HYDROCARBON POTENTIAL OF THE MESOZOIC AND BASAL TERTIARY OF THE GIPPSLAND BASIN: A STRATIGRAPHIC ANALYSIS

Abstract

A thick sequence of uppermost Jurassic, Cretaceous and basal Tertiary non-marine sedimentary rocks underlies the Gippsland area of Victoria. The older part of this sequence is extensively exposed in the west of the Gippsland area, but elsewhere it is known dominantly from well intersections. Although several hiates are recognised, palynological data indicate that a comparatively complete Cretaceous section can be compiled from this sequence in the Gippsland area.

The uppermost Jurassic to Paleocene rocks can be divided into three units. The oldest unit is uppermost Jurassic and Lower Cretaceous in age. It consists of variably compacted greywackes and lithic sandstones, minor arkoses and interbedded siltstones and mudstones. The overlying early Upper Cretaceous and Paleocene units are distinguishable paleontologically and consist of quartzose sandstones, carbonaceous siltstones and mudstones.

There is no indication of marine influence on sedimentation present in the microfossil content of any of the palynotogical preparations from samples taken throughout most of the sequence. Several species of microplankton are common in the oldest unit, but they are indicative of the lacustrine conditions under which the unit was deposited.

Minor hydrocarbon shows have been recorded from the oldest unit, but the sandstones are characteristically tight. More significant shows have been reported from the two younger units that contain relatively clean sandstones interbedded with siltstones and mudstones. These units possess the greatest economic potential of all of the pre-Eocene rocks of the Gippsland Basin.

The structural framework of the region is composed of separate series of north-easterly and easterly trending faults or monoclines and a south-easterly regional dip. Differential movements of blocks defined by this fault-monocline pattern appears to have resulted in erosion of the more prospective early Upper Cretaceous and Paleocene strata from all but two subrectangular areas respectively immediately north and south of Seaspray.