Zagros Basin

From SEG Wiki
Jump to navigation Jump to search
ADVERTISEMENT
Figure 1. The Zagros Basin is located in the Middle East, largely in Northern Iraq. (Courtesy of Dynamic Resource Corporation)

[1]This page is currently being authored by a student at the University of Oklahoma. This page will be complete by Dec. 15, 2021.

The Zagros Basin is located in the Middle East, the majority of the basin being in Iraq, Syria, and Turkey. This basin covers a large amount of land, in fact covering an area of 500,000 km2 . The Zagros Basin is bar far one of the most prolific and important basins in the Middle East regarding how many oil and gas fields have been discovered. For this reason, this basin has been a focal point of oil and gas production in the Middle East finding over 400 oil and gas fields, as well as large amounts of hydrocarbon accumulation.[1] Drilling began in the early years of the 1900's and with continued success, the Zagros Basin will continue to be a sought after explorative area.[2]

Contents

[hide]

Tectonic History [edit source]

The Zagros Basin is in a unique spot, both in the world as well geologically. The basin itself lies in between the Arabian plate and the Eurasian plate. To the northeast of the Arabian plate is the Eurasian and to its southwest is the African plate, which puts the Zagros Basin at the southwestern portion of the Zagros orogenic belt. In relation to the Persian Gulf, as well as the Mesopotamian plain the Zagros Basin is parallel to both. Given that the Zagros Basin is between the Arabian Plate it is important to note that this plate has been exposed and uplifted ever since the Proterozoic Era. Similarly, to other basins, the Zagros has undergone changes, some that have been very noticeable. There are three main tectonic periods and sedimentary developments that should be pointed out. The first of these would be the Permian-Triassic extension, leading into the Jurassic-Early Cretaceous passive continental margin, and lastly the Late Cretaceous0 Quaternary foreland basin.[3] We also know that during the Jurassic to the middle Cretaceous the subsidence and sedimentation patterns were becoming more and more complex, eventually aiding in the evolution of the carbonate shelf on the newly formed Arabian continent.[1] In summary, due to subduction and collision of the Arabian and Iranian plate, with regards to regional extension, the Zagros Mountains were formed. Due to this and the orogenic effect of the Zagros Mountains, a peripheral foreland basin was created at the continental margin.

Stratigraphy Process- High Fault Zone [edit source]

Figure 2. This image contains stratigraphic formation names of the Zagros basin and adjacent areas. (Courtesy of Koop W.J. and Stoneley R, et all, 1982)
  1. To the Northeast deep water argillaceous carbonates curtained the confined basin deposits, and to the Southwest evaporites and shelf carbonates represent Middle-Late Jurassic.
  2. Basinal shales and marls are then deposited during the Early Cretaceous.
  3. This process was then overlain by marls, limestones, and neritic carbonates. Being separated from Late Cretaceous carbonates by another unconformity.
  4. Siliciclastics from the Late Cretaceous were then deposited into the front of obudcted united of the Zagros Basin.
  5. Marginal marine marls and shales are deposited into laterally changing rigid carbonate units during the Paleogene.
  6. We then see red Eocene clay and Carbonates.
  7. In the High Folded Zone we are missing the Oligocene.
  8. The Eocene carbonates presuppose the Middle Miocene evaporitic units, clays, and limestones.
  9. This is curtained by the Zagros basin deposit of the Middle-Late Miocene a sandy, fluvial succession, and the Late Miocene-Pliocene conglomerate succession.[3]

Petroleum Geology [edit source]

Traps and Seals [edit source]

The main ways oil and gas are trapped within this basin are by evaporite and shale cap-rocks, as well as anticlines. The main evaporite cap-rock of the Zagros basin is the Miocene Gachsaran Formation, controlling 60% of the recoverable reserves of the basin. There are multiple other evaporite cap-rocks throughout the basin, all known for high percentages of recoverable reserves, but more importantly known for having minimal porosity and the ability to deform plastically making them great cap-rocks. Furthermore, the simply folded zone of the basin is controlled by shale cap-rocks, carbonate cap-rocks, and evaporite cap-rocks cumulatively adding up to 9.8% of recoverable reserves.[4] Moving onto the anticline, a major controlling factor of the oil and gas reservoir formations in the basin, there are large compressive folds in the Zagros basin due to the collision of the Arabian plate and Eurasian plate. With this, the anticline is formed and closely related in regard to the distribution of oil and gas fields to the basin. Primarily in the direction of northwest-southeast, the extension of the anticline matches extremely well.[1]

Source Rocks [edit source]

When discussing the main source rocks in the Zagros basin we need to look at the source rocks in both the Paleozoic succession, as well as the Mesozoic succession to fully understand it. It is also important to note that the Albanian event, which is the event that was responsible for the deposition of source rocks of in North Oman, Dubai and Abu Dhabi, was also the event responsible for deposition of source rocks in the Zagros basin which we see in Iran. The Paleozoic source rock that we see comes from the Silurian time period, the source rock itself is shale. The source rocks in this area are actually over-mature and reaching the graphite stage. It is also because of these shales that most believe there are huge accumulations of gas in Iran, Qatar, and Abu Dhabi. In relation to this, it is important to note that we see the widespread Silurian episode because of worldwide sea-level rise.

Figure 3. This is an example of the simply folded zone of the Zagros basin, its cap-rocks and oil and gas distribution. (Courtesy of Science Direct, 2018)

-       Silty shales 1- 4.3% TOC

In the Mesozoic succession we are able to see a different source rock, thick dolomitic limestones and thin evaporate layers. As time continued organically lean sediments remained strong, which continued to be true for successions of limestones in different formations. Through the Middle Jurassic in this area there were depositions of Sargelu shales. It is also during this time that we can see in areas, such as Lurestan, dark brown radioactive pyritic marls switch back and forth with fine grained argillaceous limestones. Throughout this time period and area we can conclude that the source rocks from the Mesozoic succession area responsible for marginal gas production and overall insignificant source rocks.

-       Black pepper shales 1.5 – 4.5% TOC

-       TOC vary from 2-9% in the marls and from 1 to 2% in the limestones[5]

Lastly, API gravity is used to measure how light or heavy petroleum liquid is compared to water. The Zagros basin condensates range from 8 to 46° API in the Jurassic reservoirs, as opposed to the Triassic reservoirs which range from 28 to 45° API.[6]

Future Risks and Uncertainties [edit source]

The finding of oil and gas in the Zagros basin have established many explorative projects. It is because of the projects, geologists have been able to acquire data regarding the basins source rocks, traps and seals, and overall tectonic history. At the end of 2017, there had been more than 400 oil and gas fields found throughout the basin, resulting in natural gas recoverable reserves of 18.3 x 10¹² m³ , petroleum recoverable reserves of 26.32 x 10⁹ t and condensate recoverable reserves of 1.31 x 10⁹ t.[1] It is because of these amazing amounts of recoverable reserves, that add so many opportunities and being the main commodity for this geographical area that makes it so important to assess the risks. Due to the location of the basin and its proximity to the Zagros mountains, the main risk is groundwater disaster. The risk of groundwater arrives because data from GIS environment plays shows water following the direction of the basin’s main faults and fractures. With this information and recent tectonic activities disasters such as sinkholes, landslides, and chemical changes in ground water have been seen in the Zagros Mountain and basin area. This is a case of change because of developments outside of the mountains, especially the oil and gas fields, sadly the more exploration and success you have outside of the area the more risk starts to travel down.[7] Lastly, it is known that oil fields dominate the foredeep zone and gas fields dominate the simply folded zone. And because of extremely compressed tectonic deformations of the imbricated zone, Mesozoic strata has been exposed on the surface in large amounts. These circumstances make it very easy for oil and gas to escape and harder for it to be preserved, the opposite of most areas for this basin and its cap-rocks. Luckily, these risk and conditions have not made it so that this basin has declined in production and the Zagros still continues to be one of the Middle East’s essential basins for oil and gas reserves. [1]

References[edit source]

  1. 1↑ Xiaobing Liu, Zhixin Wen, Zhaoming Wang, Chengpeng Song, Zhengjun He, Structural characteristics and main controlling factors on petroleum accumulation in Zagros Basin, Middle East, Journal of Natural Gas Geoscience, Volume 3, Issue 5, 2018, Pages 273-281,↵↵ISSN 2468-256X, https://doi.org/10.1016/j.jnggs.2018.11.004.
  2. 2↑ Rasoul Sorkhabi, Ph.D., et al. “The Centenary of the First Oil Well in the Middle East.” GEO ExPro, 21 Jan. 2014, https://www.geoexpro.com/articles/2008/05/the-centenary-of-the-first-oil-well-in-the-middle-east.
  3. 3↑Koop W. J. and Stoneley R., et al. “Subsidence History of the Middle East Zagros Basin, Permian to Recent.” Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 5 May 1982, https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1982.0031.
  4. 4↑Zebari, Mjahid, et al. “Structural Style of the NW Zagros Mountains and the Role of Basement Thrusting for Its Mountain Front Flexure, Kurdistan Region of Iraq.” Journal of Structural Geology, Pergamon, 11 Oct. 2020, https://www.sciencedirect.com/science/article/pii/S0191814120304260.
  5. 5↑M.L. Bordenave, R. Burwood, Source rock distribution and maturation in the Zagros Orogenic Belt: Provenance of the Asmari and Bangestan Reservoir oil accumulations, Organic Geochemistry, Volume 16, Issues 1–3,1990, Pages 369-387,ISSN 0146-6380, https://doi.org/10.1016/0146-6380(90)90055-5.
  6. 6↑Joseph M. English, Grenville A. Lunn, Luke Ferreira, George Yacu; Geologic evolution of the Iraqi Zagros, and its influence on the distribution of hydrocarbons in the Kurdistan region. AAPG Bulletin 2015;; 99 (2): 231–272. doi: https://doi.org/10.1306/06271413205
  7. 7↑Pradhan, Biswajeet. “Disasters and Risk Reduction in Groundwater: Zagros Mountain Southwest Iran Using Geoinformatics Techniques.” Disaster Advances, 3 Jan. 2017, https://www.academia.edu/30723848/Disasters_and_Risk_Reduction_in_Groundwater_Zagros_Mountain_Southwest_Iran_Using_Geoinformatics_Techniques?auto=citations&from=cover_page.

External links[edit source]

http://www.walden3d.com/iraq/031106/USGS_03_North_Geology.html

https://glossary.oilfield.slb.com/

https://oec.world/en

https://www.api.org/

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Cite error: Invalid <ref> tag; no text was provided for refs named :0
  2. Rasoul Sorkhabi, Ph.D., et al. “The Centenary of the First Oil Well in the Middle East.” GEO ExPro, 21 Jan. 2014, https://www.geoexpro.com/articles/2008/05/the-centenary-of-the-first-oil-well-in-the-middle-east.
  3. 3.0 3.1 Koop W. J. and Stoneley R., et al. “Subsidence History of the Middle East Zagros Basin, Permian to Recent.” Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 5 May 1982, https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1982.0031.
  4. Zebari, Mjahid, et al. “Structural Style of the NW Zagros Mountains and the Role of Basement Thrusting for Its Mountain Front Flexure, Kurdistan Region of Iraq.” Journal of Structural Geology, Pergamon, 11 Oct. 2020, https://www.sciencedirect.com/science/article/pii/S0191814120304260.
  5. M.L. Bordenave, R. Burwood, Source rock distribution and maturation in the Zagros Orogenic Belt: Provenance of the Asmari and Bangestan Reservoir oil accumulations, Organic Geochemistry, Volume 16, Issues 1–3,1990, Pages 369-387,ISSN 0146-6380, https://doi.org/10.1016/0146-6380(90)90055-5.
  6. Joseph M. English, Grenville A. Lunn, Luke Ferreira, George Yacu; Geologic evolution of the Iraqi Zagros, and its influence on the distribution of hydrocarbons in the Kurdistan region. AAPG Bulletin 2015;; 99 (2): 231–272. doi: https://doi.org/10.1306/06271413205
  7. Pradhan, Biswajeet. “Disasters and Risk Reduction in Groundwater: Zagros Mountain Southwest Iran Using Geoinformatics Techniques.” Disaster Advances, 3 Jan. 2017, https://www.academia.edu/30723848/Disasters_and_Risk_Reduction_in_Groundwater_Zagros_Mountain_Southwest_Iran_Using_Geoinformatics_Techniques?auto=citations&from=cover_page.