THE PETROLEUM GEOLOGY OF THE ONSHORE BONAPARTE BASIN

Abstract

The Bonaparte Basin formed in the Early-Mid Palaeozoic as a result of divergent left-lateral wrenching within the northeast trending Halls Creek Mobile Zone. The main phase of deposition in the onshore portion of the basin occurred in the Late Devonian to Early Carboniferous, when over 5 000 m of sediments were laid down. Extensive outcrops around the basin margins demonstrate complex facies relationships and the continuance of wrench faulting during deposition.

A reef complex of Late Devonian age fringes the western basin margin. In the basin centre, shales and silts of the Bonaparte Beds form the lateral equivalent of the various facies cropping out around the flanks.

A northeast trending basement high occurs in the eastern Bonaparte Basin. A barrier reef complex is predicted to cap the ridge with subtidal and estuarine sediments deposited in the shallow water conditions that existed east of the barrier.

Oil shows have been encountered in shallow core holes drilled around the basin margins. Source rock analyses have also highlighted the potential of the basinal shales and lagoonal carbonates to produce hydrocarbons. Gas has flowed on test from thin sandstones within the Bonaparte Beds in two of the wells drilled onshore.

Reservoirs are predicted to be present in sandstones of the Bonaparte Beds and also where secondary porosity is developed in carbonates, especially those of the reef complex.

Although subsurface structural data are sparse, anticlines are mapped from outcrop data and structures related to both faulting and compaction over basement highs are predicted to occur.