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Abstract

The Otway Basin is a large, multi-phase rift basin located on the Australian southern margin. Oil and gas recovered from the Otway Basin belong to the Austral petroleum supersystem and are predominately derived from Mesozoic fluvio-lacustrine source rocks. Based on the results of geohistory modeling and oil and gas family analysis, two main petroleum systems are identified in the basin, the Early Cretaceous Crayfish(!) petroleum system and the Early-Late Cretaceous Eumeralla-Shipwreck(!) petroleum system. Both of these petroleum systems are classified as “known”. The symbol (!) represents the level of certainty, in this case “known,” meaning that the hydrocarbons in the named source rock and the named primary reservoir unit have been geochemically examined and found to be genetically related.

The Crayfish(!) petroleum system is mostly restricted to a series of syn-rift half grabens in the western Otway Basin. The stratigraphic and areal location of potential source and reservoirs rocks in the Crayfish(!) petroleum system is strongly controlled by growth on major half graben faults. Multiple episodes of hydrocarbon charge characterize the Crayfish(!) petroleum system, with oil and gas emplaced during the Early Cretaceous, and a dry gas charge during the Tertiary.

Hydrocarbons from the Eumeralla-Shipwreck(!) petroleum system are sourced from coaly facies developed during the regionally extensive fluvio-lacustrine sag sequence that followed the first rifting phase. The reservoir is a fluvio-deltaic lowstand sequence that was deposited during initiation of the second rift phase. The major factors influencing the distribution of the Eumeralla-Shipwreck(!) petroleum system are Late Cretaceous deltaic loading and Oligocene-Miocene carbonate deposition. Favorable conditions for a recent expulsion phase occur in the eastern Otway Basin, where a thick Oligocene-Miocene prograding carbonate succession overlies the relatively thin Late Cretaceous succession.

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