Pannonian Basin

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The Pannonian basin is a large basin located in the southeastern part of Central Europe that covers parts of Bosnia, Croatia, Slovenia, Austria, Hungary, Slovakia, Ukraine, Romania, and Serbia and covers approximately 80,000 square miles. [1]The geomorphological term, Pannonian Plain, is used more often in the same region describing only the lowlands, the plain that remained is a result of the Pannonian Sea drying out during the Pliocene Epoch. The Pannonian basin is surrounded by many natural barriers including the Carpathian Mountains and the Alps, along with the Danube and Tisza rivers dividing the basin in half.[2] Fig. 1 is shown describing the location of the Pannonian Basin (marked III).

Fig. 1: III describes the location of the Pannonian Basin (outlined in red)

History

The Pannonian Basin is a result of a series of discrete back-arc syn-rift basins during the Middle Miocene period up until the late Pliocene around 9-20 million years ago.[1] The Pannonian Basin has a long history of Petroleum exploration going back to the first systematic exploration of natural gas and oil in the northern part of the Pannonian region in 1909. Throughout 1995, discovered petroleum totaled about 2.1 billion barrels of oil (MMBO) and 11.2 trillion cubic feet of natural gas (TCF). In just Hungary, there is an estimated 668 million barrels of recoverable oil and approximately 8.5 TCF of recoverable natural gas. As of January 1, 1995, annual Hungarian petroleum production was 11.6 MMBO and 187 BCF of natural gas with reserves in 1996 measuring 132.7 MMBO and 3 TCF of natural gas.[3] Fig. 2 shows the topography of the Pannonian Basin region bordered by the Carpathian Mountains and Dinaric Alps.[2]

Fig. 2: Topography of Pannonian Basin region.

Geologic Structure

The Pannonian Basin is characterized by having a large system of Neogene basin sediments with some inner elements of deformed and complex nappes of Mesozoic, Paleozoic, and Precambrian rocks. Basement rocks of the Pannonian Basin consists complex, faulted, and folded formations of pre-Tertiary sedimentary, igneous, and metamorphic rocks, all with Mesozoic, Paleozoic, and Precambrian origin along the Carpathian foldbelt.[3] Fig. 3 shows the primary geologic structure of the Pannonian Basin and the neighboring geologic units of the region.[4]

Risks and Uncertainty

Although the geothermal potential is high in the Pannonian Basin, this sedimentary basin is hardly competitive because of volcanic areas. Wells within this area have to be drilled deep, about 2000-4000 meters, and be able to explore 100-150 degrees Celsius (212-302 Fahrenheit) temperatures. The water salinity and gas-water ratio is often high as well as having very high pressure zones in the most prospective areas reaching up to 360 bar (5221.36 psi).[5]

Petroleum Geology

Fig. 3: Geologic structure of the Pannonian Basin (outlined in red) and neighboring geologic units.

Source Rock & Migration

The oldest known source rocks in the Pannonian Basin region are Triassic organic-rich shales and marlstones, namely the Rhaetian Kössen Marl and Carnian Veszprém Marl formations in the region. Total organic content of the Carnian Veszprém measures around 3-5% while the Rhaetian Kössen has a larger range of about 3-20%, both formations can contain type-I and type-II Kerogen. Neogene rocks, from Miocene age, are also considered to be a major source rock of the region and can scale from middle to late Miocene shale, clay-marl, and marl. Potential source rock thickness in this area can range from less than 1 meter to 4 kilometers, these rocks are generally lower quality and mainly provide type-II and type-III kerogen. The average geothermal gradient for the Pannonian Basin region is approximately 3.6 Celsius/100 meters, some areas can exceed more than 5.8 Celsius/100 meters. Because of these high geothermal gradients in the region, organic-rich source rocks prove to be good sources for oil and gas at shallow depths. Rocks around 2,000 meters are generally considered to contain immature oils wile depths around 2,500 meters have more mature oils. rocks below 5,000 meters or more are generally considered gas generative. Oil generation in the region started around 5-8 million years ago so sediments below the approximate depth of 4-5 kilometers have migrated past the oil generation window.[3]

Reservoir

Reservoir rocks throughout the Pannonian Basin region vary in lithology and age. In 1987, it was discovered that Neogene reservoir rocks make up around 61% of discovered petroleum while Paleozoic and Mesozoic reservoir make up 33% and approximately 7% from Paleogene rocks. More recent studies in 1994 concluded that 62% of total oil production was from Tertiary sedimentary rocks and 24% from Mesozoic carbonates. 70% of all natural gas production is also from Tertiary reservoirs. Production is typically associated with many different zones but particularly in anticlinal closures. 80% of all reservoirs discovered are associated with having Neogene rocks, and of those reservoirs, 95% of them are made of sandstones. Data in 1996 has discovered that the average porosities of these sandstones is approximately 16%.[3]

Traps and Seals

Producing traps in the Pannonian Basin region have a depth range from about 80 meters to 5,000 meters, with oil being prevalent between 800 meters and 3,000 meters and gas down toward 5,000 meters. Multiple combinations of traps are present throughout the region, such as compactional anticlines, fault-closed features, roll-overs that are closely associated with growth faults, and different closures in flower structures along strike-slip faults. Seals in the Tertiary sediments are associated with mudstones, marls, and fine-grained rocks.[3]

Future Petroleum Potential

Since 2017, the technical success rate of the Pannonian basin has ranged from 83 to 94%, while the commercial success rate is approximately around 50%. In 2020, natural gas flow rates have been in the range of 1.4-17.2 MCF of gas per day and typical pool size is around 20 BCF of gas. The average reservoir depth is 2,200 meters and drilling costs have gone down. Success in the region has been associated with modern technologies and techniques, namely 3D seismic, and drilling in areas where risk is low. Because of the high heat flows, deep drill depths, and high pressure, further innovations in technology would help reduce risk and uncertainty in the Pannonian Basin as of 2021. [1] Total undiscovered resources as of 2000 are estimated to be 153-631 MMBO and 1.7-7.4 TCFG.[3]

Petroleum and Facility Engineering

Hydraulic fracturing has been the most effective tool in low-permeability reservoirs found throughout the Pannonian Basin.[6] Much of the Pannonian Basin is underdeveloped because of the geologic risks associated with the region. Hydrocarbons are considerably more difficult to extract in most parts of the basin because of the deep depths of potential reserves and the excessive temperatures/pressures. Figure 4 below describes different Oil and Gas drilling operations found throughout the Pannonian Basin region, large parts of the basin are not drilled yet because of the risks and uncertainties described before.[1] Sand and water are pumped down at high pressures to crack the formation rock in order to extract Oil and Gas in wells associated with low risk.

Fig. 4: Drilling of oil and gas in the Pannonian Basin

External Links

  1. https://www.geoexpro.com/articles/2020/05/oil-and-gas-exploration-success-in-the-eastern-european-pannonian-basin
  2. http://archives.datapages.com/data/specpubs/history2/data/a113/a113/0001/0300/0319.htm
  3. https://eos.org/research-spotlights/unraveling-the-history-of-central-europes-pannonian-basin
  4. https://scholarship.rice.edu/handle/1911/16778

References

  1. 1.0 1.1 1.2 1.3 Ford, J. N. (2020, May 6). Oil and Gas Exploration Success in the Eastern European Pannonian Basin. GEO ExPro. https://www.geoexpro.com/articles/2020/05/oil-and-gas-exploration-success-in-the-eastern-european-pannonian-basin
  2. 2.0 2.1 Wikipedia contributors. (2021, April 5). Pannonian Basin. Wikipedia. https://en.wikipedia.org/wiki/Pannonian_Basin
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Dolton, G. L. (2006). Pannonian Basin Province, Central Europe (Province 4808)—Petroleum Geology, Total Petroleum Systems, and Petroleum Resource Assessment. U.S. Geological Survey. https://pubs.usgs.gov/bul/2204/b/pdf/b2204-b_508.pdf
  4. Pannonian Basin. GeoERA. (2020, January 27). https://geoera.eu/projects/geoconnect3d6/pannonian-basin/.
  5. Kujbus, A. (2009, January). Risks and opportunities of a geothermal exploration project in the Pannonian Basin in Central-Eastern Europe. Geothermal Resources Council.
  6. Carpenter, C. (2021). Fracturing With Height Control Extends the Life of Mature Reservoirs in the Pannonian Basin. Journal of Petroleum Technology, 73(01). https://doi.org/10.2118/0121-0053-jpt