Pegasus basin

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Pegasus Basin and Surrounding Basins, Stanford University. (n.d.). East Coast Basin, New Zealand. Retrieved December 13, 2020, from https://bpsm.stanford.edu/east-coast-basin-new-zealand

The Pegasus Basin is a geologic formation located off the coast of the North Island of New Zealand. The basin is to the southwest of Wellington, New Zealand, and is bounded by the East Coast Basin to its west/ northwest and the Raukumara Basin to its north. [1] The exact location can be seen in the Pegasus Basin and Surrounding Basins image to the right. There are many active seeps, pockmarks, gas chimneys (Dictionary:Gas chimney, gas cloud), etc. present within the Pegasus basin, making it an area of economic interest in terms of future petroleum production.[2] In 2012, Anadarko New Zealand Co. (affiliate of Anadarko Petroleum Corp., Houston) was awarded two separate blocks of the Pegasus basin for exploration. [3] These two blocks cover a combined 7,000 square miles of the basin. [3] WesternGeco conducted seismic research across the basin from 2014-2016, which resulted in a completely new interpretation of the formation compared to earlier data. [3] This new seismic interpretation has offered a better understanding of the area, but it still remains untested. The size and scope of the basin are not fully understood at this time, because the basin extends past the limits of the seismic data gathered to this point. Gravity data suggests that the basin could be up to 50,000 square kilometers.[4]

Region History

The east coast of New Zealand’s North Island where the Pegasus Basin is located, is believed to have potential hydrocarbon producing formations, but historically this has not been the case. [1] Shallow wells were drilled onshore near the Pegasus Basin as early as the 1870s.[1] These first wells maxed out their production at around 50 barrels of oil per day. Through the years, other exploratory wells were drilled, but none of them produced in commercial quantities. The first well to produce in paying quantities wasn’t drilled until 1990. This was the Te Hoe-1 Well drilled by Petrocorp Exploration.[1] This well was drilled into late Cretaceous sandstone and was the first sign of potential petroleum production for the formations in the area. [1] The first offshore well in the region was drilled in 1976 by a coalition of multiple companies and was named the Hawke Bay-1 Well with a total depth of 2,372 meters. [1] When predicting the petroleum-producing potential of the Pegasus Basin, these previous wells are used as reference points. However, since no wells have been drilled in the Raukumara and Pegasus Basins, the petroleum producing potential of these specific basins remains unknown. [1] Exploration in the area is low to moderate right now, which means the Pegasus Basin looks to remain untested for the foreseeable future unless the activity in the area increases.

Hikurangi Subduction Zone, LEARNZ navigation. (n.d.). Retrieved December 13, 2020, from https://www.learnz.org.nz/naturalhazards174/bg-standard-f/the-hikurangi-plate-boundary

Geologic Setting

The Pegasus Basin is located just east of the Hikurangi subduction zone between the subduction zone and the North Island of New Zealand. The location of the Hikurangi Subduction Zone can be seen in Hikurangi Subduction Zone shown to the left.[5] Over the course of its life, the basin forming mechanisms of the Pegasus Basin have been constantly changing. [6] While the basin is believed to have initially been formed at the trench of the Mesozoic subduction[5][6], it has seen a changing of mechanisms through the Mesozoic, Neogene, Cretaceous and Paleogene.[6] During the Mesozoic and Neogene, flexurally and dynamically induced subsidence were the driving force behind the formation of the basin. During the Cretaceous and Paleogene, the formation force was thermal subsidence.[6] There are 4 major units in the stratigraphy of the Pegasus Basin, and it is believed that the majority of the Pegasus Basin was formed through a marine depositional environment.[6][2] The 4 main units can be broken down into the Torlesse Composite Terrane metasedimentary basement, the Hikurangi Plateau, late Early Cretaceous and Paleogene sedimentary rocks, and Neogene sedimentary rocks.[4]

Torlesse Composite Terrane metasedimentary basement

The first unit, which forms the basement of the basin, is believed to have formed as the Hikurangi plateau subducted underneath the Chatham Rise. Remnants of the Hikurangi Plateau were scraped off on the Chatham Rise forming an accretionary wedge.[4] This wedge comprises mostly the northern border of the basin, and the rocks are believed to be composed of volcanic rock, volcaniclastics, limestone, and chert. [4] The rest of the composite consists of Torlesse rocks beneath the accretionary wedge. These are mainly sedimentary rocks (Dictionary:Sedimentary rock) that are interpreted to be deposited by marine environments.[4][6][2][5]

Hikurangi Plateau

The next unit is the Hikurangi Plateu itself. As mentioned above, it is believed to consist of volcanic rock, volcaniclastics, limestone, and chert.[4] Some samples from the plateau include lava, breccia, conglomerate, and sandstone, which have been interpreted to have been deposited by shallow-marine or sub-aerial eruptions.[4] Erosion at or above sea level is also believed to have taken place after these depositional eruptions. [4][6][2][5]

Late Early Cretaceous and Paleogene sedimentary rocks

The third unit is made mostly of sedimentary rocks from the Cretaceous and Paleogene.[4] These rocks share many characteristics of both basement rock and cover rocks.[4] However, since the rocks are not fully accreted, they are not included within the basement unit of the Pegasus Basin.[4] There are some late Cretaceous rocks within the Paleogene rocks, meaning that at some point there was uplift causing the two rocks to be combined within this cover strata.[4] Portions of this strata also indicate that there was episodic volcanism.[4] This unit ranges from roughly 1500m thick to 2500m thick, and the top of the unit ranges from 8000m deep to 1000m deep as it onlaps the Chatham Rise.[4] [6][2][5]

Neogene sedimentary rocks

This unit was formed from eroded sediments being deposited from landforms to the west, and is attributed to very strong current activity such as ocean tides.[4] The Pacific Plate shifting resulted in downwarping of the basin, which left room for these sediments to be deposited.[4] The thickness of this unit is very important in terms of petroleum prospects because it determines the thermal maturity of the underlying sedimentary layer of Late Early Cretaceous and Paleogene sedimentary rocks.[4] The thickness of this layer ranges up to 6000m, but there is a very thin layer of this unit that resides above the Chatham Rise.[4] The thicker portions of this strata would mature the hydrocarbons in the strata under it at a different rate than the thinner portions of this strata.[4][6][2][5]

Pegasus Basin Today

The Pegasus Basin is not being explored, and the exploration in the area surrounding it is minimal.[1] The majority of the exploration taking place uses onshore wells that have been drilled previously.[1] Because there has been a lack of success in the area for over a century [1], it is not a hot zone for exploration at this point in time. As mentioned above, the formation is believed to contain hydrocarbons due to the presence of active seeps, pockmarks, gas chimneys, etc. in seismic interpretations.[2] The reserves of the formation are untested and all the information available is based off of seismic interpretations that have changed at least once.[3]

Geologic Risks and Uncertainties

The main source of geologic risks and uncertainties is the fact that no wells have been drilled in the Pegasus Basin.[1] Seismic data shows that there are most likely large petroleum reserves in the Pegasus Basin in the form of gas hydrates (Dictionary:Gas hydrate), but until wells are drilled or samples of the reservoir are taken it is nearly impossible to determine if the reserves can be economically produced.[7] This means that there is extreme uncertainty in terms of the porosity, permeability, water saturation, etc. of the underlying strata. Without knowing these properties of the underlying strata, it is hard to determine the best drilling location, the best drilling depth, the best method of production, etc.

Petroleum Elements of the Basin

Reservoirs

Since the Pegasus Basin hasn't been drilled, there is no way to determine where the best reservoirs (Dictionary:Reservoir) are within the basin.[7] By using information from other basins that have been drilled, inferences can be made to decide which portions of the Pegasus Basin to test first.[7] Using this data, it is believed that the primary potential reservoirs in the basin consist of Cretaceous sandstones, Neogene sandstones, and Neogene fractured limestones.[7]

Seals

The traditional seals (Dictionary:Seal)in these potential reservoirs are believed to be interbedded shales, mudstones, and marls[7], while the base of the gas hydrate stability zone (BHSZ) is believed to be the seal for free gas accumulations within the reservoir, meaning it is a depth-based boundary between free gas and gas hydrates within the reservoirs.[7]

Source Rocks and Migration

The source rocks (Dictionary:Source rock) of the Pegasus Basin's anticipated hydrocarbons are relatively unknown.[8] Kroeger, Plaza-Favorola, Barnes, and Pecher have investigated multiple theories regarding the source rocks within the Pegasus Basin, yet have not come to a definitive conclusion due to the lack of solid data.[8] Onshore data has revealed Late Cretaceous rocks as source rocks for oil and gas, yet off the east coast of New Zealand where the Pegasus basin resides, the rock layers that coincide with that time frame are marine based deposits.[8] These rocks are believed to have a total organic content of around 2% based on similar formations samples near the Pegasus basin, which does not provide assurance that these rocks are the source of the gasses expected in the Pegasus Basin.[8] The migrations (Dictionary:Migration) of the hydrocarbons within the Pegasus Basin are a little easier to interpret due to the presence of the base of the gas hydrate stability zone predicted through seismic data.[8] This insinuates that the gas hydrates have migrated upwards within the formations of the basin until they reached the BHSZ, and then the free gas that was released continued to migrate upwards through the formation. This is corroborated by the interpretation of gas chimneys within the formation.[3]

Anticline, Anticline. (n.d.). Retrieved December 13, 2020, from https://energyeducation.ca/encyclopedia/Anticline
Anticline Conceptual, Anticline. (n.d.). Retrieved December 13, 2020, from https://energyeducation.ca/encyclopedia/Anticline

Trap

The New Zealand government released a report that identified different kinds of traps (Dictionary:Trap, Definition 2) within the Pegasus Basin consisting of anticline folds, bottom simulating reflector traps, compound structural traps, stratigraphic pinch-out traps, and stratigraphic traps.[7] Without knowing which formations within the basin are desirable reservoirs, it is impossible to assign a trap type to the hydrocarbon reservoirs in the basin. Both a real-life example and a conceptual example of an anticline, which is believed to be the most common trap type within the basin, can be seen in Anticline and Conceptual Anticline. This type of trap is associated with an upward movement of strata, resulting in an "A" shaped formation that traps oil and gas.

Future Petroleum Potential

The Pegasus Basin is believed to contain large reserves of hydrocarbons, mainly in the form of free gas and gas hydrates.[8][7] This means that the basin could offer enormous production in the future, but only if more research is done and more data is collected within the basin.[7] Since the basin is an offshore prospect, the potential to produce hydrocarbons isn't the only deciding factor when debating whether to commence operations within the basin. The production of the basin must also be economical, which isn't always easy with offshore production costs. Further research and the constant evolution of the oil and gas industry will yield a better idea of the future petroleum potential of the Pegasus basin. For now, however, the strong indicators of gas reserves with the basin make it an attractive basin for future exploration and production.[8]

Offshore Oil Rig, SurferToday.com, E. A. (n.d.). What is offshore drilling? Retrieved December 13, 2020, from https://www.surfertoday.com/environment/what-is-offshore-drilling

Petroleum and Facility Engineering in the Pegasus Basin

The exploration and production of the Pegasus basin will require some form of offshore rig to drill into the formations, as seen in Offshore Oil Rig to the left. Until this takes place, the porosity, permeability, water saturation, etc. of the basin will solely be based on interpretations. Once the basin has been drilled and the petroleum engineering aspects of the project have been lined out, there are no facilities currently in place for transporting produced hydrocarbons from the basin. Since the majority of the hydrocarbons within the basin are expected to be gas[8], the main facilities that would be needed would be gas transporting facilities. This would mean that a gas pipeline would have to be built to the rig, or the gas would have to be compressed and then transported by tanker or some other machinery to refineries. [9] Additionally, if oil is found within the Pegasus Basin, the transportation process would be very similar. The oil can be transported by either tanker ship or pipeline from the point of production.[9]

Further Reading

The geology of Pegasus Basin based on outcrop correlatives in southern Wairarapa and northeastern Marlborough, New Zealand

Pegasus Basin, eastern New Zealand: A stratigraphic record of subsidence and subduction, ancient and modern

The Submerged Continent of New Zealand


References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 [New Zealand's Petroleum Basins - Part Two. New Zealand Petroleum and Minerals. https://www.nzpam.govt.nz/assets/Uploads/doing-business/nz-petroleum-basins-part-two.pdf.]
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 [King, S. E. (2017). The Tectonic Evolution of Pegasus Basin and the Hikurangi Trench, Offshore New Zealand. Mountain Scholar. https://mountainscholar.org/bitstream/handle/11124/170990/King_mines_0052N_11245.pdf?sequence=1.]
  3. 3.0 3.1 3.2 3.3 3.4 [Petzet, A. (2010, December 14). Anadarko takes New Zealand Pegasus basin blocks. Oil & Gas Joournal. https://www.ogj.com/exploration-development/article/17274214/anadarko-takes-new-zealand-pegasus-basin-blocks.]
  4. 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 [Bland, K. J., Uruski, C. I., & Isaac, M. J. (2015). Pegasus Basin, eastern New Zealand: A stratigraphic record of subsidence and subduction, ancient and modern. New Zealand Journal of Geology and Geophysics, 58(4), 319-343. doi:10.1080/00288306.2015.1076862]
  5. 5.0 5.1 5.2 5.3 5.4 5.5 [authors, A., & Bland, K. J. (2015, November 9). Pegasus Basin, eastern New Zealand: A stratigraphic record of subsidence and subduction, ancient and modern. Taylor & Francis. https://www.tandfonline.com/doi/full/10.1080/00288306.2015.1076862.]
  6. 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 [Sæterdal, H. S. (2017, June 29). Basin Analysis of the Pegasus Basin, New Zealand - A Frontier Basin Uniquely Bounded by Two Opposing Subduction Systems. NTNU Open. https://ntnuopen.ntnu.no/ntnu-xmlui/handle/11250/2460906. ]
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 [King, S. E. (2017). The tectonic evolution of pegasus basin and the Hikurangi trench, offshore new zealand (Order No. 10267226). Available from ProQuest Dissertations & Theses Global. (1914884843). Retrieved from https://login.ezproxy.lib.ou.edu/login?url=https://www-proquest-com.ezproxy.lib.ou.edu/dissertations-theses/tectonic-evolution-pegasus-basin-hikurangi-trench/docview/1914884843/se-2?accountid=12964]
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 [K.F. Kroeger, A. Plaza-Faverola, P.M. Barnes, I.A. Pecher, Thermal evolution of the New Zealand Hikurangi subduction margin: Impact on natural gas generation and methane hydrate formation – A model study, Marine and Petroleum Geology,Volume 63, 2015, Pages 97-114,ISSN 02648172,https://doi.org/10.1016/j.marpetgeo.2015.01.020 (http://www.sciencedirect.com/science/article/pii/S026481721500046X)]
  9. 9.0 9.1 [NaturalGas.org. (2013, September 20). Retrieved December 13, 2020, from http://naturalgas.org/naturalgas/transport/#:~:text=There are three major types,wellhead to the processing plant.]