Geology and prospectivity of the northern Houtman Sub-basin

The Houtman Sub basin is an under-explored region of the northern Perth Basin, offshore Western Australia. Only three wells have been drilled in the southern part of the sub-basin, while the northern part remains an exploration frontier. Adjacent areas of the Perth Basin are proven hydrocarbon provinces with a long and continuing history of exploration, discovery and production. New seismic reflection data, obtained by Geoscience Australia in 2014–15 (GA-349) under the Australian government's precompetitive data acquisition program, has been used to reassess the tectonic evolution, structural architecture, major depositional phases and petroleum prospectivity of this frontier basin. Interpretation of these data has enabled mapping of the Moho, basement, pre-rift sequences and major syn rift sequences, along with the creation of a 3D geological model covering the survey area. As a result, this study has significantly reduced the level of exploration risk in the northern Houtman Sub-basin. Seismic interpretation, integrated with potential field modelling, shows a large variation in crustal structure beneath the study area. Beneath the central part of the northern Houtman Sub-basin continental crust is highly extended to hyperextended (<5 km thick), while further east, beneath the Bernier Platform, continental crust remains greater than 25 km thick. This extreme change in crustal thickness occurred along a major fault zone, marking the eastern boundary of the northern Houtman Sub-basin. Located outboard, along the western margin of the northern Houtman Sub-basin, is a zone of volcanic seaward-dipping reflector sequences, defining the volcanic province of the Wallaby Saddle. Regional correlation of the seismic stratigraphy across the northern Perth Basin has enabled the development of a new tectonostratigraphic framework for the Houtman Sub basin. The depocentre includes up to 16 km of a Permian–Cretaceous succession underlain by a 2–3 km thick section of pre-rift (?Paleozoic) sediments. The Permian syn rift succession is confined to a series of large half graben that are controlled by basement-involved faults that separate the northern Houtman Sub-basin from the Bernier Platform. This succession is up to 10 km thick and was mapped throughout the inboard part of the new seismic grid. A prominent unconformity at the top of the Permian syn-rift sequence is overlain by a thick (up to 1800 m) and regionally extensive seismic sequence interpreted as the Lower Triassic Kockatea Shale. The thickness of the Triassic succession (i.e. Kockatea, Woodada and Lesueur formations) ranges from about 1 km in the inboard part of the basin to up to 5 km outboard. The Late Triassic to Jurassic succession is thickest (up to 4 km) in the outboard part of the basin and is interpreted to contain sequences corresponding to the Eneabba, Cattamarra, Cadda and Yarragadee formations. An Early Cretaceous depocentre was mapped in southwestern part of the study area, and is interpreted to be a Parmelia Group equivalent. Comparison with the adjacent hydrocarbon producing regions of the northern Perth Basin highlights the likely presence of multiple petroleum source, reservoir and seal units. Potential source rocks consist of Permian to Jurassic marine and non marine carbonaceous shale and coal, the most prospective of these is the marine Triassic Kockatea Shale, especially if the organic-rich Hovea Member is present. In addition, multiple reservoir and seal intervals are likely to be present throughout the thick Permian, Triassic, Jurassic and Lower Cretaceous successions.