Using palaeogeographic reconstructions to understand lithological variability within the Early Cretaceous Gage Sandstone and South Perth Shale in the Vlaming Sub-Basin, offshore southern Perth Basin

As part of the Australian government’s National CO2 Infrastructure Plan, Geoscience Australia undertook a CO2 storage assessment of the Vlaming Sub-basin in the southern Perth Basin, offshore WA. The post-Valanginian breakup reservoir-seal pair investigated is also a viable target for petroleum exploration.

The Vlaming Sub-basin is an elongate, north–south trending Mesozoic depocentre that covers an area of 23,000 km² and contains up to 14 km of sediments. The main depocentre formed during the Middle Jurassic to Early Cretaceous extension between greater India and Australia prior to their breakup in the Valanginian (Veevers, 1975; Playford et al, 1976; Veevers, 1991; Marshall et al, 1993; Harris, 1994). During this extensional phase, up to 12 km of fluvio-lacustrine sediments accumulated in the northern and central parts of the Vlaming Sub-basin. In the Valanginian, the sub-basin experienced regional uplift and erosion with a maximum uplift in the north and the south–west. Consequently, maximum deposition occurred in the central part of the sub-basin (Spring and Newell, 1993). The post-rift succession comprises up to 1,500 m of a rapidly deposited fluvio-deltaic, shelfal and submarine fan system. Deposition was controlled by the enclosed embayment geometry of the sub-basin and variations in relative sea levels (Spring and Newell, 1993; Nicholson et al, 2008).

Hydrocarbon exploration began in the offshore sub-basin in the 1960s and contains a total of 17 wells and a large volume of 2D seismic data. Sub-economic oil was discovered at Gage Roads 1 and Gage Roads 2 (West Australian Petroleum Pty Ltd, 1969, 1971), and sub-economic gas was intersected in Marri 1 (Ampolex Ltd, 1993) (Fig. 1). These accumulations together with oil shows recorded in two other wells, indicate a working petroleum system. There are no shows in the Gage reservoir (Fig. 1). Petroleum prospectivity assessments (Miyazaki et al, 1996; Crostella and Backhouse, 2000; Nicholson et al, 2008) attributed the lack of exploration success in the Vlaming Sub-basin due to poor understanding of structure, lack of seal integrity and possible accumulation breaches.

Fig. 1.  Offshore Vlaming Sub-basin Upper Jurassic to Lower Cretaceous stratigraphy showing sequences and supersequences, lithostratigraphy and hydrocarbon shows.

Spring and Newell (1993) were the first to establish a framework for the depositional systems and sequence stratigraphy of the Early Cretaceous Warnbro Group in the Vlaming Sub-basin. Their study focused on permit WA-221-P, and the prediction of reservoir and seal facies from seismic and well data. Initial screening of Australian basins by GEODISC (Bradshaw et al, 2000) identified the Vlaming Sub-basin to be prospective for CO2 storage. Subsequently, the CO2CRC (Causebrook et al, 2006) undertook an in-depth assessment of the sub-basin, evaluating the Valanginian Gage Sandstone reservoir unit and its overall storage capacity. The study demonstrated that the Gage Sandstone (Warnbro Group) is a suitable reservoir for the long-term geological storage of CO2 with the Valanginian—Hauterivian South Perth Shale (Warnbro Group) acting as a regional seal. The study also discussed possibilities for storage site locations.

The present study is a detailed sequence stratigraphic and palaeogeographic investigation of the post-Valanginian breakup reservoir-seal pair. Particular attention was paid to mapping the good quality (effective) seal and characterising the depositional environments of the Gage reservoir unit. This information was integrated into a 3D static geological model to determine reservoir heterogeneity and CO2 storage capacity.

A revised understanding of the Valanginian—Hauterivian South Perth (SP) Supersequence sedimentary succession was developed using an integrated sequence stratigraphic analysis of 19 wells and >10,000 line km of 2D reflection seismic data. Sequence stratigraphic concepts, principally described by Mitchum (1977),Vail (1987), Posamentier and Vail (1988), Van Wagoner et al (1990), Posamentier and Allen (1999) and Embry (2009), were applied to the interpretation of this sedimentary succession.

New and revised biostratigraphic data (Ingram, 1991; Western Palynoservices, 1991; Monteil, 2005); Monteil et al, 2006; Macphail, (2012), wireline logs and lithological interpretations of cuttings and core samples were used to define a new chronostratigraphic sequence framework.

Palaeogeographic maps were prepared in time-slices by mapping higher-order prograding packages, and establishing changes in relative sea level and sediment supply to portray the development of the delta system. Five facies types, delta plain, deltaic shallow marine, delta front, pro-delta and submarine fan, were identified using seismo-stratigraphic techniques (Mitchum, 1977; Vail et al, 1987). The primary criteria for mapping the delta components from the seismic included identifying erosional features, such as feeder channels and canyons, shelf slope break, toe of slope break and onlap-downlap relationships. Identification of the delta’s toe of slope break allowed delineation between silty-slope sediments and pro-delta shale. These provide an effective seal for the underlying Gage submarine fan complex. The characteristics of the interpreted Gage lowstand fan were investigated further using detailed seismic faces mapping, including a numerical analysis of seismic attributes. The Gage lowstand fan was penetrated by nine wells and where possible, the seismic facies were verified by being tied to the wells.

The SP Supersequence is interpreted to comprise three third-order sequences, spanning 5.5 million years (see Figs 1 and 2). The palaeogeographic reconstructions shown in Fig. 3 show multiple transgressive-regressive cycles that infill the central depocentre. Variations in SP Supersequence in the north compared to the south are due to differences in palaeotopography and sediment supply. Some of the sequences are not intersected by any wells, their characteristics are derived from seismic attributes and stratal relationships with other sequences. Sequence 1 is confined to the southern and northern margins of the central sub-basin depocentre and contains submarine fan to inner shelf and highstand delta sediments (Fig. 3a). The overlying Sequence 2 contains the basal Gage lowstand fan unit, which is overlain by pro-delta shale. These grade to siltstone and sandstone as the succession transitions to highstand deltaic and shallow marine sediments (Figs 3b and 3c). The pro-delta shale seal is the thickest (around 580 m) between Gage Roads 1 and Rottnest Island, and is mostly over 200 m thick in the northern Vlaming Sub-basin and immediately south of Warnbro 1. The pro-delta shale seal is thinnest in the western and southern portions of the sub-basin. North of the study area, Sequence 3 is predominantly coarse-grained sandstone, located on the aggrading upper part of a fluvio-deltaic system (Fig. 3d). In the south, however, it is finer-grained and reflects a deeper marine setting with a more distal source to the sediment supply. By the Aptian, the sub-basin depocentre was almost completely filled.

Fig. 2.  (a) Wheeler diagram and (b) seismo-stratigraphic cross-section showing the major sequences within the South Perth Supersequence, sub-divided into depositional systems tracts and major depositional environments. Systems tracts coloured in Fig. 2b match the column with systems tracts depicted in Fig. 2a. LST = Lowstand systems tract, TST = Transgressive systems tract and HST = Highstand systems tract.

Fig. 3.  Palaeogeographic maps of the offshore South Perth Supersequence showing changes in sediment input and deposition through time by time slices. (a) South Perth Sequence 1 (b) South Perth Sequence 2: Lowstand systems tract, (c) South Perth Sequence 2: Transgressive systems tract and (d) South Perth Sequence 3: Highstand systems tract.

The Gage reservoir forms part of a sand-rich submarine fan system, similar to the model proposed by Richards et al (1998), and was sub-divided into three units. It has core porosities of 9.5–23.9% and permeabilities from 52 to 1340 mD (West Australian Petroleum Pty Ltd, 1971; Cinar, 2013), and was deposited in broad palaeotopographic lows of the Valanginian breakup unconformity. The unit represents a lowstand component of the thick deltaic SP Supersequence. It ranges from canyon-confined inner fan deposits to middle fan deposits on an inclined and undulating basin plain, and slump deposits adjacent to the palaeotopographic highs. Directions of sediment supply are complex. Initially, major sediment contributions were from a northern and southern canyon adjacent to the Mandurah Terrace. These coalesced in the inner middle fan and extended westward onto the plain, producing the outer middle fan. As time progressed, sediment supply from the east became more significant. Although, much of the submarine fan complex is not penetrated by wells, the inner fan is interpreted to contain stacked channelised high-energy turbidites and debris flows. This part of the reservoir would provide the most suitable target for CO2 storage and hydrocarbon accumulations, as it is sandiest and has good vertical sand connectivity within the channel complex (Weimer and Slatt, 2006).

The inner middle fan is a transition zone that contains a combination of channels and suprafan lobe deposits. The sands are interpreted to have good lateral and good-moderate vertical communication and should exhibit good reservoir properties. The outer middle fan deposits are predicted to contain more fine-grained material and thin, laterally connected sand sheets. The sheet-like lobes can form good reservoirs with good lateral continuity and a narrow range in grain size (Weimer and Slatt, 2006). Like the inner and inner middle fan deposits, the outer middle fan deposits in the central Vlaming Sub-basin can also provide viable targets, although well penetration is required to confirm the reservoir’s potential. Slumps were not considered as viable targets due to their chaotic bedding, poor sand connectivity, and poorer porosity and permeability due to possible de-watering and compaction during transport and emplacement (Shipp et al, 2004).

Reservoir and seal facies interpretations, combined with containment constraints were used to find suitable locations for potential CO2 storage in the offshore central Vlaming Sub-basin. The analysis shows that the most lithologically prospective sites are channel sands in canyon-confined inner fan deposits where thick pro-delta shale acts as a seal. These are located between Gage Roads 1 and Rottnest Island in the northern region of the study area, and south of Warnbro 1 in the southern region of the study area. The transition zone of the inner middle fan, and sheet sands of the middle fan may also be good reservoirs and may provide suitable targets for the CO2 storage in the offshore Vlaming Sub-basin. The reservoir property interpretation derived from this study was incorporated into a 3D static geological model of the Gage reservoir to provide a more realistic and accurate representation of its CO2 storage capacity.

Palaeogeographic reconstructions for the SP Supersequence portray a more complex early post-rift depositional history in the central Vlaming Sub-basin unlike what was previously mentioned. Additional drilling and 3D seismic data could reveal the full potential for CO2 storage and petroleum accumulations in the sub-basin. This approach proved to be effective in identifying suitable storage sites and is applicable for detailed geological studies, underpinning both CO2 storage and petroleum prospectivity investigations.