Bremer Sub-Basin

Basin Details and Geological Overview

A recent reassessment of basin terminology along the southern margin of Western Australia groups the Mesozoic succession into the Bight Basin and the Cainozoic succession into the Eucla Basin.

The Bight Basin is one of a series of Mesozoic-Cainozoic depocentres that developed along Australia's southern margin during a period of extension and passive margin evolution that commenced in the Middle-Late Jurassic (Fraser and Tilbury, 1979; Bein and Taylor, 1981; Willcox and Stagg, 1990; Stagg et al., 1990; Hill, 1995; Totterdell et al., 2000; Norvick and Smith, 2001; Totterdell and Bradshaw, 2004).

The basin contains five main extensional depocentres - the Ceduna, Duntroon, Eyre, Bremer and Recherche Sub-basins (Bradshaw et al., 2003; Totterdell and Bradshaw, 2004). A less well known extensional depocentre in the far western part of the basin has been named the Denmark Sub-basin (Bradshaw et al., 2003).

The Bremer Sub-basin referred to previously as the Albany Sub-basin of the Bremer Basin, extends over an area of 11,500 km2 in water depths of 100 to 4500 metres. The sub-basin is a frontier region for petroleum exploration as no wells have been drilled, however exploration permits are currently held. There is a regional seismic coverage and some lithological control from sea-floor grab sampling within submarine canyons.

Established ports at Albany and Esperance can provide logistic support for discoveries and ready access to the rest of Western Australia, particularly major mining operations in the eastern goldfields. The Goldfields Gas Transmission Pipeline has an extension to Esperance that could be utilized in the event of an economic gas discovery.

As part of the Australian Government’s New Petroleum initiative, Geoscience Australia undertook a study of the geology and petroleum potential of the Bremer Sub-basin. The study commenced in February 2004, with a geological and geophysical survey (GA-265) aboard the R/V Southern Surveyor.

The survey aimed to gather geological data from the Bremer Sub-basin by dredging a series of submarine canyons which have incised up to 1.5-2.0 kilometres into the Bremer Sub-basin (Exon et al., 2005).

Data acquired during this survey included 6500 kilometres of high-resolution swath data that was used to map the submarine canyons and rock samples from 45 dredge sites that provide data on the sub-surface geology of the Bremer Sub-basin (Blevin, 2005).

This was followed in October and November 2004 by a geophysical survey, which acquired 2700 kilometres of industry-standard seismic data in the Vlaming Sub-basin (Perth Basin), Mentelle Basin and Bremer Sub-basin.

Seismic data acquired during the Southwest frontiers survey includes 12 lines (1300 kilometres) in the Bremer Sub-basin. This data is now available from the Geoscience Australia Repository at the cost of transfer.

Geochemical, petrographic and palaeontological analyses of rock samples, integrated with interpretations from the new seismic data, indicate that the Bremer Sub-basin contains the essential petroleum system elements (source, reservoir and seal) and structures to generate and trap hydrocarbons (Blevin, 2005; Bradshaw, 2005).

Bremer Sub-basin Details and Geological Overview - Provinces Database

Geological setting

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 Eucla Basin extends landward from the northern margin of the Bremer Sub-basin as a thin veneer (~500 metres) of Cainozoic cool-water carbonates and siliciclastics overlying the Albany-Fraser Orogen (Hocking, 1994; Bradshaw et al., 2003; Clarke et al., 2003).

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 that are hard-linked to basement and strike perpendicular to the basin axis (See structural elements map).

Petroleum Potential

Although no wells have been drilled in the Bremer Sub-basin, its hydrocarbon potential can be assessed from seismic data and 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
• a Lower Cretaceous marine organic-rich facies

Source quality - maturity plot of Hydrogen Index versus Tmax for Bremer Sub-basin dredge samples

Many samples show good organic richness (TOC > 2% and up to 22.6% in one sample), good to excellent generative potential (S2 > 5) and moderate oil potential. Although most samples are immature for hydrocarbon generation, they record only the top two kilometres of a basin succession possibly 10 km thick. Oil flouresence and oil inclusions were noted in some samples.

Fluvio-lacustrine 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. See the Bremer Sub-basin petroleum systems chart.

Exploration Plays and Risks

The exploration potential of the Bremer Sub-basin is summarised in Geoscience Australia's basin framework and petroleum prospectivity study (Bradshaw, 2005). Three potential play types are present in the Bremer Sub-basin: anticlines, fault blocks and combined structural/stratigraphic plays.These begin in water depths of 500m and extend to over 1000m.

Individual plays are estimated to have potential to trap several hundred million barrels of oil in place. The principal risk is that hydrocarbon charge has yet to be proven.

However, trace oil inclusions have been identified in several dredge samples, while seismic interpretations indicate a significant depocentre and potential source kitchen area in the central part of the Bremer Sub-basin (see sediment thickness map). Breaching of traps by submarine canyons could be seen as a potential exploration risk, but only a few canyons cut through the Valangianin-Hauterivian regional seal interval into older strata.

Structural elements

The geometry and location of major depocentres in the Bremer Sub-basin is controlled by a southerly-dipping fault system that strikes predominantly east-northeast along the landward margin of the sub-basin.

This fault system comprises a number of smaller discontinuous, arcuate-shaped rift border faults that strike east-northeast in the eastern Bremer Sub-basin and change to an easterly strike in the western Bremer Sub-basin.

The change in strike of the rift border faults is accompanied by significant changes in structural style from west to east, as described by Stagg and Willcox (1991) and Bradshaw et al. (2003).

Basement trends have had a profound influence on the structural architecture of the Bremer Sub-basin, with the change in structure style from the western to eastern Bremer Sub-basin coinciding with a change in orientation of the Albany-Fraser Orogen from east-northeast to northeast trending (Bradshaw et al., 2003; Totterdell and Bradshaw, 2004).

A series of shear zones in the Albany-Fraser Orogen may also have exerted some control on the location, orientation and timing of basin forming and basin modifying structures in the Bremer Sub-basin (Bradshaw et al., 2003; Totterdell and Bradshaw, 2004).

Stratigraphy

Sedimentary rocks from the Bremer Sub-basin have been sampled by dredging strata exposed in the walls of submarine canyons (Blevin, 2005).

Biostratigraphic analysis of the spore and pollen, dinoflagellate cysts and nannofossil assemblages in these dredge samples indicate that the Bremer Sub-basin contains Jurassic and Cretaceous age strata, overlain by a thin Cainozoic cover from the Eucla Basin (Monteil et al., 2005).

Stratigraphic units sampled by dredging are predominantly those occurring within the upper two kilometres of the basin succession, as reflected by a large number of Cretaceous age rocks that were recovered (about 103 Cretaceous rock samples) and the low thermal maturity (<0.6% Ro; Boreham et al. 2005) of most dredge samples. Older Jurassic rocks are less common and were mainly sampled from thin basin successions over shallow basement blocks. Thus, dredge samples only provide a partial understanding of the Bremer Sub-basin's sub-surface geology.

By comparison with 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 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 siliciclastics 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 mid-Eocene and younger submarine canyon incision.

Key References