the rheology of shales in the upper part of the geologic section (temperature below 40-50°C) also should be investigated. (2) In the reviewed papers there were no thorough analysis and direct estimation of the precision of pressure determination by well-logging data. Whether or not calculated pressures are always or usually close enough to the actual values is not clear. During the geologic history, pressure abnormalities in shales and permeable formations can increase after some decline. In such a case, a definite relation between the pressure and porosity will cease to exist. Vertical fluid migration and increase in the oil and, especially,

A.E. Gurevich, G. Y Chilingar/Journal

of Petroleum Science and Engineering 13 (1995) 125-135

gas column heights can raise the pressure. Excess pressures due to the fluid column height also could reverse the pressure change trend. Vertical paths for the upward fluid migration and pressure redistribution (i.e., currently active and buried mud volcanoes, fractured zones and faults) are numerous in Azerbaijan. Upward migrating fluids, especially gas, can significantly increase the pressure in the beds they intrude. As a result, the effective stress (total overburden stress minus the fluid pore pressure) is reduced, whereas porosity remains the same. Under such circumstances well-logging methods may provide incorrect pressure values. Therefore, the effects of pressure increase, caused by vertical fluid migration, should be recognized and current pressure detection methods should be improved or, in some cases, substituted by other methods for such zones. Excess pressure, caused by the fluid column height, can also invalidate usage of ( 1) standard porosity/ pressure relations, and (2) well-logging pressure determination methods in shales lying above such pools. To confirm their validity, calculated pressures in shales were compared by investigators to “equilibrium” drilling mud pressures. Mostly this was done by calculating abnormal pressure gradients from the weight of mud columns; therefore, pressure drops due to mud movement were not taken into account although the head loss due to the friction is appreciable in most cases. In some papers, reference was made to a “static mud pressure”, which does not provide the necessary precision of pressure evaluation during drilling or a trip. Wellbore walls deformations also cannot be considered an ideal reference point. Plasticity of shales depends on ( 1) mineral composition, (2) amount and nature of bound water, and (3) the amount of “dry” contacts and crystal bonds. Although the pressure excess over the hydrostatic pressure contributes to the plasticity of shales, it is neither the primary nor the only cause of plasticity. If wellbore pressure is lower than the pressure in shales saturated with gas, expansion of gas will contribute to heaving and sloughing of shale into the wellbore. But in the case of high plasticity, shale can flow into the wellbore, even without the presence of abnormal fluid pressure, just under the geostatic pressure of the overburden. Under such circumstances “equilibrium” pressure of the drilling mud cannot be

133

used to confirm calculated pressures in shales and, thus, the validity of the well-logging methods. Calculation from well-logs show pressure gradients of 0.015-0.018 MPa/m in very thin shale beds, It is not convincing that shale beds of about l-6 m thick can sustain the pressure excess above that in the adjacent sand beds. There is a possibility that the pressure in such shales, calculated from well-log measurements, was overestimated. Calculated pore pressure values decrease with time from the beginning of production (fluid withdrawal). If they respond to an additional pressure difference during such a short period of time, how could they maintain the pressure in geologic time with a very significant difference between the shale and sand pore pressures? (3) In the Azerbaijan fields, pressures in shales are higher than pressures in the sandstone reservoirs. This is possible only if reservoirs have lateral conductivity high enough to discharge excess volume of fluids to the surface or into shallow aquifers. Therefore, for a better pressure prediction, this conductivity and total regional and local hydrodynamic scenarios should be analyzed for each reservoir much more thoroughly than it was done previously.

Acknowledgements The writers are greatly indebted to Dr. G.G. Nalewaik of Occidental International Exploration and Production Co.

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Journal of Petroleum Science and Engineering 13 (1995) 125-135

Buryakovskiy, A.A., Djevanshir, R.D. and Aliyarov, R.Yu., 1986. Geophysical Methods of Geofluidal Pressure Exploration. Elm Publ., Baku, Azerbaijan, 81 pp. Chilingtian, G.V., 1991. Discussion on: Experimental results on the influence of electric fields on the migration of oil, ionic species and water in porous media, by F. Lancelot, H. Londiche and G. de Marsily. J. Pet. Sci. Eng., 5( 3): 293-295. Djevanshir, R.D., 1987. Relationships between clay minerals, low temperatures, high pore pressures, and oil and gas reservoirs at great depths in the Baku Archipelago, U.S.S.R. J. Pet. Sci. Eng., 1:155-162. Durmishian, A.G., 1972. On the abnormal pressures role in the formation of tectonic structures and deposits of oil and gas in the South Caspian Depression. Proc. USSR Acad. Sci., Ser. Geol., 5: 114-125. Kasumov, K.A., Dergunov, E.N. and Aleksandrov, B.L., 1976. Abnormal pressure origin in sections of the Kyurovdag and Karabagly fields of Prikurinskaya Lowland. Geol. Oil Gas, 8: 3943. Khalilov, N.Yu., Kerimov, A.N., Omarov, A.K. et al., 1988. On the origin of shallow epigenetic abnormal pressures in the Bulla-Sea field. Proc. VUZ, Oil Gas, 7: 8-13. Kheirov, M.B., Daidbekova, E.A. and Djavadov, Ya.J., 1990. Reservoir and Sealing Rocks of Azerbaijan Mesozoic-Cenozoic Sediments. Reservoir Rocks at Greater Depths. Moscow, pp. 155-162. Mekhtiev, Sh.F. et al., 1988. Role of caprock permeability for oil and gas in formation and preservation of pools of hydrocarbons in the west slope of the South Caspian Basin. In: Geology, Exploration and Development of Oil and Gas Fields in Offshore Caspian Sea, pp. 57-62. Melik-Pashaev, V.S., Khalilov, E.M. and Seregina, V.N., 1983. Abnormally-High Formation Pressures. Nedra Publ., Moscow, 181 pp. Samedov, F.I., 1959. Oil Stones Field. Azerneshr Publ., Baku, Azerbaijan, 171 pp. Yakubov, A.A., Alizadeh, A.A. and Zeinalov, M.M., 1971. Mud Volcanoes of the Azerbaijan Republic (Atlas). Azerbaijan Acad. Sci. Publ., Baku, 201 pp. Yusuf-Zadeh, Kh.B., Dergunov, E.N. and Aliyarov R.Yu., 1979. Results of the Exploration and Prediction of Abnormal Pressures by Well-logging Methods in the Offshore Fields of Azerbaijan. Review Information, Ser. Oil-Gas Geol. Geophys., VNIIOENG, Moscow, 49 pp.

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