Offshore Northern Perth Basin Research Articles

Petroleum systems analysis of the northern Houtman Sub-basin

Interpretation of newly acquired seismic data in the northern Houtman Sub-basin (Perth Basin) suggests the region contains potential source rocks similar to those in the producing Abrolhos Sub-basin. The regionally extensive late Permian–Early Triassic Kockatea Shale has the potential to contain the oil-prone Hovea Member source interval. Large Permian syn-rift half-graben, up to 10 km thick, are likely to contain a range of gas-prone source rocks. Further potential source rocks may be found in the Jurassic–Early Cretaceous succession, including the Cattamarra Coal Measures, Cadda shales and mixed sources within the Yarragadee Formation. This study investigated the possible maturity and charge history of these different source rocks. A regional pseudo-3D petroleum systems model was constructed using new seismic interpretations. Heat flow was modelled using crustal structure and possible basement composition determined from potential field modelling, and subsidence analysis was used to investigate lithospheric extension through time. The model was calibrated using temperature and maturity data from nine wells in the Houtman and Abrolhos sub-basins. Source rock properties are assigned based on an extensive review of total organic carbon, Rock Eval and kinetic data for the offshore northern Perth Basin. Petroleum systems analysis results show that Permian, Triassic and Early Jurassic source rocks may have generated large cumulative volumes of hydrocarbons across the northern Houtman Sub-basin, whereas the Middle Jurassic–Cretaceous sources remain largely immature. However, the timing of hydrocarbon generation and expulsion with respect to trap formation and structural reactivation is critical for the successful development and preservation of hydrocarbon accumulations.

Exploration leads in a proven petroleum system - Offshore Northern Perth Basin

The Offshore Northern Perth Basin is a north to northwest trending basin located on the West Australian margin. Formed during the separation of Australia and Greater India in the Permian to Early Cretaceous, it includes a series of NW - SE trending sub-basins and structural highs (Figure 1). Exploration success in the basin has been sparse to date with the exceptions of the Cliff Head Field, three non-commercial discoveries and numerous hydrocarbon shows (Figure 2). Despite limited success, evidence from exploration to date provides invaluable information of a working petroleum system and the key risks to be addressed. In 2015 Spectrum acquired the modern long offset ‘Rocket’ 2D seismic survey, covering the majority of the Houtman sub-basin and extending northwards into the South Carnarvon Basin, providing further seismic coverage within licensed blocks and open acreage (Figure 1). Improved seismic imaging obtained from this dataset is expected to form an integral part in reviving interest in this basin and in helping operators to exploit this underexplored portion of the Australian margin. Integrating new modern seismic data with information provided by key wells within both the Abrolhos sub-basin and the South Carnarvon basin, points to the presence of an extensive, viable, petroleum system within the province.

Integrated hydroacoustic flares and geomechanical characterization reveal potential hydrocarbon leakage pathways in the Perth Basin, Australia

Geoscience Australia (GA) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) conducted a marine survey including monitoring hydroacoustic flares in order to understand the natural leakage pathways in the offshore northern Perth Basin. 186 hydroacoustic contacts were encountered and classified and thirty two were interpreted as possible seeps or expulsion of gases from the subsurface. The contacts were typically distributed above areas of interpreted subsurface faulting. In the survey site Da (15 km2), nine probable seeps and sixteen other contacts were interpreted and are aligned with a fault segment (A2) interpreted on 2D seismic reflection data. The segment A2 is part of a major N–NNW trending fault system intersecting the sedimentary sequence from the near seabed to the Permian units, including the Kockatea Shale source rock located in the oil window. Evaluation of the stress state on the fault segment A2 suggests that it is not critically stressed and therefore not likely prone to reactivation and dilation and vertical leakage under the modelled stress field. We propose that fault segment A2 acts as a baffle delimiting a migration pathway in the wall rock and permitting hydrocarbons generated from the source rock to overcome capillary entry pressures of the overlying marginal seals. The interpreted seeps could therefore be associated with intraformational vertical migration in the wall rock focused by the faults.

A reassessment of the petroleum systems in the offshore northern Perth Basin

The discovery of commercial oil in the Cliff Head-1 well in 2001 set an important milestone in the exploration history of the offshore northern Perth Basin. The region had been less explored before then, partly due to the perception that the main source of onshore petroleum accumulations, the Late Permian-Early Triassic Hovea Member, had only marginal potential offshore. The typing of the Cliff Head oil to the Hovea Member provided evidence that the key onshore petroleum system extends offshore and has revitalised exploration with 13 new field wildcat wells drilled since 2002. A reassessment of the hydrocarbon generative potential in the offshore northern Perth Basin confirms the widespread occurrence of good to excellent oil-prone Hovea Member source rocks in the Beagle Ridge and Abrolhos Sub-basin. The Early Permian Irwin River Sequence and several Jurassic Sequences are also recognised as prime potential source rocks offshore, mostly for their gas-generative potential. The unique hydrocarbon assemblages exhibited by the Hovea Member extracts are shared by the oils recovered from Permian reservoirs in the offshore Cliff Head-3 and Dunsborough-1 wells, indicating the Hovea Member as the primary source charging these accumulations. Geochemical correlation of oil stains from Hadda-1 and as far north as Livet-1 provides evidence for a working Early Triassic petroleum system across much of the Abrolhos Sub-basin. In this area, the Hovea Member was shown to be both of limited quality and only marginally mature for oil generation, which suggests the occurrence of effective source kitchens in the adjacent Houtman Sub-basin.

New exploration opportunities in the offshore Houtman and Abrolhos sub-basins, northern Perth Basin, WA

The 2013 Acreage Release Areas W13-19 and W13-20 in the offshore northern Perth Basin, Western Australia, cover more than 19,000 km2 in parts of the Houtman, Abrolhos, Zeewyck and Gascoyne sub-basins. The Release Areas are located adjacent to WA-481-P, the only active offshore exploration permit in the Perth Basin, granted to joint venture partners Murphy Australia Oil Pty Ltd, Kufpec Australia Pty Ltd and Samsung Oil and Gas Australia Pty Ltd in August 2012. Geoscience Australia recently undertook a regional prospectivity study in the area as part of the Australian Government’s Offshore Energy Security Program, which provides fresh insights into basin evolution and hydrocarbon prospectivity. A sequence stratigraphic framework, based on new biostratigraphic sampling and interpretation, and an updated tectonostratigraphic model, using multiple 1D burial history models for Permian to Cenozoic sequences, have been developed. New geochemical studies of key offshore wells demonstrate that the oil-prone source interval of the Lopingian–Lower Triassic Hovea Member extends regionally offshore into the Abrolhos Sub-basin and potentially as far as the Houtman Sub-basin. This is supported by fluid inclusion data that provide evidence for palaeo-oil columns within Permian reservoirs in wells from the Abrolhos Sub-basin. Oil trapped in fluid inclusions in Houtman-1 can be linked to Jurassic source rocks, suggesting that multiple petroleum systems are effective in the Release Areas. The presence of active petroleum systems is also supported by the results of a recent marine survey. Potential seepage sites on the seafloor over reactivated faults correlate with hydroacoustic flares, pockmarks and dark colored viscous fluids that were observed over the areas. This may indicate an active modern-day petroleum system in the Houtman Sub-basin. Finally, a trap integrity analysis was undertaken to mitigate exploration risks associated with trap failure during Early Cretaceous breakup and provides a predictive approach to prospect assessment. These results provide strong support for the presence of active petroleum systems in the offshore northern Perth Basin and upgrade the prospectivity of the Release Areas.

Geoscience Australia seismic survey 310: revealing stratigraphy and structure of the outer northern Perth Basin margin

Geoscience Australia acquired seismic survey GA 310 in 2008–09, across the southwest margin of Australia, as part of the Australian government’s Energy Security Program. Deep reflection seismic and potential field data were recorded across sparse 2D grids located on the Wallaby Plateau in the north, Mentelle Basin in the south, and the intervening Houtman and Zeewyck sub-basins of the northern Perth Basin. The offshore northern Perth Basin extends for about 700 km along the Western Australia margin, from the towns of Carnarvon in the north to Cervantes in the south. The largely Paleozoic-Mesozoic tectonostratigraphic framework is dominated by Permian and Early-Middle Jurassic rifting, followed by Late Jurassic-Early Cretaceous rifting leading to Valanginian breakup between Australia and Greater India. Underlying Precambrian Pinjarra Orogen structuring, in conjunction with rifting, has resulted in the development of several complex depocentres and basement highs. A recent re-evaluation of the offshore northern Perth Basin well-based lithostratigraphy into a new chronostratigraphic sequence framework has been carried outboard, on the GA 310 seismic lines, into the margin bounding the Zeewyck and northern Houtman sub-basins. The main sequences hosting source rocks—Kockatea and Cattamarra—are widely present in the expansive northern Houtman Sub-basin, and are likely to be present in the deep Zeewyck Sub-basin. The mapping of a thick Late Jurassic Yarragadee Sequence in the Zeewyck Sub-basin indicates a major pre-breakup locus of relatively rapid deposition. The structural interpretation across the sub-basin highlights breakup-drift unconformities and strike-slip faulting and suggests a probable along-margin sheared crustal sliver; tectonic elements commensurate with an evolving rift-shear breakup margin.

Depth to magnetic sources in the offshore northern Perth Basin

During 2009-11 Geoscience Australia completed a petroleum prospectivity study of the offshore northern Perth Basin. Basement in the northern Perth Basin is deep and generally not resolved in the reflection seismic data. Recent improvements to the magnetic ship-track database and magnetic anomaly grid allowed an assessment of depth to magnetic sources, and estimation of sediment thickness, providing new insight into basement depth and trends. 2D models along seismic transects and analysis using spectral methods indicate that penetration of the deepest sediments by high-susceptibility bodies is probable. The reflection seismic evidence for these bodies is not clear, though in some cases they may be associated with faults and structural highs. Where the modelled bodies penetrate the sediments they are mostly below or within the Permian strata. A moderate positive magnetic anomaly (the Turtle Dove Ridge) is modelled by massive bodies whose tops are 5-15 km below sea floor. The depth to magnetic basement map highlights sub-basins and structural highs within the northern Perth Basin, with up to 12 km of sediment in the Zeewyck sub-basin.

Potential-field data for structural interpretation in the northern Perth Basin, Australia

New compilations of levelled marine and onshore gravity and magnetic data are facilitating structural and geological interpretations of the offshore northern Perth Basin. Multi-scale edge detection applied to these data helps the mapping of structural trends within the basin and complements interpretations based on seismic reflection data. Together with edge detection, magnetic source polygons determined from tilt angle aid in extrapolating exposed basement under sedimentary basins and, therefore, assist in the mapping of basement terranes. Three-dimensional gravity modelling of crustal structure indicates deeper Moho beneath the onshore and inboard parts of the Perth Basin and that crustal thinning is pronounced only under the outboard parts of the basin (Zeewcyk Sub-basin).

Trap integrity framework for the offshore northern Perth Basin

The presence of hydrocarbon accumulations and several palaeo-oil columns in Permian reservoirs across the offshore northern Perth Basin attests to a widespread charge system and suggests that trap integrity is a critical exploration risk. The local structural evolution between the Beagle Ridge and the Wittecarra Terrace and the impact of renewed mid-Jurassic extension on a series of prospects (Cliff Head, Dunsborough, Lilac and Morangie) is assessed to define a regional trap integrity framework. The focus is on the potential development of pathways through the Early Triassic Kockatea Shale regional top seal caused by reactivation of trap bounding faults and/or interaction with newly formed overlying structures. A series of 3D geo-mechanical models are constructed to simulate the Mid-Jurassic deformation and to evaluate the first-order factors controlling the partitioning of reactivation stresses/strains and the development of seal bypass. In parallel, the local fault seal potential is assessed by focusing on the likelihood of the fault surfaces to reactivate and conduct fluid within the Jurassic stress regimes. The stress data are also used to compute the risk of reactivation in terms of the increase in pore fluid pressure required to bring a fault segment to a state of instability. The overall risk of top seal bypass is primarily driven by the reservoir-bounding faults orientation and dip. The distribution of jogs, the interactions between fault tips, the fault density, the location of regional major fault, the fault propagation mechanisms and top seal brittleness also control locally the integrity of the top seal.

Detection of palaeo-oil columns in the offshore northern Perth Basin: extension of the effective Permo-Triassic charge system

A widespread charge system for oil accumulation in the offshore northern Perth Basin, Western Australia, is revealed by specialised fluid inclusion technologies. Palaeo-oil columns are detected in about three of four exploration wells, including those at the Cliff Head, Dunsborough, Frankland and Perseverance fields, and in dry wells at Flying Foam–1, Hadda–1, Houtman–1, Leander Reef–1, Lilac–1, Livet–1, Mentelle–1 and Morangie–1. A high incidence of palaeo-oil charge into Permian reservoirs below the Kockatea Shale confirms that the conventional oil shows are, in part, residues of palaeo-oil. Oil migration is suggested at Vindara–1 and Leander Reef–1 and is below detection limits in Batavia–1, Charon–1, Fiddich–1 Geelvink–1A, Gun Island–1 and South Turtle Dove–1B, Twin Lions–1 and Wittecarra–1. New geochemical data from fluid inclusion oil at Hadda–1 shows evidence for a contribution from the Hovea Member of the Kockatea Shale, including: high wax content; low pristane/phytane ratio; high abundance of extended tricyclic terpanes; and, the highly diagnostic C33 n-alkylcyclohexane biomarker. This key component of the petroleum system acted as both source and seal, and extends further offshore than previously realised. Possible co-sourcing from terrestrial organic matter is indicated by high abundances of C29 steranes and diasteranes, C19 tricyclic, and C24 tetracyclic terpanes, which may be sourced from Permian rocks. The high incidence of palaeo-oil and residual columns suggests that trap integrity is likely to be an important preservation risk, with elements of gas displacement. Screening of prospects for structural and hydrocarbon charge characteristics, which are favourable for retention of oil, is key in future exploration of the offshore northern Perth Basin.

New exploration opportunities in the offshore northern Perth Basin

The petroleum prospectivity of the northern Perth Basin has been assessed by Geoscience Australia (GA) as part of the Australian Government’s Offshore Energy Security Program, in support of the 2011 offshore acreage release. This assessment includes the first published synthesis of data from fourteen new field wildcat wells drilled in this part of the basin since the Cliff Head 1 discovery (2001), and the interpretation of new regional 2D seismic data acquired during GA survey 310. Most petroleum accumulations in the northern Perth Basin are associated with Permian and Triassic source and reservoir intervals, and are found onshore and nearshore (ie. Cliff Head, Frankland, Dunsborough and Perseverance discoveries). In addition to the technical and commercial successes, numerous wells in the offshore part of the basin have intersected residual oil columns indicative of trap breach. New and legacy palynological data from Permian to Cretaceous strata in offshore wells have been used to provide age constraints for a sequence stratigraphic framework for this part of the basin. New seismic data show Permo-Triassic strata that are stratigraphic equivalents of the productive onshore and nearshore Perth Basin petroleum system, also occur within Permian half-graben in the outer Abrolhos and Houtman sub-basins. Source rock, oil stain and fluid inclusion sampling from this interval suggest that the proven onshore petroleum system is also effective in the offshore. A refined tectono-stratigraphic model for the offshore basin provides insights into basin evolution, prospectivity and contributing factors driving trap breach. Geochemical sampling in the context of the new detailed sequence framework, including from recently dredged rock samples from incised canyons, has also provided insight into the potential effectiveness of a Jurassic/Cretaceous petroleum system in the Houtman and Zeewyck sub-basins.