TECTONICS
During Late Jurassic to Early Cretaceous time, the Gippsland Basin was part of a rift complex several hundred kilometers wide that formed between the Australian Plate/Tasman Fold Belt and the Antarctic Plate (Rahmanian and others, 1990; Etheridge and others, 1987; Falvey and Mutter, 1981). This Bassian Rift System included three major components from west to east, the Otway, Bass and Gippsland Basins (Mehin and Bock, 1998). By mid-Late Cretaceous time, this rift complex was the site of the separation of Gondwana along what is now the southern margin of Australia (Rahmanian and others, 1990). Although the Late Jurassic rift complex was located between Australia and Tasmania, Late Cretaceous separation and creation of ocean crust occurred to the west of Tasmania (Rahmanian and others, 1990; Mehin and Bock, 1998). Separation and creation of ocean crust in the latest Cretaceous Tasman Rift event progressed along the eastern side of Tasmania and Australia, separating (1) the Australian Plate from the continental crust of New Zealand/Lord Howe Rise, and (2) the Antarctic Plate from continental crust of the Campbell Plateau (Rahmanian and others, 1990), leaving Tasmania attached to Australia. Regional extension and development of syn-rift troughs and volcanism, beginning at approximately 96 million years ago (Ma), occurred in the Gippsland Basin and began a history separate from the other two components of the earlier Bassian Rift System (Mehin and Bock, 1998; Hill and others, 1995; Lowry and Longley, 1991).

STRUCTURE 
The Gippsland Basin trends east-west and is comprised of a deep central depression symmetrically bounded by faulted terraces and stable platforms on the north and south. The Northern, or Lakes Entrance Platform, is bounded on the south by the Lake Wellington fault and the Southern Platform is bounded on the north by the Foster fault system (Fig. 3 and Fig. 4). Sediments of the basin crop out in the west and are deeply buried offshore and to the east. The deep Central Depression (Central Deep) continues onshore as the Seaspray Depression (Fig. 3 and Fig. 4). It lies between the Rosedale fault, which defines the southern edge of the Northern Strzelecki Terrace, and the Darriman fault, which defines the northern edge of the Southern Strzelecki Terrace (Fig. 3).

Normal faults trending northwest to southeast, of Early Cretaceous to Early Eocene age, characterize the central depression (Fig. 3) (Ozimic and others, 1987). The major fields are located in anticlinal traps formed by compressional events and shear faults associated with the opening of the Tasman Sea (Fig. 1). Late Eocene to Early Oligocene and Late Miocene age anticlines are generally oriented southwest to northeast and are located in the central depression and on the northern terrace (Ozimic and others, 1987).

A variety of Paleozoic rocks underlies the Gippsland Basin Province. Much of the area was covered by glaciers that came from Antarctica, located to the southwest, in Early Permian time (Bowen and Thomas, 1976). The Strzelecki Group is described at various locations as unconformably overlying Permian sediments, Upper Devonian rocks, Lower Devonian rocks, Devonian granite, and in other locations as faulted against Lower Devonian or Silurian beds (Douglas, 1976). At one onshore well location on the Northern Strzelecki Terrace near the Lake Wellington fault, a post-glacial, marginal marine to deltaic Lower Permian sandstone, shale, and siltstone section more than 180 m thick was described overlying folded Ordovician rocks (Bowen and Thomas, 1976; Douglas, 1976). Both crystalline basement and metamorphic basement have been described in onshore wells on the Lakes Entrance Platform. Extrusive igneous rocks were reported in the most southerly well in the province, which is located on the Southern Platform offshore Flinders Island. Jurassic age rocks were reported in wells located onshore on the Southern Strzelecki Terrace in the westernmost area of the province, and on the Northern Strzelecki Terrace near Lake Wellington (Petroconsultants, 1996).

DEPOSITION
Strzelecki Group
The Gippsland Basin includes Lower Cretaceous continental and lacustrine clastics deposited in an extensional rift trend that extended along the modern southern margin of Australia and which had formed between the then connected Australia Plate and the Antarctic Plate of Gondwana. These deposits are known as the Otway Group in the Otway Basin area of the southern margin, which is located northwest of the Bass Basin Province 3907 (Fig. 1), and the Strzelecki Group in the Gippsland Basin.

The known extent of the nonmarine Strzelecki Group (Korumburra Group, near offshore; Koonwarra Fish Beds and Tyers Group, Rhyll Arkose, onshore) (Douglas, 1976), is roughly between the Foster fault system to the south and the Lake Wellington fault to the north and from the Gippsland Highlands east, to where the strata are interpreted from seismic data (Fig. 3 and Fig. 4) (DPIE, 1998; Mehin and Bock, 1998; Douglas, 1976). The group crops out onshore in uplifted fault blocks of the Gippsland Highlands (Strzelecki Ranges) and northeast of Yallourn, with exposures covering more than 3,900 km² (Douglas, 1976).

The Strzelecki Group is of Early Cretaceous age and was deposited in an east-west trending rift basin complex that began as a pre-breakup depression and failed rift approximately 130 Ma (Mehin and Bock, 1998). The rift complex was the eastern extension of the Otway and Bass Basins, which also received sediments of similar composition and depositional environment beginning in the Late Jurassic (Bradshaw, 1993). The correlative group in the Otway Basin is a good coaly source rock that sources gas in at least two fields (M. Woollands, written personal commun., 1999).

Exploration in the Gippsland Basin has thus far considered the nonmarine Strzelecki Group to be economic basement and unprospective (M. Woollands, written personal commun., 1999), but new research is currently reconsidering its potential (Mehin and Bock, 1998). The group occurs in the subsurface to the north and south of the productive Upper Cretaceous trend (discussed below), on the Northern Strzelecki Terrace and the Southern Strzelecki Terrace (Douglas, 1976) but has not been explored under most of the productive trend in the offshore Central Depression because of drilling depths that generally exceed 5-7 km (Fig. 4).

Thickness of the Strzelecki Group generally ranges from a few hundred meters to more than 2,600 m (Douglas, 1976, McPhee, 1976) but estimated thickness in some areas is as much as 6,000 m (McPhee, 1976). The group is composed of nonmarine greywackes, mudstones, sandstones, conglomerates, coals, and volcanoclastics (Douglas, 1976; Mehin and Bock, 1998). Depositional environments include lakes, swamps and floodplains.

Subdivision and correlation of units within the group have proven difficult, although outcrops were described as early as 1856 in connection with evaluations of coal resources (Douglas, 1976; Gloe, 1976; Knight, 1976). The lower portion of the Strzelecki Group consists of the Lower Neocomian Tyers Conglomerate and finer grained equivalents deposited on a Paleozoic age erosional surface on granites and metamorphics (Mehin and Bock, 1998). The upper Strzelecki Group consists of coals, shales, siltstones, and volcanic sandstones of the Wonthaggi Formation (Mehin and Bock, 1998). Medium- to fine-grained feldspathic sandstones make up more than 40% of the outcrop area (Douglas, 1976). Lenticular and cross-bedded sandstones 60 m thick are common, and sandstone sequences are as much as 140 m thick in some outcrops (Douglas, 1976). The sandstone contains oligoclase, orthoclase, perthite, microcline, quartz, biotite, hornblende, andesite and calcite (Douglas, 1976). Mudstone beds are as much as 130 m thick and are mineralogically similar to the sandstone except that the mudstone is dominantly potassic whereas the sandstone is dominantly sodic (Douglas, 1976).

Potential sandstone reservoirs within examined intervals of the Strzelecki Group are of poor quality (McPhee, 1976), having poor porosity due to diagenetic clays and alteration of volcanic rock fragments. Better reservoirs and good exploration targets may be defined with an exploration program aimed specifically at rift-basin style stratigraphy and traps. The source-rock potential of the coals and lacustrine shales in the group is poorly known (Mehin and Bock, 1998), although the subsurface edges of the depositional extent of the group appear to be in the oil window along the northern and southern terraces (Ozimic and others, 1987), and methane occurrences at the margins of the basin may have been sourced by the Strzelecki Group (Mebberson, 1989).

The Lower Cretaceous Strzelecki Group was deformed and eroded by a 96 Ma event that may have included compressional realignments of the breakup of Gondwana (Fig. 2) (Duff and others, 1991). Strata of latest Early Cretaceous through Cenomanian age are not present. The overlying Late Cretaceous Central Depression, developed between the northern and southern terraces, has buried the Strzelecki Group offshore to depths of more than 8 km (Ozimic and others, 1987) and currently into the overmature range.

Golden Beach Group
The Golden Beach Formation of the Latrobe Group was described by T. R. Haskell in 1972 from the Golden Beach 1A well drilled in 1967 (Lowry, 1987). This formation was described as continental sandstone and shale overlying the Strzelecki Group and underlying the unconformity within the N. senectus palynomorph zone (Lowry, 1987). Since 1990 the lower Latrobe has been described in the literature as the Golden Beach Group (Fig. 2). The Golden Beach Group was deposited in a rapidly subsiding, extensional rift trend of a slightly different orientation that overprinted and formed within the trend that existed during deposition of the Strzelecki Group. The rift was occupied by deep-water lakes and, following the breakup of the southern Australian margin, was flooded by marine waters (Duff and others, 1991; Partridge, 1996). Where the Golden Beach Group is present, it rests unconformably on the Strzelecki Group (Fig. 4). Mineralogy changes from volcanogenic in the Strzelecki Group to quartzose in the Golden Beach Group (Mehin and Bock, 1998).

The Golden Beach Group is divided into the Turonian age Emperor sub-group and the Santonian to Campanian age Golden Beach sub-group (also referred to as the Golden Beach Formation or Chimera Sandstone) (Partridge, 1996; Mehin and Bock, 1998; DPIE, 1998). An unconformity of Turonian to Santonian age (duration of 5 million years (m.y.)) separates the two sub-groups (Fig. 2) (Partridge, 1996; DPIE, 1998).

The Kipper Shale of the Emperor sub-group was deposited in lakes (Partridge, 1996). This lake complex is described as deep and persistent through time (perhaps as long as 3 m.y.) with lake surface areas estimated at 5,000 km² (Partridge, 1996). Thick sections of lacustrine shale have been drilled (Moore and others, 1992). In the Manta and Basker fields area, the lower Latrobe Group is described as alluvial sandstones with interspersed volcanic flows of andesitic to basaltic composition (Clark and Thomas, 1988).

More than 600 m of Golden Beach Group is preserved in the Central Depression offshore area, as well as in parts of the adjacent onshore Seaspray Depression (Fig. 3) (Partridge, 1996; Mehin and Bock, 1998). Normal faulting along the Rosedale and Darriman faults truncated and then confined deposition of the group to the rapidly subsiding depression between the Northern and Southern Strzelecki Terraces (Mehin and Bock, 1998). Development of these terraces coincides with opening of the Tasman Sea beginning at 80 Ma (Rahmanian and others, 1990). The first marine influences in the Gippsland Basin occurred in middle Santonian time during deposition of the Golden Beach sub-group (Partridge, 1996) and the transgressions are interpreted by some to have come from the southwest (Megallaa, 1993). The Campanian rocks are described as sandstones and overbank shales that were deposited across a coastal plain by a meandering river system with north to south flow (Clark and Thomas, 1988).

An unconformity developed on top of the sediments of the Golden Beach sub-group approximately 80 Ma, with subsequent latest Campanian volcanics resting unconformably on this surface (DPIE, 1998; Partridge, 1996). The Golden Beach and Strzelecki were tilted to the east-southeast and eroded (Mehin and Bock, 1998). This Campanian tectonism and associated volcanism accounts for major displacements on intra-Golden Beach sub-group faults, and for the extrusion of the surface basalt flow that serves as a seal for gas and oil traps in Golden Beach sandstones at Kipper Field (Fig. 5) (DPIE, 1998; Sloan and others, 1992).

Latrobe Group
Late Cretaceous (Campanian) tectonism, which ended deposition of the Golden Beach Group, resulted in (1) the separation of Gippsland Basin from the Bass Basin by the Bassian Rise, and (2) the emergence of the Southern Platform (Duff and others 1991; Megallaa, 1993) (Fig. 3). Paleogeographic maps portray the basin during latest Cretaceous Latrobe deposition as a gulf facing the opening Tasman Sea to the east with two structurally controlled lows traversing the Gippsland Rise (Fig. 3) (Fielding, 1992; Megallaa, 1993). The Gippsland Rise is shown as sometimes emergent with circulation from the sea through the structural lows (Megallaa, 1993). By the end of the Cretaceous, the offshore Gippsland Basin began to subside as a marginal sag (DPIE, 1998). Compression became the dominant tectonic style from Eocene to Miocene time during the passive margin stage related to the continuing opening of the Tasman Sea (Megallaa, 1993).

The Latrobe Group consists of sandstones, siltstones, mudstones, shales, coals, and volcanics that were deposited in alluvial, shoreline and shallow to shelf marine settings and deep-water low-stand cut-and-fill channels; the thickness of the sequence totals more than 4,500 m within the Central and Seaspray depressions (McPhee, 1976; Featherstone and others, 1991). The Cretaceous to Paleocene portion of the Latrobe Group is interpreted to have been sand-rich coastal plain and barrier sediments deposited in an aggradational setting (Fielding, 1992). Multiple transgressions and regressions of coastal plain and barrier settings are recorded in the Late Paleocene to Eocene portion of the group (Fielding, 1992). Paleoshorelines trended generally southwest to northeast and were located, for the most part, between Mackerel and Blackback fields (Fig. 5) (Rahmanian and others, 1990; Fielding, 1992; Megallaa, 1993; Moore and others, 1992; Gross, 1993). These paleogeographic reconstructions show a wide zone of migrating shoreline environments backed by an extensive coastal plain surrounded by alluvial and fluvial settings within the confines of the faulted basin during deposition of the lower Latrobe section (Douglas, 1976; Fielding, 1992). Clastic sediments were sourced from the Australian continent to the north and the Bassian Rise and Southern Platform to the southwest and south of the basin as well as from the west along the basin axis. Westward marine transgressions and eastward regressions of the Tasman Sea alternated with periods of continental clastic sedimentation (Douglas, 1976; Fielding, 1992), but no major deltaic sequences have been described. These sediments progressively onlapped the terraces to the north and south and onlapped to the west during Late Cretaceous through Oligocene time. Onshore in the northwest area, the Strzelecki Group is overlain by a series of Latrobe Group formations: an Eocene or Paleocene sandstone, Thorpdale Volcanics consisting of Eocene basaltic lavas, Oligocene to Miocene age nonmarine clastics, and Latrobe Valley Coal Measures (Mehin and Bock, 1998). In the onshore Seaspray Depression, Latrobe Group rocks are described as Upper Cretaceous to Oligocene non-marine sandstones, siltstones, mudstones, coals and shales (Mehin and Bock, 1998). The Maastrichtian-Paleocene upper Latrobe Group rocks in the offshore area are described as having been deposited in coastal plain, shoreline, and nearshore settings featuring a series of thick barrier bar facies (Clark and Thomas, 1988). Eocene paleogeographic reconstructions show as much as a 40 km wide coastal barrier fronted by wide shallow marine environments and backed by a coastal plain that was no longer confined by the faulted basin (Fielding, 1992).

Tectonic uplift of the northeastern basin in Eocene time along with sea level lowstands, caused deep submarine channeling into strata of the Latrobe Group and into anticlines involving these rocks (Douglas, 1976; DPIE, 1998). Three of these erosional unconformities occur at the Blackback field anticline (Gross, 1993). Major channeling events include a Maastrichtian event, the early Eocene Tuna/Flounder channel that was filled by the Flounder Formation and the Eocene Marlin Channel filled by the Turrum Formation (Fig. 2) (Douglas, 1976; Moore and others, 1992). The current Bass Channel, or Bass Canyon, occupies the southern structural passage across the Gippsland Rise and erodes into the Latrobe Group. The Gurnard Formation, where present, is generally placed at the top of the Latrobe Group and consists of offshore (Eocene) to onshore (Oligocene) glauconitic sandy siltstone (DPIE, 1998). Further onshore in the Latrobe Valley, deposition of the Latrobe Valley Coal Measures continued through the Pleistocene (Mehin and Bock, 1998; McPhee, 1976; DPIE, 1998).

Another tectonic event formed an extensive erosion surface on top of the Latrobe Group 50 Ma (Duff and others, 1991). The overlying Seaspray Group consists of shales and marls that are the regional seal for the majority of fields in the Gippsland Basin (Fig. 2). Unconformities and at least two mid-Miocene channel cut-and-fill events are present within the Seaspray Group. Compression during Seaspray time is represented by gentle unfaulted anticlines in contrast to older faulted anticlines (Duff and others, 1991).

HISTORY OF PETROLEUM EXPLORATION
Oil and gas were discovered onshore at Lake Bunga No. 1 in 1924 (Fig. 5) (Ozimic and others, 1987). This discovery, near Lakes Entrance, started exploration interest in the Gippsland Basin. Cumulative production to 1956 of 10,000 barrels of biodegraded, heavy crude oil from an Oligocene sandstone in the onshore Lakes Entrance Field was overshadowed by later activity in the offshore areas. In December of 1964 the drill ship Glomar III began exploration in offshore Gippsland Basin drilling Esso Gippsland Shelf No. 1, in what is now known as Barracouta Field (Fig. 5). Gas production from Barracouta began in early 1969 from Late Eocene Latrobe Group clastic reservoirs in a faulted anticline. Oil was also discovered and later produced from the same field (Ozimic and others, 1987).