A Geochemical Overview of Some Gippsland Basin Hydrocarbon Accumulations

In the 50 years since the first commercial discovery in 1965 at Barracouta-1, and 46 years since production commenced from the Barracouta field, a total of 16.5 TCF of gas, 4026 MMbbl of oil, 385 MMbbl of condensate and 752 MMbbl of LPG have been found in the Gippsland Basin (Estimated Ultimate Recovery, as at the end of 2012). Despite these extensive resources, all from CretaceousPaleogene Latrobe Group reservoirs, there are questions regarding the effective petroleum systems, contributing source rock units, and the migration pathways between source and reservoir. Resolution of these uncertainties is essential to improve our understanding of the remaining prospectivity and for creating new exploration opportunities, particularly in the eastern, less explored part of the basin, but also for mitigating risk for the potential sequestration of carbon dioxide along the southern and western flanks. Geochemical fingerprinting of reservoir fluids has identified that the oil and gas originate from multiple sources. The most pervasive hydrocarbon charge into the largely produced fields overlying the Central Deep has a terrestrial source affinity, originating from lower coastal plain facies (Kingfish, Halibut, Mackerel), yet the oils cannot be correlated using source-related biomarker parameters to source rocks either within the Halibut Subgroup (F. longus biozone) at Volador-1, one of the deepest penetrations of the Upper Cretaceous section, or to older sections, penetrated on the flanks of the basin. However, within the underlying SantonianCampanian Golden Beach Subgroup an oil-source correlation has been established between the Anemone-1A oil and the marginal marine Anemone Formation (N. senectus biozone) at Anemone-1/1A and Archer-1. A similar correlation is indicated for the Angler-1 condensate to the Chimaera Formation (T. lilliei biozone) in the deepest section at Volador-1 and Hermes-1. In the Longtom field, gas reservoired within the Turonian Emperor Subgroup, potentially has a source from either the lacustrine Kipper Shale or the Albian portion of the Strzelecki Group. The molecular and carbon isotopic signatures of oil and gas from the onshore Wombat field are most similar to hydrocarbons sourced from the AptianAlbian Eumeralla Formation in the Otway Basin, also implicating a Strzelecki source in the Gippsland Basin. These results imply that sediments older than the Paleocene are significant sources of petroleum within the basin. Presented at the the AAPG/SEG 2015 International Conference & Exhibition set in Melbourne