THE SNAPPER DEVELOPMENT, GIPPSLAND BASIN

Abstract

The Eocene N-I reservoir at the top of the Latrobe Group at Snapper is the second largest gas accumulation discovered to date in the Gippsland Basin. Oil reserves exist in a four to eight metre oil leg below this gas pool and in various small intra-Latrobe Group reservoirs.

Development drilling took place in two phases, between 1981 and 1987, with exploitation of the N-I gas reserves being the long term aim. However, initial emphasis has been to maximise production from the N-I oil column. This was the first significant development of a thin oil column in the Gippsland Basin and required detailed study of the reservoir stratigraphy, accurate mapping and the drilling of a number of costly, ultra-high angle wells.

The N-I oil leg required intensive development because each well provides only limited drainage, despite the generally excellent reservoir quality. Recovery is limited by gas and water coning, shale and coal units that act as barriers to drainage and, in some areas, by the presence of dolomitic cement in the reservoir.

After all 27 conductors had been used for development drilling, 5 unsuccessful or depleted wells were redrilled to additional N-I oil development targets. The Federal Government granted a 'Substantial New Development' classification to these wells before they were drilled. This provided a reduction in excise on part of the oil produced from them. The targets were small and difficult to reach and would not have been viable without this reduction.