Dr Tom Bernecker and Dr Geoffrey O’Brien
GeoScience Victoria

Following the partial relinquishment of VIC/P41 by Bass Strait Oil & Gas in 2006, Victoria’s DPI and the Commonwealth’s DITR have decided to gazette three areas in the offshore, north-eastern Gippsland Basin (Figs. 1 & 2). These areas provide explorers with the opportunity to explore an as yet untapped region within this prolific hydrocarbon province, at a time when exploration activity is reaching new heights in Victoria. Discoveries along the Rosedale Fault System and northern basin margin have provided the impetus to further examine the stratigraphically deeper parts of the Latrobe Group which can be accessed within the gazetted blocks. Formal acreage release and invitation for bidding is scheduled for 16 April 2007 at the APPEA Conference in Adelaide.

The lack of exploration wells in the three gazettal blocks requires that prospectivity assessments for the areas be made by examining the working petroleum systems elements that are located further to the west. The currently producing Basker/Manta field is an example of an intra-Latrobe petroleum system. The source for both the gas and oil appears to be located within the eastern Central Deep, in the thick section of coal-bearing, lower coastal plain sediments of the Volador Formation. 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). Top sealing in both hydrocarbon-bearing intervals is provided by a series of interbedded claystones and siltstones.

The Rosedale Fault System controls the distribution of hydrocarbons along the northern margin of the Central Deep, as exemplified by the Latrobe-sourced Kipper and Longtom discoveries. Exploration failures along the Rosedale Fault Zone have been ascribed to either the presence of variable sealing lithologies or to the complex nature of fault seal integrity in the area.

Further to the north, on the Northern Terrace, the commercial discoveries are exclusively dry gas and are believed to have been sourced locally from the underlying, thermally mature Strzelecki Group. The Sole gas field overlies part of a rotated Strzelecki Group half-graben that developed during the Albian-Aptian and was reactivated during the Turonian 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. The trap itself appears to be a Late Tertiary inverted hanging wall feature, which has clearly been charged in the Late Tertiary. The gases in the Sole field occur at shallow depths (approximately 800 m) and have been strongly biodegraded, which has further decreased the heavier hydrocarbons in the reservoir gases.

No discoveries have been yet been made on the offshore Northern Platform. Only thin intervals of Strzelecki and Latrobe group sediments are preserved across the Northern Platform and these rapidly thin out towards the present day coastline. A local mature Strzelecki source is lacking in this area and hydrocarbons would have to have migrated relatively long distances from the Northern Terrace or even from south of the Rosedale Fault System (in the Central Deep). A viable play in the central and southern part of the Northern Platform may be the stratigraphic pinch-out of thin sandy Latrobe Group sediments that overlie 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.

AREA V07-1

Area V07-1 lies on the shallow shelf (water depths 20-200 m) south of the State’s Three-Mile Zone; the area partly surrounds the retention lease (VIC/RL3) around the Sole gas field. Most of the area extends across the Northern Platform (Fig. 2), where shallow Palaeozoic basement is overlain by a thin veneer of Early Cretaceous (Strzelecki Group) and the Paleocene (Latrobe Group Kingfish Formation) sediments.

The main play type on the Northern Platform would appear to be the stratigraphic pinch-out of the Latrobe Group over basement. The pinch-out is interpreted to occur immediately north of Northright-1 and hence much of Area V07-1 falls within the Latrobe pinch-out play fairway (Fig. 2). Reservoir quality, Latrobe Group sandstones are known to exist on the Northern Platform and hydrocarbon charge would either be derived from the mature Strzelecki Group sources on the Northern Terrace (as in the Sole field) or via longer distance migration from Latrobe Group sources south of the Rosedale Fault. The south-western corner of gazettal block V07-1 lies on the Northern Terrace, between the Rosedale and Lake Wellington fault systems, and is part of the top-Latrobe play fairway (Fig. 2), where top-Latrobe gas accumulations appear to have been sourced via short-distance migration from the Strzelecki Group. Seismic mapping should allow the lateral continuation of these half-grabens to be defined and should readily delineate any hanging wall inversion structures that may have received a gas-charge from the Strzelecki source system. An analogue for this scenario is provided by the Sole gas field.

AREA V07-2

Area V07-2 spans the easternmost part of the Northern Platform and Terrace. Water depths range principally between 70 and 200 m, though they increase to 300 m in the southernmost part of the block. Area V07-2 has not been explored since the 1970s, when Shell acquired seismic survey GS70A. Aeromagnetic data indicate the presence of a magnetic low in the area, which has been interpreted as a shallow sub-basin (Fig. 1).

Basement is very shallow over the northern portion of Area V07-2 and thus the Latrobe pinch-out play-type is the principal play present (Fig. 2). Multiple Latrobe pinch-out traps could potentially be developed around the rugose basement surface.

AREA V07-3

Area V07-3 is located to the north of the Bass Canyon and covers part of the easternmost Northern Terrace and the Central Deep, where water depths exceed 1,000 m. This area contains the main four play-fairways is hence the most geologically diverse of the 2007 Gippsland gazettal blocks (Fig. 2). Regional seismic lines have outlined the distribution of basement highs that are flanked by sediment-filled grabens (Fig. 3). Several basin-wide unconformities can be recognised and should allow correlation between the principal basin-fill sequences.

An area of shallow basement in the northern part of V07-3 contains the Latrobe pinch-out play fairway (Fig. 2). The nature of the top-Latrobe play fairway is less clear in the area due to limited seismic availability, but would rely on the presence of a rotated, Strzelecki-Emperor-filled half graben, similar to the one which underlies the Sole gas field.

The Golden Beach-Emperor play fairway (Fig.2) is only a valid target in the western part of the area as submarine erosion associated with the development of the Bass Canyon has removed parts of the deeper Latrobe Group. The westernmost region of Area V07-3 is crossed by the East Gippsland Rise, a basement high that defines the eastern boundary of the Gippsland Basin.

The intra-Latrobe play fairway is only present in the south-western corner of the area. Access to the intra-Latrobe petroleum system (? Volador Formation source and ?Golden Beach/intra-Volador reservoirs, ?intra-Volador seals) appears viable; however, lateral facies variations could mean that the entire Volador section further east is dominantly marine. This may potentially produce improved reservoir characteristics, with lower coastal plain shaly sealing units being less frequently developed.
The Gippsland Basin has enjoyed resurgence in recent years, with a number of new explorers undertaking exploration and development programmes in the region. The 2007 gazettal blocks offer opportunities within largely unexplored parts of the basin. Clearly, a significant amount of new seismic data and an improved understanding of hydrocarbon migration and sealing processes, are required to adequately assess the hydrocarbon potential of these frontier blocks.