North Carnarvon Basin

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The North Carnarvon Basin is located in the North-West shelf of Australia and is the premier hydrocarbon basin of Australia. The North Carnarvon Basin is compromised of many sub-basins with the two most notable being the Barrow and Dampier. The basin is mostly offshore and covers roughly 500,000 Km2 and is the most extensively explored area in the country[1]. Exploration in the North Carnarvon Basin began in 1953, when oil was discovered in the first well drilled in the basin. following the discovery of oil there were a few follow-up discoveries of oil in 1964 and gas in 1971.

[2]Location of the North Carnarvon basin

History of the North Carnarvon Basin

[3]locations of the sub-basins found within the Carnarvon

The North Carnarvon Basin is located in the North-West shelf of Australia and is mainly on offshore basin with onshore exploration being minimal. The formation of this basin happened with a rift system that relates to the breakup of Australia and Great India. Exploration began in 1953 when the first well drilled in the basin discovered oil. after the discovery of oil in the basin, there were a few follow-up discoveries of oil at Barrow Island in 1964 and gas in the North Tryal Rocks in 1971. Following a steep decline in exploration activity in 2001 and 2002, exploration activity has increased in recent years.

Provincial Geology

Tectonics

The North Carnarvon Basin derives from an intracontinental basin that was present in the late Paleozoic time period, which we know today as the breakup of Australia and Great India. through time the basin evolution and stratigraphy has changed and According to the Australia Government [4] the stratigraphy can be related through six tectonic phases, which relate to the continental break up of the West Australian Margin through Middle–Late Jurassic and Early Cretaceous.

1.         Pre-rift (Silurian to Toarcian)

2.         Early syn-rift (Toarcian to earliest Callovian)

3.         Main syn-rift (earliest Callovian to Berriasian)

4.         Late syn-rift Barrow Delta (Berriasian to Valanginian)

5.         Post-breakup subsidence (Valanginian to mid-Santonian)

6.         Passive margin (mid-Santonian to present)

Depositional History

[5]Stratigraphy map of sub-basins found within the Carnarvon Basin

In the early Jurassic time period a series of extensions created the four main depocenters which include the Exmouth, Dampier, Barrow, and Beagle sub-basins that contain up to 15 km of sedimentary infill[6]. Through the Triassic to early Cretaceous time period a large amount of the deposition was siliciclastic deltaic to marine, and the Mid-Cretaceous to Cainozoic time periods deposition was largely dominated by slope and shelfal marls and carbonates. The sediments found within the sub-basins were deposited as a northwestward prograding wedge when the region was cooling and subsiding[6] . This deposit of sediments resulted in the burial of the underlying Mesozoic source rock and reservoir sequences in parts of the basin.

Petroleum Geology

Source Rocks

[7]Source Rock Characteristics of the Exmouth Sun-basin

With the North Carnarvon Basin being so large and having several sub-basins there are many source rocks that have been found over the years. While oil has been found in the basin, much of its source rocks are type lls-lll which are gas-prone source rocks. The gas-prone source rocks that can be found in the Barrow, Dampier and Exmouth sub-basins are believed to be a Triassic fluvial-deltaic sediment of the Mungaroo Formation, with a secondary contribution from the overlying Lower–Middle Jurassic marine and deltaic Murat Siltstone[8].

Pictured above are three sets of source rocks that can be found in the Exmouth Sub-basin and the characteristics of the source rocks found. The first set being the Mungaroo Formation or the orange triangles is made up of marine shale and coal bearing clay rock and has a total organic carbon (TOC) range from 0.5%-1.5% with a hydrocarbon index (HI) around 30-303 mg/g. The second set of middle-lower Jurassic or the green dots is made up of marine terrestrial transitional facies and carbonaceous mudstone. This set of source rocks has a TOC range of 1.0%-2.5% with a HI of 50-350 mg/g. The third set of lower cretaceous or purple squares is largely composed of marine shales which have a TOC range of 0.5%-3.0% and has a HI of 40-200 mg/g[7].

Seals and Traps

There are multiple reginal and intraformational seals throughout the North Carnarvon Basin and its sub-basins. The two most notable seals include the Muderong Shale and the Windalia Radiolarit shale both of which come from the late Cretaceous time period[10]. Many locations in the basin are shown to have a marine claystone reginal seal which has contributed to the high exploration success rates in the basin and its sub-basins. Within the basin itself lies the Barrow Group which is comprised of a large set of intraformational seals that is largely made up of marine claystone found in the Carnarvon basin itself. In recent years there has been some speculation about the seal integrity of the basin with late-stage faulting and inadequate migration pathways causing breaches in the seal that has led to many failed wells.

With the Carnarvon Basin being so large there are many types of traps, with the two most common being structural and stratigraphic. The Carnarvon basin is comprised of structural traps that have been observed to be Horsts, tilted fault blocks, drapes and fault roll-over anticlines and stratigraphic traps found within the basin include basin-floor and turbidite fans, unconformity pinch-outs and onlaps[11].

Risks and uncertainties

There are many risk associated with drilling in deep water basins, but a lot of the risk associated with the Carnarvon basin come from the sub-basins found within the Carnarvon. In the Dampier sub-basin risk in the deep water reservoirs include trap integrity and reservoir preservation, while the inboard risk of the sub-basin include migration distance, biodegrading of oil, and the absence of a seal[11]. Within the Barrow sub-basin the risk associated with exploration include reservoir occurrence and quality, trap geometry, and gas flushing. With the Exmouth Plateau being the least explored area of the deep water part of the basin the full extent of the petroleum systems and risks have yet to be determined. However, seismic and well data has shown that the seal in the northern part Exmouth Plateau have been breached, which has been related to hydrocarbon-bearing structures that are not filled to the spill point.

Future production

Future production in the North Carnarvon Basin is expected to increase as there are numerous oil and gas fields all through out its sub-basins. Many believe that the sub-basins of the Carnarvon may hold the key to future production, as there might be many undiscovered reserves within them. While there has been some risk associated with the basin future production looks bright especially in the Barrow sub-basin as it has great reservoir characteristics and has a faulted anticlines as a structural trap through out the sub-basin. The source rock found with in the sub-basin is estimated to produce 8 BBL of oil[12]. While many parts of the Carnarvon Basin have been explored and are know to produce oil and gas, many regions in the basin have been unexplored. The majority of the basin is under deep water but there lies a portion of the basin that can be accessed onshore and has received little to no exploration. Throughout the basin more and more discoveries are popping up in well know hydrocarbon-rich sub-basins and less explored sub-basin such as a near shore sub-basin that is known to be highly prospective, but it left relatively unexplored due to difficulties with seismic and drilling operations in the shallow water[12].

References

  1. Northern carnarvon basin. Department of Mines, Industry Regulation and Safety. (2015, November 4). Retrieved April 27, 2022, from https://www.dmp.wa.gov.au/Petroleum/Northern-Carnarvon-Basin-10990.aspx
  2. Regional Geology of the Northern Carnarvon Basin. Regional Geology of the Northern Carnarvon Basin | Geoscience Australia. (n.d.). Retrieved April 27, 2022, from https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/acreagerelease/northerncarnarvon
  3. Carnarvon Basin. Carnarvon Basin | Geoscience Australia. (n.d.). Retrieved April 27, 2022, from https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/offshore-northwest-australia/canarvon
  4. Regional Geology of the Northern Carnarvon Basin. Regional Geology of the Northern Carnarvon Basin | Geoscience Australia. (n.d.). Retrieved April 27, 2022, from https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/acreagerelease/northerncarnarvon
  5. Regional Geology of the Northern Carnarvon Basin. Regional Geology of the Northern Carnarvon Basin | Geoscience Australia. (n.d.). Retrieved April 27, 2022, from https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/acreagerelease/northerncarnarvon
  6. 6.0 6.1 Carnarvon Basin. Carnarvon Basin | Geoscience Australia. (n.d.). Retrieved April 27, 2022, from https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/offshore-northwest-australia/canarvon
  7. 7.0 7.1 Feng, Y.-wei, Ren, Y., Zhang, G.-cheng, & Qu, H.-jun. (2021, January 16). Petroleum geology and exploration direction of Gas Province in deepwater area of North Carnarvon Basin, Australia. China Geology. Retrieved April 27, 2022, from https://www.sciencedirect.com/science/article/pii/S2096519220302068#:~:text=The%20results%20show%20that%20there%20are%20three%20sets,Barrow%20Group%20and%20marine%20shale%20in%20Muderong%20Formation.
  8. Regional Geology of the Northern Carnarvon Basin. Regional Geology of the Northern Carnarvon Basin | Geoscience Australia. (n.d.). Retrieved May 11, 2022, from https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/acreagerelease/northerncarnarvon
  9. Regional Geology of the Northern Carnarvon Basin. Regional Geology of the Northern Carnarvon Basin | Geoscience Australia. (n.d.). Retrieved April 27, 2022, from https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/acreagerelease/northerncarnarvon
  10. https://core.ac.uk/download/pdf/354409298.pdf
  11. 11.0 11.1 Feng, Y.-wei, Ren, Y., Zhang, G.-cheng, & Qu, H.-jun. (2021, January 16). Petroleum geology and exploration direction of Gas Province in deepwater area of North Carnarvon Basin, Australia. China Geology. Retrieved April 27, 2022, from https://www.sciencedirect.com/science/article/pii/S2096519220302068#:~:text=The%20results%20show%20that%20there%20are%20three%20sets,Barrow%20Group%20and%20marine%20shale%20in%20Muderong%20Formation.
  12. 12.0 12.1 Northern carnarvon basin. Department of Mines, Industry Regulation and Safety. (2015, November 4). Retrieved April 27, 2022, from https://www.dmp.wa.gov.au/Petroleum/Northern-Carnarvon-Basin-10990.aspx