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The Cenozoic East Java Basin is aprolific petroleum basin in Western Indonesia. It has been explored and producing for 125 years Cenozoic. The proven source rocks are carbonaceous shales of middle Eocene fluvio-deltaic to marginal marine Ngimbang Formation, feeding various intervals of reservoirs from pre-Eocene to Pleistocene ages. The processes of petroleum generation-migration-charging, stratigraphic-structural trapping, and preservation of accumulation occurred in the Paleogene and Neogene. New tectonic reconstructions argue that various Australian-derived microcontinents collided with SE and East Sundaland during mid-Cretaceous times. This argument is supported by deep seismic sections in offshore areas of South Makassar, East Java Sea, and South East Java Forearc, which show a considerable pre-Cenozoic sedimentary section of probable Mesozoic and Paleozoic age. In addition, comparison between bulk properties, biomarkers and isotopes of Sepanjang oils and general East Java oils, lead to a conclusion that there is another proven source rock which has actively generated oils since the Miocene, namely Lower Cretaceous marine shales, charging the Sepanjang field in the Kangean area. This impliesan emergence of Pre-CenozoicCenozoic petroleum system in East Java Basin and its surrounding areas. Therefore, the Mesozoic and Paleozoic sections of the microcontinents may contain important source rocks, reservoirs, seals, and overburdens, as shown by their counterparts in NW Shelf of Australia from where these microcontinents are proposed to have originated. Trap formation, in addition to the generation, migration and preservation of petroleum from the pre-CenozoicCenozoic source rocks, is thought to have occurred sequentially during the Cenozoic following tectonic episodes in the East Java Basin. The presence of actively-generating Lower Cretaceous source rocks within the pre-Cenozoic section of the East Java Basin – in addition to other basins underlain by Australian-derived fragments - warrants further exploration. CenozoicCenozoic