Overburden is thin in the Petrel sub-basin, and consists of shallow shelf and continental clastic deposition changing to clastic and carbonate deposition as subsidence shifted to the northern portions of the province during Mesozoic time. Some of the offshore areas, as well as all of the onshore Petrel sub-basin, were characterized by nondeposition or erosion during the Cenozoic (Mory, 1988; McConachie and others, 1996). The post-Paleozoic section thickens in a wedge shape from the head of Joseph Bonaparte Gulf northward to where it is overprinted by the opposing Mesozoic Malita-Vulcan structural trend. Thickness of this wedge ranges from less that 2,000 m to more than 8,000 m resulting in maturation of older source rocks progressively toward the south.

The Malita graben is overlain by Neocomian to Holocene sediments. The Goulburn graben in the easternmost portion of province 3910 consists of pre-Cambrian basement, folded and faulted Paleozoic rocks, and undeformed middle Jurassic to Holocene rocks. Seismic data in the Goulburn graben indicates progradation from the south and southeast (McLennan and others, 1990).

The Cretaceous Bathurst Island Group is both seal and overburden in the Vulcan graben area (Pattillo and Nicholls, 1990). Some good reservoir-quality sandstones occur in this group. These sediments were deposited in a late Campanian lowstand submarine fan complex, sourced from the southeast, that prograded across the Vulcan graben to the Ashmore platform following an Albian to Campanian highstand (Pattillo and Nicholls, 1990).

UNDISCOVERED PETROLEUM
The Bonaparte Gulf Basin Province (3910) contains three proven petroleum systems with numerous styles and ages of exploration targets and considerable under-explored potential. The eastern arm of the Mesozoic petroleum system (Malita assessment unit, 39100301) contains one gas discovery, Evans Shoal-1 (Fig. 10), and several shows thought to be sourced from this portion of the Malita graben indicating the presence of mature source rocks in the eastern area of the province (DPIE, 1998). Numerous exploration targets may be present within the Malita graben and on the faulted edges north and south (Fig. 3 and Fig. 4 A-B). This eastern area also contains potential exploration targets involving Jurassic and Cretaceous sandstones onlapping basement rock along the northern edge of the Darwin shelf (DPIE, 1998). These onlapping stratigraphic and drape anticline type of prospects should be targets around the entire edge of the Darwin shelf. Some of these types of plays have been drilled on the western side of the Bonaparte Gulf, but most of the area of the Darwin shelf and the Moyle platform on the eastern side of the gulf adjacent to the mature Mesozoic system and the mature Paleozoic system remains unexplored (Miyazaki, 1997). On the west side of the Joseph Bonaparte Gulf along the Berkeley platform and the Londonderry high, where drilling has also been sparse, similar onlapping stratigraphic and paleotopographic drape style traps may occur. This play has been successful in the Browse Basin Province (3913) on the western edge of the Londonderry high with discoveries at Gwydion and Cornea (Bishop, 1999).

Fault traps on the margins of the Londonderry high, Laminaria high, Flamingo high, Sahul platform and Troubadour high, adjacent to mature source rocks in the synclines, have been successful exploration targets and could be the sites for additional discoveries (Fig. 3). Lowstand, shallow-water and highstand, deep-water sandstones in the synclines offer numerous under-explored drilling targets. In the Vulcan graben, possible inversion structures similar to Jabiru, along with lowstand valley-fill clastics are potential targets.

Similar lowstand stratigraphic and paleotopographic traps might be encountered in the Paleozoic systems (391001 and 391002) along both the east and west sides of the gulf. High-quality, mature source rocks are predicted adjacent to the Moyle platform on the east side of the Petrel assessment unit (39100201) (Fig. 8) and shows occur to the west on the Plover-Lacrosse terrace and Londonderry high (Edwards and others, 1997).