Bass Basin Regional Geology

Intracontinental extension between Australia and Antarctica during the Jurassic and Early Cretaceous resulted in the formation of a series of rift basins along the southern margin of Australia—the Southern Rift System (SRS; Stagg et al, 1990). The Bass Basin, which forms the easternmost element of the SRS, was initiated in the Early Cretaceous (Figure 2). Extension in the Bass Basin preceded eventual breakup between Australia and Antarctica (Blevin, 2003b), which commenced in the Bight Basin to the west in the latest Santonian–earliest Campanian (83 Ma; Sayers et al, 2001). To the east, commencement of breakup in the Tasman Basin in the Campanian was approximately coeval with breakup along the southern margin (Symonds et al, 1996; Blevin, 2003b). Blevin (2003b) argued that the location of the Bass Basin at the junction between two Cretaceous rift systems (the Southern Rift System and Tasman Basin) had a strong control on the evolution of the basin and resulted in the area experiencing multiple periods of deformation. Although extension progressed to breakup and seafloor spreading in the adjacent Southern Ocean and Tasman basins, the Bass Basin remained a ‘failed rift’ basin where breakup did not occur (Blevin, 2003b).

The Bass Basin is characterised by generally northwest to north-northwest trending half graben (Etheridge et al, 1985; Williamson et al, 1985; Smith, 1986; Blevin, 2003b; Cummings et al, 2004; Blevin et al, 2005). Half graben are bounded by large displacement normal faults that generally dip to the southwest to west-southwest in the Cape Wickham Sub-basin and northeast to east-northeast in the Durroon Sub-basin. The Cape Wickham and Durroon sub-basins are separated by the north-northeast-striking Chat Accommodation Zone (Figure 1). Blevin (2003b) suggested that the location of this feature and the structural architecture of the two sub-basins are likely to have been controlled by the underlying basement fabric, with the Durroon Sub-basin overlying the deformed sediments and granites of the Lachlan Fold Belt, and the Cape Wickham Sub-basin overlying an older fold belt terrane.

Cape Wickham Sub-basin

The Cape Wickham Sub-basin consists of a series of large northwest- to north-northwest-trending Cretaceous–Cenozoic half graben informally named the Cormorant, White Ibis, Yolla, Dondu and Pelican troughs (Lennon et al, 1999; Blevin et al, 2003, 2005; Cummings et al, 2004; Figure 1). Rift bounding faults generally dip to the southwest to west-southwest. The Cormorant Trough has a more of full graben architecture, with faults on both sides of the rift (Figure 3). The fault on the southern side of the trough partly defines a distinctive northwest-trending intra-basin horst block that is a key feature of the sub-basin. This horst is offset along-strike across a north-northeast striking feature, the Yolla Accommodation Zone, the location of which may reflect the underlying basement fabric (Blevin et al, 2003; Cummings et al, 2004; Blevin and Cathro, 2008). Fault throws in the sub-basin are in the order of 3–5 km, with the total sedimentary succession (syn-rift and post-rift) reaching a thickness of 8–10 km in the main depocentres. The sub-basin has been variably affected by compressional episodes in the Cenozoic that resulted in partial inversion of structures such as the Cormorant Trough (Figure 3).

The southwestern margin of the Cape Wickham Sub-basin is characterised by a structural ramp that dips basinward, while the northwestern margin is generally bounded by a steep fault that has been focus of reactivation during mid-Miocene compression. The northwestern margin of the sub-basin is heavily affected by volcanic intrusions and extrusions.

Durroon Sub-basin

The structural style of the Durroon Sub-basin differs significantly from that of the Cape Wickham Sub-basin. The Durroon Sub-basin (Baillie and Pickering, 1991; Das and Lemon, 2000) is characterised by a series of northwest- to north-northwest-trending half graben, bounded by northeast- or east-northeast-dipping faults, which are generally narrower and more intensely faulted than those in the western part of the basin (Blevin, 2003b; Figure 4). Some inversion structures are observed on the northern and eastern margins of the sub-basin. Blevin and Cathro (2008) proposed that these structures developed as a result of compressional deformation in the Cenomanian and mid-Eocene, prior to the deposition of the Flinders Sequence. The sub-basin contains a Cretaceous–Cenozoic rift and post-rift succession up to 7000 m thick (Baillie and Pickering, 1991).

The evolution of the Bass Basin was dominated by a series of distinct rift phases. The two earliest phases affected the entire basin, while the final period of crustal extension was concentrated in the Cape Wickham Sub-basin. The first phase of extensional deformation, the Otway rift phase, is generally interpreted to have occurred in the Early Cretaceous (Barremian–earliest Cenomanian; Figure 2), coeval with extension in the adjacent Otway Basin (e.g. Blevin, 2003b; Blevin et al, 2005). Cummings et al (2004) presented an alternative interpretation of basin evolution, suggesting that the initial rift phase in the Cape Wickham Sub-basin was older (Berriasian–mid-Barremian), equivalent to the syn-rift Crayfish Group of the Otway Basin.

The subsequent Turonian–Campanian Durroon rift phase (Figure 2) has been attributed to extensional stresses that preceded Tasman Basin break-up to the east (Cummings et al, 2002; Blevin and Cathro, 2008). During this period of extension, half graben initiated in the Otway rift phase continued to develop and expand, forming a system of linked depocentres (Blevin et al, 2005). The syn-tectonic successions deposited during the first two rift phases (Otway and Durroon sequences) are well imaged within the deeper half graben on seismic data (Figure 3, Figure 4), but have rarely been penetrated by drilling due to the depth of burial. Durroon 1 and Chat 1, in the Durroon Sub-basin, were the only wells to recover Lower and mid-Cretaceous sediments (Baillie and Pickering, 1991; Blevin, 2001; Partridge, 2003). Extension in the Durroon Sub-basin largely ceased in the mid-Campanian with sediments of Late Cretaceous to Miocene age deposited under dominantly sag (thermal subsidence) conditions (Blevin, 2003b; Figure 2).

A final phase of extension (Bass rift phase) occurred in the Campanian–early Eocene (Smit, 1988; Blevin, 2003b; Blevin and Cathro, 2008; Figure 2). This phase of extension focused particularly on structures within the Pelican, Yolla and Cormorant troughs (Figure 3), and resulted in episodic reactivation of existing Lower Cretaceous half-graben structures from the Campanian until the early Eocene. In the Cape Wickham Sub-basin, the oldest sediments penetrated by drilling are Campanian in age (Bass Rift Phase).

Until the late Eocene, deposition of non-marine sediments in the Bass Basin was controlled by internal drainage systems that migrated from the uplifted flanks of the basin into the developing half-graben structures. Following the cessation of rift-related activity in the latest early Eocene, accommodation was controlled by normal post-rift subsidence and eustatic fluctuations. Widespread marine flooding of the basin occurred from west to east in the late middle Eocene, although indications of periodic marine incursions are recorded as early as the Paleocene. Multiple periods of post-rift tectonic reactivation, including several episodes of Cenozoic inversion, have been recognised in the Bass Basin (Hill et al, 1995; Holford et al, 2011). The contractional events formed large scale anticlines within the syn- and post-rift successions, with several of these structures targeted by exploration drilling (e.g., Cormorant 1; Figure 3). Polygonal faulting is widespread throughout the basin (Das, 2001), but is generally limited to fine-grained late Oligocene to early Miocene successions (Blevin, 2003b; Blevin et al, 2005).

The Bass Basin and adjacent areas of mainland Tasmania and southern Victoria have been affected by multiple periods of volcanic activity during the syn- and post-rift phases of basin evolution. Offshore, these events are marked by the widespread emplacement of intrusive (sills and dykes) and extrusive rocks (hydrothermal vents, volcanic mounds and flows), particularly in the southern and western parts of the basin (Blevin and Cathro, 2008; Holford et al, 2012; Figure 3). Volcanic activity appears to have been focused along large-scale faults and accommodation zones in the basin (Etheridge et al, 1985; Cummings et al, 2002, 2004; Blevin and Cathro, 2008).

Stratigraphy

The Bass Basin contains sediments ranging in age from Early Cretaceous to Quaternary (Figure 2). Several stratigraphic nomenclature schemes have been proposed for the basin (Smith, 1986; Maung et al, 1993, Lennon et al, 1999; Cummings et al, 2002). These schemes vary mainly in their recognition of subdivisions within the Cretaceous to middle Eocene succession (e.g. Eastern View Group/Coal Measures/Formation).

In the early 2000s, a study aimed at integrating biostratigraphy, sequence stratigraphy and seismic stratigraphy to identify accommodation cycles and related environments, resulted in a revised sub-division of the stratigraphic succession (Boreham et al, 2003; Blevin et al, 2003, 2005). Six depositional sequences— mainly megasequences and supersequences—were recognised by this study and correlated to phases of tectonic activity and eustatic fluctuations. The following summary of the tectonostratigraphic development of the basin is based largely on the results of those studies. A recent analysis of the palaeogeographic evolution of the basin (Briguglio et al, 2013) provides additional detail about depositional systems in the area.

The key sequences of potential economic interest in the Bass Basin are the Bass, Aroo and Flinders sequences, which correspond approximately with the middle and upper sections of the Eastern View Formation (EVF). Thin successions of the older Durroon (equivalent to the lower portion of the EVF) and Otway sequences onlap the margins of the rift basin and may also be viable drilling targets in these locations, although they are probably of greater economic importance as potential source rocks in the deeper flanking half graben. Stratigraphic targets may also be present in the younger Torquay Sequence.

The mid-Campanian to lower Eocene Bass Megasequence was deposited during the final extensional phase in the western Bass Basin. This megasequence, formerly known as the Middle Eastern View Coal Measures (Lennon et al, 1999), is largely terrestrial in origin with intermittent brackish/marine influences evident during the early Paleocene and early Eocene. Sedimentation continued within an internal drainage basin setting, with fluvial systems feeding into the slowly subsiding depocentres from the uplifted basin flanks. The Bass Megasequence consists of the following component sequences: the mid-Campanian to Maastrichtian Furneaux Sequence, the late Maastrichtian to late Paleocene Tilana Sequence, and the late Paleocene to early Eocene Narimba Sequence. Each sequence was deposited during episodic periods of rift activity that were focused in the central and western Bass Basin (Cape Wickham Sub-basin). Within the depocentres, these sequences consist of expanded sections of aggradational fluvio-deltaic sandstones and siltstones, and finer-grained lacustrine sediments. On the rift flanks, sediments accumulated as thinner, stacked successions of fluvial-deltaic sandstones and interbedded over-bank shales, mudstones and thin coal beds. The systems of lakes that formed during the late Maastrichtian to early Eocene (Figure 5a) and their importance as source rocks in the Bass Basin are discussed in Boreham et al (2003) and Blevin et al (2005).

In the latest early Eocene to middle Eocene, tectonic activity in the Bass Basin began to wane. This marked a period of transition from rift to post-rift subsidence conditions, and resulted in a rapid decrease in the rate of accommodation and a distinct change in depositional style. At this time, the basin had relatively low relief and sediments were deposited in low-energy, meandering fluvial environments flanked by floodplains and ephemeral lakes. From the latest early Eocene until the early middle Eocene (Upper M. diversus and P. asperopolus spore/pollen zones), coal beds up to 25 m thick accumulated in peat mires that fringed the margins of the former rift depocentres. Lakes were formed in areas of maximum subsidence overlying the half graben, particularly in the northern and western parts of the basin. This coal-rich section is the Aroo Sequence (formerly the upper Eastern View Coal Measures; Lennon et al, 1999).

By the middle Eocene, faulting related to rift activity and sediment loading had largely ceased. Fluvial environments continued to dominate in the Durroon sub-basin, while thin coals and fine-grained sediments accumulated in lower delta plain and paralic environments further to the west (Flinders Sequence). Lakes and restricted lagoons filled the central and western parts of the basin, as marine water began to encroach from the west (Figure 5b). Third and fourth-order fluctuations in sea level (base level) resulted in rapid shifts of sediment facies along the basin margins, although sedimentation within the deeper parts of the basin was probably continuous. In the late middle Eocene (Middle N. asperus spore/pollen zone), a short regression led to incision across much of the basin and the deposition of a widespread sand facies. Locally, the sandy facies associated with the regression is known as the ‘Boonah Sands’. This sequence represents the ‘top of reservoir’ across most of the basin. A rapid flooding of the basin occurred in the late Eocene resulting in the deposition of the thick regional sealing shales of the Demons Bluff Formation. Shallow bay siltstones and shales were deposited until the latest early Oligocene. The predominantly terrestrial nature of sedimentation in the Bass Basin until the middle Eocene has meant that most reservoir and seal facies occur as interfingering and interbedded units.

The upper Oligocene to Quaternary Torquay Sequence consists of a lower argillaceous (mud and marl) succession up to 650 m thick, overlain by a calcarenite-dominated succession up to 860 m thick and ranging in age from late Miocene to Quaternary (Williamson et al, 1985).

There has been extensive and widespread magmatic activity in the Bass Basin, particularly in the southeast and western regions. Volcanic and intrusive rocks are intersected in many wells and imaged in seismic data (Figure 3).

Regional petroleum systems

The hydrocarbons discovered to date on the southern margin of Australia have been assigned to the Austral Petroleum Supersystem based on the age of their source rocks and common tectonic history (Edwards et al, 1999). Bass Basin oils are derived from Upper Cretaceous–Cenozoic source rocks and are therefore assigned to the Austral 3 system (Edwards et al, 1999). In the Cape Wickham Sub-basin of the Bass Basin, exploration drilling has confirmed accumulations at Yolla, Pelican, White Ibis, Cormorant, Trefoil and Rockhopper, reservoired in sands of Paleocene to Eocene age (L. balmei to M. diversus; Figure 2). At present, only the accumulation at Yolla has been developed (Origin Energy-operated BassGas Project). Exploration in the Durroon Sub-basin has not yet been successful. The hydrocarbon accumulations discovered in the Cape Wickham Sub-basin are assigned to the Austral 3 subsystem.

The Bass Basin oils and gases have a terrestrial source affinity and are geochemically similar to the Gippsland Basin oils (Summons, 1996); some geochemical differences are attributed to differences in source rock palaeoenvironment, palaeogeography and biota between the two basins (Summons et al, 2002). The Yolla 1 oil is a medium-gravity (46°API) crude with a correspondingly high wax content, whereas the Cormorant 1 oil is a heavy crude (21°API) as a result of biodegradation (Edwards et al, 1999). Geochemical analyses of the hydrocarbons indicate an input of predominantly land-plant material to the source kerogens, and there is no evidence of marine organic matter (Edwards et al, 1999).

Source rocks and maturation

Potential source rocks in the Bass Basin are non-marine and include Maastrichtian to middle Eocene coals and carbonaceous claystones (Boreham, 2003; Boreham et al, 2003; Figure 2). These successions were deposited in low-energy fluvial, deltaic and lacustrine environments (Blevin et al, 2003; Lang et al, 2003). The coals are between 5 m and 25 m thick, and are rich in extractable organic matter with significant petroleum generation potential. Coals within the upper Bass and Aroo sequences (upper EVF) generally have high hydrocarbon indices and contain abundant exinite and vitrinite (Miyazaki, 1995). Differences in the organic character and relative abundance of coals have been recognised, and are attributed to fluctuations in regional water tables and sea level during deposition (Baillie, 1992). In general, coals are thicker and occur more frequently within the younger Aroo Sequence (upper M. diversus and P. asperopolus spore/pollen zones). In addition, the younger coals are richer in exinite and poorer in inertinite than coals within the underlying stratigraphic succession (Smith, 1986).

3D petroleum systems modelling by Arian et al (2010) indicates that maturity values are highest in the Cape Wickham Sub-basin and the Bark Trough of the Durroon Sub-basin; the rest of the Durroon Sub-basin is less mature. Paleocene and older source rocks are mature for hydrocarbon generation and expulsion across much of the basin, particularly the deeper central areas in Cape Wickham Sub-basin. Deeper undrilled source rocks may be present in the Durroon and Otway sequences. Arian et al (2010) suggested that there was early expulsion of hydrocarbons from Lower Cretaceous source rocks in the Otway Megasequence; these hydrocarbons are modelled to have migrated upwards via permeable faults prior to the deposition of a regional seal.

Some early studies (Nicholas et al, 1981; Miyazaki, 1995) proposed that liquid hydrocarbons were generated from disseminated organic matter within carbonaceous claystones, while gases were sourced mainly from coaly facies. However, oil-to-source correlations undertaken at Geoscience Australia (Boreham et al, 2003) have shown that the oils were sourced by lower Paleogene coals, particularly those concentrated within the Paleocene to lower Eocene succession (Boreham et al, 2003). Oils in the basin are a single oil population (Yolla and Pelican accumulations), while in-reservoir biodegradation of the Cormorant oil has resulted in its statistical classification as a separate oil family (Boreham et al, 2003). Fluid inclusions studies (Kempton et al, 2002) have identified palaeo-oil zones at Yolla 1 and Cormorant 1, along with suspected zones at King 1 and Pelican 5. In addition, several potential palaeo-hydrocarbon zones were identified at Yurongi 1, Chat 1, Seal 1, Tilana 1 and Squid 1. These zones are characterised by aqueous inclusions containing small amounts of oil (Kempton et al, 2002).

Gaseous hydrocarbons have a similar source to the oil, but have been generated over a wider maturity range. Boreham (2003) reported that Yolla gases are associated with Yolla oils and both were generated from the same source and at similar maturities; White Ibis gases were generated at a higher maturity than the Yolla gases.

While Paleogene coals are abundant in the Cape Wickham Sub-basin, in the Durroon Sub-basin, the equivalent section is either missing or the facies are sand-prone (DITR, 2004). Therefore, in this part of the basin potential source rocks are likely to be older, occurring within the Barremian–Maastrichtian section.

Geochemical analysis and maturity modelling (Boreham, 2003; Boreham et al, 2003; Cummings and Tingate, 2003; Radlinski et al, 2003) suggests that the basin-wide onset of oil generation and expulsion in the Bass Basin occurs at a depth of 2450–3000 m. Blevin et al (2005) reported that the key events in the process of petroleum generation and migration from source rocks were:

• Onset of oil generation occurred at a VR of 0.65% (2450 m in Pelican 5)
• Onset of expulsion (primary migration) occurred at a VR of 0.75% (2700–3200 m across the basin; 2850 m in Pelican 5)
• Main oil window occurred between VR of 0.75% and 0.95% (28500–3300 m in Pelican 5)
• Main gas window occurred at VR>1.2% (>3650 m in Pelican 5)

A zone of overpressure below about 2800 m (Miyazaki, 1995) and a permeability barrier at depths greater than 2900 m are probably related to a decrease in pore space as a result of oil moving from smaller to larger pore space (Radlinski et al, 2003). This depth correlates to a VR of about 0.75%, the onset of primary oil migration.

Geohistory and maturation modelling indicates that key potential source rocks (Paleocene–lower Eocene coals) entered the oil generation window just prior to and following deposition of the Demons Bluff regional seal (Flinders sequence), and after the major structural trap forming events in the Late Cretaceous and early Eocene (Cummings and Tingate, 2003: Williamson and Pigram, 1986). Paleocene to early Eocene units continued to pass into the oil expulsion window during and after Miocene structural events. A petroleum systems modelling study by Arian et al (2010), suggested that expulsion from these oil-prone source rocks (Paleocene–early Eocene Narimba Sequence) in the deeper parts of the Yolla Trough only commenced in the last 5 million years. Migration from mature source rocks into available traps may have occurred via local fault-related conduits and intra-stratal fairways. The overlying upper Eocene and younger marine succession (Flinders and Torquay sequences) is immature across the basin. Overall, the pattern of hydrocarbon distribution in the basin shows that hydrocarbon generation has occurred in the Cormorant, Yolla and Pelican troughs with migration into structures within the depocentres and on the adjacent flanks.

Reservoirs and seals

Proven petroleum-bearing reservoirs in the Bass Basin occur within the Paleocene and lower Eocene succession (Bass and Aroo sequences; Figure 2). These reservoirs are predominantly fluvio-deltaic sandstones with a net thickness of 20–50 m. Facies analysis and correlation of cores with wireline logs suggests that reservoir sandstones were deposited in a range of terrestrial, paralic and shallow marine environments (Baillie et al, 1991; Brooks et al, 2006). The lateral continuity of these facies is expected to vary widely. Porosity analyses on cores and sidewall cores at Cormorant 1 and Yolla 1 recorded average values of 15–25%. In general, porosity declines moderately with increasing depth (approximately 5.7 porosity units per 1000 m; Meszoly et al, 1986), while permeabilities range from 16 mD to 308 mD.

A study of the nature and distribution of porosity by Meszoly et al (1986) suggests that present-day reservoir quality is highest between 1900 m and 2700 m subsea over the extent of the basin. Two depth-related diagenetic zones have been recognised within the quartz-rich siliciclastics of the Bass and Aroo sequences—an upper zone characterised by carbonate authigenesis from 1950 m to the top Aroo Sequence; and a lower zone lying below 2000 m, characterised by dissolution, compaction, quartz overgrowth cementation, and authigenic illite and kaolinite (Meszoly et al, 1986).

Hydrocarbon accumulations at the Yolla and White Ibis fields and in the Pelican wells are reservoired in fault blocks and associated anticlinal structures within the Bass and Aroo sequences. These traps are sealed by intraformational shales that were deposited in lacustrine and interdistributary to floodplain environments. The upper Eocene Flinders Sequence (Demons Bluff Formation) contains a marginal marine to shallow marine shale that blankets much of the Bass Basin. The shale forms a thick regional seal over the underlying terrestrial reservoir successions. Seal analyses of recent wells such as Barramundi 1, have highlighted the negative implications of late stage reactivation events on trap integrity that has often resulted in localised fracturing and faulting of the regional seal. Details of the sealing capacity of mudstone and siltstone units from Bass Basin wells is presented by Daniel et al (2003).

Play types

Release Areas T17-1 and T17-2 cover most of major depocentres of the Bass Basin (Figure 1). In the western Cape Wickham Sub-basin (T17-1), the Cormorant and White Ibis troughs are known to have generated hydrocarbons, as shown by gas and condensate accumulations at the White Ibis, Bass, Trefoil and Rockhopper fields, and at Cormorant 1 (Lennon et al, 1999; Blevin et al, 2003; Boreham et al, 2003). To the east, accumulations at Yolla and Pelican attest to the presence of a petroleum system in the Yolla and Pelican troughs. Play types for the basin include rotated fault blocks and overlying drape closure, as well as extensive reactivation structures such as inversion anticlines, crestal collapse fault blocks and deformation associated with the intrusion of igneous rocks (e.g., Trefoil 1). Exploration of similar plays in the Durroon Sub-basin has so far been unsuccessful.

Wells in the western Bass Basin have tested a range of plays, including:

1. onlap onto or structural closure (drape) over the crest of intra-basin fault blocks located updip from deep flanking half-graben (Yolla 1, White Ibis 1, Aroo 1, Rockhopper 1 and Tilana 1);
2. Miocene and younger inversion structures and fault blocks associated with crestal collapse (Cormorant 1, Toolka 1A, Spikey Beach 1);
3. Eocene anticlines within the hanging wall of half graben and associated crestal collapse fault blocks (Pelican wells, Peejay 1);
4. low-relief fault blocks within the basin associated with late-stage growth faults (Poonboon 1, Tarook 1);
5. footwall fault blocks along the basin margin with stratigraphic onlap (Tasmanian Devil 1, Flinders 1).

Many of these plays have experienced late stage enhancement of structural closure (Eocene, Oligocene and Miocene) that may have partially breached in-place accumulations (White Ibis 1, Cormorant 1). The most successful plays have been those located updip or along the flanks of Cretaceous half graben where growth faulting continued until the early Eocene.

In the Durroon Sub-basin (T17-2), play types include rotated faults blocks with structural closure extending upward into onlapping and overlying strata, with charge provided by mature source rocks in the deepest and thickest successions in the half graben. Stratigraphic traps are also possible in sandy facies along the basin margin (alluvial fans), and lacustrine deltaic and turbidite facies associated with the “Durroon Lake” near the base of the Durroon Megasequence. Mature source rocks are likely to occur in Eumeralla Formation-equivalent coals in the Otway Megasequence. Potential long-range migration from the western Bass Basin is also possible given the westward-thickening, wedge-like geometry of Upper Cretaceous and younger strata across the basin (DITR, 2004).

A range of plays based on 3D petroleum systems modelling is presented by Arian et al (2010).

The Bass Basin is a moderately explored basin with 45 wells drilled since 1965 (Figure 1). Similar to other basins along the Australia’s southern margin, exploration has occurred in phases, although development of the discovered reserves and production from the Yolla Field has only commenced in recent years.

Hydrocarbon exploration in the Bass Basin began in the early 1960s with permits awarded to Hematite Petroleum Pty Ltd (BHP) and Esso Exploration and Production Australia Ltd (Esso) (Robinson, 1974; Smith, 1986). Bass 1, the first well in the basin, was drilled by Esso in 1965. Esso’s subsequent exploration program included 15 wells drilled from 1966 to 1974; Hematite drilled four wells between 1974 and 1982. During this exploration phase, gas and condensate accumulations were discovered at Pelican 1 and Pelican 2, while numerous gas shows were recorded at several wells (including Bass 3, Cormorant 1, Pelican 4 and Aroo 1). Only one well was drilled in the eastern Bass basin during this period, Durroon 1. The well was unsuccessful, failure being attributed to minimal structural closure, poor reservoir quality, poor regional sealing facies and a lack of hydrocarbon charge (Trigg et al, 2003).

In 1985 Amoco Australia Petroleum Company made a significant gas discovery at Yolla 1, a crestal test of the Eastern View Formation (EVF) within a four-way dip and fault-closed structure. The well encountered five separate gas columns in the middle Eastern View Formation, as well as a separate hydrocarbon column (wet gas over an oil leg) in the upper EVF (Brooks et al, 2006). The stacked hydrocarbon accumulations within the EVF at Yolla are sealed by relatively thick and laterally continuous intraformational lacustrine shales (Lennon et al, 1999).

The following year, Amoco Australia drilled Pelican 5 in the southern part of the basin to test the production potential of the lower Eocene sands (M. diversus spore/pollen zone) of the middle EVF, which had good shows of gas and condensate at Pelican 1, 2 and 4, but had not been tested for production. Numerous hydrocarbon shows were noted while drilling through the EVF. Gas was produced during two tests, and wireline log evaluation indicated the presence of moveable hydrocarbons at various levels (Amoco, 1987). Hydrocarbons were found in limited quantities, with the low permeability of EVF reservoirs preventing the accumulation of economic quantities of hydrocarbons. In the Durroon Sub-basin, Chat 1 was drilled by Bridge Oil to test a large fault block with overlying anticlinal closure located near the western boundary of the sub-basin; however, no significant hydrocarbon shows were encountered during drilling, nor were any indicated from log analysis.

In 1998, Boral Energy Resources Ltd and partners drilled two wells in the central Bass Basin—Yolla 2 and White Ibis 1. Both wells encountered gas columns in the Paleocene to lower Eocene succession, with liquids-rich gas recovered from reservoirs in wireline tests (Lennon et al, 1999). Yolla 2 was an appraisal well that confirmed a resource of 450–600 Bcf (12.74–17.0 Bcm) of liquids-rich gas and 70 MMbbls (11.1 MMcm) of oil in-place (OIP). The White Ibis 1 well was a crestal test over a large basement high up-dip of Bass 3. Formation pressure data suggests the presence of a thin oil rim, with the well encountering a gas accumulation with an estimated 85 Bcf (2.4 Bcm) of gas in-place. The well was suspended for possible future production (Premier Oil, 1998).

In mid-2004, a consortium of Origin Energy Resources Limited (Permit Operator and 32.5% participant), Origin Energy Northwest Limited (5%) and joint venture partners, AWE Petroleum Pty Ltd (30%), CalEnergy Gas (Australia) Limited (20%) and Wandoo Petroleum Pty Ltd (12.5%) commenced a three-well drilling campaign in the western part of the basin. The first two wells were the Yolla 3 and Yolla 4 development wells in production licence T/L1 (Figure 6). These wells successfully encountered and tested several gas zones, and an oil and gas saturated sand in the upper EVF.

The final well in the three well drilling campaign was Trefoil 1, located in the White Ibis Trough, 37 km west of the Yolla production platform. The well intersected approximately 50 m of net gas pay over eight zones in good quality sandstone reservoirs within the EVF and was confirmed as a wildcat gas-condensate discovery; the well was cased and suspended for possible future production (Brooks et al, 2005). Early estimates for in-place resources for the field ranged from 200–300 Bcf (5.6–8.5 Bcm) of gas and 14–21 MMbbls (2.2–3.4 MMcm)  of associated liquids (AWE, 2004).

The most recent phase of exploration (2005–12) saw the acquisition of several 3D and 2D seismic data sets in both the Cape Wickham and Durroon sub-basins, and the drilling of 6 exploration wells and one side-tracked well. Spikey Beach 1 and Peejay 1 were drilled by Beach Energy in the Pelican Trough, to test anticlinal closures in the upper EVF for predicted oil accumulations; however, both were dry. Origin drilled Trefoil 2 to appraise the accumulation encountered in Trefoil 1, with the well providing additional control on reservoir and hydrocarbon distribution (Hall, 2012a). This was followed by the successful drilling of Rockhopper 1. The well (and its side-tracked appraisal well Rockhopper 1 ST1) intersected up to 28 separate hydrocarbon (gas condensate and oil) bearing intervals within Paleocene and Cretaceous sandstones of the EVF, with a total net hydrocarbon pay of 28.9 m. The accumulations at Rockhopper 1 were interpreted to have been sourced from mature source rocks in the Yolla Trough to the east, and the Cormorant Trough to the north (Hall, 2012b).

The most recent exploration wells drilled in the basin—Silvereye 1 drilled by Origin in the southwestern part of the Cape Wickham Sub-basin, and Craigow 1 drilled by TAP on the northeastern edge of the Cormorant Trough—were both unsuccessful. In 2014-15, a further two production wells were drilled and brought online at Yolla—Yolla 5 and Yolla 6.

While most recent exploration activity, including all drilling, has been focused on the Cape Wickham Sub-basin, 3D Oil and Bass Strait Oil also carried out exploration programs in the Durroon Sub-basin. New 2D and 3D seismic data sets were acquired and prospects identified; however, no wells were drilled and the three permits in the sub-basin were relinquished in 2012.

Currently, one production lease and four retention leases are held in the Bass Basin (Figure 6).

Gas, condensate and LPG are currently produced by the BassGas project from the Yolla Field. The BassGas project (a joint venture between Origin Energy and AWE Petroleum, and later partners) was formed to develop the Yolla field, and is the first production of hydrocarbons from the Bass Basin. The production platform was installed in 2004, a pipeline to Victoria was constructed, and first gas was produced in June 2006 (Offshore-Technology.com, 2017).

In June 2016, AWE reported that average gross daily production rate at BassGas was 51 TJ/d (AWE, 2016b). Total production from BassGas in 2015-2016 was 18.25 PJ natural gas, 611.5 kbbls (97.2 Mcm)  condensate and 54.7 kt LPG (AWE, 2015a, 2015b, 2016a, 2016b).

In June 2016, Origin Energy reported that net proven and probable (2P) reserves for the Yolla field were 50 PJe, down from 90 PJe in June 2015 (Origin Energy, 2015, 2016a); this equates to a gross 2P for the field of 117 PJe in 2016, down from 211 PJe in 2015, based on Origin’s 42.5% share in BassGas. This downgrade followed 12 months production to June 2016 and revisions based on the results of Yolla 5 and Yolla 6 (Origin Energy, 2016a).

The production facility is currently undergoing a Mid-Life Enhancement project designed to fully develop the field and ensure that production continues into the next decade; tie-in and commissioning of compression and condensate modules on the Yolla platform is expected to be completed in 2017 (Origin Energy, 2016b; AWE, 2017).

Introduction

This section provides an overview of the environmental characteristics of the Acreage Release areas and the general setting of the region overlying the Bass Basin (Figure 7), together with information focused on the environmental geoscience characteristics of individual acreage release blocks.

The Bass Basin lies beneath the Bass Strait off the northern coast of Tasmania. Release Areas T17-1 and T17-2 cover mostly shelf and basin geomorphic features within the Bass Basin. The southwestern edge of T17-1 and the southeastern edge of T17-2 extend onto geomorphic plateaus associated, respectively, with the King Island High surrounding King Island in the west, and the Bassian Ridge and Flinders Island in the east.

Industrial ports in the area include Burnie, Devonport, Bell Bay and Melbourne. Five Commonwealth and three Tasmanian Fisheries operate in the area.

No Commonwealth Marine Reserves (CMR) overlap the Bass Basin Release Areas, however the Boags and Beagle CMRs are located west of T17-1 and north of T17-2, respectively.

Climate of the region

The region is characterised by a cool temperate climate, with precipitation throughout the year and few temperature extremes. Mean minimum and maximum annual temperatures are 8.3°C and 17.0°C for the period 1991–2017 (http://www.bom.gov.au/). Mean annual rainfall recorded over 52 years at Devonport Airport is 773 mm (http://www.bom.gov.au/).

Based on a 20-year average, sea surface temperatures (SST) in this region are comparatively cool for Australian waters (around 15°C), although there are high levels of interannual variations in SSTs often linked to warm eastern incursions from the East Australian Current (R. Przeslawski, Geoscience Australia, pers. comm., March 2017). SSTs are predicted to increase by 0.5°C per decade in the immediate future (Foster et al, 2014).

Oceanic regime

Bass Strait is a 250 km wide, shallow microtidal seaway between mainland Australia and Tasmania. The granitic King Island High to the west and the Bassian Rise to the east isolated the basin (the offshore Bass Basin) at the 60 m isobath during times of glacial low sea level (Blom, 1988; Blom and Alsop, 1988). Li et al (2007) defined the complexity and seasonality of the most significant geostrophic currents that interact to affect Bass Strait: the Flinders, Zeehan, East Australia, and Antarctic Circumpolar currents. Persistent and strong westerly winds also result in uniform velocity profiles through the strait and drive the eastward flowing Westwind Drift throughout the year (Blom and Alsop, 1988). In general, salinity and temperature are consistent throughout the water column, indicating high levels of vertical mixing (Huang and Przeslawski, 2016).

Maximum significant wave heights (MSWH) approaching the western coast of Tasmania are some of the largest in Australia (<9 m), however, Bass Strait is partly protected from these larger swells and MSWH over the western (5–6 m) sill reduces to 4–6 m across the central to eastern Bass Basin and Strait (Li et al, 2007). Maximum tidal speeds are exerted over these flanking sills (0.3 m/s to >0.6 m/s), and tidal currents in the central Basin are much reduced (<0.3 m/s) (Li et al, 2007).

Passlow et al (2004) presented modelled wave to tidal ratios throughout Bass Strait and determined that, while tidal mobilisation dominates a portion of the central western Bass Basin, the remainder of the basin is dominated by waves only. This pattern is reflected in the rates of sediment accumulation whereby the central basin is accumulating at twice the rate (<12 cm/1000 yr) of the basin margins (<6 cm/1000 yr) (Blom and Alsop, 1988), and generally high energy conditions transport fine-grained sediments away from the basin rim and deposit them in either the lower-energy central basin or on the shelf (Fandry, 1981; Harris, 1994; Harris and Coleman, 1998).

The river catchment sediment yield rate in the southeastern Australian region is in the order of 10–30 t/sqkm per annum (Harris, 1995). Li et al (2007) identified the 18 most influential rivers discharging into the Bass Strait region, and of these the Tamar River (Tasmania: low sediment load; flow discharge 6600 GL/a), Snowy River (Victoria: medium sediment load; flow discharge 4000 GL/a), Latrobe River (medium sediment load; flow discharge 2250 GL/a) and Arthur River (Tasmania: medium sediment load; flow discharge 2100 GL/a), are the largest. By global standards, Australian rivers discharge only modest sediment loads into the sea (Blom and Alsop, 1988; Bax and Willams, 2001; Bax et al, 2001; Roy et al, 2001) and, even by Australian standards, the Bass Strait rivers have a low sediment yield (Harris et al, 2002).

Seabed environments: regional overview

The central portion of Bass Strait forms a basin with a maximum depth near its centre of 83 m. Shelf encircles the basin and is, in turn, bounded to the east and west by shallow (55 m) sills and plateaus (Heap and Harris, 2008). Tidal sand waves and sand banks have accumulated atop these shallower features (Malikides et al, 1988, 1989), which coincide with the enhanced tidal velocities generated by ocean current constriction between the land masses (Malikides et al, 1988, 1989; Morrow and Jones, 1988; Black and Hatton, 1992). Amongst these sand waves and banks, seabed erosion has been modelled (Li et al, 2007). Evidence for erosion is provided by the presence of gravel lags (Passlow et al, 2004) and current-parallel depressions (Harris et al, 2003).

Li et al (2007) modelled likely seabed change as a result of climate change scenarios. Their work suggests that, while small changes in climate will result in large (-54% to +24%) changes in total sediment delivery to the Bass Strait region, the absolute contribution of the fluvial sediment to the seabed will remain small and it will be largely overwhelmed by stronger marine forces (waves and currents). Indeed, they assert that climate change may exacerbate sea bed erosion already occurring on the topographic highs in the Bass Strait as a result of potentially strengthened waves and currents. They also identified potential turbidite activity in the central-southern Bass Basin under present and predicted future climate scenarios.

Passlow et al (2004) have summarised Bass Strait seabed sedimentology (see also Jones and Davies, 1983; Blom and Alsop, 1988). In general, the seabed is characterised by cool-water carbonates with a low terrigenous content. Fine-grained sediments (muds and silty sands) are restricted to the deeper waters of the Bass Basin. Gravels and sands cover the remainder of the shelf (Passlow et al, 2004; James et al, 2008). Fine shelly sands occur along the inner shelf of the south-eastern Victorian coast and north of Flinders Island. Moderately well and well sorted sediments are restricted to nearshore environments and to areas between Flinders Island and Mornington Peninsula. Sediments over the remainder of Bass Strait are poorly to very poorly sorted and include quartzose sands, and bryozoan sands and gravels. Carbonate content within the Bass Basin generally exceeds 80% of seafloor sediments. With the exception of restricted occurrences of limestone and beach rock, the carbonate fraction in coarse sediments consists of recognisable skeletal debris derived from bryozoans, molluscs and foraminifera (Passlow et al, 2004; James et al, 2008). Sediments in the Bass Basin also contain a high proportion of carbonate mud derived from the breakdown of carbonate material and containing the remains of coccoliths, benthic foraminifera and tunicate spicules. The sand fraction of these sediments contains benthic foraminifera, bryozoans and molluscs, with minor amounts of echinoids, brachiopods, ostracods and worm tubes, while the gravel fraction is dominated by bryozoans. The non-carbonate fraction generally consists of quartz, with minor amounts of lithic fragments, feldspar and ferromagnesian minerals. Silica, in the form of sponge spicules, is present in small quantities throughout. Quartz and clay are present also in the mud fraction of basin sediments. Distinction of modern and relict material, both in the biogenic and siliciclastic fractions, is difficult in many sediment types. Relict material, characterised by poor sorting and multi-modal grain size distribution, may comprise up to 50% of some sediments.

Ecology

The Bass Basin is part of the Southeast Marine Bioregion which extends from the far south coast of New South Wales to Kangaroo Island. This region hosts high levels of biodiversity and endemism, particularly of macroalgae (Womersley 1990), and possibly represents one of the most diverse marine floral assemblages in the world (Commonwealth of Australia, 2005). At a regional scale, habitats include kelp forests, temperate seagrass meadows, and rocky reefs, as well as extensive soft sediment ecosystems. These marine habitats support a diverse range of temperate marine plants, invertebrates, fishes, and larger marine animals such as whales, dolphins and seals. Humpback Whales, seabirds and other threatened or migratory animals can migrate through this region on their way to breeding grounds, and other species such as Blue Whales may use this region as a feeding ground (Gill et al, 2011). At the smaller scale of provincial bioregions (IMCRA, 2006), the Bass Basin is included in the Bass Strait IMCRA Province, which is an area of fauna, flora and ocean conditions distinct from adjacent areas (Commonwealth of Australia, 2005).

Recent geological history

The Bass Basin formed during the Late Cretaceous and Cenozoic during the break-up of Australia and Antarctica (Veevers, 1984). Paleozoic granitic basement ridges form the eastern and western highs that partly enclose the basin (Blom and Alsop, 1988). Quaternary sea level fluctuations have repeatedly exposed the eastern Bassian Rise, which lies at the modern 55 m isobath, forming a large marine embayment with an opening to the northwest. When sea level dropped still further to the modern 67 m isobath, the King Island High to the west was also exposed and the basin formed a shallow saline lake (Blom and Alsop, 1988). Facies changes in cores have been dated and linked directly to these changes in sea level. A succession of highly calcareous, poorly sorted muddy bryozoan sands towards the base of several cores described by Blom (1988) likely relate to deposition during the latter stages of Marine Isotope Stage 3 when sea level was at least 60 m below present levels (Murray-Wallace, 2014). The later Pleistocene–Holocene stratigraphy of the Bass Basin is broadly described as lacustrine facies overlain by brackish-water (embayment) facies (approximately 10–12 ka BP), overlain by fully marine Holocene facies (approximately 9 ka BP), demonstrating the flooding of the basin by rising post-glacial sea level (Blom, 1988; Blom and Alsop, 1988; Murray-Wallace, 2014).

Commonwealth Marine Reserves

Boags Commonwealth Marine Reserve

https://www.environment.gov.au/topics/marine/marine-reserves/south-east/boags

Major conservation values

• Ecosystems, habitats and communities associated with the Bass Strait Shelf Province, and associated with plateau and tidal sandwave/sandbank sea-floor features
• Important foraging area for Shy Albatross, Australasian Gannet, Short-tailed Shearwater, Fairy Prion, Black-faced Cormorant, Common Diving Petrel and Little Penguin

The reserve is zoned Multiple Use Zone IUCN Category VI (Figure 7). Management plans for the South-east Marine Reserves Network are in place. Mining operations, including exploration are allowed.

Beagle Commonwealth Marine Reserve

https://www.environment.gov.au/topics/marine/marine-reserves/south-east/beagle

Major conservation values

• Ecosystems, habitats and communities associated with the Southeast Shelf Transition, and associated with basin, plateau, shelf and sill sea-floor features
• Important migration and resting on migration area for Southern Right Whale
• Important foraging area for Australian Fur Seal, Killer Whale, White Shark, Shy Albatross, Australasian Gannet, Short-tailed Shearwater, Pacific and Silver gulls, Crested Tern, Common Diving Petrel, Fairy Prion, Black-faced Cormorant and Little Penguin

The reserve is zoned Multiple Use Zone IUCN Category VI (Figure 7). Management plans for the South-east Marine Reserves Network are in place. Mining operations, including exploration are allowed.

Fisheries

The following fisheries occur in the Bass Basin area:

• Scallop Fishery. The Tasmanian zone is between the high tide mark, excluding bays and inlets, out 20 . The fishery was closed in 2016 (http://dpipwe.tas.gov.au/sea-fishing-aquaculture/commercial-fishing/scallop-fishery). The Commonwealth zone is between the Tasmanian and Victorian fisheries that lie within 20  of their respective coasts (http://www.afma.gov.au/fisheries/bass-strait-central-zone-scallop-fishery/). The fishing season runs 1 April–31 December.
• Eastern Tuna and Billfish Commonwealth Fishery operates in the Australian Fishing Zone and adjacent high seas. The season runs for 12 months starting 1 March (http://www.afma.gov.au/fisheries/eastern-tuna-and-billfish-fishery-page).
• Small Pelagic Commonwealth Fishery, western sub-area. The season runs for 12 months starting 1 May (http://www.afma.gov.au/fisheries/small-pelagic-fishery/).
• Southern and Eastern Scalefish and Shark Fishery, Commonwealth Southeast Trawl Sector and Commonwealth Scalefish Hook Sector. The season runs for 12 months starting 1 May (http://www.afma.gov.au/fisheries/southern-eastern-scalefish-shark-fishery/).
• Southern Bluefin Tuna Commonwealth Fishery. The season runs for 12 months beginning 1 December. (http://www.afma.gov.au/fisheries/southern-bluefin-tuna-fishery/).
• Rock Lobster Fishery extends to 20  from the coastline (http://dpipwe.tas.gov.au/sea-fishing-aquaculture/commercial-fishing/rock-lobster-fishery).
• Commercial Scalefish Fishery is a diverse multi-species, multi-gear fishery. The licensing year runs from 1 March. (http://dpipwe.tas.gov.au/sea-fishing-aquaculture/commercial-fishing/scalefish-fishery/commercial-scalefish)

Information resources

National Conservation Values Atlas http://www.environment.gov.au/webgis-framework/apps/ncva/ncva.jsf

Boags Commonwealth Marine Reserve https://www.environment.gov.au/topics/marine/marine-reserves/south-east/boags

Beagle Commonwealth Marine Reserve https://www.environment.gov.au/topics/marine/marine-reserves/south-east/beagle

AFMA Commonwealth Fisheries http://www.afma.gov.au/fisheries/

Tasmanian Commercial Fisheries http://dpipwe.tas.gov.au/sea-fishing-aquaculture/commercial-fishing

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