The influence of salt structure movement on potential reservoir occurence in the Bonaparte Basin, Northwest Shelf, Australia

Abstract

By integrating seismic stratigraphy concepts and salt tectonics it has treen possible to predict likely zones of anomalous porosity induced by salt tectonics within the stratigraphic sequence in the offshore, southern Bonaparte Basin, I, Western Australia. Due to movement along basin-forming faults and associated subsidence, which commenced in the early Carboniferous, late Devonian - early Silurian salt was withdrawn into pillows. This initial salt withdrawal coincided with a period of 1ow sea level during which time a large sea floor mound was deposited forming the local base to the Milligans Formation. Overlying this sea floor fan are lower fan sediments of a low stand wedge. These fans are predicted to be zones of significant porosity. In response to a rise in sea leve1 the distal portions of transgressive systems tract sediments of Mi2 were deposited. The deeper water sediments of Mi2 are unlikely to contain significant zones of anomalous porosity in the study area due to its distal position from high porosity zones. A subseguent halt in sea level rise resulted in the progradation of the high stand systems tract sediments of Mi3. Withdrawal of salt into pillows ceased during this period. Due to combined effects of slope and gravity, the peripheral sinks caused by salt withdrawal are predicted to contain zones of significant porosity. As the basin shallowed, the blanketing siliciclastics of the 'lower' Tanmurra Formation gave way to the carbonate shelf sediments of the 'upper' Tanmurra Formation. Post-rift subsidence buried the salt withdrawal influenced sediments and the upper Tanmurra Formation. The combination of relative topographic highs over the crests of the salt pillows and low sea level during deposition of the Tanmurra Formation suggests that the immediate flanks of the salt structures are favourably areas for significant porosity. Early to late Permian reactivation of rifting resulted in the deposition of a thick sequence of siliciclastics of the Kulshill Group. Diapirism may have commenced at this time and has certainly continued either continuously or episodically, through to the present. Winnowing of deeper water sediments over the topographic highs resulting from salt diapirism is predicted to have resulted in areas of significant porosity in the deeper water post Tanmurra sequence on the salt structure crests.