VIMP Report 88 - Hydrocarbon Prospectivity of Areas V06-2, V06-3 and V06-4, Southern Offshore Gippsland Basin

Category: Victorian Initiative for Minerals and Petroleum Reports	Product Code: MP-R-34467
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Executive Summary
The Gippsland Basin is situated in southeastern Australia, located about 200 km east of the city of Melbourne, and is well serviced by roads and population centres. About two thirds of the basin lies offshore representing one of Australia's most prolific hydrocarbon provinces. Most of the discoveries to date have been hosted within the Late Cretaceous to Early Tertiary Latrobe Group with Initial Proven Reserves estimated at 4.3 billion barrels of oil (Bbbl) and 13.5 trillion cubic feet (Tcf) of gas. The remaining 2P reserves have been assessed at 400 MMbbl of liquids and 7 Tcf of gas. Around 80,000 line km of 2D seismic data and thirty 3D seismic surveys have been acquired, and over 300 exploration wells have been drilled in the basin’s 46,000 km². Thus, although a mature basin by comparison with other Australian basins, by world standards, the Gippsland Basin is still relatively under-explored.

A network of pipelines transports produced hydrocarbons to the onshore petroleum processing facilities at Longford and Orbost. From here, pipelines deliver Gippsland gas to Sydney in New South Wales, to Adelaide in South Australia and to Tasmania. The potential for additional discoveries is expected to ensure continued interest in the region, which will also be maintained by an increasing market demand for natural gas in the southeastern Australian region.

The areas described in this report were partially explored by previous operators who identified several prospects and leads. All areas offer significant opportunities for hydrocarbon entrapment in several different play types.

The Southern Platform areas (V06-2 and V06-3), have hitherto been deemed unprospective, because exploration in the late 1960s had not met with success. All five wells drilled on the platform targeted a pinch-out play, involving rapidly thinning Latrobe Group sediments over Palaeozoic basement capped by the regional seal of the Seaspray Group. All wells were dry and, although Groper-2 did have some indications of a petroliferous odour in cores, the play was no longer pursued. Since that time, however, oil and gas has been discovered along the southern margin of the basin’s Central Deep providing encouragement for a new appraisal of the platform’s prospectivity.

A re-evaluation of seismic, aeromagnetic, well-log, drill-core and geochemical data has identified distinct variations to the basic pinch-out play. All three plays listed below depend on communication with source rocks within the Central Deep or on in-situ biogenic gas generation.

Conventional Latrobe Group Pinchout Play - Where developed, the Latrobe Group is represented by coal bearing lower-middle coastal plain sediments of the Burong Formation overlain by mudstones of the Lakes Entrance Formation The Burong Formation is known to comprise fluvial sandstones of excellent reservoir quality, similar to those encountered in the Perch oil field on the Southern Terrace. An untested top-Latrobe basement pinch-out may exist west and updip from the Groper-2 well in areas of no or limited seismic coverage.

Basement drape and inliers - The rugose basement surface also provides some untested targets. In addition to possible drape of either thin Burong Formation or Oligocene sandstones in the lower Seaspray Group, it is likely that the depressions on the basement unconformity surface were exclusively filled with clastic material before a marine transgression reached the southern basin margin and blanketed the fluvial and coastal plain siliciclastics with marine sediments of the Gurnard Formation. These inliers or drapes would be sealed by the Seaspray Group. Hydrocarbon accumulations in this basement-drape play may be small but could be developed as multiple targets across a broad area.

Oligocene Sandstones above Latrobe Unconformity - Well correlations, supported by seismic sections and biostratigraphy, have identified the presence of a sandy interval within the lowermost Seaspray Group above the Latrobe Unconformity. This Oligocene sand unit, time-equivalent to the Cunningham Greensand Member that is oil bearing in the Lakes Entrance Oil field on the northern margin, appears to represent the erosional detritus from the Bassian Rise and marine reworking has contributed to high reservoir qualities. The interpreted thickness and facies distribution of this Early Oligocene interval highlights two untested basement pinch-out plays on the basement flank also sealed by the fine-grained calcareous marls mudstones of the basal Seaspray Group.

Area V06-4 in the basin’s deep water area is still an exploration frontier. To date only limited seismic and aeromagnetic data are available for the delineation of valid structures. The prospectivity in this area is largely dependant on the development of traps along the fault terrace marking the southern margin of the basin in a setting analogous to the Archer-Anemone discovery. Basin-ward dipping fault blocks are likely to have shed erosional sediments into marine depocentres, possibly including occasional turbidites. Although illdefined with the existing seismic small top-Latrobe closures have been mapped along the terrace that might allude to larger closures at depth within the Golden Beach subgroup. Clearly more seismic is required to confirm the structure of this block and to assess its depositional setting and source potential.

All three blocks require additional work in order to define the trapping mechanisms and timing as well as the maturation and migration setting. All three blocks have demonstrable play fairways and are adjacent to a proven petroleum system. This in itself should ensure that the blocks provide exciting and challenging opportunities for potential explorers.

Bibliographic reference
Bernecker, T., Thomas, J.H. & O'Brien, G.W., 2006. Hydrocarbon Prospectivity of Areas V06-2, V06-3 and V06-4, Southern Offshore Gippsland Basin, Victoria, Australia. Victorian Initiative for Minerals and Petroleum Report 88, Department of Primary Industries.

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