Great Australian Bight Basin is a superimposed basin, which evolved from J2-K1 rifting period, through K1-2 transitional period to K2-Q drifting period. Five sets of source rocks are distinguished in the eastern sea area of basin. These source rocks commonly are Cretaceous lacustrine mudstones, deltaic coal-measures and marine mudstones in lithology. It is shown that four sets of reservoir-cap assemblages and four associated plays develop also in the eastern sea area. The plays are Lower Cretaceous Bronze Whaler Group play and Upper Cretaceous White Pointer Group, Tiger Group and Hammerhead Group plays, in which the Upper Cretaceous Hammerhead play is regarded as the best potential zone for exploration in offshore Ceduna Depression because the Hammerhead delta front sandstone is identified as a good reservoir and the Hammerhead prodeltaic mudstone as a good source rock.