SEQUENCE STRATIGRAPHY, BASIN-FLOOR FANS AND PETROLEUM PLAYS IN THE DEVONIAN – CARBONIFEROUS OF THE NORTHERN CANNING BASIN

Abstract

A sequence stratigraphic study of 4 000 kilometres of seismic data and fifty wells from the northern part of the Canning Basin has improved vasdy our knowledge of the evolution of the basin succession and has identified new petroleum play concepts. The Devonian to Lower Carboniferous succession can be subdivided into at least sixteen 'Vail type' sequences. This shows two contrasting phases of development; a reef complex in the Frasnian—Early Famennian, and a ramp in the Late Famennian-Tournaisian. The reef complex was characterised by reciprocal sedimentation with thick clastic lowstand wedges deposited in the Fitzroy Trough and thinner transgressive and highstand carbonate sediments deposited on the Lennard Shelf. The reef complex shows an early phase of backstepping, related to rapid drowning of the platform, and a later phase of marked progradadon when relative subsidence was lower. The ramp, in contrast, has only poorly developed lowstand wedges. Early ramp sequences are lenticular and successively offlap basinward; later sequences are more tabular and progressively onlap landward.Sequence stratigraphy has provided a new perspective on the setting and evolution of the traditional Devonian reef play. It emphasises the attractiveness of the Frasnian cycle of reef growth where source, seal and reservoir potential appear to be better than in the younger, much more extensively drilled Famennian cycle.New and untested stratigraphic targets have been identified in Famennian-Tournaisian highstand ramps and in lowstand fans. These new stradgraphic targets are generally basinward of the more traditional target areas and need to be assessed by either wildcat or stratigraphic drilling. The lowstand fans, especially, are large and extensive and favourably located to trap migrating hydrocarbons. Several prospects, each with the potential to contain in excess of 100 million barrels of recoverable oil, have already been identified on seismic data recorded basinward of the Lennard Shelf.