Canterbury Basin

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A topographical map of New Zealand's Petroleum Basins. Canterbury Basin is pictured on the east side of the South Island, above Great South Basin.[1]

The Canterbury Basin is located along the east coast of New Zealand's South Island, with most of the basin lying offshore; the remaining portion of the basin is situated below the inland Canterbury Plains[1] The basin takes up an area of approximately 50,000 km^2.[1] The basin is neighbored by the Great South Basin, to the south, and the Pegasus Basin, to the north. The Clipper sub-basin is contained within the Canterbury Basin and contains one of few discovery drill sites, Clipper-1.[2]

The Canterbury Basin is a frontier basin with no developed petroleum operations and a small amount of exploration. Only nine sites have been drilled in the prospective basin for exploration purposes, with only five of them located offshore.[1]

Provincial Geology

Tectonic and Depositional History

The depositional and tectonic history of the Canterbury Basin is closely related to New Zealand's continental geologic history. New Zealand and the Canterbury region has a long history of faulting, volcanism, orogeny, and other tectonic activity.

Tectonic Timeline

Cretaceous

  1. Formation of the Canterbury Basin began during the Cretaceous Period with the breakup of Gondwana
  2. Mid-Cretaceous Volcanoes appear in Canterbury
  3. Clipper sub-basin bounded to the east by a growth fault

Paleogene

  1. More volcanism occurred, Endeavor Volcanics site, and basalt intrusions appeared because this activity
  2. Some minor tectonism occurred in the form of faulting and erosion in Northwest Canterbury in the Late Eocene
  3. The Alpine Fault plate boundary originated during the late Oligocene and was accompanied by extensional or transtensional tectonism
  4. Tectonism leading to inception of the Alpine Fault also causes formation of Northeast trending ridge along Canterbury Bight and subsidence on both sides of the structure

Neogene

  1. Increased tectonic activity in Canterbury due to rising of the Southern Alps
  2. Increasing amount of folding and faulting activity in West Canterbury during Pliocene
    Chronostratigraphic chart of the Canterbury Basin, showing age and place of deposition for each rock unit.[3]
  3. Pliocene era East Canterbury had little to no tectonic activity, as seen by undisturbed beds
  4. Extreme deformation in North Canterbury, with Hikurangi Trough section of plate boundary migrating south
  5. In Canterbury Bight, little to no tectonism with the basement rock structures remaining undisturbed
  6. In West Canterbury some of the faults became active again and reversed movement direction
  7. Some volcanic activity occurred during this period but nothing major or formative in the region[1]

Depositional Timeline

Cretaceous

  1. Sediments of the Torlesse Supergroup deposited, making up the basement of newly formed basin
  2. Clipper Formation, made up of coal measures and paralic sediments, deposited over Canterbury Bight; these are the largest group of potential reservoir and source rocks at maturity, made up of sand and mudstone layers
  3. Sediments now part of mudstone Katiki Formation deposited over Clipper Formation
  4. Transgression begins in Canterbury region, lasting until Oligocene

Paleogene

  1. Paleocene deposition of sediments forming carbonaceous mudstones and black shales, now making up another potential source rock
  2. Mudstones and micrites forming sediment deposited during Eocene and Early Oligocene; transgression still occurring during this time
  3. New Zealand continent submerged during the Oligocene
  4. Sediments forming limestones deposited and development of regional unconformity during Oligocene

Neogene

  1. Clastic sedimentation increased in region as a result of erosion of pre-Neogene cover sediments and older basement rock; sediments from this erosion formed a sediment wedge, now forming the continental shelf in the Canterbury region
  2. Kowai Formation sediments, now made of conglomerate sheets, deposited in Pliocene West Canterbury
  3. Siltstone deposition in tectonically undisturbed Pliocene East Canterbury[1]
Geologic time scale: Notable Canterbury Basin geologic events began 145 million years ago, during the Cretaceous Period.[2]

Petroleum Geology

Source Rock

The Canterbury Basin has one main form of hydrocarbon source rock from the Cretaceous period, and another potential source rock from the Paleocene. The main source rock in the Canterbury basin are made up of Cretaceous coal measures. Coal Measures make up a large division of Upper-Carboniferous rocks, and as the name describes, generally produce large amounts of coal. Coal measures are made up of repeating layers of marine and nonmarine strata beds, with the marine layers made of black carbonaceous shales and fossiliferous limestones, and the nonmarine layers composed of nonmarine shales and mudstones. The nonmarine strata makes up the majority of Coal Measures, however the marine strata contains the most productive coal deposits.[4] In the differing layers of Coal Measures there marine fossils which are often shelled creatures in the marine layers, and freshwater plant and animal fossils in nonmarine strata.

The Canterbury Basin's main source rock, Mid-Cretaceous coal measures, are mainly made up of coal and coaly mudstones. These primary coal measures were deposited coastally, along with facies of sandstone and siltstones, some of which contain a small portion of coal measure source rocks. The primary hydrocarbon source in the Canterbury Basin, the mid-Cretaceous source rocks have reached thermal maturity, and can expel hydrocarbons. The rocks are believed to have reached their highest temperatures during the late-Paleocene due to climate warming, and modelled igneous intrusions in particular areas, such by the Clipper sub-basin. The climate warmed further during the Eocene, rising the basin and source rock temperature further. The temperature is modelled to have fallen by the late Eocene through Early Oligocene as a result of cooling climate, water temperature drop, and erosion. Because of models showing these temperature changes and a predicted temperature of >1500 C for source rock below 85 million years old, mid-creataceous source rock should be in the gas window. Late cretaceous source rocks in Western Canterbury are predicted to be in the early oil window mostly, with some in the oil window. [5]

Along with Mid-cretaceous coal measures, the Paleocene Tartan Formation is being considered a potential source rock. The source rock of this formation went through the same temperature changes as the majority of the cretaceous source rock, however it did not have as long to mature, making the majority of this formation unable to generate hydrocarbons. There may be small areas of mature Paleocene source rock below the Neogene foreset progradation, or in areas heated well by igneous intrusions.[5]

Canterbury Petroleum Systems chart depicting the rock components of the basin system, tectonic events, and when they occurred.[6]

Reservoir

Reservoir rocks are an important part of hydrocarbon generation and storage, as these rocks hold organic material that eventually create hydrocarbons. Stratigraphic records of the Canterbury Basin show potential reservoir rocks throughout the region. At the Galleon-1 drill site in the Canterbury Basin, gas condensate was discovered in Cretaceous non-marine and shallow marine sandstone reservoir rock. This reservoir rock is aged from Late Cretaceous to the Miocene and was deposited in fluvial, paralic, shelf, and turbidite depositional environments.[7]

Trap

Tectonism before and during the Cretaceous Period allowed for the Canterbury Basin to produce traps to hold reservoir rocks. Most of the reservoir rocks in the basin were deposited during the Late Cretaceous and Micocene, in Cretaceous rifts and rift basins. These rifts allowed for fluvial systems to flourish, and for their sediments to be deposited in the syn-rift basins; these basins filled with over three kilometers of nonmarine rock and sediment in the clipper Sub-basin. The traps of the Canterbury Basin are made through normal faulting or basement drape. [7]

Seal

Modelling in the Canterbury Basin suggests Shelfal Mudstone seals made up of around 70% Shale and 30% Silt, sit atop reservoir rock in the Canterbury Basin. Most seals over the basin have been in place since the Late Paleocene, around 84 million years ago, and are still present today. In West Canterbury, seals were not in place over late Cretaceous reservoirs until the Late Miocene. Because seals were not in place until the Late Miocene, most potential hydrocarbon content was expelled during the Paleocene and Eocene. The expelled hydrocarbons escaped through vertical migration, and were later trapped by Neogene foresets; these hydrocarbons likely accumulated in late Cretaceous reservoirs in West Canterbury. Modern day reservoirs in West Canterbury are predicted to have little hydrocarbon content though.[5]

Migration

The hydrocarbons of the Canterbury Basin have not migrated much aside from vertical migration. Hydrocarbons migrate in either a liquid or gas state as shown in Figure 4, with the green arrows representing liquid migration and the red arrows representing gas hydrocarbon migration. Some migration took place as there was no proper seal in place, as shown by Late Cretaceous reservoirs in West Canterbury, which were uncovered by a seal until the Late Miocene, allowing much of the hydrocarbon content to escape.[5] Rising temperatures also played a role in vertical migration, with hydrocarbons heating up and moving vertically in a vapor state. The migration of hydrocarbons within in the Canterbury Basin could be characterized as short distance, because most of the hydrocarbons moved less than a kilometer.

Figure 4: Model showing oil and gas migration from Cretaceous source rock near Clipper-1 Drill.[3]

Geologic Risks and Uncertainties

While there have been proven stores of oil and natural gas in the Canterbury, there is large uncertainty about how much the basin is holding. There has never been any commercial production in the basin, and few exploratory drilling sites. There are also concerns about the maturity of the source rock in Western Canterbury.

Petroleum and Facility Engineering

There are currently no active petroleum operations in the Canterbury Basin. Beach Energy, an Australian company, was planning exploration of the Canterbury basin during the mid 2010s to into the end of last year. The company ultimately ran out of time to drill the exploration wells and their permits were relinquished.[8]

Future Assessment

There will likely be no production in the Canterbury Basin for the next decade, as Beach Energy's permits to explore expired and there appears to be no other companies who wish to explore the region at the moment. As petroleum prices rise, there is a good chance oil companies will seek to explore again and find the true commercial viability of the basin, but it does not appear this will happen in the close future.

References:

  1. 1.0 1.1 1.2 1.3 1.4 Samuel, Sanjay. (2016). Depositional History of Paleocene Sediments in the Offshore Canterbury Basin, New Zealand.
  2. S. D. Killops, R. A. Cook, R. Sykes & J. P. Boudou (1997) Petroleum potential and oil‐source correlation in the Great South and Canterbury Basins, New Zealand Journal of Geology and Geophysics, 40:4, 405-423, DOI: 10.1080/00288306.1997.9514773
  3. Samuel, Sanjay. (2016). Depositional History of Paleocene Sediments in the Offshore Canterbury Basin, New Zealand.
  4. Britannica, T. Editors of Encyclopaedia (2008, August 8). Coal Measures. Encyclopedia Britannica. https://www.britannica.com/science/Coal-Measures
  5. 5.0 5.1 5.2 5.3 Sahoo, Tusar & Kroeger, Karsten & Thrasher, Glenn & Munday, Stuart & Mingard, Hugh & Cozens, Nick & Hill, Matthew. (2015). Facies Distribution and Impact on Petroleum Migration in the Canterbury Basin, New Zealand. 520-520. 10.1190/ice2015-2148698.
  6. https://www.nzpam.govt.nz/
  7. 7.0 7.1 New Zealand's Petroleum Basins. (2014). Retrieved May 12, 2021, from https://www.nzpam.govt.nz/nz-industry/nz-petroleum/resource-potential/
  8. Wilkonson, R. (2021). Beach Energy quits permits offshore South Island NZ. Retrieved April 22, 2021, from https://www.ogj.com/general-interest/article/14196105/beach-energy-quits-permits-offshore-south-island-nz

External Links

https://www.britannica.com/science/Coal-Measures

https://www.nzpam.govt.nz/