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• Location Map
• Basin Details and Geological Overview
• Structural Elements 
• Stratigraphy
• Regional Cross-sections
• Petroleum Systems and Hydrocarbon Potential
• Key References

Location Map

Map of South West Australia showing Vlaming Sub-basin in relation
to Australia
© Geoscience Australia

Basin Details and Geological Overview

 

Figure 1. Regional setting of
the Vlaming Sub-basin
© Geoscience Australia

The Vlaming Sub-basin is an elongate, north-south trending depocentre that lies on the shelf and upper continental slope (20–1000 m water depth) in the offshore southern Perth Basin (Figure 1). It is bordered by the Mandurah Terrace in the east, Edward’s Island Block in the north and in the west it is separated from the Mentelle Basin by the Yallingup Shelf.

Among the Perth Basin main depocentres the Vlaming Sub-basin is the major Jurassic-Early Cretaceous depocentre. Structurally it belongs to an extensional system on Australia’s southwestern margin that formed during the Palaeozoic to Mesozoic rifting of eastern Gondwana. The architecture of the Vlaming Sub-basin is a result of the northeast-southwest extension during the Permian followed by predominantly east-west extension during the Middle Jurassic–Early Cretaceous.

The Vlaming Sub-basin contains more than 12 km of sedimentary sec­tion, the bulk of which is Jurassic to Cainozoic in age. No wells in the central Vlaming Sub-basin have penetrated below the Middle Juras­sic Yarragadee Formation. Permian rocks have, however, been recovered on the flanks of the sub-basin.

The Vlaming Sub-basin contains an active petroleum system with several source rocks generating hydrocarbons from the latest Jurassic–Early Creta­ceous to the present day. The basin contains multiple good quality reservoirs a range of regional and intraformational seals. Possible structural and stratigraphic traps have been identified throughout the basin at multiple stratigraphic levels.

• ProvExplorer - Vlaming Sub-basin Details and Geological Overview 

Structural Elements

Figure 2 - Major structural elements
of the Vlaming Sub-basin
© Geoscience Australia

The Vlaming Sub-basin has complex architecture resulting from the multiple extensional and transtensional episodes combined with the influence of the pre-existing basement structures. Protero­zoic igneous and metamorphic rocks of the Pinjarra Orogen, an interconti nental mobile belt that separated the Australian and Indian parts of east­ern Gondwana, underlie the sub-basin. The Pinjarra Orogen is characterised by major long lived north-south and northwest-southeast trending crustal structures that probably controlled the segmen­tation of the Vlaming Sub-basin and other depocentres along Australia’s southwestern margin.

The northwest-southeast trending Harvey Transfer is a major accommodation zone separating northern and southern parts of the Vlaming Sub-basin. The other major structural elements of the Vlaming Sub-basin include the Roe and Parmelia highs, Bathurst Syn­cline, Edward’s Island Block, Rottnest Trough, Peel Arch, Sugarloaf Arch and Badaminna Fault System (Figure 2). These structures are northerly to north-westerly trending, and formed during Middle Jurassic–Early Cretaceous extension (Crostella and Backhouse, 2000; Bradshaw et al, 2003). Syn-rift faulting in the sub-basin produced a series of half graben and associated anticlinal rollovers with collapse sys­tems on the updip flanks (Spring and Newell, 1993). The 20–25 km wide Peel and Sugarloaf arches are examples of these crestal collapse structures. Their complex architecture can be partly at­tributed to breakup-related fault re­activation.

Major fault reactivation, block rota­tion, uplift and erosion occurred across the sub-basin during the Valanginian. Between 250 and 2,000 m of syn-rift section was removed with maximum erosion in the west and northwest. Compaction related subsidence in depocentres on either side of the Harvey Transfer formed two palaeo-lows on the Valanginian uncon­formity and focussed post rift sedimen­tation. After the breakup, most of the syn-rift faults remained inactive, with the exception of minor reactivation on the sub-basin flanks to accommodate increasing sag fill.

Stratigraphy

Figure 3 - Total sediment thickness
in the Vlaming Sub-basin
estimated from seismic mapping
© Geoscience Australia

The Vlaming Sub-basin contains more than 12 km of sediments (Figure 3). During the Jurassic–Early Cretaceous syn-rift extension, more than 10 km of fluvio-lacustrine sediments accumulated in the northern and cen­tral parts of the Vlaming Sub-basin. The syn-rift accumulation stopped in the Va­langinian as a result of the breakup between Australia and India. Up­lift prior to the breakup led to wide-spread erosion creating a prominent, often angular unconformity. Significant parts of the Mesozoic section were eroded and in these areas older syn-rift sequences outcrop below the breakup unconformity.

Figure 4 - Tectonostratigraphic chart
of the Permian to Upper Cretaceous
succession in the Vlaming Sub-basin
(after Nicholson et al,2008)
© Geoscience Australia

The postrift sequence (Late Va­langinian to Campanian) is thickest (up to 2,500 m) in the central part of the Vlaming Sub-basin. In the Early Cretaceous, two major deltaic systems operated in the region: one originated in uplifted areas in the northern part of the sub-basin and prograded southward while a smaller one drained uplifted areas adjacent to the Leeuwin Block and prograded to the north. Regionally, a significant part of the post-rift sec­tion shows deltaic affinities commonly exemplified by prograding clinoforms on seismic images.

Interpretation of seismic and well data, major tectonic phases and petroleum system elements are summarised in the tec­tonostratigraphic chart (Figure 4). For more information on stratigraphy of each megasequence download Nicholson et al., 2008.

Regional Cross-sections

• 

  Figure 5 - Cross-section trough the northern Vlaming Sub-basin featuring Jurassic half-graben (Bathurst Syncline) and eroded
  crestal collapse structure (Peel Arch). © Geoscience Australia

•  

  Figure 6 - Cross-section trough the southern Vlaming Sub-basin showing Permian section intersected by Felix 1 and transition to predominantly Jurassic depocentre to the east © Geoscience Australia

Petroleum Systems and Hydrocarbon Potential

Hydrocarbons discoveries in the Vlaming Sub-basin include a non-commercial oil accumulation (Gage Roads 1) and small gas accumulation (Marri 1). Five of the 17 exploration wells drilled in the basin recovered oil/and or gas shows (eg. Tuart 1, Gage Roads 2, Araucaria 1).

 

Figure 7 - Source and depositional
environment predictions based on
the relative distribution of C₂₇, C₂₈
and C₂₉ αββ20R steranes for potential
source rocks from the Vlaming
Sub-basin and representative oils
from the Perth Basin
© Geoscience Australia

So far ex­ploration activity has been focussed in the northern and central parts of the sub-basin, where South Perth Shale provides regional seal for potential plays. A more structurally complex southern part of the basin is less explored. It has a higher proportion of sandstones in its syn-rift section and the seals are intraformational.

Three potential source rock inter­vals have been identified in wells from the Vlaming Sub-basin:

1. Lower Cretaceous marine shales of the South Perth Shale
2. Lower Cretaceous lacustrine shales in the Parmelia Group (Otorowiri and Carnac Fm)
3. Middle–Upper Jurassic non-marine coals and carbonaceous shales in the Yarragadee Forma­tion

The Yarragadee Formation is con­sidered the principal source rock unit based on the quantity, quality and maturation of carbonaceous shales (Miyazaki et al, 1996). A recent study of oil-source correlations (Boreham, 2008) has shown that the predominant source input of terrestrial and aquatic (non-marine) organic matter to the Vlaming Sub-basin oils (Gage Roads 1 oil and the Araucaria 1 oil stain) are from the Early-Middle Jurassic (lower Yarragadee Fm) source rocks (Figure 7).

Good reservoir rocks exist through­out Late Jurassic and Early Cretaceous strata in the Vlaming Sub-basin. These include the Leederville Formation, the Gage Sandstone, the Charlotte Sand­stone, the Jervoise Sandstone, and the upper part of the Yarragadee Forma­tion (Miyazaki et al, 1996). Several seal intervals are present in the Vlaming Sub-basin above and be­low the Valanginian breakup unconfor­mity. The best sub-unconformity seal potential relates to mudstone-dominat­ed, amalgamated lacustrine shales and siltstones that were deposited during at least three intervals in the Berria­sian Parmelia Group. These intervals are mappable, regionally consistent mudstone-dominated packages of 40 to 50 metres in thickness and include the Otorowiri and Carnac formations.

Figure 8 - Conceptual play diagram
for the Vlaming Sub-basin
(after Nicholson et al, 2008)
© Geoscience Australia

Multistage rifting and complex structuring in the Vlaming Sub-basin has led to the development of a variety of possible structural and stratigraphic plays. Most of these plays are associated with the Valanginian breakup unconformity (sub-unconformity truncations, palae­otopographic highs and post-breakup pinch-outs) with the South Perth Shale forming a top seal to sandstones of the Parmelia Group beneath the uncon­formity, or the Gage Sandstone above the unconformity (Figure 8). Seismic mapping has also shown that fault block plays occur at different strati­graphic levels where Jurassic and Early Cretaceous lacustrine shales provide effective seals. These shaly intervals may provide a seal in sub-unconformity anti­clinal rollovers and horst-block plays.

Key References