THE INTRA-LATROBE PLAY : A CASE HISTORY FROM THE BASKER/MANTA BLOCK (VIC/P19), GIPPSLAND BASIN

Abstract

In 1981 Shell and its co-venturers were the successful applicants for gazettal block V80-3, now permit VIC/P19, on the north-eastern margin of the Gippsland Basin where, subsequently, 10 exploration wells were drilled. At the time of the award, the offshore Gippsland Basin had been explored for some 20 years, and the conventional 'Top Latrobe' play even then was considered to be largely exhausted although a few small traps of doubtful economic viability remained untested. Thus, from the outset, exploration in VIC/P19 also emphasised the intra-Latrobe play. While the existence of the intra-Latrobe play has long been recognised, its extent and limiting parameters are less well documented.

Recent exploration results in the southern part of VIC/P19 indicate that the main phase (peak) of generation and expulsion of hydrocarbons from intra- Latrobe source rocks occurs at depths of 4 to 5 km for oil and 5 to 6 km for gas. Large-scale vertical migration of hydrocarbons (up to 2 km or more) is probably occurring in many parts of the central Gippsland Basin and has charged faulted intra- Latrobe traps, such as found at Basker-1 and Manta-1. These accumulations are characterised by multiple stacked reservoirs (fluvial and alluvial sandstones) and thin hydrocarbon columns. However, similar adjacent traps are dry, despite the presence of equivalent reservoirs (e.g. Basker South-I and Chimaera-I), probably due to inadequate lateral fault seals. Lateral sealing may occur along the fault plane itself or by favourable juxtaposition of lithologies, both cases being demonstrated in the Manta accumulation. The petrology and reservoir characteristics of Latrobe Group sandstones vary considerably, depending largely on sedimentary facies. Sands of fluvial or alluvial origin are generally inferior to those deposited in shoreline environments. An effective intra-Latrobe reservoir 'floor' lies at around 4 km burial depth for both fluvial and alluvial sandstones.