Fluid Dynamics of Oil and Gas Reservoirs

By M. Z. Rachinsky and V. Y. Kerimov

Whether as a textbook for the petroleum engineering student or a reference for the veteran engineer working in the field, this new volume is a valuable asset in the engineer’s library for new, tested methods of more efficient oil and gas exploration and production and better estimating methods.  In this book, the authors combine a rigorous, yet easy to understand, approach to petrophysics and how it is applied to petroleum and environmental engineering to solve multiple problems that the engineer or geologist faces every day.  Useful in the prediction of everything from crude oil composition, pore size distribution in reservoir rocks, groundwater contamination, and other types of forecasting, this approach provides engineers and students alike with a convenient guide to many real-world applications. 

Fluid dynamics is an extremely important part of the extraction process, and petroleum geologists and engineers must have a working knowledge of fluid dynamics of oil and gas reservoirs in order to find them and devise the best plan for extraction, before drilling can begin. This book offers the engineer and geologist a fundamental guide for accomplishing these goals, providing much-needed calculations and formulas on fluid flow, rock properties, and many other topics that are encountered every day.

The approach taken in Fluid Dynamics of Oil and Gas Reservoirs is unique and has not been addressed until now in a book format. Readers now have the ability to review some of the most well-known fields in the world, from  the USA to Russia and Asia.  

Useful for the veteran engineer or scientist and the student alike, this book is a must-have for any geologist, engineer, or student working in the field of upstream petroleum engineering.

• Language: English
• Publisher: Wiley
• Release date: Feb 2, 2015
• ISBN: 9781118999042

Book preview

Chapter 1

Geology and Oil and Gas Occurrences in the Alpine Mobile Belt Basins

The geology and oil and gas occurrences in the Alpine mobile belts were covered in numerous studies included on the list of publications at the end of the book.

1.1 Intermontane Troughs

1.1.1 The South Caspian Basin

The present-day South Caspian Basin (SKB) (the Pliocene-Quaternary structural-facies stage) includes the South Caspian Sea and the adjacent areas of the Eastern Azerbaijan, Western Turkmenistan and Northern Iran (Figure 1.1). It is bounded in the north by the Derbent-Krasnovodsk deep-seated fault, in the west by the Talysh-Vandam gravity maximum, in the east by the Aladag-Messerian tectonic step and in the south by the folded mountainous structure of Elburs. The basin’s areal extent is about 160,000 km², the volume of its sedimentary fill is approximately 2.7–2.9 million km³, the thickness of the Meso-Cenozoic sequence overlying the pre-Jurassic substrate ranges between 7–8 and 28–30 km and the total clay content of this sequence is up to 85–90%. The Jurassic thickness is 4 to 7 km, Cretaceous 6 to 8 km, Paleogene-Miocene 3 to 5.5 km and the Pliocene-Quaternary 3 to 8 km. The Mesozoic clastic portion of the section is mostly flysh, the Paleogene-Miocene part comprises a clay facies of the typical geosynclinal schliere and the Pliocene-Quaternary section is mostly molasses.

Figure 1.1 Location map: South Caspian Depression, structure inventory, oil and gas occurrences and regional tectonics. 1. Oil, gas and condensate fields; 2. Unexplored structures; 3. Structures with negative appraisal results; 4. Regional faults; 5. Unpromising areas. Deep-seated faults: I. Derbent-Krasnovodsk; II. North Apsheron; III. Apsheron-Balkhan; IV. Sangachaly-Ogurchin; V. Mil-Chikishlyar; VI. Pre-Caucasus Minor; VII. Elburs; VIII. West Caspian; IX. East Azerbaijan (Yamshin); X. Shah-Azizbeck; XI. Sefidrud-Karabogaz; XII. Central Caspian; XIII. Ogurchin-Chikishlyar; XIV. West Turkmen; XV. Aladag-Messerian; XVI. Ajikabul-Mardakyany; XVII. Abich Swell.

The basin’s structure is a complex system of tectonic steps and individual macro- and micro-blocks. The blocks are separated by the intersecting deep-seated faults varying in size and orientation and are sequentially subsiding from the external frameworks of the basin into the internal South Caspian Depression (see Figure 1.1). The sub-longitudinal faults are, west to east:

The West Caspian (VIII–VIII), East Azerbaijan (Yashmin) (IX–IX), Shah-Azizbeck (X–X), Sefidrud-Karabogaz (XI–XI), Central Caspian (XII–XII), Ogurchin-Chikishlyar (XIII–XIII), West Turkmen (XIV–XIV), Aladag-Messerian (XV–XV).

The sub-latitudinal faults are, north to south: Derbent-Krasnovodsk (I–I), North Apsheron (II–II), Apsheron-Balkhan (III–III), Sangachal-Ogurchin (IV–IV), Milsk-Chikishlyar (V–V), Pre-Caucasus Minor (VI–VI), Elburs (VII–VII); and the diagonal faults are: Adjikabul-Mardakyany (XVI–XVI) and the Abich Swell fault (XVII–XVII).

The indicated lineaments bind and define the following major tectonic elements in the SKB structure. On the west flank, onshore (west to east): the Lower Kura Depression, Alyat Ridge, Kobystan with the Djeirankechmes Depression, the Apsheron Peninsula folding; offshore: South Apsheron trough (South Apsheron shelf), the Baku Archipelago folded zone. On the north flank, folding of the Caucasus Major meganticlinorium southeastern plunge: onshore, North Apsheron Depression, North Apsheron zone of uplifts, Artem-Kelkor Trough; offshore: the western and central parts of the Apsheron-Balkhan sill. On the eastern flank, the Balkhan area of uplifts (eastern onshore part of the Apsheron-Balkhan sill), the KyzylKum trough, the Gorgandag-Chikishlyar zone of uplifts – onshore and offshore – the Turkmen structural terrace. On the southern flank of the basin, the Lenkoran-Gorgan trough offshore and the onshore portion of the Pre-Elbrus trough. And in the central part of the basin, the folding of the deep-water South Caspian Depression with the Abich Swell and the Sary-Chikishlyar zone of folds (see Figure 1.1).

The general SKB geology displays a number of characteristic features. The structural architecture of the Mesozoic structural-formational stage does not match that of the Cenozoic one. A typical feature of the basin is geologically frequent change in the direction and sign of the regional tectonic motions. The basin has a step-block structure. Different stratigraphic complexes, intervals and lithofacies contact each other across the regional faults. The section is quite variable in terms of lithofacies and reservoir properties; some Cenozoic intervals were formed in the environment of avalanche deposition. The PT-KT (Productive Sequence-Red-Bed Sequence) section is rhythmical, with clay varieties periodically replaced in the section by sands. The PT-KT stratigraphic components regionally pinch-out both up- and down-dip of the general folding (Figure 1.2) thereby forming lenses. There are clear indications of the paleotectonic and neotectonic stress. Widespread is a powerful fault, diapir, fracture and nappe overthrust tectonics with common mélange-like twisted lamination of the crushed clays, carinate and overturned folded forms.

Figure 1.2 South Caspian Depression, western flank. Regional lithotectonic map of the Middle Pliocene Productive Sequence: 1. Clay content contours, percent; 2. Major faults; pinch-out lines: 3. Productive Sequence, 4. Pereryv (lacuna) formation (SP), 5. Post-Kirmaki clay formation (NKG), 6. Post-Kirmaki sand formation (NKP), 7. Kirmaki formation (KS), 8. Sub-Kirmaki formation (PK), 9. Kalin formation (KaS).

In some regions, density inversion of the depositional section is observed with thick unconsolidated (sometimes even quasi-liquefied) high-porosity water-saturated plastic (mostly montmorillonite) clays. Some seismic velocity inversions are encountered with chaotic seismic events on the cross-sections accompanied by consistent negative gravity, electric and magnetic anomalies. There are in the section tubular subvertical geologic bodies to a depth of 10–12 km, sometimes deeper (Mamedov, 2001; Ivanov and Guliyev, 2002). Very common are mud volcanoes and zones of tectonic fracturing.

High micro- and macro-seismicity is common; peculiar, often inverted hydro-geochemical profile and AHFP in the reservoirs and AHPP in the pore space of impermeable varieties are widespread.

The main commercial oil- and gas-saturation interval in the region is a thick (up to 5 km) clastic (sandy-clayey) Middle Pliocene series called Productive Sequence (PT) in Azerbaijan and Iran and Red-Bed Sequence (KT) in the Western Turkmenistan. It includes almost 95% of all appraised hydrocarbon reserves in the region. As of today, oil and gas occurrences in all other stratigraphic intervals of the sedimentary section have been discovered by individual wells on a limited number of prospects. They are sporadic and of minor commercial value.

Certain patterns have been identified in the hydrocarbon accumulation occurrences of the PT-KT. They may be summarized as follows:

1. The discovered accumulations form three zones differing in the phase state of their hydrocarbons. The upper zone (weighted average depth, 480 m) is of a smaller significance due to its strong deformations; the accumulations are mostly gas. The middle zone includes mostly oil accumulations at the weighted average depth of 1,350 m. The lower, gas-condensate zone has average weighted depth of 4,500 m.

2. The oil and gas accumulation zones are controlled by the major faults and are spatially associated with them.

3. As the folded system gradually dips to the center of the basin and as the local structures become less faulted, oil and oil-gas accumulations give place to gas-oil and gas-condensate accumulations.

4. The section’s gas-saturation increases with depth (both stratigraphic and physical).

5. Over 80% of the discovered accumulations are found within the depth interval at depths to 3.5 km.

The largest fields in the South Caspian Depression are: onshore, Balakhany-Sabunchi-Ramany, Surakhany, Bibieybat, Kyurovdag, Cheleken, Koturtepe, Barsagelmes; offshore, Neftyanyye Kamni (Oil Rocks), Gyuneshli, Chirag, Azeri, Shahdeniz, Sangachaly-More– Duvanny-More–Bulla Is. (Figure 1.3).

Figure 1.3 The South Caspian Depression. Oil and gas field location map. a. Regional faults; fields: b. Oil; c. Gas (gas-condensate); d. Gas- (gas-condensate) oil. List of the fields: 15. Kalamadyn, 16. Mishovdag, 17, Kyurovdag, 18. Karabagly, 19. Kyursangya, 20. Kalmas, 21. Pirsagat, 22. Durovdag, 23. Khilly, 24. Neftechala, 25. Adjiveli, 26. Umbaki, 27. Duvanny, 28. Dashgil, 29. Garasu, 30. Sangachaly-More - Duvanny-More - Bulla Isl., 31. Bulla-More, 32. Kergez-Kyzyltepe, 32a. Shongar, 33. Karadag, 34. Lokbatan-Puta-Kushkhana, 35. Karaeybat, 36. Gyuzdek, 37. Masazyr, 38. Sianshor, 39. Binagady, 40. Chakhnaglyar, 41. Sulutepe, 42. Shabandag-Shubany-Yasamal Valley-Atashkya, 43. Bibieybat, 44. Bukhta Ilyicha, 45. Kyurdakhany, 46. Kirmaku, 47. Balakhany-Sabunchi-Ramany, 48. Surakhany, 49. Karachukhur, 50. Zykh, 51. Gousany, 52. Buzovny-Mashtagi, 53. Kala, 54. Zyrya, 55. Peschany-More, 56. Bakhar, 57. Apsheron Bank, 58. Darwin bank, 59. Atrem Isl., 60. Gyurgyany-More, 61. Yuzhnaya, 62. Yuzhnaya-2, 63. Zhiloy Isl., 64. Azi Aslanov, 65. Gryazevaya Sopka (Mud Volcano), 66. Neftyanyye Kamni, 67. 28 April, 68. Kaverochkin, 69. 26 Baku Commissars, 70. Promezhutochnaya, 71. Livanov West Bank, 72. Livanov Central, 73. Livanov east bank, 74. Barinov bank, 75. LAM Bank, 76. Zhdanov Bank, 77. Pricheleken Dome, 78. Cheleken, 79. East Cheleken, 80. Koturtepe, 81. Barsakelmes, 82. Burun, 83. Nebitdag, 84. Kumdag, 85. Kyzylkum, 86. Karatepe, 87. Kuydjik, 88. Erdekli, 89. Gograndag, 90. Ekiz-Ak, 91. Bugdayli, 92. Nogay, 93. South Bugdayli, 94. Korpedzhe, 95. East Kamyshldzha, 96. Kamyshldzha, 97. Okarem, 98. Keymir, 99. Ak-Patlaukh, 100. Chikishlyar; 101. Shahdeniz.

1.1.2 The Padan Depression

The depression is the western portion of the Adriatic oil and gas basin. It is positioned between the folded Alpine and Apennine mountains and is open eastward, toward the Adriatic Sea (Figure 1.4). The section includes thick Permotriassic (up to 5,000 m), Jurassic (up to 400 m), Cretaceous (up to 500 m), Paleogene (up to 3,700 m), Neogene (up to 8,500 m) and Pleistocene (up to 3,000 m) deposits. Two lithostratigraphic complexes are identified within the sediment cover: the carbonate Eocene-Mesozoic and the clastic Oligocene-Quaternary. A typical feature of the clastic series is its drastic facies variability causing the formation of lens-shaped reservoir intervals, their common pinch-outs and replacement along the strike by impermeable varieties. Average thickness of the sediment cover is 13.5 km; its total clay content is about 65%.

Figure 1.4 Padan Depression. Location map of oil and gas fields. a. Regional faults, b. Oil and gas fields; Fields: 1. Sergnasno, 2. Caviaga, 3. Corneliano, 4. Ripalta, 5. Corrazina, 6. Bordolano, 7. Cortemaggiore, 8. Corregio, 9, Ravenna, 10. Alphonsine, 11. Cotignola, 12. Desana, 13. Pontenure, 14. Imola, 15. Santerno, 16. Podenzano, 17. Piadena, 18. Ravenna-Mare, 19. Porto-Corsini-Mare.

Tectonically, the region is a typical geosynclinal intermontane depression with a median massif at the base. The depression includes two major structural elements, Apennine and Alpine foredeeps separated by the Ferrara horst. In the Apennine Foredeep, the deposition is continuous during the entire Neogene-Quaternary. In the Alpine Foredeep, the Pliocene is transgressive over the eroded substrate of the Upper cretaceous through the Lower Miocene, with a Miocene lacuna in between.

Disjunctive tectonics formed the step-block structure of the region. Two major fault types have been identified. The cross-type mostly longitudinal faults cut the Apennine and Alpine folding. The lengthwise faults extend parallel to these mountain ranges (see Figure 1.4). Some scientists (Bakirov, Varentsov, Bakirov, 1970; Gortani, 1965; Rocco, Dzhaboli, 1961) believe that the cross-type faults reflect basement blocks, whereas the lengthwise faults do not penetrate below the sediment cover. The largest faults reach 100 km in length with the throw of over 4,000 m. Usually they are found on the north flanks of the local highs making them downthrown.

The folds in the region are genetically fault-related. They have high amplitudes; they are asymmetric and faulted. They form individual linear zones parallel to the Apennines and Alps Mountains. The closer to the mountains, the stronger is deformation of the local structures; some intervals show the indications of neotectonic nappes (Beka, Vysotsky, 1976; Vysotsky, Olenin, Vysotsky, 1984; Gortani, 1965).

The commercial production in the depression comes from the clastic and carbonate reservoirs. The entire penetrated section is mostly gas-saturated. The major fields are associated with the Pliocene deposits. Small gas accumulations are found in the Miocene sandstones (Cortemaggiore, Desana and Vigarro fields), which also include small oil fields (Cortemaggiore, Podenzano, Valenza and Salsomaggiore fields). In recent years, commercial hydrocarbons were discovered in the Triassic limestones and dolomites where rather large gas and condensate accumulations (about 50 BCM) have been identified. The hydrocarbon accumulations basically form three zones: the upper gas zone, the middle oil zone and the lower gas-condensate zone. Over 80% of the discovered reserves are shallower than 2,000 m.

1.1.3 The Viennese Depression

The depression is a folded graben bounded on all sides by deep-seated faults. The basin’s boundaries in the north and northwest are Flysh and Limestone Alps, in the east Small Carpathians and Heinburg Mountains, and in the south and southeast northeastern offspurs of the Central Alps (Rosalien and Leita Mountains) (Figure 1.5). The Lower Miocene (Burdigalian, Helvetian) through the Pliocene-Quaternary interval of the basin comprises mostly sandy-silty deposits up to 6 km thick. In the northwestern region, this sequence overlies the eroded surface of the Cretaceous-Paleogene flysh and in the southwest it overlies intensely deformed Jurassic and Triassic carbonates of the Ötscher, Luntz and Frankenfels nappes. In the southeast, the Neogene is underlain by Paleozoic metamorphic rocks. The average total sediment cover thickness is 9.5 km; the clay content is close to 50%.

Figure 1.5 The Viennese Basin. Oil and gas field location map. a. Crystalline rocks of the Bohemian Massif; b. Washberg clip zone; c. Flysh zones; d. Limestone zones; e. Alpine-Carpathian zone of the Eastern Alps; f. Basin boundaries; g. Overthrusts; h. Normal regional faults; i. Gas and oil fields; j. Political boundaries; k. Profiles. Fields: 1. Wazenowizi; 2. Godenin, 3. Zhizhkov, 4. Luzhitsi, 5. Bilovice, 6. Breslov, 7. Gbely, 8. Mühlberg, 9. Sankt-Ulrich, 10. Win-Zickl, 11. Houskirchen, 12. Gesting, 13. Moustrenk, 14. Rag, 15. Zistendorf, 16. Geiselberg, 17. Spanberg, 18. Hohenruppelsdorf, 19. Matzen, 20. Zwerndorf, 21. Aderlkaa, 22. Enzersdorf.

A clear pattern is observed in the Neogene facies and thicknesses. The central areas of the basin with maximum thicknesses are composed by clays and marl rocks. The border areas are dominated by sandstone and conglomerate series.

The section of the region is subdivided into two structure-lithostratigraphic stages. The lower one includes mostly carbonates and metamorphic rocks of the pre-Neogene series. The upper one is composed by Neogene (Pliocene and Miocene) clastics. The major lineaments in the region are large regional normal faults (the Steinberg, Leopoldsdorf, Aderclaa and Lab-Shashtin). The amount of throw on these faults reaches 1,000 to 1,500 m, sometimes 2,000 m. The cross-faults (Mühlberg, Zisterdorf, Danube, etc.) cause step-block descending of the Alpine basal complex.

Local highs in the region are mostly high-amplitude intensely faulted fault-line structures. They are clearly linear due to association with regional faults.

In terms of its oil and gas occurrences, the Viennese Basin is a unique example of the hydrocarbon saturation associated with neotectonic faults of the sediment cover. All known accumulations are controlled by large faults with which oil and gas accumulation zones are connected.

Typical of the basin is the section’s high oil- and gas-saturation compared with its relatively small size. Over 90% of the discovered accumulations are found in the upper section (depths up to 2,000 m). Almost the entire clastic Neogene and Paleogene-Triassic of the lower structure-facies stage include commercial oil and gas accumulations. What is interesting is that wherever the upper complex is commercially productive the lower stage is always hydrocarbon-saturated.

A certain zoning is established in the oil and gas accumulation distribution. Mesozoic reservoirs of the lower stage include mostly gas (gas-condensate) accumulations with the oil accumulations associated with erosion buttes. In the Neogene series, the Miocene (the middle zone) includes mostly oil and the Pliocene rocks, mostly a methane gas. The upper gas-saturated zone is strongly faulted so its value is limited. The oldest productive horizons are discovered in the Jurassic and Triassic rocks. The Triassic includes a number of gas (Aderklaa, Baumgarten, Schönkirchen-Über Tief and Reiersdorf) and oil (Schönkirchen-Tief and Protess-Tief) accumulations. In the Jurassic, small oil accumulations are discovered at the Aderklaa, Breitenlee and Strasshoff-Tief prospects. In the Cretaceous, gas accumulations are found at Aderklaa, Breitenlee, Protess-Tief and Schönkirchen-Über Tief and oil accumulations at the Protess-Tief. The Eocene flysh rocks, which are also included into the lower structure-facies complex, are productive in the Sankt Ulrich-Hauskirchen field.

The largest hydrocarbon accumulations in the upper stage are discovered in the Matzen, Zwerndorf, Schönkirchen, Aderklaa and Mühlberg fields. The lower stage accumulation are mostly massive-type; stacked fields of the upper stage as a rule include fault- and facies change-bounded anticlinal accumulations.

1.1.4 The Irrawaddy-Andaman Basin

This basin is a sub-longitudinally elongated intermontane trough. It is separated from the adjacent mountains of Arakan-Yoma-Naga (in the northwestern part of the basin), of the Pusat Gayo Range on the plunge of the Sumatra anticlinorium, of the Pematangsiantar highlands in the south, the Shan Plateau and the Malacca Peninsula ridges in the east by large regional faults defining a step-block structure of the area (Figure 1.6).

Figure 1.6 Irrawaddy-Andaman Basin. Oil and gas field location map. a. Regional faults, b. Oil fields, c. Gas fields. Fields: 1. Indou, 2. Seib, 3. Yenangyat, 4. Mani, 5. Laniva, 6. Chauk, 7. Yenangyang, 8. Minbu, 9. Palanion, 10. Yathaya, 11. Yenaima, 12. Pyaya, 13. Prome, 14. Taung-Yangve, 15. Mayang, 16. Henzada, 17. Payagon, 18. Paseh, 19. Idi, 20. Djulo-Rajeu, 21. Perlak, 22. Gyeongdog, 23. Rantau, 24-30. Serang-Jaja, Tenang, Telaga-Said, Damar, Pulau-Panjang, Pangkalan-Susu.

The sediment fill of the basin is mostly Cenozoic. The sediment cover is 9 to 10-km thick with about 70% being clays.

The Paleocene series in the northern (Irrawaddy) portion of the region reaches 1,200 m. Most of the section is composed by the Eocene clastics with conglomerate beds (total thickness of up to 8,000 m). The Oligocene series (up to 3,000 m thick) comprises complexly alternating sandstones, clays, marls and limestones. The uppermost, Neogene section (up to 6,000 m thick) includes continental (including deltaic) type rocks: loose sandstones, conglomerates and pebble stones. The rocks in the southern area (the North Sumatra Depression) are mostly marine. A specific feature of the section in the Irrawaddy-Andaman Basin is strong facies variability causing lens-shaped nature of reservoirs. The clay content of the section increases toward the subsided parts of the depression.

The region has asymmetric structure with its axial most subsided portion offset westward, toward the Alpine folding. As a result, the western part of the basin forms a narrow trough with the steep geosynclinal flank. The larger eastern part of the basin is a broad complexly structured flat-folded slope facing the Shan Massif. The sediment thickness there is much reduced.

The transverse highs and the Pegu-Yoma anticlinorium subdivide the northern part of the basin into a number of troughs: the Northern, Chindwa, Minbu, Delta and Sittang.

Based on drilling and seismic results, a number of high-amplitude anticlines are identified within the region. They are grouped into linear zones and are cut with regional faults mostly parallel to the general trend of the depressions. Local highs are usually asymmetric, with low-angle western and steep eastern flanks. Very common are crosscutting faults making the anticlines into block structures. Numerous anticlines in the north of the basin have mud volcanoes usually associated with large faults over the anticlinal crests. Furthermore, there are intense neotectonic processes and associated diapir folding due to the Oligocene clay plasticity.

Oil and gas occurrences in the Irrawaddy zone are associated with the Middle Eocene through Lower Miocene stratigraphic interval. The production in the Sumatra portion of the basin comes from the Pliocene sandstones and Miocene reef limestones. With the exception of reefs, the natural reservoirs are sheet-type and are represented by sandstone beds, 3 to 15-m thick. The number of productive intervals may reach 35 to 50. Most of the discovered accumulations are found at depths shallower than 2,000 m.

There are certain specifics of the hydrocarbon saturation of the region: oil and gas accumulations are spatially associated with intensely faulted portions of the local structures; the gas-saturation of the oils increases with depth; the section is predominantly oil-saturated; the upper gas-saturated zone is practically nonexistent due to the faulting and mud volcanism. The largest fields of the region are Chauk-Laniva, Yenangyang (the Irrawaddy region) and Rantau, Arun and Gyeongdong (the Sumatra part of the basin).

1.1.5 The Los-Angeles Basin

The basin is a typical intermontane depression surrounded by the Santa Monica and San Gabriel Mountains from the north and northeast, the Santa-Ana Mountains from the east and Palos Verdes Hills from the southwest. The basin’s tectonic boundaries are large deep-seated regional faults at the feet of these mountains. In the north, it is the Santa Monica and San Gabriel fault system, in the east the Elsinore-Chino fault system, in the south Christianitos Fault, and in the west Palos Verdes Fault (Figure 1.7).

Figure 1.7 Los-Angeles Basin. Oil and gas field location map. a. Regional faults, b. Oil and gas fields. Fields: 1. Beverly Hills, 2. Cheviot Hills, 3. Salt Lake, 4. Los-Angeles, 5. Englewood, 6. Playa del Rey, 7. El Segundo, 8. Bowdini, 9. Montebello, 10. Santa Fe Springs, 11. Rosecrans, 12. Dominguez, 13. Torrance, 14. Wilmington, 15. Long Beach, 16. Seal Beach, 17. Hantington beach, 18. Brea Olinda, 19. West Coyote, 20. East Coyote, 21. Yorba Linda, 22. Kramer, 23. Richfield, 24. Olive, 25. San Clemente.

The section is represented by Upper Cretaceous through Pleistocene rocks overlying the Jurassic basement. The western part of the basement is most uplifted. There, the Cretaceous and Paleogene are missing from the section and the crystalline basement is overlain by Miocene sediments. Average thickness of the sediment cover in the basin is 6 km; the section’s clay content is 46%. The Upper Cretaceous is composed mostly of arkose sandstones overlying the basal conglomerates with total maximum thickness of up to 2,700 m. The Paleogene formation comprises argillites, variegated clays, sandstones and in conglomerates of the upper section (total thickness of up to 4.600 m); the Miocene complex is composed of the sequences of alternating red-bed clays and sandstones, up to 3,500 m thick. The Pliocene deposits, up to 2,100 m thick, comprise alternating sands, silty clays, silts and clayey rocks. The Quaternary interval is generally represented by coarse-grained material dominated by sands and pebble-stones; the thickness is up to 800 m. The major lithofacies pattern in the basin is increase in the clay content toward the center of the depression.

Structurally, the depression is a complex combination of individual blocks and steps subsiding toward the center along the lengthwise regional faults. The major lineaments are normal faults Newport-Englewood, Wittier and Norwalk. Local highs in the area usually group within the band of these faults; they are quite elongated and substantially faulted by smaller faults. Three types of local highs are identified. In the areas of a shallow crystalline basement (the western part of the depression – Torrance-Wilmington zone) prevail structures draping basement highs. In the areas of a deeper basement most typical are the highs formed by the tangential forces. Within the band of Newport-Englewood fault system are developed the folds formed due to vertical motions of the basement blocks. Typical for the region is a substantial neotectonic activity associated with horizontal displacements along the San Andreas Fault running close to the depression’s eastern boundary.

Commercial oil accumulations are discovered in the Miocene (Puente and Topanga Formations) and Pliocene (Pico and Repeto Formations). Small oil accumulations are also discovered in the basement rocks in the Wilmington and Playa del Rey fields and small gas accumulations in the El Segundo Field basement rocks and in the Yorba Linda Field Pleistocene sandstones. Over 90% of the discovered accumulations are found at depths shallower than 2,000 m. Most hydrocarbon accumulations are fault-trapped; most reserves are concentrated in the most faulted zones. All fields are stacked; the number of productive intervals (mostly sheet-type anticlinal and fault-trapped accumulations) is 10 to 15. The main fields are Wilmington, Santa Fe Springs, Huntington Beach and West Coyote. The Los Angeles Basin is among the richest world regions in terms of oil reserves per unit volume of the sedimentary fill. Due to intense deformation of the local structures, the upper gas zone is absent from the fields.

1.1.6 The Maracaibo Basin

The basin is bound from the north by a latitudinal deep-seated fault Oka, from the west by regional faults along the mountain system Sierra de Perija, and from the east and southeast by large faults of the mountain of Andes de Merida and Falcon Lara (Figure 1.8).

Figure 1.8 Maracaibo basin. Oil and gas field location map. a. Regional faults, b. Oil and gas fields. Fields: 1. Amana, 2. Mara, 3. Netic, 4. La Paz, 5. Concepcion, 6. Tatumo, 7. Boscan, 8. Los Claros, 9. Macoa, 10. San Jose, 11. Rio de Oro, 12. Tibu, 13. Los Manueles, 14. West Tarra, 15. Tarra, 16. Sardinata, 17. Petrolea, 18. Carbonera, 19. Rio Sulia, 20. Urdanetta, 21. Sibucara, 22. Lamar, 23. Centro, 24. Sauta, 25. Bolivar, 26. Lagunillas, 27. Bochakero, 28. Mene Grande, 29. Motatan, 30. Baruya, 31. Mene de Mauroa, 32. Medio, 33. Pintado, 34. Las Palmas, 35. Tiguaje, 36. El Mamon, 37. Cumarebo.

The sediment cover in the region is a thick Meso-Cenozoic sequence overlying a Precambrian-Paleozoic basement. Average sediment cover thickness is 10.5 km and the clay content is 55%. The lithofacies are clearly distinct between the upper portion of over 5,000 m Oligocene-Neogene clastics and the lower one, over 2,000 m of Paleocene and Cretaceous clastic-carbonate formations. Numerous stratigraphic complexes such as the Miocene, Oligocene, Eocene and Paleocene are separated by unconformities. At the base of the sediment cover is found a thick (up to 1,000 m) Cretaceous-Jurassic coarse-grained sediment sequence. It includes fragments of the basement rocks and is unconformably overlain by Cretaceous limestones. The general lithofacies pattern is increasing clay content in the direction of regional subsidence of the folding.

The basin is an asymmetric structure with the axis zone offset to the southeast. Its eastern and central portions are occupied by the so-called Maracaibo Platform; it is a stable block with large platform-type brachianticlines and dome-like highs. The region is substantially faulted (the faults parallel the framework mountain buildups) and is subjected to strong neotectonic processes associated with the Andes inversion. Local highs that extend along the regional faults are represented by anticlines intensely cut by normal and reverse faults and over thrusts branching from the major lengthwise fault forming the anticlinal zone (belt). The throw of the largest faults reaches and sometimes exceeds 1,000 m and cross-faults reach 200 m.

Commercial oil and gas occurrences are established in all intervals of the sedimentary complex and at some prospects in the crystalline basement (the La Paz, Tatumo and Mara fields). The productive intervals are included in the fractured limestones Apon, Copacho (Lower Cretaceous) and La Luna and in the Mito Juan Formation sandstones in the western part of the basin, and also in the Miocene complex all over the region. The main oil and solution gas reserves are associated with the clastic reservoirs of the Cenozoic formation. Up to 65% of the discovered accumulations are found at depths shallower than 2,000 m. Several regional oil and gas accumulation zones are identified within the depression. They include the Bolivar zone with unique reserves of 4.3 billion tons, the Mene Grande Motatan zone (the Bolivar Falcon oil and gas area), the Mene de Mauroa-Las Palmas zone, the Mara– San Jose zone and the southern zone of uplifts (the Western oil and gas area).

The Oligocene-Miocene and Eocene accumulations in the Bolivar zone are mostly fault- and facies change-trapped. At the top of the Eocene are also found stratigraphic accumulations associated with the unconformably overlying Oligocene deposits. The Upper Miocene-Pliocene accumulations are discovered in the sand members, pinching-out updip the monocline and sealed in a number of cases with asphalt and kir (weathered asphalt). The dominant type of the Eocene hydrocarbon accumulations near the south portion of the basin’s east flank and in the central part of Lake Maracaibo is sheet-type anticlinal. There are some fault-trapped accumulations in the Cretaceous limestones and facies-change trapped accumulations in the Miocene sandstones. The western flank is dominated by the fault-trapped, stratigraphic and sheet-type anticlinal accumulations. The region contains mostly oil accumulations, caused by intense faulting of the local structures.

The discovery in recent years of a number of gas and condensate accumulations at depths exceeding 4,500 m may suggest that lower gas-condensate is present in the basin. The largest fields are Bolivar, Lama, La Paz, Mara, Lamar, Boscán, Urdaneta, La Concepcion and Mene Grande.

1.2 Foredeeps

1.2.1 The Carpathian Foredeep

The foredeep is positioned between the Volyn-Podolsk edge of the Russian Platform and the Carpathian folded mountains. It is bounded in the north-northwest by a large regional Rava-Russian Fault, in the southeast by a band of faults extending along the north slope of the Bukovina cross-uplift and in the south-southeast by the Beregovoy overthrust of the Eastern Carpathians (Figure 1.9).

Figure 1.9 Carpathian Foredeep. Oil and gas field location map. a. Regional faults, b. Oil fields, c. Gas fields. Fields: 1. Kokhanovskoye, 2. Svidnitskoye, 3. Rudkovskoye, 4. Khodanovichskoye, 5. Sadkovinchskoye, 6. Pynyanskoye, 7. Malogorozhanskoye, 8. Medynichskoye, 9. Opary, 10. North Bilche-Volitskoye, 11. Bilche-Volitskoye, 12. Kavskoye, 13. Ugerskoye, 14. Dashavskoye, 15. Bolokhovskoye, 16. Kadobnyanskoye, 17. Grynovskoye, 18. Bogorodchanskoye, 19. Kosovskoye, 20. Krasnoilskoye, 21. Strelbichskoye, 22. Naguevichskoye, 23. Borislav, 24. Skhodnitskoye, 25. Ivanikovskoye, 26. Orlov-Ulichanskoye, 27. Stynyavskoye, 28. Dolinskoye, 29. North Dolinskoye, 30. Strutynskoye, 31. Spasskoye, 32. Olkhovskoye, 33. Rypnyanskoye, 34. Rosilyanskoye, 35. Kosmachskoye, 36. Gvizdetskoye, 37. Bitkovskoye.

Based on the sum of tectonic features, stratigraphy, lithofacies and the geologic history, two tectonic zones are identified within the foredeep, the Internal and the External ones. The Internal zone is the geosynclinal folded flank and the External zone is the platform flank. The zones are separated by the Stebnik overthrust on which the folded flank is thrust over the platform flank by up to 20 km.

The Internal zone comprises Cretaceous, Paleogene and Miocene (pre-Sarmatian) molasses, up to 12-km thick. The Paleozoic basement is overlain by the Upper Cretaceous (Turonian-Danian) Stryy series of flyshoid rhythmically alternating sandstones, siltstones, argillites, marls and limestones, over 1,000-m thick. The Paleogene sequence includes massive Paleocene-Lower Eocene sandstones and a sandy-clayey alternation of the rest of the Eocene. The Oligocene complex (the Menilite series) is an up to 2,000 m thick alternation of shales, argillites and sandstones. The lower portion of the Miocene is a thick (up to 800–1,000 m) sequence of Aquitanian evaporites, variegated Burdigalian rocks (up to 2,500 m) and Helvetian grey-colored sandy-clayey rocks (up to 3,200 m). The External zone section begins with variegated Middle Jurassic clastics (up to 500 m). The Upper Jurassic is mostly reefs, up to 1,500-m thick. Out of the Cretaceous, only the upper series is represented. It is over 800-m thick; carbonates in the section dominate clastics. Eroded Cretaceous rocks are overlain by the Lower-Tortonian marls with tuff, argillite and calcareous clay interbeds (up to 250 m); by the Lower-Tortonian salt-bearing Tirassa Fm. (up to 200 m) and sandy-clayey alternation of the Kosov Fm. (up to 1,200 m); and by the Lower-Sarmatian calcareous clays with siltstone and sandstone interbeds (the Dashava Fm., up to 3,500-m thick overlying the eroded Tortonian complex).

The foredeep is highly asymmetric in a cross-section. Its geosynclinal flank is narrow and steep; its platform flank is broad and low-angle. The axial portion is noticeably offset toward the Carpathian Mountains. Numerous lengthwise and crosscutting faults were mapped. They cause a step-block structure of the region.

Two tectonic subzones are identified in the Internal zone of the foredeep, the Borislav-Pokut and the Sambor subzones. The former is a complex anticlinorium filled with linear high-amplitude folds extending along the faults. The folds are often overturned and thrust over one another; they are cut by overthrusts and crosscutting faults. As a result, the brachian-ticlinal portions of anticlinal zones may turn out to be in different fault-blocks offset in the cross direction. The edge southwestern blocks (internal or Borislav) form ledges separated by large overthrusts (1,500–2,000 m and greater).

The Sambor subzone forms a synclinorium of several linear rows of folds, which are thrown one over the other and on the whole over the External zone of the Foredeep. The Internal zone is an area of strong neotectonic activity. Local structures in the Internal zone are intensely faulted; nappe tectonics and significant horizontal offsets are common. It causes drastic disagreements between the structure of different stratigraphic intervals in individual step-blocks.

The External zone is simpler: the local structures are not as strongly faulted, and the structures of different stratigraphic intervals are almost coincide. The general tectonic plan of the folding is mostly a northwesterly monoclonal dip.

The commercial oil accumulations in the Internal zone are mostly associated with nonuniform discontinuous Oligocene and Eocene lens-shaped reservoirs. The External zone includes mostly natural gas accumulations in the Mesozoic interval. The fields in the Internal zone usually comprise stacked sheet-type, fault-trapped accumulations. The largest fields in the Internal zone are Borislav, Dolinskoye and Bitkovskoye, and in the External zone, Rudki, Khodnovichi, Dashava and Ugerskoye.

The general distribution pattern of the hydrocarbon accumulations in the Internal portion of the foredeep is a strongly reduced (by faulting) upper gas zone and lower oil and gas zone; over 90% of the accumulations are discovered within the interval shallower than 2,000 m.

1.2.2 The Indol-Kuban Foredeep

This is a large Alpine mega-structure in the south of the Russian Federation. It is bounded by regional deep-seated faults. These faults are: in the north, the Novotitarev-North Kerch; in the east, the Ust-Lanin structural isthmus (an element of the Yeysk-Berezansk Swell) and a flexure along the western flank of the Adygey basement salient; in the south, the Akhtyr-Parpach suture zone separating the foredeep from the Caucasus and Crimean mountains; and in the west, the West Kerch Fault (Figure 1.10).

Figure 1.10 The Indol-Kuban Foredeep. Oil and gas fields and potential structures. Location map. a. Regional faults: I. West Kerch, II. Marthovsky, III. E. Crimean, IV. Djiginsky, V. Crimean, VI. Gelendzhikian, VII. Afipian, VIII. Tsitsinian, IX. Kurdjipian, X. Novo-Titarev – N. Kerchen, XI. Anastasiyev, XII. Akhtyr-Parpachian; b. Prospects/potential prospects; c. Oil and gas fields. Fields and prospects: 1. Tambovskaya, 2. Vladislavovskoye, 3. Frontovoye, 4. Kharchenkovskaya, 5. Andreyevskaya, 6. Korolevskaya, 7. Slyusarevskaya, 8. Belokamenskoye, 9. Mysovoye, 10. Karalarskaya, 11. Malo-Babchinskoye, 12. Borzovskoye, 13. Chistopolskaya, 14. Andreyevskaya, 15. Glazovskoye, 16. Priozernoye, 17. Moshkarevskoye, 18. Kuybyshevskoye, 19. Seleznevskoye, 20. Alagayevskaya, 21. Krasnopolskaya, 22. Opukskaya, 23. Pogranichnaya, 24. Korenkovskoya, 25. Vyshesteblinskoye, 26. Strelchanskoye, 27. Kurchanskoye, 28. W. Anastasiyevskoye, 29. Anastasiyevsko-Troitskoye, 30. Ust-Chekupskoye, 31. Varenikovskoye, 32. W. Adagumskoye, 33. Adagumskoye, 34. Kudako-Kiyevskoye, 35. Kukolovskoye, 36. E. Medovskoye, 37. N. Crimeasn, 38. Crimean, 39. Abino-Ukrainskoye, 40. Ukrainskoye, 41. Levkinskoye, 42. Akhtyrsko-Bugundyrskoye, 43. Chernomorskoye, 44. Zybza-Gluboky Yar, 45. Afipskoye, 46. E. Afipskoye, 47. E. Severskoye, 48. Azovskoye, 49. Novo-Dmitriyevskoye, 50. Kaluzhskoye, 51. Kolinskoye, 52. Stavropolskoye, 53. Abkhazskaya, 54. Sergeyevskaya, 55. Generalskaya.

The region has a Pre-Jurassic basement overlain by mostly clastic rocks from Jurassic to Quaternary. Average thickness of the sediment cover is 11 km; the clay content is 53%. The Mesozoic complex over most of the region is deep and poorly studied. The Jurassic was penetrated in a few wells on the foredeep’s periphery (over the southeastern plunge, in the Kerch-Taman area). It is composed there of Oxfordian-Kimmeridgian limestones. It is assumed by analogy with the adjacent areas on the northwestern plunge of the Caucasus Major meganticlinorium that the Jurassic is up to 3,000 to 6,000 m thick. The Lower Cretaceous complex with a maximum thickness of 4,000 m is mostly a clay facies with interbeds of compact sandstones and siltstones. The Upper Cretaceous is mostly carbonate up to 1,500-m thick. The Paleocene series (includes the Tsitse, Goryacy Klyuch and Ilsk formations) is mostly composed of flyshoid rocks up to 1,600m thick, alternating with clay, marls, sandstones and siltstones. The Eocene complex (except for the Kuma Horizon of the Upper Eocene) is mostly represented by a clay facies (the Zybin, Kutaisi, Kaluga, Khadyzhen and Beloglin formations), up to 700-m thick. The Kuma Horizon (maximum 400-m thick) includes dark bituminous clays and marls and a clastic flysh, which is one of the major reservoir intervals in the region. The Oligocene-Lower Miocene interval and the entire overlying section are mostly clayey with some reservoir (sandstone and siltstone) members, which are discontinuous and are replaced by clays at short distances. Its total thickness reaches 6,500 m. The Meothic Stage forming a thick natural reservoir in the central part of the region has elevated sand content.

The general lithofacies pattern in the Indol-Kuban Foredeep is the lens-shaped reservoir geometry and the increase in clay content toward the axis of the region.

The Foredeep is a complex structure. Its general architecture is defined by the lengthwise and cross-faults causing the step-block nature of its sediment cover over a block basement. The following cross-faults (anti-Caucasus cuts) are identified in the Foredeep, east-to-west: the Kurdzhip, Tsitsin, Afip, Gelendzhik, East Crimean, Zhigin, Marthov and West Kerch. These faults determine the step profile of the region. The former five faults provide for the sequential subsidence of the steps they form into the sublongitudinal (superimposed) Kerch-Taman trough from the east, and the latter two, from the west. Three major lengthwise faults of the general Caucasus trend are established in the foredeep. North-to-south they are: the Novotitarev-North Kerch Fault and Anastasiyev and Akhtyr-Parpach suture zones.

Both types of faults may exceed 1,000 m of throw.

The Indol-Kuban Foredeep has a drastically asymmetric profile. Its external northern flank adjacent to the Scythian Platform is broad and low-angle with a monoclonal Tertiary complex. Its internal geosynclinal (southern) flank is narrow and steep, intensely folded and faulted.

The southern flank comprises two structural stages. The lower one includes the Mesozoic and Lower Paleogene and is deformed into a system of narrow, often overturned folds. The upper one begins with the Middle Maikopian and is mostly monoclonal.

Within the flank are identified several fault-associated sub-latitudinally-trending anticlinal zones. These zones are the Kaluga (Seversk), Azov, Levkin, Crimea-Seversk, Varenikov, Anastasiyev and Kerch-Taman. Each of them includes clearly individualized local structures.

The Kaluga zone anticlines are buried under the monoclonal Pliocene-Quaternary deposits and are clearly identified large, with relatively low-angle slopes structures in the Upper Eocene cut with lengthwise and cross faults with the throw of up to 100 m. The structures in the Azov and Levkin zones flatten in the Lower Maikopian; these are typical fault-associated asymmetric, often eroded high amplitude strongly faulted (block-type) anticlines, sometimes with overturned flanks. The highs in the Crimean-Seversk zone are diapirs and crypto-diapirs; they display strong morphologic expression up to the Quaternary. The Varenikov zone folds are relatively weakly deformed brachi-anticlines. The structures in the Anastasiyev zone are low-angle in the east portion and in the west, and in the next, Kerch-Taman zone; they are clearly diapiric, with mud volcanism and intense faulting.

The region, especially its western portion, is affected by neotectonics (active growth of diapire structures, present-day seismic activity, mud volcanoes, etc.).

Commercial oil and gas occurrences are found in the stratigraphic range of the Upper Cretaceous (in the Kerch-Taman zone) through the Cimmerian Stage (Middle Pliocene). The hydrocarbons are mostly oil. The number of productive intervals in the fields reaches and sometimes exceeds ten. The major identified resources are concentrated within the Kuma Horizon (Upper Eocene) and the Miocene interval. About 70% of the discovered accumulations are concentrated within depths shallower than 2,000 m. The Foredeep includes a number of oil and gas accumulation zones (whose names are the same as the zone names) with individual dominating trap and accumulation types.

The Kaluga zone includes sheet-type anticlinal accumulations. In the Azov zone, the lower section contains sratigraphic-type, fault-trapped and sheet-type accumulations and the upper section, facies-change trapped accumulations. Accumulations in the Anastasiyev zone are associated with the diapir plugs. The hydrocarbon accumulations in the Kerch-Taman area Neogene are also found with the diapir-type structures, and in the Upper Cretaceous reservoirs, they have morphology of the massive accumulations.

A typical feature of the oil occurrences in the region is their spatial association with major deep-seated and their feathering smaller faults. The largest fields are Zybza-Gluboky Yar, Anastasiyev-Troitsk, NW Afip, Novo-Dmitriyev, Akhtyr-Bugundyr and Abino-Ukrainskoye. The region as a whole has three different hydrocarbon type distribution zones: the upper gas, middle oil and the lower gas-condensate.

1.2.3 The Tersk-Caspian Foredeep

The basin is separated in the north from the Scythian Platform by the Tersk-Caspian edge fault, in the east by the regional faults in the axial zone of the Dagestan Piedmont, in the south by the Chernogor fault and flexure system from the Caucasus Major mountains and in the west by the Mineralovodsky basement high (Figure 1.11).

Figure 1.11 Tersk-Caspian Foredeep. Oil and gas fields and prospective structures location map. a. Regional faults: I. Baksan, II. Chegem, III. Nalchik, IV. Cherek, V. Urukh, VI. Ardon, VII. Tskhinvali-Kazbek, VIII. Argun, IX. Aksay, X. Datykh-Akhlov, XI. Benoy-Eldar, XII. Gudermes-Mozdok, XIII. Krayevoy (Priterechny), XIV. Cherkes, XV. Sredinny (Terskoy), XVI. Argudan-Sunzhen, XVII. Chernogorsky; b. Oil fields; c. Prospective structures. Fields and structures: 1. Dykhatakaya, 2. Syurekskaya, 3. Predgroznenskaya, 4. Benoy, 5. N. Benoy, 6. Oktyabrskaya, 7. Andreyevskaya, 8. Starogroznenskaya, 9. N. Oktyabrskaya, 10. Sernovodskaya, 11. Karabulak-Achaluki, 12. Zamankul, 13. Kardzhin-Zmeyskaya, 14. N. Zamankul, 15. Kharbizhin, 16. N. Sernovodskaya, 17. S. Yastrebinskaya, 18. Argun, 19. Khankal, 20. Belorechenskaya, 21. E. Oktyabrskaya, 22. Sayasan, 23. Zandag, 24. Granichnaya, 25. Nozhay-Yurt, 26. Gilyan, 27. Masketin, 28. Novolakslaya, 29. E. Gudermes, 30. Koshkeldin, 31. W. Gudermes, 32. Bragun, 33. Goryacheistochnenskaya, 34. Khayan-Kort, 35. Mineralnaya, 36. N. Mineralnaya, 37. Eldar, 38. Eldar, 39. Malgobek-Gorskaya, 40. N. Malgobek, 41. Akhlovskaya, 42. Aral-Dalaterek, 43. Novo-lvanovskaya, 44. Prokhlandnenskaya, 45. Pravoberezhnaya (N. and S.), 46. N. Bragun, 47. Chervlennaya North, 48. Chervlennaya South, 49. Komsomolskaya, 50. Salkushi, 51. Alpatovskaya, 52. Argudan, 53. Kurskaya, 54. Sovetskaya, 55. Cherek-Baksan, 56. Maryinskaya, 57. Lesnaya.

The sediment cover overlying a Paleozoic basement includes Permotriassic through Quaternary rocks. Average thickness of the sediment cover is 12 km; the clay content is 45%. Several lithostratigraphic complexes are identified in the section. They are: the lower-Middle Jurassic clastic complex (total thickness up to 2,000 m); the Upper Jurassic-Valanginian carbonate complex (about 2,000 m thick); the Lower Cretaceous clastic complex (800–1,100 m), the carbonate Upper Cretaceous-Eocene complex (350–650 m) and the mostly clastic Upper Eocene-Quaternary complex (over 5,000 m thick). The clastic complexes are composed of granular reservoir members alternating with clays; the carbonate complexes include fractured limestones, marls, etc.

The highest clay content is identified in the Lower-Middle Jurassic, Albian, Maikopian and Pliocene rocks.

The foredeep is asymmetric. Its northern external flank facing the platform is broad and low-angle with monoclonal rock dip. Its internal southern geosynclinal flank is narrow and steep, substantially affected by faults and intense folding. The region is cut by large regional faults into individual tectonic blocks and steps generally subsiding from north to south and from east to west. The lengthwise faults are the Tersk-Caspian, Middle-Tersk, Argudan-Sunzha and Chernogor. The cross-faults are Cherkassky, Malkin, Baksan, Chegem, Ardon, Kazbek and Argun. And the diagonal faults are Nalchik, Datykh-Akhlov, Benoy-Eldar, Gudermes and Samur. The lengthwise faults have throws of up to 2,000 m; the cross-faults, up to 1,000 m; and the diagonal faults up to 1,500 m. The major tectonic elements of the deep in a north-south cross-section are: Trans-Terek anticlinal zone; Trans-Terek trough; Terek anticlinal zone; Terek syncline zone; Tersk anticlinorium; Alkhanchurt-Petropavlovsk syncline zone; Sunzha anticlinorium; Beslan-Sunzha Depression; Chernogor monocline; and in the eastern part of the region the Dagestan salient (wedge) and the anticlinorium of the Dagestan Piedmont.

Two major stages differing in their structural plans are identified in the section. The lower, Mesozoic, with the local structures relatively big, high-amplitude, complex, block-type; these are brachianticlines delimited by tapering down conoidal faults (normal faults and overthrusts) with substantial amounts of throw up to 2,500 m (the Sunzha anticlinorium). The upper, Post-Eocene one includes narrow (compressed) eroded crypto-diapir and diapir folds intensely faulted (normal faults and overthrusts with the throw of up to 2,000 m); the faults die-out down the section; the folds are often overturned and showing indications of nappe folding mechanism.

The region is neotectonically active. The manifestations are present-day horizontal