South Caspian

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The South Caspian Basin (SCB) is one of the deepest basins in the world, laying at a depth of 20 to 25 km (12.4 to 15.5 miles). As shown in figure 1, it is located in the southern part of the Caspian Sea, including areas in eastern Azerbaijan, western Turkmenistan, and northern Iran. For more than 150 years, the South Caspian Basin has been crucial to the economy with the production of oil and gas in the region, especially in Azerbaijan, since it is rich with petroleum resources. Shallow-marine, deltaic to lacustrine deposits of the middle Pliocene period are where the major oil reserves found, ranging in depth of 2,500 to 3,500 m (8202 to 11483 ft). More than 600 oil and gas fields have been discovered in formations ranging in age of Miocene to Quaternary, and less than a dozen of these fields produce from both Miocene and Quaternary reservoirs[1].

In the South Caspian Basin, part of the sedimentary fill is allochthonous and folded, overlying a ductile detachment zone within the Makop shale (Oligocene–early Miocene) and is greatly deformed. The Upper Pliocene-Quaternary neo-autochthonous sediments are overlying the folded succession with an unconformity in between. The collision between the Arabian and Eurasia plates helped depositing the Pliocene-Quaternary sediments within a compressional tectonic setting. The close by Russian Platform and the surrounding orogens (Caucasus, Alborz and Kopet-Dagh) are the main sources of the Pliocene-Quaternary sediments, and they are found in a rapidly subsiding basin. These sediments are nearly 10 km thick[2].

History of the basin

Figure 1 The South Caspian Basin Province of Azerbaijan, Iran, and Turkmenistan. (Courtesy of Smith-Rouch, L. 2006)

Tectonic history

Platforms

The South Caspian Basin is surrounded by two platforms: The Turan Platform that is located in the northeast of the basin, and the Scythian Platform that is located in the northwest of the basin. During the Late Triassic period, both of these platforms were affected by Eo-cimmerian (Late Triassic) events. As a result, the Kopet-Dagh and the Great Caucasus orogens in the southern parts of these platforms became the foreland areas of the basin[2].

Orogenic belts

Figure 2 shows that the South Caspian Basin is surrounded by four orogens, and they are the Great Caucasus, the Alborz, the Kopet-Dagh, and the Great Balkhan (GB). Firstly, during the Alpine collisional event due to the north-south collision between the Arabian and the Eurasian plates, the Great Caucasus was developed from a Mesozoic back-arc rifted continental basin into a doubly vergent intracontinental orogenic wedge. During early Jurrasic, the orogen's crust was originally extensively stretched and intruded, then later was inverted. Also, the orogen is bordered by deep flexural molasse-type basins extending from the north to the south[3].

The Alborz orogen is located in the southern border of the South Caspian Basin. The orogen has a late Precambrian basement covered by Devonian to middle Triassic sedimentary layers with a thickness reaching 2 to 3 km (1.24 to 1.86 miles). This cover was only slightly affected by the Eo-cimmerian orogeny during the middle to late Triassic[2].

The Kopet-Dagh is located in the east of the South Caspian Basin and boarders the north of the easternmost part of the Alborz. The Kopet-Dagh was affected by Eo-cimmerian deformation and is separated from the Alborz by fragments of the Palaeo-Tethys suture (Agdharband). Subsequently, a thick sedimentary basin developed within its margins with an almost continuous succession of Jurassic to Tertiary, about 10 km (6.2 miles), of marine deep-water to continental sediments[2].

The Great Balkhan to the northeast of South Caspian Basin is considered as the northwestern prolongation of the Jurassic Kopet-Dagh basin. It is also considered as an eastern prolongation of the Great Caucasus Bajocian (middle Jurrasic) basin[2].

Figure 2 Geological map showing geological features and the locations of the platforms and the orogenic belts. (Courtesy of Brunet, M.-F., Korotaev, M. V., Ershov, A. V., Nikishin, A. M. 2001)

Depositional history

Figure 3 Sequence of stratigraphic units in the South Caspian Basin region. Columns on the right show average thicknesses, which units include source and reservoir rocks and stratigraphic seals, which units are petroleum bearing, and the timing of trap formation. (Courtesy of Smith-Rouch, L. 2006)

The rock's age in the South Caspian Basin ranges from Early Permian to Quaternary. The stratigraphic sequences (fig.3) in the South Caspian Basin is divided into “suites”, the term “suite” is a lithologic unit that is smaller than a "formation"[1].

  1. Low Permian: Limestones, marl, and carbonaceous siltstones indicating a marine depositional environment.
  2. Jurassic: Volcaniclastic rocks with minor claystones, argillaceous shales, limestones, and calcareous sandstones deposition.
  3. Cretaceous: Tuffaceous, clastic, and calcareous rocks in some areas and shale with interbeds of marl and sandstone east to west.
  4. Paleocene-Eocene: The lower Eocene of the Koun Suite consists of shale with interbedded sandstone, marl, limestone, and volcanic tuff. Middle Eocene is tuffaceous and terrigenous rocks with interbedded shale and marl. Upper Eocene is argillaceous.
  5. Oligocene-Miocene: Maykop and Diatom Suites comprised of claystone, sandstone, organic-rich shale, and tuffaceous sandstone. Maykop Suite is the thick marine shale source rock that was formed in early Miocene.
  6. Pliocene: The Pliocene Productive Series has thick fluviodeltaic rocks that created some of the main hydrocarbon reservoirs in the South Caspian Basin Province. Large volume of sediments were deposited during middle Pliocene period.
  7. Quaternary: Sandstone, limestone, siltstone, and shale deposited in deltaic environments.

Geological Risks and Uncertainties

Exploration of the South Caspian Basin dates all the way back to 1848. Successful exploration continued through out the 19th century and 20th century with major fields discovered both onshore and offshore. Expectations were high for major oil companies, however, success has been limited in terms of these expectations despite the used of top-tier drilling and technology.

There are some factors that challenge exploration in the South Caspian:

- Reservoir prediction

- Reservoir quality

- Phase prediction

- Overpressure

- Geohazards

- Drillability

- Commerciality

- Lack of exploration infrastructure

These are exploration challenges and understanding more of these will reduce risks.

Reservoirs

The major reservoirs lie in the Miocene to Pliocene in age and all relate to a major fall in relative sea-level which caused incision of river systems and migration of their deltas to the margins of the South Caspian Basin. Palaeo-Volga, Palaeo-Amu Darya and Palaeo-Kura are some of the major river systems.

Despite considerable research over recent years on the palaeogeography of the reservoir succession, conflict exists on understanding which palaeo-river systems were dominant during the period of reservoir deposition. Resolving this has profound implications for better understanding prediction of reservoir presence and reservoir quality[4].

Outcrops of surrounding reservoirs help understand depositional setting, vertical and horizontal heterogeneities and the prediction of sands down dip. The best reservoirs are likely to be on the palaeo-delta plain, with little sand reaching the palaeo deep water region [5].

Overpressure and geohazards

The extremely rapid sedimentation with gas saturated sedimentation in the last 5 million years has led to overpressures of pressures exceeding 100MPa at typical reservoir depths. [5]. The Paleao-Volga delta provides lateral carrier beds that relieve pore pressure and allows for the development of thicker hydrocarbon columns.

The risk presented by geohazards is so great in the South Caspian Basin because the hazards are likely to be ecnountered by mud volcanoes and breccia that allows formation of weak sediment, shallow gas, slope failure, gas hydrates, and gas chimneys. All this provides limitation to well-positioning.

Economic considerations

In many prospects, gas is more likely to be present. This is uneconomic since the pipeline is full. Overpressure inhibits drilling at the crests unless the cfrests are eroded away. Therefore the flanks must be drilled, increasing costs and reducing deliverability [4].This also inhibits well casing. Therefore, cost of drilling is highand requires high rates of production such as 10,000 barrels/day for economic returns. A robust assessment of all available data must be done in order to make good predictions of thick, good quality sands for success in the South Caspian.

Petroleum system elements

The following paragraphs discuss the five petroleum systems, summarized in figure 4.

Source rock

Figure 4 Shows the geologic events for Maykop/Diatom suits (Oligocene - Miocene) and the petroleum system elements involved. Abbreviation: Ak-Ap, Akchagylian and Apsheronian units. (Courtesy of Smith-Rouch, L. 2006)

The marine Maykop (Oligocene to lower Miocene) and the Diatom (upper Miocene) Suites are the main source rocks of the South Caspian Basin. Amorphous Organic Matter is found mainly in the middle Maykop Suite as well as the richest Total Organic Content (TOC) with a value larger than 10%. The organic facies in this source rock is an algal marine clastic and moderately calcareous, which is where most oils are originated from. Kerogen types II and III are found in these source rocks[1].

Generation & Migration

Early Pliocene time is when the earliest hydrocarbons have been generated in the source rocks as a result of tectonism in the Caucasus Mountains and subsidence of the adjacent Lower Kura Depression (easternmost of the Kura basin). It is believed that the middle Pliocene time is when the major hydrocarbon migration began and it is still migrating. Deep-water structured oil seeps have been seen in the southern part of the Caspian Sea, and that shows that oil is still migrating and supplying the Pliocene-Pleistocene reservoirs with hydrocarbons[1].

Reservoir

The reservoir units were deposited during Pliocene to Pleistocene. The Volga paleodelta source of sediments deposited quartz-rich rocks from the Russian Platform, located at the northwest of the basin, that has the best reservoir properties. The main reservoir rocks in the middle Pliocene Productive Series are created by fluviodeltaic clastic rocks. There are four types of reservoirs were identified based on their facies associations, stratigraphic architecture, cementation, and faulting patterns. The best reservoirs were deposited in the fluvial facies where rocks have no major porosity or permeability barriers. The main reservoirs are in the Balakhany Suite (Upper Productive Series), which consists of more than 300 m of interbedded siltstone and sandstone[1].

Trap

The types of traps in the South Caspian Basin are uncertain. It is believed that two-thirds of the traps are stratigraphic, and the rest is structural. A model for the Azerbaijan part of the South Caspian Basin was created to show the location of hydrocarbon in structural and stratigraphic traps. This model showed that the majority of trap formations and tectonic movement happened during early Pleistocene and the minority of traps were formed in early - middle Pliocene. The Maykop Suite shales were tightly folded and faulted anticlines. Neogene and Quaternary formations were extremely compressed in steep to overturned and faulted anticlines in the Lower Kura Depression and adjacent shelf areas[1].

Seal

The middle Pliocene reservoirs are effectively sealed by the transgressive shale sequences within the Productive Series and Akchagylian (uppermost Pliocene) and Apsheronian (lower Pleistocene) units. The discontinuity of some thin seals and the active tectonic compression reduces the quality of sealing and make the basin kind of leaky. Sealing thickness varies throughout the basin, depending on the different sources of sediments that were part of the seal and reservoir evolution. The Kalinsk Suite (lower Pliocene) is effectively sealed by the Podkirmaku and Nadkirmaku Suites (lower - middle Pliocene)[1].

Hydrocarbon Assessment

The South Caspian basin is known for its abundant petroleum resources which result in a gas and oil production for over 150 years. The major oil reserves are mostly concentrated in shallow marine, deltaic to lacustrine deposits of middle Pliocene age. Five assessment units (AUs) have been designated for the purpose of assessing the hydrocarbon-resource potential in the Oligocene–Miocene of the South Caspian Basin Province of Azerbaijan, Iran, and Turkmenistan [1]:

  1. Apsheron-Pribalkhan Zone AU (11120101)
  2. Lower Kura Depression and Adjacent Shelf AU (11120102)
  3. Gograndag-Okarem Zone AU (11120103)
  4. Central Offshore AU (11120104)
  5. Iran Onshore-Nearshore AU (11120105)
Figure 5 Oligocene-Miocene Maykop/Diatom, Total Petroleum System, probability (including both geologic and accessibility probabilities) of at least one field equal to or greater than the MFS. Results shown are fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas liquids) category. F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation.

The units, which are shown in figure 1, are separated by different structural styles, as discussed in the following.

Apsheron-Pribalkhan Zone - Assessment Unit 11120101

The Apsheron-Pribalkhan Zone is between Azerbaijan and Turkmenistan (Fig.1). It is divided into four subparallel lines of elliptical and domal folds. Overall, the 11120101 unit contains one of the thickest accumulations of sedimentary strata (20 km) in the South Caspian Basin Province.  Large volumes of sediment from the Volga and Amu Darya paleodeltas and the Caucasus Mountain alluvial fan are discovered in this zone. Oil fields in the unit range in size from approximately 2 to 300 million barrels of oil (MMBO). Reservoir porosities range from 13 to 35 percent, and permeabilities range from 7 to 1,600 millidarcies (mD). The undiscovered resources were also estimated (Fig.5). 3,091 MMBO and 7,908 Billions cubic feet gas are still not discovered in this area[1].

Lower Kura Depression and Adjacent Shelf - Assessment Unit 11120102

The Lower Kura Depression and Adjacent Shelf lie in Azerbaijan (Fig.1).The unit sits in the western sector of the South Caspian Basin south of the Greater Caucasus and is bordered on the south by the Lesser Caucasus accretionary complex (Fig.1). Folds characterize this area: they are overturned, faulted and commonly have steep flanks. Altogether, the reservoirs, consisting mostly of volcaniclastic rocks and feldspathic sandstones, are primarily in the middle Pliocene Productive Series. Reservoir porosities range from 14 to 33 percent, and permeabilities are from 10 to 1,400 mD[1]. Traps are about 95 percent structural (anticlines, some of which are recumbent) and 5 percent stratigraphic. The undiscovered resources are also listed in Fig.5.

Gograndag-Okarem Zone - Assessment Unit 11120103

This zone lies almost entirely in Turkmenistan with a small part in Iran (Fig.1). Anticlines cored by shale diapirs and mud volcanoes are mostly present in this field. The relatively cool nature of the temperature gradient in the southeastern part of the South Caspian Basin result in the oil-generating window for source rocks in the Maykop and Diatom Suites (Fig.2). The most important hydrocarbon-producing zones are in the area of the Gograndag-Okarem zone. Reservoir permeabilities range from 10 to 710 mD while porosities are 17 to 23 percent. Both structural and stratigraphic traps are found. Most of the oil fields exceed 1 MMBO; while gas field sizes are between 10 and 900 BCF.The largest field discovered to date has estimated reserves of 670 MMBO and 2,140 BCFG[1]. An average estimate of 4,764 MMBO and 47,515 BCFG are still undiscovered (Fig.5).

Central Offshore - Assessment Unit 11120104

Assessment Unit 11120104 which lies within the known area of source rock deposition in the South Caspian Basin Province do not have known fields. The unit is offshore with water depths ranging from 300 to 1,000 m. it is believed that oil is currently seeping from deep-water structures, suggesting that hydrocarbon generation and migration are occuring. The Central Offshore is separated into two sections divided by a left-lateral strike-slip fault (Fig.1). The western section is characterized by buckle folds, shale diapirs, and mud volcanoes, whereas the eastern section is a zone of slumps, growth faulting, and mud volcanoes[1]. An average estimation of undiscovered resources were also quantified (Fig.5 ).

Iran Onshore-Nearshore - Assessment Unit 11120105

Little information is available from this zone; as a result, it is not possible to quantify the assessment unit’s potential. The future petroleum potential of the area is discussed. It is believed that five potential fields can be discovered in the unit, on the basis of

paleogeographic maps[1].

Future assessment

To assess the level of the Caspian Sea, called Caspian Sea Level (CSL), we use climate model‐predicted precipitation (P), evaporation (E), and observed river runoff (R) to reconstruct long‐term CSL changes and show that PER (PE + R) flux predictions agree very well with observed CSL changes[6]. As this basin is an exclosed basin, CSL vriation is primarily controlled by water inflow: rivers, precipitation, evaporation into the Kara‐Bogaz‐Gol (KBG) Bay. Long term CSL fluctuations has dramatically altered coastal ecosystems.

The better access there is to geological information, the easier it is to assess potential in the area. For example, the Apsheron-Pribalkhan Zone has information on conformities, lithology, and petrophysical properties that helps determine how much oil can be produced from this certain reservoir or area, hence helping in future assessment of the area. On the other hand, zones like Iran Onshore-Nearshore has very little information on it. This makes it harder to predict the future potential for this area. Innovative technologies will be useful to assess areas that are hard to reach. Sophisticated data acquisition, processing and visualization used from the start of exploration to the end which is safe final plugging of wells. Also, technologies that can enhance reservoir recovery factors via stimulation is also useful.

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Smith-Rouch, L. S. (2006). Oligocene–Miocene Maykop/Diatom Total Petroleum System of the South Caspian Basin Province, Azerbaijan, Iran, and Turkmenistan. Retrieved from https://pubs.usgs.gov/bul/2201/I/pdf/B-2201-I_508.pdf
  2. 2.0 2.1 2.2 2.3 2.4 Brunet, M.-F., Korotaev, M. V., Ershov, A. V., Nikishin, A. M. (2001). The South Caspian Basin: a review of its evolutionfrom subsidence modelling. Retrieved from https://reader.elsevier.com/reader/sd/pii/S0037073802002853?token=A3C72B1421A8F635FB62A71A26D5D1D7BDF5569671F3E724BC0C3C3A16A338A7CAD762D7770DF479A7FEDF904F584F3B
  3. Mauvilly, J., Koiava, K., & Gamkrelidze, I., & Mosar, J. (2016). Tectonics in the Georgian Greater Caucasus: a structural cross-section in an inverted rifted basin setting. Retrieved from https://www.researchgate.net/publication/310600163_Tectonics_in_the_Georgian_Greater_Caucasus_a_structural_cross-section_in_an_inverted_rifted_basin_setting
  4. 4.0 4.1 Simmons, M. D. (n.d.). Geological challenges to exploration in the South Caspian Basin. Retrieved from https://www.researchgate.net/publication/300120766_Geological_challenges_to_exploration_in_the_South_Caspian_Basin.
  5. 5.0 5.1 Green, T., Abullayev, N., Hossack, J., Riley, G and Roberts, A.M. [2009] Sedimentation and subsidence in the South Caspian Basin, Azerbaijan. In: Brunet, M.-F., Wilmsen, M. and Granath, J.W. (eds.) South Caspian to Central Iran Basins. The Geological Society, London, Special Publications, 312, 241-260.
  6. Chen, J. L., Pekker, T., Wilson, C. R., Tapley, B. D., Kostianoy, A. G., Cretaux, J. F., & Safarov, E. S. (2017, July 12). Long‐term Caspian Sea level change. Retrieved October 23, 2019, from https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL073958.

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