Northern Houtman Sub-basin prospectivity preliminary results

Transcription

1 Northern Houtman Sub-basin prospectivity preliminary results Irina Borissova, Chris Southby, George Bernardel, Jennifer Totterdell, Robbie Morris & Ryan Owens Geoscience Australia, GPO Box 378, Canberra ACT, 2601, Australia Extended abstract presented at the 2016 APPEA Conference, 5-8 June 2016, Brisbane, Australia Abstract In Geoscience Australia acquired 3,300 km of deep 2D seismic data over the northern part of the Houtman Sub-basin (Perth Basin). Prior to this survey, this area had a very sparse coverage of 2D seismic data with km line spacing in the north and an industry grid with 20 km line spacing in the south. Initial interpretation of the available data has shown that the structural style, major sequences, and potential source rocks in this area are similar to those in the southern Houtman and Abrolhos subbasins. The major difference between these depocentres, however, is in the volume and distribution of volcanic and intrusive igneous rocks. The northern part of the Houtman Sub-basin is adjacent to the Wallaby Plateau Large Igneous Province (LIP). The Wallaby Plateau and the Wallaby Saddle, which borders the western flank of the Houtman Sub-basin, had active volcanism from the Valanginian to at least the end of the Barremian. Volcanic successions significantly reduce the quality of seismic imaging at depth, making it difficult to ascertain the underlying thickness, geometry and structure of the sedimentary basin. The new 2D seismic dataset across the northern Houtman Sub-basin provides an opportunity for improved mapping of the structure and stratigraphy of the pre-breakup succession, assessment of petroleum prospectivity, and examination of the role of volcanism in the thermal history of this frontier basin. Introduction The Houtman Sub-basin is the largest depocentre in the offshore Perth Basin, extending for 700 km from the Turtle Dove Ridge in the south to the Wallaby Saddle in the north (Figure 1). The southern part of the sub-basin has been explored for hydrocarbons since the 1970s; however, the prospectivity of the northern part remains unknown. Seismic coverage is very sparse and there are no exploration wells in this part of the sub-basin. The main depocentre of the Houtman Sub-basin is marked by a prominent residual gravity low (Figure 1) that extends to the northern boundary of the sub-basin, suggesting the presence of a significant depocentre. A few Geoscience Australia 2D seismic lines (survey s310) traverse this depocentre and these show that it contains at least 7 km of pre-breakup sediments. These data show evidence for a significant input of volcanic and intrusive material, especially in areas adjacent to the Wallaby Saddle. In 2014 Geoscience Australia initiated a new seismic acquisition program in the northern Houtman Sub-basin with the aim of gaining a better insight into its petroleum prospectivity. Between November 2014 and January 2015, Geoscience Australia acquired 3,300 km of deep (12 sec) broadband 2D seismic data that provide the first regional grid across this frontier area (s349 survey, Figure 1). As part of the Houtman Sub-basin study, existing data and interpretations were reviewed and a regional Extended abstract. APPEA Conference, 5-8 June 2016, Brisbane, Australia 1

2 basin-scale interpretation of all industry 2D seismic data was undertaken. This work provides the basis for tying the newly-acquired data to the existing coverage and building an understanding of regional variations in structure and stratigraphy within the Houtman Sub-basin. Figure 1. Location of the 2D seismic lines acquired in the northern Houtman Sub-basin between November 2014 and January 2015 (survey s349). The background image is residual Bouguer gravity showing a gravity low (blue colours) that coincides with the main depocentre of the Houtman Sub-basin. Background The Houtman Sub-basin was formed during Palaeozoic Mesozoic rifting between Greater India and Australia. Its sedimentary succession is estimated to be up to 12 km thick (Johnston and Petkovic, 2 Extended abstract. APPEA Conference, 5-8 June 2016, Brisbane, Australia

3 2012), comprising Lower Permian Lower Cretaceous sediments, with a thick Middle Upper Jurassic succession, overlain by an 800 1,000 m thick Cretaceous Recent post-rift succession. The northern part of the Houtman Sub-basin lies adjacent to the Wallaby Plateau Large Igneous Province (LIP) and exhibits signs of abundant volcanism, including Seaward Dipping Reflector Sequences (SDRS), large sill and dyke complexes, and lava flows on the breakup unconformity (Symonds et al, 1998), which puts it in the category of a volcanic margin. The western part of the Houtman Sub-basin had active volcanism from the Valanginian to at least the end of the Barremian. Many characteristics of this basin have been influenced by its location on the giant Wallaby-Zenith sheared margin. In the Valanginian, when India and Australia started to break up, the Wallaby Plateau remained attached to Australia. The Wallaby-Zenith Fracture Zone (WZFZ) accommodated spreading in the Perth Abyssal Plain opening to the south of the Wallaby Plateau for another My (Figure 2; Gibbons et al, 2012). Strike-slip structures of the WZFZ extend to parts of the Houtman Sub-basin and add complexity to its structure. Extended abstract. APPEA Conference, 5-8 June 2016, Brisbane, Australia 3

4 Figure 2. Valanginian plate-tectonic reconstruction showing the landlocked part of the Houtman Sub-basin after breakup (after Gibbons et al, 2012). H Houtman Sub-basin; CAP Cuvier Abyssal Plain; PAP Perth Abyssal Plain; WS Wallaby Saddle, WP Wallaby Plateau. New findings Structure Regional interpretation of legacy industry 2D seismic data in the Houtman Sub-basin indicates that the structural style of the basin is characterised by the presence of two interlinked fault systems (Figure 3). The older basement-involved faults control the geometry of the Permian rift basins. These Permian 4 Extended abstract. APPEA Conference, 5-8 June 2016, Brisbane, Australia

5 structures are imaged on the seismic in the inboard part of the basin, and range from small basins with less than 1,000 m of sediments to very large half-graben with up to 6,000 m of synrift sediments. The orientation of these grabens is predominantly north south to north-northwest south-southeast, similar to the Permian structures in the onshore Perth Basin. In the Houtman Sub-basin the Jurassic Early Cretaceous succession is 3,000 to 5,000 m thick. The structure of this succession is controlled by northwest southeast oriented faults, which is consistent with the direction of spreading after the breakup. Some of the major Jurassic faults started as reactivated Permian faults, whereas others show no correlation to the Permian structures and dip in the opposite direction to the Permian faults. In areas close to basement highs, the Triassic succession and Late Permian unconformity are often cut by steeply-dipping Jurassic faults, whereas in the deeper parts of the basin, Jurassic faults are often separated from deeper faults by seemingly unfaulted Triassic Kockatea Shale. Jurassic faults are often listric in nature and sole out in the Kockatea Shale. The seismic data also suggests that pre-existing inter-basin basement highs and Permian rift basins played an important role in the formation of the Jurassic structures. The difference in the extensional direction during the Jurassic-Cretaceous in comparison to the Permian led to transpression on a number of faults and creation of large inverted structures. Provided these structures are sealed, they could act as large, good-quality hydrocarbon traps. Volcanostratigraphy On all lines extending to the Wallaby Saddle, seismic data clearly shows the presence of SDRS (Figure 3). They appear to dip west-northwest and are thicker in the south (up to 3 s TWT). Drilling results from other volcanic margins have shown that SDRS are composed of thick interbedded subaerial volcanic flows, volcaniclastic and non-volcanic sediments with an average velocity of 4 5 km/s, indicating that the Wallaby Saddle volcanic successions are about 4,000 6,000 m thick. Seismic data suggest that the outboard part of the Houtman Sub-basin extends laterally for at least another km beneath the SDRS (Figure 3). In this part of the sub-basin the Valanginian breakup unconformity appears to form the base of the SDRS succession, while a younger breakup unconformity observed on the Wallaby Saddle (Barremian-Aptian?) defines the top of the succession. Part of the basin immediately adjacent to the SDRS also shows a number of large sill and dyke complexes (Figure 3). Previous studies of volcanic margins based on drilling results show that SDRS form in a relatively short geological period of about only 2 My (Franke, 2013). Data from the Wallaby Plateau and Cuvier Basin, however, suggest that volcanic activity associated with this LIP continued until the end of the Cretaceous (Dadd et al, 2015). The crust is highly extended beneath the Wallaby Saddle, and possibly in the adjacent part of the Houtman Sub-basin, therefore heat from the long-lived Wallaby Plateau LIP may have significantly impacted the thermal history of the basin and, consequently, source-rock maturation. Extended abstract. APPEA Conference, 5-8 June 2016, Brisbane, Australia 5

6 Figure 3. Seismic transect across the northern Houtman Sub-basin and the adjacent part of the Wallaby Saddle showing architecture and potential thickness of the basin in deep water. The eastern part of the transect shows preliminary pre-stack time migrated data from the GA Houtman Sub-basin survey (s349). Source rocks and potential plays The most prospective source rock in the Houtman Sub-basin is the oil-prone Triassic Kockatea Shale (Jones et al, 2011). In the Abrolhos Sub-basin and on structural highs within the Houtman Sub-basin, the Kockatea Shale has been intersected by all wells and is, on average, m thick. The author s interpretation suggests that the Kockatea Shale is present in the main depocentre below the thick Jurassic succession and extends into the northern part of the sub-basin. Its thickness in the deep part of the depocentre is interpreted to be about m. The overlying Jurassic succession extends to the middle of the new seismic grid and may contain prospective source rock intervals such as Cattamarra Coal Measures, Cadda Formation shales, and Yarragadee Formation coals (Jones et al, 2011). Permian rift structures appear to be present only in the south-eastern part of the new seismic grid, where they correlate with a separate north south trending gravity low (Figure 1). Permian source rocks, equivalent to Irwin River Coal Measures, may be present in this graben. The Houtman Sub-basin has a number of potential plays that include the following. Stratigraphic and structural plays where Upper Permian Dongara Formation reservoir units overlie source rocks in the Permian synrift and are sealed by the Kockatea Shale; inversion anticlines in the Jurassic Early Cretaceous succession, with hydrocarbons generated by the Triassic Kockatea Shale source rocks and sealed by Cadda marine shales or intraformational shales of the Yarragadee Formation; fault-block plays in the Jurassic-Early Cretaceous succession with variable combinations of source rocks, reservoirs and seals; and, sub-unconformity plays below the Valanginian unconformity. Conclusions The new 2D seismic data collected by Geoscience Australia in the northern part of the Houtman Subbasin, together with regional interpretation of existing 2D data, provide new insights into the prospectivity of this area. The new data show the presence of a thick basin in this frontier area. 6 Extended abstract. APPEA Conference, 5-8 June 2016, Brisbane, Australia

7 Preliminary interpretation suggests that several source rock intervals are likely to be present in the northern part of the Houtman Sub-basin and that a variety of structural and stratigraphic plays may work in this area. Extra heat generated by the proximity to the Wallaby Plateau LIP potentially increased the thermal maturity of source rocks. Validity of the plays is likely to depend on the timing of hydrocarbon generation in relation to trap formation, as well as trap integrity. The new seismic data is available from Geoscience Australia, at the cost of transfer, from ausgeodata@ga.gov.au Acknowledgements The authors wish to thank everyone who provided support during planning and running the seismic survey in the northern Houtman sub-basin. We also thank Tanya Fomin and Merrie-Ellen Gunning for managing the seismic processing. This paper is published with the permission of the CEO, Geoscience Australia. Copyright Commonwealth of Australia (Geoscience Australia) 2016 With the exception of the Commonwealth Coat of Arms and where otherwise noted, this product is provided under a Creative Commons Attribution 4.0 International Licence. ( References DADD, K.A., KELLERSON, L., BORISSOVA, I. AND NELSON, G., 2015 Multiple sources for volcanic rocks dredged from the Western Australian rifted margin. Marine Geology, 368, FRANKE.D.,2013 Rifting, lithosphere breakup and volcanism: Comparison of magma-poor and volcanic rifted margins. Marine and Petroleum Geology, 43, GIBBONS, A.D., BARCKHAUSEN, U., VAN DEN BOGAARD, P., HOERNLE, K., WERNER, R., WHITTAKER, J.M. AND MULLER, R.D., 2012 Constraining the Jurassic extent of Greater India: Tectonic evolution of the West Australian margin. Geochemistry Geophysics Geosystems, 13 (5), JOHNSTON, S. AND PETKOVIC, P., 2012 Offshore northern Perth basin 2D and 3D models of depth to magnetic basement. Record 2012/39. Canberra, Geoscience Australia. JONES, A.T., KENNARD, J.M, NICHOLSON, C.J., BERNADEL, G., MANTLE, D., GROSJEAN, E., BOREHAM, C.J., JORGE NSEN, D.C. AND ROBERSTON, D., 2011 New exploration opportunities in the offshore northern Perth Basin. The APPEA Journal, 51, SYMONDS, P. A., PLANKE, S., FREY, O. AND SKOGSEID, J., 1998 Volcanic evolution of the Western Australian continental margin and its implications for basin development. In: Purcell P. G. and Purcell R. R (eds.) The sedimentary basins of Western Australia 2: Proceedings PESA Symposium, Extended abstract. APPEA Conference, 5-8 June 2016, Brisbane, Australia 7