SEQUENCE STRATIGRAPHY AND HYDROCARBON PROSPECTIVITY OF THE CAMPANIAN TO EOCENE SUCCESSION, NORTHERN BONAPARTE BASIN, AUSTRALIA

Abstract

The Campanian to Eocene succession of the Northern Bonaparte Basin contains a number of siliciclastic reservoirs which provide alternative targets to the Callovian structural plays that have dominated exploration to date. The succession is part of the Passive Margin Megasequence which extends from the Aptian to the Pliocene, and is traditionally subdivided into the Turnstone, Johnson and Grebe Formations.

Prograding deltaics of the Turnstone Formation swamped an incipient Early-Campanian carbonate ramp following a second-order sequence-boundary. Five third-order sequences are recognised within the Turnstone Formation, each dominated by Lowstand (shelf-margin wedge) and Highstand Systems Tract components. A coeval basinal carbonate system resulted in the deposition of marls and lutites distal of the clastic deltaics. In the Early Paleocene, drowning of the clastic system led to the establishment of a productive carbonate ramp. Rare lowstand siliciclastic reservoirs are developed within carbonate-dominated prograding complexes, as incised valley-fill, and possibly within prominent slope canyons. In the Late Paleocene, a third-order transgression drowned the carbonate system. The Early Eocene Grebe sandstones were then deposited as a second-order lowstand package upon a prominent sequence-boundary. Subsequent flooding of the siliciclastic system resulted in the re-establishment of the prograding carbonate ramp system.

The morphology of the passive-margin was strongly influenced by the interplay between sediment-supply and subsidence. The predominantly ramp-like geometry of the margin promoted the development of numerous shallow-marine lowstand reservoirs. The hydrocarbon prospectivity of each of these reservoirs is primarily controlled by the magnitude of the subsequent flooding events: Only the largest transgressions resulted in sufficient reduction of depositional energy to isolate the lowstand siliciclastics.

Vertical migration remains the critical risk for all passive margin plays, as the reservoirs are separated from the Late Jurassic and Early Cretaceous source kitchens by up to one kilometre of claystone dominated sequences. None-the-less, the widespread occurrence of shallow hydrocarbon shows in the greater Bonaparte Basin indicates that Neogene faulting does provide locally valid migration pathways into post-rift reservoirs.