Hydrocarbons sourced from basins along the southern margin of Australia have been assigned to the Austral Petroleum Supersystem (Bradshaw, 1993; Summons et al., 1998). Within this supersystem, three petroleum sub-systems related to the Otway Basin and other southeastern margin basins have been recognised (Edwards et al., 1999). Each sub-system correlates geochemically to distinct oil families and related source rock facies with differences primarily related to the type of depositional environment. The three sub-systems are:
Six Austral Petroleum System oil families (A1F1, A1F2, A1F3, A1F4, A2 and A3) have been previously identified in the Otway Basin (Edwards et al., 1999). However, a recent integrated geochemical study by Boreham et al. (2004) shows that the A1F1 oil family (e.g., bitumens from Crayfish A1 and Zema 1), originally interpreted as being sourced from saline lacustrine source rocks, and the A3 oil family (e.g., Wilson 1), originally interpreted as being sourced from marine source rocks, are most likely drilling contaminants, rather than true oil families. Therefore, indigenous, uncontaminated oils in the Otway Basin are interpreted as belonging to four families (A1F2, A1F3, A1F4, and A2) within the Austral 1 and Austral 2 Petroleum Systems (Boreham et al., 2004):
There are strong stratigraphic and geographic controls on oil families within the Otway Basin. Oils in the west and onshore belong to Austral 1 families and were sourced from Late Jurassic - Early Cretaceous, syn-rift, dominantly fluvio-lacustrine organic facies. Oils in the east belong to an Austral 2 family derived from Early Cretaceous, post-rift coaly organic facies. Oils in the central part of the basin have a mixed source affinity, but are predominantly from Eumeralla Supersequence sources (Boreham et al., 2004).
Natural gases in the Otway Basin show clear geochemical differentiation between those from the western and eastern parts of the basin. The western gases (e.g., Jacaranda Ridge 1, Katnook 2, Ladbroke Grove 2, Redman 1 and Troas 1) belong to the Late Jurassic - Early Cretaceous, Crayfish Supersequence-sourced Austral 1 Petroleum System (Boreham et al., 2004). The eastern gases (e.g., Thylacine 1, Geographe 1, La Bella 1, Minerva 3, Casino 1, Casino 2) belong to the Aptian-Albian, Eumeralla Supersequence-sourced Austral 2 Petroleum System (Boreham et al., 2004). Gases from the central Otway Basin (e.g., Port Fairy 1, Caroline 1) are the products of mixing from both sources within local depocentres.
Multiple charge histories in the natural gas reservoirs are evident from the widespread influx of overmature, dry gas to an initially in-place wet gas, particularly in the western Otway Basin. Both gas charges have the potential to displace and/or alter the composition of any reservoired oil. In the east, however, most natural gases (e.g., Geographe 1, Thylacine 1, La Bella 1, Lavers 1) are interpreted as the result of a single gas charge (Boreham et al., 2004).
Otway Basin natural gases show a strong geochemical association with their respective oils, suggesting that both are generated from the same source (Figures 8 and 9). Also, the gases and oils and their effective source rocks have a strong stratigraphic and geographic relationship, indicating mainly short- to medium-range migration distances from source to trap (Boreham et al., 2004).