Mentelle basin

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The Mentelle Basin spans a small south west corner of Western Australia, near Cape Naturaliste and it is a deep to extremely deep water frontier basin, this class of basin is considered within the range of 500 to 4000 meters depth.[1] Though, as far as exploration for petroleum goes, this basin is rather untapped. This basin is located in an area of a plethora of geological features, it neighbors the Perth Basin (also known as Vlaming Sub-basin) only separated by a basement block, the Yallingup Shelf, in the east. As well as bordered by the Naturaliste Plateau in the west. Due to differences in the water depth at this basin, it is commonly separated into the West and East Mentelle basin (WMB & EMB).[1]

Figure 1: A map view of the Mentelle Basin.[1]

Though, seemingly more speculating has been done rather then acting about this basin's potential, that doesn't mean nothing has been done at all. Geological assessment has been carried out in this region and has lead to speculation of thick sedimentary units that are near in age to those in the Perth basin, hinting this basin to be worth checking for hydrocarbon capacity. Seismic data was collected about this basin in 2004 by Geoscience Australia in order for petroleum potential assessment to take place, with more to follow up in 2008-09. Studies were then done using this data collected and what was found was promising. From what was found of these studies by Geoscience Australia, the earliest there seems to be impactful geologic activity was rifting occurring in the early Permian period. Followed by thermal subsidence in the Triassic to early Jurassic. Successions aged near the Permo-Triassic have been interpreted up to 7 kilometers in depth in the eastern Mentelle. It was also during the breakup of Gondwana that the basin was near a triple junction of Australia, Antarctica, and India and sedimentary structures nearing 7-9 kilometers in thickness accumulated in "half-graben depocentres". This breakup occurred near the early Cretaceous, a time also known for extensive volcanism near this area.[1]

Assessment of this basin began in the first place largely due to its proximity to the Perth basin and its own stratigraphy. Given this, it was easy to suggest that this basin has plenty of its own source rocks in the form of coals and carbonaceous shales. On top of this, petroleum system modelling would indicate the source rocks present would be thermally mature and have generated hydrocarbons of its own. Given this, the Mentelle offers plenty of potential in wide ranges of plays, with plenty of different anticlines and fault blocks, as well as fold and unconformity plays.[1]

Provincial Geology

Tectonic History

Figure 2: A cross section through the middle of the basin.[1]

It is thought that the basin is a product of the movement of the Gondwana supercontinent as a part of a fluvial pathway and the breakup of India during the Late-Jurassic to Early-Cretaceous.[2] Though this does not seem to be all that is know of this basin's history, as new data has suggested that the initial rifting of this basin actually began in the Early Permian due to the Perth Basin extensional system.[3] This same dataset suggests this Permian subsidence was then followed by Late Permian to Early Jurassic thermal subsidence. This led to half graben with Permo-Triassic fills in the EMB and in the WMB, a Mid-Jurassic to Early Cretaceous extension in the Perth Basin and on the southern margin. As well as the northern part of the WMB having its major structures trending north to south and its southern structures trending northeast to southwest in line with the orientation of its extensional basins on the southern margin.[4]

While what may have gotten the basin to where it is today may be understood, there is still much to learn and much to collect from this basin and the region surrounding at large. By many standards of petroleum geology, this basin is under-explored as is, again, its surroundings. It is indeed this Perth Basin that those research the Mentelle must go off of for the most part when interpreting much about the basin. The evolution this basin has experienced tectonically takes place mostly in the Late Paleozoic and Mid Cretaceous and formed as it is known during the aforementioned Gondwana breakup. Though, this is not to say that older sediments related to earlier evolutionary phases of the western Australian margin are not present within the basin.[5]

Development History

Figure 3: The deposition of each of the layers of the Mentelle Basin and their approximate geologic age.[1]

The basin is divided into 12 main supersequences and were laid in such order:[1]

Mentelle 1A- Presumed to contain Permian coal similar to Perth Basin and correspond to Early Permian Deposition.

Mentelle 1B- Correlated to a phase of thermal subsidence, possibly Upper Permian- Lower Triassic to Triassic.

Mentelle 1C- Correlated to Lower Triassic, extremally deformed, faulted, and eroded during the second phase of rifting during the Middle Triassic.

Mentelle 2- Deposited during the Middle Jurassic to Early Cretaceous major extensional phase in the basin (mid-Bajocian - Kimmeridgian part of the Yarragadee Formation in the Vlaming Sub-basin).

Mentelle 3- Deposited during the Middle Jurassic to Early Cretaceous major extensional phase in the basin [(Tithonian - Berriasian) is correlated to the upper Yarragadee Formation and lower part of the Parmelia Group)].

Mentelle 4- Deposited during the Middle Jurassic to Early Cretaceous major extensional phase in the basin [(Berriasian) is correlated to the upper part of the Parmelia Group in the Vlaming Sub-basin].

Mentelle 5A- Thick, basalt igneous unit throughout The Western Mentelle.

Mentelle 5B- Sedimentary package, laid on top of and sometimes intermingling with the basalt unit, leaks into eastern portion of basin. Corresponds to early Cretaceous.

Mentelle 6- Deposited during post-breakup thermal subsidence, correlated to interbedded marine sandstones and mudstones of the Leederville Formation in the Vlaming Sub-basin. Age: Hauterivian to Albian.

Mentelle 7A- Same as Mentelle 6.

Mentelle 7B- Still Hauterivian to Albian in age, though associated closer with the Albian black marine mudstones.

Mentelle 8- Albian to Cenomanian in age, DSDP 258 is able to effectively constrain the geology and age of this supersequence. Linked to an establishment of open marine environment and development of a deep intra-slope basin system in the WMB.

Mentelle 9- Aged Cenomanian to Campanian, corresponds to deep marine, carbonate sedimentation. Near Molecap Greensand, Gingin Chalk and Poison Hill Greensand in the southern Perth Basin in age.

Mentelle 10- Cenozoic Strata, difficult to differentiate between last three supersequences, though unconformities can help show that there are roughly three unique Cenozoic deposits, this is a middle Paleocene-lower Eocene chalk.

Mentelle 11- Cenozoic Strata, this is the upper Eocene ooze.

Mentelle 12- Cenozoic Strata, this is the upper Miocene-Holocene ooze.

Petroleum Geology

Reservoir Rocks

Most of the assessment of the Mentelle Basin and its petroleum prospectivity was initially correlated to the stratigraphy of the southern Perth Basin. This indicated that the bulk of the source rocks within the Mentelle would be coals and carbonaceous shales, with reservoirs in the fluvial strata. Petroleum systems modelling has indicated that these source rocks are thermally mature and possess their own hydrocarbons. Within the WMB, the potential for reservoirs lie within the fluvial sandstones and within the west's sub-basalt anticlines are Lower Cretaceous fluvial reservoirs sealed by thick syn-breakup volcanics. The marine sandstone and turbidite units are good reservoir intervals in the western basin as well. In the east of the basin, the possible reservoirs lie in the pre-breakup succession and Lower Permian coals that are likely mature source rocks.[1]

Traps and Seals

The Mentelle Basin possesses many different kind of traps and play types within its two sides: The Eastern Mentelle Basin and The Western Mentelle Basin. In The Western Mentelle Basin trap types include dome structures and forced folds which originate from Cretaceous and Cenozoic igneous events, inversion anticlines that formed during the Cenozoic margin tilting and subsidence, as well as drape structures. Along the boundary that separates the two halves of the basin are combined structural-stratigraphic traps, this hinge zone is where Lower Cretaceous sandstones onlap on pre-rift strata. This zone has a favorable location and timing for charge from syn-rift source rocks in the western half of the basin, making it an excellent candidate for future exploration plays.[1]

As for The Eastern Mentelle Basin, it also includes three main trap types: faulted anticlines, high-side fault blocks, and sub-unconformity traps. Within both the east and west of the basin, there is potential for stratigraphic traps, these are made up by turbidite sandstones sealed and encased by slope mudstones.[1]

Source Rocks

Much of what is known about the Mentelle Basin is merely an educated guess based on what is known by the neighboring, more well known, Perth Basin, this seems to still be the case for its source rock. Though based on this postulation, this basin likely does contain hydrocarbon bearing source rocks in the form of coals and carbonaceous shales. Petroleum systems modeling- aforementioned in the Petroleum Geology section- fortunately indicates that if and when these source rocks are truly utilized, they are already thermally mature and have generated hydrocarbons. These source rocks likely began generating during the Early to Middle Jurassic in the EMB and the Early Cretaceous in the WMB. These source rocks likely continued to create and expel these hydrocarbons following breakup, thus charging any existing and newly created traps.[1] Precise weight precents and ages for the source rock can unfortunately not be recorded as of December 7, 2022, though research is continually furthering and more is hoped to be known about the basin's source rock matrix in each subsequent survey of the area.

Hydrocarbon and Maturation

Due to the traps types and their location in the WMB, syn-rift source rocks are likely charging said traps. In the EMB, Lower Permian coals are well enough matured in the pre-breakup succession.[1] What exactly these source rocks are is currently unknown.

Future Assessment

There is much more assessment needing to be conducted on the basin; though, being as it is close to an ultra-deepwater basin[5], exploration is not as easy as a typical basin much closer to sea level. To find out much more about this basin's actual hydrocarbon prospectivity, much more research is needed.

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 “Mentelle Basin.” Mentelle Basin | Geoscience Australia, https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/offshore-southwest-australia/mentelle-basin#heading-2.
  2. Author links open overlay panelAlessandroMaritatiaPersonEnvelopeJacqueline A.HalpinaJoanne M.WhittakeraNathan R.DaczkobCarmine C.Wainmanc, et al. “Provenance of Upper Jurassic–Lower Cretaceous Strata in the Mentelle Basin, Southwestern Australia, Reveals a Trans-Gondwanan Fluvial Pathway.” Gondwana Research, Elsevier, 10 Feb. 2021, https://www.sciencedirect.com/science/article/pii/S1342937X21000629?casa_token=SNyo1XIvhsAAAAAA%3Ab2MfuGqvLkJU5JHCrZk23CrcBF1FKcUb1LUajrtPhND_QMkrf5zs2oOLzBgJci2_mzI0-LzOjA.
  3. “Mentelle Basin – Tectonic Evolution Controlled by of the Combined Extensional History of the Southwestern and Southern Australian Margins.” Taylor & Francis Online: Peer-Reviewed Journals, https://www.tandfonline.com/doi/abs/10.1081/22020586.2010.12041981?cookieSet=1.
  4. Borissova I. , Bradshaw B. , Nicholson C. , Payne D. Struckmeyer H. (2010) Mentelle Basin ? tectonic evolution controlled by of the combined extensional history of the Southwestern and Southern Australian margins. ASEG Extended Abstracts 2010, 1-4.
  5. 5.0 5.1 Evidence for Non-Marine Jurassic to Earliest Cretaceous Sediments in the Pre-Breakup Section of the Mentelle Basin, Southwestern Australia. https://www.tandfonline.com/doi/full/10.1080/08120099.2019.1627581.