Areas V07-1, V07-2 and V07-3
Gippsland Basin
Petroleum Potential
Prospectivity
From a regional geological perspective, the 2007 release areas cover a tectonically diverse and, in parts, complex terrain. The total magnetic intensity image (Figure 12) highlights the location of the Sole gas field atop a basement high that is fault bounded in the north and south. This basement feature dominates the southern part of Area V07-1 and extends into the westernmost parts of release areas V07-2 and V07-3. A shallow sub-basin is the main feature of Area V07-1. Information from the Northright 1 well suggests that this sub-basin may contain several hundred metres of Strzelecki Group sediments over Palaeozoic basement. Another sub-basin expressed as a magnetic low extends across the boundary between the V07-2 and V07-3 release areas. Due to the limited amount of available seismic data (see below), the nature of the basin fill here can only be speculated on. The extensive basement feature in the southeastern part of Area V07-3 relates to the East Gippsland Rise, which is the limit of prospective sedimentary sections in the eastern offshore basin.
Despite the lack of exploration wells in the three release areas, prospectivity assessments for the areas can be made by examining the existing petroleum systems elements further west. At least two different petroleum systems operate in the vicinity of the release areas, the geological context of which is highlighted in the seismic correlation between the Basker/Manta and Sole fields (Figure 7).
The currently producing Basker/Manta field is an example of an intra-Latrobe petroleum system. The source for both the gas and liquids is the thick section of coal-bearing, lower coastal plain sediments of the Volador Formation (Figure 4) in the eastern Central Deep. These rich source rocks are believed to have charged the major hydrocarbon traps in the central part of the offshore basin. Reservoir units in the Basker/Manta field occur in fluvial sandstones of the Chimaera Formation (Golden Beach Subgroup) and in lower coastal plain channel sandstones of the Volador Formation (Halibut Subgroup). Seals in both hydrocarbon-bearing units are provided by a series of interbedded clay- and siltstones, as well as by cross-faulting.
In the Hammerhead area, the Rosedale Fault System controls the distribution of hydrocarbons at the northern margin of the Central Deep, as exemplified by the Latrobe-sourced Kipper and Longtom discoveries (Figure 5). The failure of Hammerhead 1 can partly be ascribed to the presence of different sealing lithologies than were expected (see above), but mainly to the complex nature of the fault system in which seal integrity is only very poorly understood.
Further to the north, on the Northern Terrace, the commercial discoveries are exclusively gas and are believed to have been sourced locally from the underlying, thermally mature Strzelecki Group (O’Brien and Bernecker, in preparation). The Sole gas field is part of a rotated Strzelecki Group half-graben that developed during the Albian–Aptian and was reactivated during the Turonian Latrobe rifting event; a relatively thick sequence of sandstones and lacustrine shales of the Emperor Subgroup was deposited in the hanging wall of the Strzelecki fault block. The Sole field is an example of a Strzelecki-Halibut petroleum system, broadly similar to the one responsible for the Patricia/Baleen gas accumulation; the only difference is that the reservoir unit there is part of the Cobia Subgroup, above the Marlin Unconformity (Figure 4). The trap itself appears to be a Neogene inverted hanging wall feature, which has clearly been charged very late.
To date, no discoveries have been made on the offshore Northern Platform. The very limited well control across the northern basin margin indicates that only thin units of Strzelecki Group and Latrobe Group sediments are preserved and these rapidly thin out towards the present day coastline (Figure 6). A local mature Strzelecki source is lacking and hydrocarbons would have to have migrated relatively long distances from the Northern Platform or even south of the Rosedale Fault System. A viable play in the central and southern part of the Northern Platform may be the stratigraphic pinch-outs of thin (>100 m), sandy Latrobe Group sediments that overlie the Strzelecki Group and Palaeozoic basement. However, the sealing lithologies are very thin across the Northern Platform and parts of the area are probably located north of the terminal edge of the regional seal.
Risk
Exploration risks for the 2007 Gippsland release areas fall into two categories:
- The amount of modern seismic data is very limited, especially in the eastern areas. The one drilled exploration well, Northright-1, provides insight into the geology of only the Northern Platform. Exploration in the region will require the acquisition of significant amounts of seismic data.
- The understanding of source rock distribution, hydrocarbon generation and migration is critical and largely unknown in the eastern offshore basin. It will be important to ascertain whether the Strzelecki Group in the east is coal-bearing and whether the sediments are sufficiently thermally mature to generate hydrocarbons. It is nevertheless likely that the generated hydrocarbons will be gas only, based on the discoveries made to date that can be related to a Strzelecki source. The classic Latrobe Group source rock kitchen area of the Central Deep (Volador Formation) may lie too far west to enable charging of the potential reservoir units in the release areas.
Distribution and effectiveness of sealing facies is not seen as a major risk over most of the areas. An effective regional seal – the marls of the lower Seaspray Group – is likely to be present over the majority areas, though the exact lithologies in the eastern offshore areas remain unknown. The quality of intra-formational seals depends very much on the overall facies associations and their variations through time. Well control in the Central Deep and on the Northern Terrace indicates that the Latrobe Group sediments tend to have more marine affinities in the eastern-most areas.
Uncertainties also relate to the behaviour of the main fault systems and related structures. The transition between the Central Deep and the Northern Terrace is controlled by a complex fault system (the Rosedale Fault System) that has juxtaposed reservoir and sealing facies in several locations, but is also known to act as migration pathway in others. Fault seal integrity is certainly one issue that requires detailed attention by any explorer.
Summary
The Gippsland Basin has enjoyed resurgence in recent years, with a number of new explorers undertaking exploration and development programmes in the region.
The three 2007 release areas are located in the northeastern offshore part of the basin and provide opportunities to explore untapped parts of this prolific hydrocarbon province. Recent discoveries, especially along the Rosedale Fault System and northern basin margin, have provided a new impetus to further examine the stratigraphically deeper part of the Latrobe Group.
Areas V07-1 and V07-3 are geologically more diverse than Area V07-2, within which only the Latrobe pinch-out play fairway is likely to be viable. Area V07-1 is attractive because it contains both the pinch-out fairway and also the top-Latrobe fairway within which the Sole and Patricia-Baleen gas discoveries occur. Access to a mature and generative Strzelecki source is guaranteed in the southwestern part of the block; there reservoir and sealing components are not seen as posing significant risks. However, the more northerly parts of Area V07-1 lie on the Northern Platform, where uncertainties associated with the top seal quality and hydrocarbon migration processes may be important. Area V07-3 is geologically the most diverse (four play fairways have been identified) block and would also be the most attractive, providing that the Golden Beach and intra-Latrobe fairways are located in areas of shallow water depth. Access to mature source rock is possible from several graben depocentres within Area V07-3, although the nature of reservoir and sealing units is poorly defined.
Clearly, a significant amount of new data – especially seismic data – are required in order to adequately assess the hydrocarbon potential of these new blocks within the northeastern part of the Gippsland Basin. These new seismic data, when combined with an improved understanding of the hydrocarbon generation-migration and sealing processes in the areas, should provide the keys to unlocking the untapped potential of these frontier areas.