Pre-Caspian Basin

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Introduction

Pre-Caspian basin map[1]

The Pre-Caspian Basin is one of the oldest basins in the world, and is located in Russia, Kazakhstan, and the Northern part of the Caspian Sea. The basin spans about 500,000 km2, and reaches depths of 20 km below the Earth’s surface.[2] The Pre-Caspian Basin is one of the largest hydrocarbon provinces in the world and exploration of the hydrocarbon-rich basin began in the early twentieth century.[3]

Geological history

Salt domes in the Pre-Caspian Basin[4]

This basin was formed by liner rifts that intersected in Russia and Kazakhstan. Intersecting rifts create a circular area in the oceanic crust which can be problematic because rifts are usually angular bends. The Pre-Caspian basin is estimated to have formed between the Paleozoic and Cenozoic times.[5] It lays over two large pre-Permian depressions. Around the Ordovician time period, sediments began filling the basin followed by a thick layer of salt that is 4-5 km thick. The salt was buried by more sediments creating large salt domes underneath the surface. It is difficult to pinpoint when exactly this basin was formed because the depth of the center of the basin is so deep that the calibration of seismic data with well data from the basin margins glitches. Researchers found that the oldest, deepest sediments are predicted to be from Riphaean or Devonian.[6]

Source rock

The primary source rocks present in the Pre-Caspian basin are basinal black-shale facies coinciding with upper Paleozoic carbonate platform deposits on the basin margins. Because of how deep the basin is, the geochemical characteristics of source rocks are not documented well. While source rocks' presence and organic carbon content are greatly disputed, it is generally understood that they have total organic carbon (TOC) content ranging from 1.3% to 10%. High TOC content is typical of deep-water anoxic black-shale facies.[7]

Traps

Under the surface of the Pre-Caspian Basin[8]

The Pre-Caspian Basin is filled with many different traps. After research on seismic acquisition and geophysical and geological data, the main traps found in this basin are faulted anticline, salt overhang, stratigraphic, unconformity, and some pinch-out traps. The size and depth of the basin make it possible for so many kinds of traps to occur. It also makes it more challenging to drill because there are so many different types of traps a producer can encounter. The Pre-Caspian Basin is unique because it has a large quantity of salt, much of this salt creates traps. Halokinesis, the movement of salt and salt bodies, led to more than 1200 salt dome structures formed in this basin. Some of these structures include salt pillows, salt diapirs, walls, and ridges.[9] The 4 km thick salt layer sits over Devonian reef complexes forming a seal that prevents hydrocarbons from escaping.[7]

Production

There are three main oil fields in the Pre-Caspian Basin. They are the Tengiz, Karachaganak, and Astrakhan fields. These fields have discovered reserves of about 19.7 billion barrels of oil and NGL and 157 trillion cubic feet of natural gas. In a 1996 study, Petroconsultants found 45.8 billion BOE of discovered hydrocarbons in the Basin, and of that, 57% is gas. Most hydrocarbons are in these super oil fields with reserves greater than 5 BBOE each.[7] Historically, the Soviet Union drilled in this basin for their energy needs, but after finding oil-rich land in Siberia, they abandoned production here.[7]

While there is an abundance of hydrocarbons in the Pre-Caspian Basin, exploration costs are too much for most producers. The Northern part of the Caspian Sea is shallow and freezes for half of the year, making it more challenging and expensive for offshore drilling. The oil and gas fields are also far away from refineries and export markets. To get hydrocarbons to the export markets, most companies would have to rely on Russian pipelines.[10]

Future production and engineering challenges

Rig in the frozen Caspian Sea[11]

One of the greatest potentials for growth in this basin is the Kashagan field in Kazakhstan. It was discovered in 2000 with estimated reserves of more than 13 billion barrels of oil and natural gas. While there is a great potential for future production, the cost, and difficulty that comes with drilling in this area. The Kashagan oil reservoir lies 2.6 miles below the seafloor at 770 psi, and has an extremely high H2S content at about 19%. Since H2S is extremely poisonous, explosive, flammable, and corrosive, there are some environmental concerns about drilling here and producers must exercise extreme caution when drilling. The shallow waters and extreme cold in the Caspian Sea make fixed floating platforms and other conventional offshore drilling techniques nearly impossible. A drilling vessel that can withstand the cold weather, shallow water, and deep reservoir costs about $25 million alone.[12]

References

  1. https://www.sciencedirect.com/science/article/abs/pii/S026481722030194X
  2. Ulmishek, Gregory F. “Petroleum Geology and Resources of the North Caspian Basin, Kazakhstan and Russia.” USGS Publications Warehouse, 23 Nov. 2016, https://pubs.usgs.gov/bul/2201/B/.
  3. Zhanserkeyeva, A. A., and A. K. Kassenov. “Prospect Evaluation Based on Integrated Petroleum System Analysis: Block E Case Study, South-Eastern Edge of Precaspian Basin (Kazakhstan) - Journal of Petroleum Exploration and Production Technology.” SpringerLink, Springer International Publishing, 7 Feb. 2022, https://link.springer.com/article/10.1007/s13202-022-01466-5/tables/2.
  4. http://historyoftheearthcalendar.blogspot.com/2014/08/august-27-north-caspian-basin.html
  5. Krylov, N A, et al. “Structure of Pre-Caspian Depression and Major Oil and Gas Fields of the Region.” AAPG Bulletin (American Association of Petroleum Geologists); (United States), 1 Mar. 1991, https://www.osti.gov/biblio/5802595-structure-pre-caspian-depression-major-oil-gas-fields-region.
  6. Brunet, Marie-Françoise, et al. “The Geodynamic Evolution of the Precaspian Basin (Kazakhstan) along a North–South Section.” Tectonophysics, Elsevier, 11 Jan. 2000, https://www.sciencedirect.com/science/article/pii/S0040195199001912.
  7. 7.0 7.1 7.2 7.3 Ulimishek, Gregory. “Petroleum Geology and Resources of the North ... - USGS.” USGS, 2001, https://pubs.usgs.gov/bul/2201/B/b2201-b.pdf.
  8. https://pubs.usgs.gov/bul/2201/B/b2201-b.pdf
  9. Akhmetzhanov, Aitbek, et al. “Post-Salt Trapping Mechanism of South-East Pre-Caspian and Its Application to Petroleum Exploration - Journal of Petroleum Exploration and Production Technology.” SpringerLink, Springer International Publishing, 9 Aug. 2020, https://link.springer.com/article/10.1007/s13202-020-00971-9#Sec4.
  10. “U.S. Energy Information Administration - EIA - Independent Statistics and Analysis.” International - U.S. Energy Information Administration (EIA), 2013, https://www.eia.gov/international/analysis/regions-of-interest/Caspian_Sea.
  11. https://www.vice.com/en/article/wnjngz/kashagan-the-worlds-most-toxic-money-pit
  12. Daly, John. “Kazakhstan's Kashagan Oil Fields Faces Problems and Soaring Costs.” Kazakhstan's Kashagan Oil Fields Faces Problems and Soaring Costs, 21 May 2014, https://www.cacianalyst.org/publications/analytical-articles/item/12981-kazakhstans-kashagan-oil-fields-faces-problems-and-soaring-costs.html.