Bremer Sub-basin - Bright Basin

The Bremer Sub-basin is a structurally complex series of perched half-graben depocentres that contain up to 11 km of predominantly Jurassic and Lower Cretaceous sedimentary rocks above Proterozoic rocks of the Albany–Fraser Orogen.

The southern boundary of the sub-basin appears to be a progressive downstepping of fault blocks from the Bremer Sub-basin to the Recherche Sub-basin. The western and eastern boundaries are north- to northeast-striking, near-vertical faults. Integration of available data suggests that the deeper depocentres are prospective, with mature rocks, valid seals and reservoirs, and potential traps.

In comparison to the Eyre Sub-basin, the Bremer Sub-basin was probably initiated by Late Jurassic extension, although the oldest strata have not been sampled. This was followed by Berriasian-Hauterivian thermal subsidence and extension, with bounding faults active at varying times across the basin. A thick succession of sandstone, siltstone, organic-rich claystone, and coal accumulated in fluvial, lacustrine and paralic conditions.

Thermal subsidence continued through the Hauterivian to breakup from Antarctica in the Santonian, and into the Maastrichtian following breakup. Restricted marine conditions developed in the Hauterivian to Aptian, with open marine conditions prevailing from the Aptian to Santonian after continued thermal subsidence and eustatic transgression.

During breakup, many older faults were reactivated and some new intra-basin faults formed. Major uplift and erosion was restricted to the western Bremer Sub-basin, where rift-flank uplift produced a major angular unconformity. A thin succession of calcareous sediments and siliciclastic rocks was deposited after breakup, indicating low sediment supply and low subsidence rates. A carbonate-dominated passive-margin phase defines the overlying Eucla Basin, and is associated with pronounced Middle Eocene
and younger submarine canyon incision.

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Exploration in the Bremer Sub-basin has been limited and sporadic. Some seismic data were acquired in the 1970s by Teledyne, Continental Oil, Shell Australia, Esso Australia and the Bureau of Mineral Resources (BMR, now Geoscience Australia), but there was no followup drilling despite the identification of significant anticlinal structures by Esso Australia.

Geoscience Australia completed a program of seafloor mapping, seismic and seafloor dredge sampling in early 2004, prior to acquisition of a regional seismic grid in late 2004. The results of this program were released in late 2005, as a series of Geoscience Australia Records.

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Although no wells have been drilled in the Bremer Sub-basin, its hydrocarbon potential can be assessed from seismic data, dredge samples recovered in 2004 by Geoscience Australia, and the known successions in the southern Perth Basin to the northwest and Jerboa 1, in the Eyre Sub-basin to the east.

The essential elements for a petroleum system (source, reservoir and seal intervals) are present and possible traps are apparent on seismic data, making the Bremer Sub-basin (and by extension the Denmark, Eyre and possibly the Recherche Sub-basins) prospective for hydrocarbons.

Samples recovered from seafloor dredging in early 2004 identified three diverse oil-prone potential source rock units in the Bremer Sub-basin: a Lower Cretaceous continental land plant (coaly) organic facies; a Jurassic – Lower Cretaceous lacustrine organic facies; and a Lower Cretaceous marine organic-rich facies. Many samples show good organic richness, good to excellent generative potential and moderate oil potential.

Although most samples are immature for hydrocarbon generation, they record only the top two kilometres of a basin succession that is possibly 10 km thick and older samples are from near basin margins, rather than from more mature, more deeply buried parts of the basin.

Two possible petroleum systems are present. Oil fluorescence and oil inclusions were noted in some samples. Fluviolacustrine Berriasian–Hauterivian sandstones are potentially good reservoir rocks, with porosities as high as 24 to 34%. A major lacustrine phase in basin deposition, beginning in the Valanginian and extending into the Hauterivian, is evident from dredge samples and seismic data.

Lacustrine mudstones deposited at this time are widespread and form a potential regional sealing unit. Younger Cretaceous marine sandstones and mudstones also have reservoir and seal potential. Three potential play types are present in the Bremer Sub-basin: anticlines, fault blocks and combined structural/stratigraphic plays. The principal risk is that hydrocarbon charge has yet to be proven.

Breaching of traps by submarine canyons could be seen as a potential exploration risk, but only a few canyons cut through the Valanginian– Hauterivian regional seal interval into older strata.

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