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Tectonic Subsidence, Thermal History And Petroleum System Modelling Of The Offshore Northern Perth Basin, Western Australia

Pfahl, Matthew

Geoscience Honours Degree, 2011

University of Adelaide

Abstract

A new evaluation of the tectonic history of the offshore northern Perth Basin, based upon tectonic subsidence modelling constrained by stratigraphic and thermal data from well intersections has resulted in a reassessment of the timing of Jurassic extensional and post-rift thermal subsidence phases. Previous research primarily based upon seismic reflection data had indicated that second and third extensional phases (following initial Permian extension) occurred during the Jurassic, with the second extensional phase occurring during deposition of the Eneabba Sequence (204-181 Ma), and the third extensional phase occurring during deposition of the Yarragadee Sequence (169-145 Ma).

Results based upon tectonic subsidence curves indicate that extension during deposition of the Eneabba Sequence did not occur extensively across the basin and was primarily restricted to the onshore regions and northern Abrolhos Sub-basin. The third extensional phase is interpreted to have occurred earlier than previous research has indicated during deposition of the transgressive Cattamarra and Cadda Sequences (181-169 Ma). Basin modelling indicates that this extensional phase was primarily concentrated in the Houtman Sub-basin (β = 1.45), with less lithospheric stretching in the Abrolhos Sub-basin (β = 1.08). The final (fourth) extensional phase may have also taken place slightly earlier (150 Ma), than previously thought (145 Ma) and was characterised by a small stretching factor (β = 1.06). Mantle strengthening may well explain the variation in the spatial distribution and magnitude of the various rifting events across the offshore northern Perth Basin.

The thermal history derived from thermal maturity data, present-day temperatures and tectonic subsidence curves subsequently mirrors the re-evaluated tectonic history, with palaeo-heat flow higher in the Houtman Sub-basin during the third extensional phase in the Jurassic (181-169 Ma). Palaeo-heat flow and modelled maturity profiles during final extension in the Early Cretaceous are higher toward the eastern flanks of the basin. This can be attributed to induced lateral heat flow which is common in narrow rift basins that display pure shear extensional dynamics.

The re-evaluated tectonic phases also have a major impact on the highly prospective Permian-Triassic petroleum system. The third extensional phase in the Jurassic (181-169 Ma) instigated maximum hydrocarbon expulsion from the Hovea Member in the majority of the Houtman Sub-basin. However it remains unclear if structural traps had the potential to accumulate hydrocarbons, as expulsion and trap development occurred during a similar time interval. 2D basin modelling suggests oil derived from the Hovea Member in palaeo-oil columns in the northern Wittecarra Terrace is allochthonous and was derived from the Houtman Sub-basin, whereas oil in palaeo-oil columns in the central Wittecarra Terrace was derived autochthonously.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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