VIMP Report 87 - An assessment of the hydrocarbon prospectivity of 2006 Otway Basin acreage release areas

Category: Victorian Initiative for Minerals and Petroleum Reports	Product Code: MP-R-34466
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Synopsis
This report presents the results of a study by GeoScience Victoria’s Energy Geoscience Group on four acreage release areas within Victoria’s Otway Basin:

Area VIC/O-06(1), which is located in the onshore Tyrendarra Embayment, east of Portland.
Areas VIC/O-06(2) and VIC/O-06(3), which are located onshore within, and around the flanks of, the Port Campbell Embayment, and on the northern limit of the Shipwreck Trough.
Area V06-1, which is located offshore, in Commonwealth waters, on the Prawn Platform, immediately to the east of the Shipwreck Trough.

The primary purpose of this report is to provide explorers with an overview of the petroleum prospectivity of these areas. The report also provides within its appendices relevant technical information on key wells in and around the gazettal blocks, including well summaries, well-log motifs, stratigraphy, palynology and interpreted depositional facies. Lists of relevant seismic surveys in and adjacent to the gazetted areas are also provided.

The Otway Basin is a Mesozoic rift-basin that developed in response to rifting along eastern Gondwana between the Late Jurassic and the Late Cretaceous (Maastrichtian), which eventually led to the separation of Australia and Antarctica. There are two proven petroleum systems within the Otway Basin, the Austral 1 and 2 systems, which are of most relevance to the gazetted areas. The Austral 1 System consists of Late Jurassic through to mid-Early Cretaceous (Barremian) source rocks in the Casterton, the Pretty Hill and the Laira formations. These consist mostly of fluvio-lacustrine shales as well as some coals and carbonaceous mudstones. Source rock quality can be good and the products of this system are mostly gas, but it has also generated some oil. The principal reservoir units, which can capture the hydrocarbon inventory produced from the Austral 1 System, are the sandstones of the Pretty Hill Formation and the basal Barremian-Aptian sandstones of the Windermere Member of the Eumeralla Formation. Both units are recognised as the main hosts for most of the hydrocarbons in the western Otway Basin. The Austral 2 System comprises the Early Cretaceous fluvial and coaly facies of the Albian-Aptian Eumeralla Formation. This system is recognised as having sourced most of the gases in the eastern Otway Basin and it is considered to also have some condensate and oil generative capacity. The reservoirs which have received most of the hydrocarbon inventory from the Austral 2 System are the sandstones of the Late Cretaceous Waarre Formation, which immediately overlies the Eumeralla Formation, the Albian Heathfield Sandstone Member (intra-Eumeralla) and perhaps the basal Eumeralla Formation (Early Aptian) Windermere Sandstone Member.

VIC/O-06(1) is located in the onshore Tyrendarra Embayment. The principal targets are sandstones within the Eumeralla Formation, such as the Heathfield Sandstone Member and the Windermere Sandstone Member as well as older sandstones within the underlying Crayfish Subgroup. Structures are typically fault-related and comprise an element of faulting which is oriented both east-west and northwest-southeast. There are three principal exploration uncertainties in the area: these are fault seal integrity, maturation and charge history, and reservoir quality. Four untested leads have been recognised that will require further evaluation to better assess their potential and associated risks. It is encouraging that these prospects are located along east-west trending fault systems, which probably have higher inherent fault-seal integrity. These leads are also located close to a number of distinct, thermally mature, ‘sweet-spot’ source rock kitchens and hence migration from these kitchens into the identified leads would typically require only very short-range migration. Much of the hydrocarbon inventory generated in VIC/O-06(1) has probably come from the Austral 1 petroleum system, though a contribution from the Austral 2 system is also possible. Oil that was recovered on test from the Heathfield Sandstone Member at Windermere-1 and the relatively wet gases that were recovered in the Windermere Member at Windermere-2 were probably sourced from the Austral 1 petroleum system. The Windermere, Koroit and Warrong troughs are all potential source rock kitchens and any principally east-west trending leads or traps, located either around or within these troughs, would appear to be well-placed to both capture and preserve any hydrocarbons that are migrating into them.

Areas VIC/O-06(2) and VIC/O-06(3) are located onshore within, and around the flanks of, the Port Campbell Embayment, and on the northern limit of the Shipwreck Trough. The main play in the southern part of these areas is the Waarre Formation which has been successfully tested and proven to be gas- and gas-condensate bearing in a number of tilted fault blocks. Additional plays have been identified and include the Eumeralla Formation which flowed gas in Skull Creek-1 and the Pretty Hill Formation reservoir to the north of the permits. While the Pretty Hill Formation is a productive unit in the South Australian portion of the basin, this reservoir has scarcely been targeted in the region, though Gavroc-1 recorded oil shows from this unit.

Other plays which are present in release areas VIC/O-06(2) and VIC/O-06(3) include:

Waarre sandstone hanging-wall plays which involve cross-fault seal against the Eumeralla Formation, as well as clay smear seal along the fault. In all other respects, the play is similar to the up-thrown play.
Waarre sandstone structural/stratigraphical plays along the back limbs of the fault blocks.
Waarre sandstone pinch-out plays along the basin margin.

In the northern part of VIC/O-06(2) and VIC/O-06(3), the Waarre Formation is absent and the main plays are sands such as the Heathfield and Windermere Members of the Eumeralla Formation (providing they are well-developed in this area) and the underlying Crayfish Subgroup reservoirs. A high trend exists between the Port Campbell Embayment and the Elingamite Trough that could potentially provide a focus for exploration within the deeper parts of the Otway Group.

The principal exploration uncertainties in these two release areas are fault seal integrity, maturation and charge history and reservoir quality, though none of these appear to be as pronounced as they were in VIC/O-061(1). There are a significant number of hydrocarbon fields in this area, indicating that fault seal integrity is probably not a significant issue. This may be because the sealing Flaxman Formation and the Belfast Mudstone are well-developed, which assists in preservation of the accumulations.

The maturation history of the Port Campbell Embayment indicates that it may be one area within the Otway Basin where a mixture of Austral 1 and 2 sourced gases could easily occur within the same accumulation. Overall, the Port Campbell gases have low CO₂^- concentrations, with three exceptions. The Port Campbell Embayment gases in a number of the fields also have unusually high C₂⁺ contents compared to typical Austral 2 gases sourced from the Eumeralla Formation. This increased wetness could be due to a number or combination of factors:

The source rock facies of the Eumeralla Formation in this area might be richer than is typical and/or the gases may have been generated at a different maturity levels.
It may be also be possible that the Austral 1 system, which overall appears to be geochemically wetter than the Austral 2 system, is contributing to the hydrocarbon inventory in the Port Campbell Embayment.
Given the gas wetness, it may even be that there is oil within the Port Campbell Embayment, which may have been displaced progressively towards the basin margins by the gas charge. At the very least, relatively wetter gas represents a more attractive exploration opportunity than does a drier gas.

Area V06-1 is located offshore in Commonwealth waters, on the Prawn Platform, immediately to the east of the Shipwreck Trough. There are several significant exploration uncertainties that are associated with the assessment of the prospectivity of V06-1. It seems clear that the exploration and exploration plays within release area V06-1 fall into two distinct categories:

a Waarre Formation play
a pre-Waarre Formation play, which might target the Heathfield or Windermere Members of the Eumeralla Formation or sandstone intervals within the Crayfish Subgroup.

The eastward onlap of the Waarre Formation onto the surface of the Otway Unconformity renders the eastern part of the block non-prospective with respect to the Waarre play. Moreover, the Flaxman Formation is not a valid seal on the platform because of its lateral discontinuity and it appears to be absent altogether near the Waarre pinch-out. It is likely that the traditional Waarre play may only work in the far southwestern corner of the permit where the sealing facies may be better developed. This area also has access to the hydrocarbon charge (Austral 2) generated within, and migrating from, the Shipwreck Trough. The gases in La Bella, Geographe and Thylacine are relatively wetter than those present in Minerva and Pecten, though they also have significantly higher CO₂^- contents. It may be that the very significant Late Tertiary deposition along the outer margin in this area has pushed the Austral 3 petroleum system (probably the Turonian interval) into the oil window, this has contributed some wetter gas. Alternatively, the wetter compositions of the more basin-ward wells could also be due to a progressive lateral change in kerogen type. Whatever the reasons for this variation in gases composition, it seems that any traps located in the southwestern corner of V06-1 will most likely contain gas that is relatively wetter than typical Eumeralla-sourced (Austral 2) gas; a significant(~6-12%) CO₂^- content for these gases is also possible.

As only a small portion of V06-1 can work for a traditional Waarre play, deeper plays will need to be explored across much of this release area. Such plays might be within the Heathfield or Windermere Members of the Eumeralla Formation or the sands of the Crayfish Subgroup. The majority of the Eumeralla Formation and the entire pre-Aptian Otway Group remains untested on the Prawn Platform. The Austral 1 petroleum system is likely to be thermally mature across most of V06-1, though significant uncertainties exist with respect to the reservoir quality of the Otway Group across the Prawn Platform. Nevertheless, large parts of V06-1 do present the opportunity to test the Early Cretaceous section in an area and geological setting where it has never been tested before.

Three leads have been identified along Late Cretaceous to Early Tertiary inversion features that trend NE-SW through the western part of V06-1. Four-way dip closures enhanced by sealing faults or stratigraphic pinch-out may provide targets for both, the Waarre and pre-Waarre plays.

Bibliographic reference O'Brien, G. W., Bernecker, T., Thomas, J. H., Driscoll, J.P, & Rikus, L., 2006. An Assessment of the Hydrocarbon Prospectivity of Areas VIC/O-06(1), VIC/O-06(2), VIC/O-06(3) and V06-1, Eastern Onshore and Offshore Otway Basin, Victoria, Australia. Victorian Initiative for Minerals and Petroleum Report 87, Department of Primary Industries.

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