Roebuck Basin

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Figure #1: Map showing the location of Roebuck Basin in relation to Australia @Geoscience Australia

The Roebuck Basin is one of the least explored offshore oil and natural gas basin in the Weststralian Super-Basin. It was previously known as a part of the Offshore Canning Basin prior to 1994, when it as renamed and acknowledged as its own separate basin in the region. Located on the continental shelf offshore waters of North West Australia (Figure 1) between the Northern Carnarvon Basin south of it, the Browse Basin in the north of the Roebuck basin, and the Canning Basin to the East. This basin spans approximately 93,000 square kilometers and has a currently unknown depth. This basin is quite old geologically as it started its formation during the Permian period, and its reservoirs continued growing until the Cretaceous period. It was formally discovered in 1980 after the Phoenix 1 well first produced oil from its rock strata. Since then it has produced much oil and is still only 30% mapped. However, there has been great successful in unlocking the basin as of 2019 with several wells coming up with non-commercial amounts of oil and gas. but there has been very promising outcomes in the last three wells drilled in the region, because even though they made non-commercial amounts the combination of all the findings shows that there are hydrocarbons in the areas. With more data about the region and better technologies it will become extremely easy to unlock the hydrocarbons in the basin, possibly turning the Roebuck basin into the "Black Gold" mine of Australia. This has led to the government and many investment groups attempting to lure companies in creating wells in the region. Resulting in a great increase in overall production in crude oil and gas in Australia.

Figure #2: Cross section of the Roebuck Basin

History

Geologic History

Figure #3: the age of the roebuck basin in relation to the other basins in the region

The Roebuck Basin initially started to form in the late carboniferous period. this was spurred on by the northwest to southeast directional extension of the Australian plate due to divergent plat boundaries separating and adding new material to it. this increased the are of the Westralian Super basin and continued on into the early Permian period of Australia's geologic history. In the late Permian the whole are began to uplift as the mantle began to heat up and push the crust higher up in this region [1]. At the same time regional faulting began to rise in the region which was associated with increased volcanism in the crust and surrounding oceans of the northwestern landmass of the continent (Figure 2). This continued on until the Triassic period when the are began to suffer from thermal sagging causing the curst to then sink lower in the mantel creating a "bowl" shaped basin. This was followed by the deposition of marine claystone which where then transgressed and overlaid by sediment deposits from the surrounding deltas on the continent[2]. This also began the first deposits of organic material for future petroleum production though the sediments that were being deposited containing the bodies of countless algae, sea plants, plankton, and other marine microorganisms. This was supplemented by a series of transgressional events in the early Jurassic period. In the early Jurassic the are once again was subject to extension ultimately leading to a Callovian breakup in the Argo Abyssal plane that existed adjacent to the basin[2]. Middle and late Jurassic periods were known for depositing fluvial and deltaic sediments from the mainland environments (Figure 3). these sediments carried decayed plant and animal material from the interior and allowed them to build up in the basin as they were deposited covered. This began to stack up layer upon layer[3]. Sediments present in the late Jurassic and cretaceous period grade into a thick section of Cainozoic carbonates. However due to the limited availability of data the dating of the major events in the Basin's history has been very difficult . Though increased surveying and study if the region has increased to correlate with the increased interest in the region due to its lucrative returns in the recent two decades.

Discovery

The basin was discovered in the 1980s after it was separated from the surrounding established basins in the west Australian Super Basin. this was due to new seismic and stratigraphic data showing that it exists in a sperate region of the super basin. Supported by faults and shifts in the layering of the rocks. It was further renamed to the Roebuck from the Canning Basin in 1994.

Exploration

The Roebuck basin is the least understood and least explored of the 4 major western Australian basins. it has been sparsely explored beginning in the 1960s. Initially prospectivity was rated highly as the seismic and gravity data from the time was interpreted as good for hydrocarbon creation and petroleum production. the first wells were drilled in the 70s and 80s but none of them came back with anything of substance[6]. The Pheonix-1 well was the first well ever drilled in the region that actually came back with promising returns, with minor gas being produced from a Triassic aged rock. This was followed up by the Perindi-1 well which showed minor oil from a Permian sandstone. Even after the promising returns that were created by these two well exploration in the are all but halted in 1984 following negative reviews of the area. However drilling and greater exploration started u again in 1993 after the Japanese National Oil Company acquired 11,000 square kilometers of the basin to get seismic data from. They along with the WA Department of Mines and the AGSO gridded the deeper portion of the basin, in total 80,000 kilometers across 14 lines in the basin. Also in the early 90s several other companies were awarded exploration permits to seek out petroleum in a possible Triassic play. While drilling there the first sub economic oil discover was discovered in 1994 which generate new interest in the region[11]. By 1999 the acreage of the area got new 2d seismic data at the behest of the Australian government. In the 2000s many regions were closed off to exploration due to the presence of the Mermaid, Lerke, and Imperieuse reef areas which are protected by the Australian government. Following poor results and the presence of the reefs exploration once again stopped though several well have been drilled over the past two decades with the last one being drilling in 2019, most of these have come up dry or with non-commercial amount of oil and gas. However, the creation of the Phoenix South, Roc, and Dorado-1 wells in the late 2010's have proven that there are extractable amounts of commercial oil and gas in the region. This has made the bounties and plays of the area far more valuable to future investors; especially in light of the fact that Australia's crude oil production has drastically fallen in the 21st century and only started to recover with basins in the Westralian Super basin plays, including the Roebuck.

Development

The Roebuck basin is practically not developed at all, this is the case because no oil companies or the government have set up any permanent commercial drilling operations. Though in the pass there have been several efforts to begin development though the drilling of appraisal wells and the conduction of a wide variety of geological tests to understand all of the characteristics of the basin. There has been a huge rush to get more wells drilled in the area after the the hug findings of the Dorad-1 well, the Phoenix South well and the Roc wells. That is because these wells have multiplied the potential hydrocarbon production in the region by more than 20 times over, as seen in the 3 major jumps in total oil and gas produced in the region. This has led to the construction and development of processing facilities and pipeline, though not too many of these are in existence. This means that most of the oil and gas producing in the rea are shipped via ship or rail to the facilities to the north and/or south of the roebuck in the region of the Super basin that have been far more successful in regards to development. But with the current outlook pertaining to petroleum production its only a matter of time before the area has its own dedicated pipeline and refineries in order to accommodate the drastically increase the amount of crude oil be pulled from the Earth.

Petroleum Properties

Source Rock

Source rocks are extremely important to the successful formation of hydrocarbons and the creation of productive basins. that is because these are the rocks that hold extremely dense deposits of organic material that are eventually heated and compressed to the point of petroleum production. Along wit this the source rock also has to be porous and permeable enough to allow future wells to successfully move the hydrocarbons out of the source our and to the surface. Sadly the Roebuck basin does not have any official proven petroleum source rocks[5]. However, geological simulations, sequence stratigraphy correlation, seismic mapping, geochemical analysis and sedimentary modelling have been used in order to produce 10 potential source rocks in the basin. Of the 10, 2 of them where deposited in the Triassic period due to rapid thermal sag, another 2 were deposited in the Jurassic period due to gentle thermal sag and infill, and the last 3 were deposited in the cretaceous period during rift and post-rift sedimentation along with passive margin conditions. However the Triassic aged sandstone deposit are currently seen as the potential source rock that has the highest chance of oil petroleum production. this is because there have been 10 succefully appraisal wells over the past 3 decades that have hit either oil and/or gas and oil 10 of these have been struck in the Triassic age range, specifically the Keraudren. this has resulted in the development of three successful plays since 2014 all of them Triassic aged.

Reservoir

Several reservoir units were identified in a study conducted by the Australian National center for Petroleum Geology and Geophysics in 1999. they concluded that these seven aged in the ranges of early Triassic to Tertiary. However there was a pervious study was conducted in 1991 which identified a potential Early Permian reservoir, which also doubled as a seal. All of the potential reservoirs are clastic in nature and more specifically fluvial sandstones. These sandstones seem to have great porosity, but the true nature of the reservoirs are unknown over most of the basin due to a lack of well data.

Migration

Migration is characterized as the movement of liquid and gaseous petroleum products from the source and reservoir rocks into new rock strata do to the permeability of both interacting rocks. Migration is both a blessing and a curser in the industry of petroleum extraction for good migration indicates high permeability which makes it easier to ump the petroleum out of the rock but on the other hand leads to a great chance of the petroleum "leaking" out of your well. In the Roebuck basin their isn't much migration at all with the main form being one pathway which leads petroleum from mature Triassic sources into Jurassic reservoirs. and this migration is further limited the further you move away from the margins of the faults in the basin. There is also minute movement of oil into the early Cretaceous reservoir unties along the basin margins.

Trap

Figure #4: the formation and shape of a standard pinch out trap

Petroleum traps are geological structures which affect the reservoirs rock and the capstone of a petroleum system by preventing the migration and loss of hydrocarbons into the water table, to the surface, or most likely into rocks outside of the play. This allows for the accumulation of hydrocarbons in the reservoir. there are two types of traps the exist: stratigraphic and structural; the structural seals form by changes in the structure of the subsurface due to tectonic, gravitational, and/or compactional processes. The stratigraphic traps are ones generated by features in the rock layer geometry that seal away hydrocarbons. When looking at the Roebuck basin structural movement associated with rifting in the region resulting in the creation of structural traps throughout the Mesozoic era[2]. This trap formation continued on into the more recent times as the presence of Miocene collisions reactivated the formation of new traps along the flanks and margins of the basin. The main type of traps present here are pinch-out traps, pinch-out traps are characterized by the termination, thinning, and tapering out of reservoir rocks against a nonporous rock. This creates a subsurface are in which hydrocarbons are able to pool and stabilize (Figure 4).There are also a plentitude of faults in the region caused by the tectonic convergence and sagging during the Mesozoic period, these have proven to be great area of hydrocarbon accumulation. Many of the successful appraisal wells have tapped in or around these faults.

Seal

Seals are relatively impermeable sold rock that form a barrier between the petroleum rich source and reservoir rocks from the surround sub strata. these prevent the migration do petroleum away from the source rock and helps maintain the purity and volume of the petroleum basin, making them extremely important. Several potential seals were developed at various stages of this basins development and maturation[2]. A thick early Cretaceous marine claystone forms the regional seal over the entire North west Shelf of Australia. It acts as a seals specifically over the middle and late Jurassic sandstones in the western part of the basin, However, this basin becomes increasingly sandy (seen though the Broome sandstone) in structure as you move east along the basin's area. Though in the depuch formation of the basin is intersected by a series of middle Jurassic aged claystone. There is also a thick redbed claystone present in the Broome Formation of the basin which separate the late Triassic and early Jurassic layers, this was found by the initial probing of late Jurassic beds by the Phoenix-1 well.

Risks

The major risk that is present the the roebuck basin is the unpredictable nature of the basin due to the lack of historical data in the region, this could mean that the great returns and production we see is only accounted for in a small region of the basin and would run out faster than expectable. This would be horrible for the industry because the money spent ton producing wells, pipelines, ports, and refineries would be a waste as there would be now petroleum products for them to deal with, making their existence unnecessary.

Petroleum and Facility Engineering

Figure #5: the drilling of the Roebuck basin

Given the size of the basin and howl long it has been known about the Roebuck is extremely underdeveloped. This is proven by the fact that over the past 5 decades there have only been 21 wells drilled into it at all starting with the Bedout 1 well in 1971 and ending in more recently the Dorado 3 in 2019. Of these wells about half of these are dry wells with no hope in producing any type of petroleum. And to add insult to injury even when looking at the wells that did have oil and/or gas return none of them were permanent well and in fact were just exploratory and appraisal wells which were funded by a wide variety of oil companies both form Australia and abroad. These well drilling have been supplemented by countless tests and simulations posed to study the geochemical seismic activity, and subsurface structure of the basin. All of this data is being combined with historical scientific accounts in order to produce a real idea about the potential productivity and viability of the basin as a oil and gas producing region. However their are promising trends in regard to the petroleum geology as the Dorado 3 driving did find oil and gas condensate in the Bedout sub basin (Figure 5). This proves that there is an active hydrocarbon province there and that future production is highly probable. It's so probable that there have been 7 more contracts given out all of which are set to expire by the year 2025. of the 7 they have all committed to produce 2 wells a piece[8]. After the success of the dorado and restimulated crude oil and gas production here has been great interest tin the construction of pipelines and refineries in the region resulting in at least one on shore facilities being contruscted.

Future Petroleum Potential

Figure #6: The oil and gas production of the basin over the years.

Commercial production hasn't even begun to start in the offshore regions of the Roebuck Basin, however they have begun commercial production of petroleum in the onshore regions of the northern Canning Basin which is adjacent to the Roebuck basin and is a member of the same Super-Basin. Drilling efforts to date have made it very clear that digging source rock and potential seals will be a difficult endeavor. These aspects indicate that the area might have low petroleum potential, thus making all assessments of hydrocarbon productivity very speculative. So the most essential aspect of future production would be the successful location of source rocks in the basin. To this end wireline logs, seismic mapping, geochemical analysis, and stratigraphic correlation is still currently being used to locate potential source rocks resulting in the location of the beforementioned 10 potential strata. it is because of this lack of flattering data in the basin that so few companies have made significant efforts in developing the plays and why so little funding has been put into the region. Thus spurred on by the fact that the nearby basins are proven commercial production, meaning that companies are more likely to invest in those regions before they migrate over to the Roebuck. But in recent years there have been a few oil booms in the areas after wells came up with enough oil and gas to attract new investors and interest from the government (Figure 6). Culminating in the acreage bidding procedures of 2019-2020 where there were 7 plots of the basin up for grabs in the region, all of which are due to the high probability of monetary return from the oil and gas that would be produced (Figure 7). A great deal more exploration, appraisal, and study must be conducted in order to truly comprehend the viability of the basin in regards to commercial petroleum production. However, as the conventional plays of the region and the world at large are drained and it becomes harder and harder for groups to find petroleum producing regions the area will definitely become a place of great intrigue. Though this might all be thwarted by the fact that there are several protected modern day reef areas and marine parks in the area that prevent the drilling and development. Overall, the Roebuck Basin has taken a while to unlock, but it seems the key is finally beginning to turn on this immature basin[8].

Figure #7; How, where, and what leases are distributed in the Roebuck Basin
Figure #8: Ideal Drilling system for the roebuck using the dorado wells

References

  1. Bernecker, T. (2019). Australia: Update on current and emerging oil & gas opportunities
  2. Hocking, R. M., Mory, A. J., & Williams, I. R. (1994, August). An atlas of Neoproterozoic and Phanerozoic basins of Western Australia. In The sedimentary basins of western australia: Proceedings of petroleum exploration society of australia symposium, perth (Vol. 1994, pp. 21-43)
  3. Kelemen, S. (2016). 2015 PESA industry exploration review. The APPEA Journal, 56(1), 505-514
  4. Molyneux, S., Goodall, J., McGee, R., Mills, G., & Hartung-Kagi, B. (2016). Observations on the Lower Triassic petroleum prospectivity of the offshore Carnarvon and Roebuck basins. The APPEA Journal, 56(1), 173-202
  5. Nguyen, D., Rollet, N., Grosjean, E., Edwards, D. S., Abbott, S., Orlov, C., ... & Buckler, T. (2019). The Roebuck Basin, Beagle and Barcoo Sub-basin well folio. The APPEA Journal, 59(2), 920-927
  6. O’Brien, G. W., Cowley, R., Lawrence, G., Williams, A. K., Webster, M., Tingate, P., & Burns, S. (2003). Migration, leakage and seepage characteristics of the offshore Canning Basin and Northern Carnarvon Basin: Implications for hydrocarbon prospectivity. The APPEA Journal, 43(2), 149-166
  7. Pedley, T., Collins, I., & Pandya, J. (2015). Western Australia: North Carnarvon-Roebuck Basins
  8. Quinn, M., & Hulbert, F. (2019). Australian Exploration Review 2018 reveals encouraging signs of revival
  9. Smith, S. A. (1999). The phanerozoic basin-fill history of the Roebuck Basin/author, Stuart A. Smith (Doctoral dissertation)
  10. Smith, S. A., Tingate, P. R., Griffths, C. M., & Hull, J. N. F. (1999). The structural development and petroleum potential of the Roebuck Basin. The APPEA Journal, 39(1), 364-385
  11. Xu, M. (2002, March). Mesozoic Sequence Stratigraphy and Implication for Hydrocarbon Exploration of North Eastern Roebuck Basin, North West Shelf, Australia. In AAPG Annual Meeting 2002

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