The Hydrocarbon Prospectivity of the Emperor Subgroup, Gippsland Basin
Honours Degree 2004
University of Adelaide
The Gippsland Basin used to be Australia’s premier hydrocarbon producing region, with recent declines in production attributed to the ageing of existing producing fields and a lack of recent exploration success of commercial size. Deeper and relatively unexplored hydrocarbon plays, such as the Late Cretaceous Emperor Subgroup, need to be targeted. The Emperor Subgroup forms the basal part of the Latrobe Group and 16 wells have so far penetrated parts of the Emperor Subgroup succession on the northern margin of the Gippsland Basin. Deposition of the Emperor Subgroup occurred during the Turonian and Coniacian and represents the final rift-basin phase of the Gippsland Basin. The majority of the Emperor Subgroup is dominated by fine-grained lacustrine sediments deposited in deep rift-valley lakes, however deltaic, fluvial and alluvial fan successions interfinger with the lacustrine successions, presenting potential reservoir targets.
This study provides a detailed examination of the hydrocarbon prospectivity of the Emperor Subgroup. Interpretation of the Northern Margin 3D seismic survey shows that the Emperor Subgroup was deposited in numerous half-graben structures syn-tectonically. New biostratigraphic interpretations have resulted in successions being reassigned to the Emperor Subgroup that were previously assigned to the younger Golden Beach Subgroup. This has led to a better understanding and an expansion of the aerial extent and thickness of the Emperor Subgroup on the northern margin, in comparison to previous interpretations.
The depositional model developed for the Emperor Subgroup on the northern margin displays the syn-depositional nature during the final phase of rifting of the Gippsland Basin. Lacustrine conditions prevail for the bulk of the Emperor Subgroup deposition, with differing amounts of subsidence due to pulses of rifting inducing deltaic and fluvial conditions into the lacustrine systems. The depositional model may also provide an analogue for the time-equivalent Emperor Subgroup sediments on the southern terrace of the Gippsland Basin.
Relatively deep burial and the immaturity of Emperor Subgroup sediments (due to proximity to provenance) both contribute to the generally poor reservoir quality of the Emperor Subgroup. This is evidenced by exploration wells, such as Judith-1, where significant amounts of hydrocarbon were encountered, but poor reservoir quality was attributed to the economic failure of the well. Petrographic descriptions of Emperor Subgroup sands reveal the majority are lithic-sandstones, with abundant kaolinite clay matrix and minor feldspars. The high amount of clay is attributed to the alteration of lithics with depth and the high lithic content is attributed to the re-working of the Strzelecki Group provenance. Lithic alteration and compaction greatly impact porosity and permeability of the Emperor Subgroup with deep burial.
This study has identified two principle play fairways. The primary play fairway is located south of the Rosedale Fault and displays all the necessary petroleum system elements. The secondary play fairway is located north of the Rosedale Fault, however previous exploration wells in this area have shown an absence of hydrocarbon charge and/or seal. For the purpose of this study, the play fairways are limited in depth by a reservoir floor based on 10% porosity. The lack of porosity/permeability data for the Emperor Subgroup has required that a ‘rule of thumb’ 10% porosity be used to estimate the reservoir floor. Exploration target zones are in close proximity to the palaeo-lake shoreline, where incursions of sandier deltaic and fluvial successions are more likely to occur within with lacustrine shale sequences.
Potential plays within the fairways include intra-Emperor Subgroup rollover structures, combination normal and antithetic faults creating closure, combined structural/stratigraphic plays and purely stratigraphic plays. Excellent intra-formational sealing potential is provided by lacustrine shales (lateral and top). Sealing at the top Emperor Subgroup may be provided by basal Golden Beach Subgroup volcanics. Cross-fault lateral seal is provided by tectonic juxtaposition of either the Strzelecki Group volcaniclastics or the Kipper Shale.