2018 Release Areas

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Acreage Release 2018: Gippsland Basin

The Gippsland Basin is recognised as one of Australia’s premier hydrocarbon provinces, having continually produced oil and gas for the southeast Australian energy market for over 50 years. In March 2017, remaining reserves were estimated at 2,2 Tcf (62.3Gm3) of natural gas and ethane, and 284 MMbbls (45.2 GL) of oil and natural gas liquids (EnergyQuest, 2017). Several petroleum systems operate in the basin, with the largest oil and gas fields hosted by top-Latrobe Group (Eocene) shallow marine barrier sandstones, and additional discoveries made in intra-Latrobe (Upper Cretaceous–Paleocene) coastal plain and deltaic channel sandstones. The 2018 Acreage Release offers opportunities in the southern, central and eastern part of the basin, the latter of which has remained largely underexplored.

Release area V18-4, Southern Gippsland Basin, Victoria

Highlights

Bids close 18 October 2018

  • Shallow water, 50–100 m

  • Access to southern migration pathways

  • Gas discovery with minor oil show at Omeo 1

  • Release Area fully covered by Southern Flank 2D MSS

  • Further guidance available, refer to 2018 Special Notices

Release Area V18-4 consists of 15 graticular blocks that cover parts of the Southern Terrace and southern portion of the basin’s Central Deep in water depths between 50 m and 100 m (Figure 1). Nine exploration wells provide geological control (Table 1) of which only one (Omeo 1) encountered hydrocarbons. Exploration initially focussed on top-Latrobe Group plays but no discoveries were made in this section of the southern basin flank, with both Gurnard 1 and Bullseye 1 (drilled by Esso Australia in 1969 and 1973, respectively) being dry. A decade later, Australian Aquitaine Petroleum (later Elf Aquitaine Petroleum Australia) drilled several other top-Latrobe targets, with Edina 1 and Speke 1 also being unsuccessful. In light of these results, their Omeo 1 well was designed to test the deeper intra-Latrobe Group level and recorded a small gas pool in sediments of the Golden Beach Subgroup and minor oil shows within both the Golden Beach Subgroup and Strzelecki Group (Figure 2). This non-commercial gas discovery was not substantiated by Omeo 2/2A (drilled in 1985); however, the results of Omeo 1 indicated that the intra-Latrobe Group play was prospective, as has been confirmed by ongoing exploration in the eastern part of the basin.

Australian Aquitaine Petroleum continued to be innovative, with Tarra 1 being the first well drilled in the offshore Gippsland Basin to target the Lower Cretaceous Strzelecki Group (Figure 2), the stratigraphic equivalent of the hydrocarbon-bearing Otway Group in the Otway Basin. Thick sections of these sediments are preserved on the early extensional platforms and terraces where they are overlain by thin veneers of the Latrobe Group. Although Tarra 1 intersected a sequence of fluvial sandstones, including coarse-grained intervals, the reservoir facies was characterised by very low porosities and permeabilities due to the large clay- and silt-content.

Exploration in this area was abandoned until 2001, when Bass Strait Oil, drilled Melville 1 to further evaluate the Golden Beach Subgroup. The well failed due to a lack of intra-formational seals but did encounter feasible reservoir intervals developed as part of a fluvial-deltaic system. The last well drilled in the area, and one of the last exploration wells to date in the basin, is Apache’s Speke South 1 which was designed to evaluate the hydrocarbon prospectivity of the stratigraphic section beneath the top-Latrobe interval, unsuccessfully tested by Speke 1. In a similar result to Melville 1, potential reservoir facies were recorded but the pore-space was water-saturated. It was concluded that the well is located outside a migration pathway.

The above well results appear rather disappointing. However, it should be noted that most wells in the area were drilled during the early stages of exploration in the Gippsland Basin, before the commercial and sub-commercial discoveries in the deep intra-Latrobe Group along the northern margin (Kipper, Longtom, Basker/Manta) and in the eastern Central Deep (Archer/Anemone, Angler). The data coverage over Release Area V18-4 (Figure 3) is represented mainly by vintage 2D surveys, but new seismic data from the Southern Flank 2D MSS is now available (Gunning et al, 2016). The survey was acquired to image the structural development of the southern Central Deep around the Darriman and Foster fault systems with the aim of allowing better predictions about migration pathways and seal facies distribution in the context of possible CO2 storage.

Future exploration success will hinge on the acquisition of 3D seismic and a comprehensive evaluation of the facies associations in the intra-Latrobe Group. The delineation of the palaeo-coastline during the Santonian and Campanian, contemporaneous with the opening of the Tasman Sea, is fundamental for the interpretation of the presence and distribution of reservoir lithologies.

Release Area V18-5, Central Gippsland Basin, Victoria

Highlights

Bids close 18 October 2018

  • Shallow water, 50–100 m

  • Underexplored part of the Central Deep

  • Surrounded by giant oil and gas fields

  • Hydrocarbon shows within intra-Latrobe Group

  • Further guidance available, refer to 2018 Special Notices

Release Area V18-5 consists of 12 graticular blocks that are surrounded by the giant oil and gas fields Barracouta, Snapper, Marlin/Turrum, Fortescue, Kingfish and Bream (Figure 4) and contains eleven exploration wells. These wells were drilled between 1970 and 2009 (Table 2), none of which was a commercial discovery and only three of them recorded hydrocarbon shows. The lack of exploration success has established the term “Hole-in-the-Doughnut” for this part of the Central Deep.

Well failure interpretation of early exploration, represented by Esso Australia’s Cod 1 and Salmon 1 which pursued top-Latrobe targets, suggested that both well locations did not receive any hydrocarbon charge. Nannygai 1 was designed to test the possible extension of the Kingfish play and recorded oil shows, but no intact hydrocarbon-bearing structure was identified.

In the mid-1970s until the end of the 1990s, both top- and intra-Latrobe Group continued to be Esso Australia’s exploration focus in this part of the basin. All their wells in this area (Swordfish 1, Rockling 1, Drummer 1, Conger 1 and Sawbelly 1) were dry, partly because seismic mapping failed to recognise sedimentary facies changes, and partly because either seal lithologies were absent or fault seals were not developed. Moreover, the seismic velocity anomalies related to extensive channelling within the overlying Miocene carbonate sequence of the Seaspray Group had led to misinterpretations of structural horizons at the top-Latrobe Group surface. Veilfin 1 was the only well that encountered gas shows within the intra-Latrobe Group, in the lower coastal plain facies of the lower Kingfish Formation (Figure 2).

The last two wells drilled within this area were ZaneGrey 1/ST2, which was another attempt to identify an extension to the Kingfish gas field and was deemed a technical success, and Bazzard 1, which was the last well trying to find hydrocarbons in the deeper intra-Latrobe Group but was a failure.

It has been ten years since exploration was undertaken in the deeper part of the Central Deep. Although several wells did target the intra-Latrobe Group, none were drilled deep enough to intersect the Golden Beach Subgroup, which is known to host hydrocarbons outside of the Central Deep. Veilfin 1 missed the top-Golden Beach horizon by approximately 300 m. Any new exploration effort should consider the validity of a Golden Beach play, which would involve the drilling of a well deeper than 4000 m. Exploration uncertainties revolve around the facies assemblage of Golden Beach sediments in the deeper Central Deep, the type and maturity of potential source rocks (intra-Golden Beach Subgroup, Kipper Shale and/or Strzelecki Group). From a data point of view, the area is in need of new 3D seismic surveys (Figure 5). New technologies that allow the imaging of sub-coal and sub-volcanic sections, while also addressing the seismic velocity anomalies in the basin, could be tested in this geological setting.

Release Areas V18-6 and V18-7, Eastern Gippsland Basin, Victoria

Highlights

Bids close 21 March 2019

  • Proven world class petroleum province with ready access to the growing southeast Australian energy market.

  • Access to southern migration pathways

  • Shallow water, 150–2000 m

  • Release Area fully covered by Southern Flank 2D MSS

  • Further guidance available, refer to 2018 Special Notices

Release Areas V18-6 and V18-7 consist of 7 graticular blocks and 9 graticular blocks respectively. Both areas are located on the northeastern flank of the Bass Canyon in water depths ranging from 150–2000 m (Figure 6). Geological control is provided by several commercial oil and gas fields, including Flounder, Grunter, Basker/Manta/Gummy, Kipper and Blackback. Both Release Areas contain three wells, which were drilled during the later exploration phase in the basin and dominantly targeted intra-Latrobe Group plays.

In Release Area V18-6, the two deep wells Bignose 1 and Culverin 1 represent technical successes, confirming the presence of both gas and oil, respectively, in intra-Latrobe sediments. However, neither well is deemed a commercial discovery because the sediments did not show good quality reservoir characteristics. Great White 1 was designed to test a possible Blackback equivalent but post-drill analysis indicated no access to hydrocarbon migration pathways.

Release Area V18-7 is located on the far eastern part of the Central Deep and has remained underexplored. Hammerhead 1 was drilled along the downthrown side of the Rosedale Fault System and targeted a potential Kipper analogue. Shark 1 is the easternmost exploration well in the basin and was designed to evaluate the presence of an intra-Latrobe play similar to the Basker/Manta gas and oil accumulation. The well was unsuccessful showing no evidence for hydrocarbon migration in the area; however, it intersected good quality reservoir facies in the Golden Beach Subgroup. Whaleshark 1 is the easternmost test of a top-Latrobe Group target, involving possible channel facies at the edge of the Bass Canyon. Well failure was ascribed to breach of the top seal, which is not surprising as the regional seal of the Lakes Entrance Formation thins out rapidly in an eastward direction and is absent over much of the Bass Canyon.

Data coverage over both Release Areas is of mixed quality but includes some 3D surveys (Figure 7). New data is needed to image the sedimentary sequence development along the eastern and northern flanks of the Bass Canyon. The exploration history indicates that the hydrocarbon potential of both Release Areas is considerable. For example, the area around the shallower flanks of the Bass Canyon offers multiple exploration targets within the intra-Latrobe Group, including the Golden Beach and Emperor subgroups near the Rosedale Fault System in the northern part of Release Area V18-7 (Figure 8).