Addressing exploration uncertainties in the southern Bonaparte Basin: enhanced stratigraphic control and post drill analysis for upper Permian plays

Keywords: biostratigraphy, Bonaparte Basin, North West Shelf, Permian–Triassic boundary, Petrel Sub-basin, post drill analysis, reservoir, stratigraphy.

The late Permian to early Triassic stratigraphy of the southern Bonaparte Basin (SBB) includes the Hyland Bay Subgroup, which incorporates the Torrens, Pearce, Cape Hay, Dombey and Tern formations, and the overlying Mount Goodwin Subgroup that includes the Penguin, Mairmull, Ascalon and Fishburn formations (Fig. 1; Gorter 1998; Gorter et al. 2009). The Permian–Triassic stratigraphic boundary (PTB) occurs at or around the transition from the Hyland Bay Subgroup to the lowermost Mount Goodwin Subgroup. The sedimentary succession spanning this interval was deposited during a prolonged marine transgression and shows a transition from the estuarine Cape Hay Formation, to the offshore carbonate-dominated Dombey Formation, to the marginal to shallow marine siliciclastic-dominated Tern and Penguin formations.

Fig. 1.  Upper Permian to lower Triassic lithostratigraphy in the southern Bonaparte Basin, Australian generalised spore pollen zones and East Australia spore pollen datums. Geologic Time Scale after Gradstein et al. (2020).

Upper Permian reservoirs in the Hyland Bay Subgroup constitute the P50 play interval in the SBB (Phillips et al. 2019). The P50 play interval is generally deeply buried where it has been the target for exploration (e.g. reservoir depths of >2450 and >3550 m in the Tern and Petrel fields, respectively). The preservation of porosities and permeabilities within these reservoirs is dependent on the presence or absence of clay coatings around detrital quartz grains, which act to inhibit siliceous cementation during diagenesis (Bhatia et al. 1984; Saïag et al. 2016). Recent work shows the presence of these clay coatings is linked to the depositional environment of reservoirs (Wooldridge et al. 2019), highlighting the need for refined depositional models to reduce the risk and uncertainty for the P50 play interval.

Studies of Triassic and Permian basins in onshore and offshore Australia rely heavily on the Australian Spore-Pollen Zonation scheme that is based largely on the work of Kemp et al. (1977), Foster (1982), Helby et al. (1987), Price (1997) and Mantle et al. (2010). The chronostratigraphic calibration of these zones has been updated as a result of radiometric dating (Laurie et al. 2016; Fielding et al. 2019; Mays et al. 2020) and updates to the Geological Time Scale (Gradstein et al. 2020).

This work presents a review of the biostratigraphic data from wells that intersect the Permo-Triassic succession in the SBB, which have been supplemented with additional samples and palynological analyses. New and revised biostratigraphic data is integrated with well log interpretations and correlations to subdivide the upper Permian and lower Triassic succession into broad parasequences that record changes in lithofacies and capture the evolution of depositional environments across the basin. Aspects of the biostratigraphic review and the depositional history of the succession around the PTB in the SBB are discussed using the updated biostratigraphic control and parasequence interpretations from the field areas of Prometheus/Rubicon, Tern, Petrel and Blacktip. A post drill analysis of the upper Permian Hyland Bay Subgroup (P50 play interval) has also been undertaken to provide insights into the main play risk elements.

A preliminary post drill analysis of the P50 play interval (Tern, Dombey, Cape Hay and Pearce formations) has been undertaken. The P50 play extends across much of the offshore SBB, but was either not deposited or subsequently eroded over the southern-most flanks of the Petrel Sub-basin, the Berkley Platform and the Darwin Shelf (B. Bradshaw, pers. comm., March 2022). Eleven proven or interpreted occurrences of gas have been intersected in the P50 play across the study area (Ascalon 1A, Defiant 1, Fishburn 1, Frigate Deep 1, Penguin 1, the Petrel Field, Plover 1, Polkadot 1, Prometheus 1, Rubicon 1 and the Tern Field). Successful gas tests in the P50 play interval are associated with four-way dip-closed anticlines and high-side fault-block traps.

The analysis identified that the primary risk for the P50 play is reservoir effectiveness (Fig. 2), with fault seal and hydrocarbon charge representing secondary risks. Assessment criteria for reservoir presence and effectiveness are based on geological cut-offs rather than commercial cut-offs, which are beyond the scope of this study. Results for reservoir presence and effectiveness would be markedly different if commercial considerations were included in the assessment. The P50 play has generally not been a primary or secondary exploration target, and has rarely been evaluated in detail through logging and/or testing. Our reservoir evaluation of the P50 interval is therefore largely qualitative in nature and based on available data from company well completion reports. Interpreted porosities are good to excellent (from 15% to 30%), however permeability tends to be low (in the tens to low hundreds of mD) as a result of silica and/or calcareous cements or high clay contents. At these relatively low permeabilities, commercial flow rates may be challenging, as shown by the drilling campaign of the Tern Field, which highlights reservoir effectiveness as a risk for this play.

Fig. 2.  Map of the southern Bonaparte Basin showing preliminary outcomes of the post drill analysis of reservoir effectiveness for the P50 play interval (Hyland Bay Subgroup). Base map shows depth to basement (OZ SEEBASE^®; Geognostics 2020). Field outlines from GPinfo petroleum database.

As part of our study, MGPalaeo were contracted to review available biostratigraphic data and undertake additional sampling and analysis (Mantle and Hannaford 2021). The revised palynological data enables updating of zonal assignments. Wells included in this work were: Ascalon 1, Blacktip 2, Blacktip P1, Petrel 1A, Petrel 2, Petrel 3, Petrel 4, Petrel 5, Petrel 6, Prometheus 1, Rubicon 1, Tern 1, Tern 2, Tern 3, Tern 4, Tern 5 and Torrens 1 (Fig. 2). Palynofacies analyses were also carried out on key intervals in selected wells.

The better-resolved and updated zonal assignments allow for improved resolution of the Dulhuntyispora parvithola, Playfordiaspora crenulata and Protohaploxypinus microcorpus zones through the studied wells. The new dataset also permits the ‘APP5’ subzones of Price (1997) to be discerned in many instances. The transitions into the P. crenulata and P. microcorpus zones – of particular interest due to being synchronous with the onset of the End Permian Extinction (EPE) event and PTB in eastern Australian basins (Fielding et al. 2019; Mays et al. 2020) – are further elucidated by the palynofacies analyses.

In the studied wells, the upper Permian to lower Triassic succession shows a progression from the estuarine Cape Hay Formation, to the shallow marine carbonate-dominated Dombey Formation, to the siliciclastic offshore to shoreface Tern Formation sandstones, and mud basin siltstones and shales of the Penguin Formation. In detail, there is variability in the nature and arrangement of the parasequences that make up the succession, reflecting subtle differences in depositional setting and evolution. The general progression can be summarised as follows:

1. Deposition of the transgressive Cape Hay Formation in a protected to restricted estuarine embayment (Kloss et al. 2004). Initial sequences are largely characterised as tide-dominated sand and mud flats with a fluvial component. Upper sequences become increasingly sand-dominated, showing a stronger marine influence, as the transgression intensifies, and include tidal channel and bar complexes and sandy tidal flats (Kloss et al. 2004).

2. Marine flooding of the estuarine embayment and deposition of the shallow marine to shelfal carbonates of the Dombey Formation. A concurrent reduction in fluvial/terrestrial sediment input would have also been important to establish conditions that were conducive to carbonate-dominated deposition.

3. Resumption of siliciclastic deposition and the establishment of a progradational shoreface system across the SBB. The offshore to lower shoreface siltstones and sandstones of the lower Tern Formation indicate relatively stable eustatic sea-level conditions, with repeated filling of accommodation delivering at least two cycles that coarsen-up to upper shoreface sandstones.

4. Deposition of restricted mud basin deposits from the lower Penguin Formation, which represent continued progradation of the coastal and shallow marine depositional system. In the Tern wells, an intervening transitional section interpreted to represent flood tide delta and tidal channel facies is preserved in the uppermost Tern Formation (Bann et al. 2004).

5. Marine transgression resulting in flooding of the restricted mud basin and deposition of marine siltstone and mudstones of the upper Penguin and Mairmull formations in an open marine embayment.

Integrating high-resolution biostratigraphic control with the interpreted upper Permian to lower Triassic parasequences highlights the diachronous nature of these deposits across the SBB (Fig. 3). This indicates the parasequences and associated facies, that form the building blocks of the upper Permian and lower Triassic lithostratigraphic units, are highly sensitive to changes in balance between accommodation and sediment supply. Within the framework of a broad regional transgression during the late Permian and early Triassic, the high input of clastic coastal and fluvial sediments produced a prograding system of stacked parasequences, with the progression into coastal depositional environments occurring later in deeper basin areas. While our interpretation implies the main direction of the deepening paleo-bathymetric trend was to the northeast in the late Permian, there is also considerable lateral variation in depositional environments indicating local variations in paleo-bathymetry as well as switching in sediment sources. Spatial variability in the generation of accommodation is considered to be a contributing factor controlling the diachronous nature of the lithostratigraphic units, which reflects variations in subsidence rates across the basin potentially related to salt tectonism in the Petrel Sub-basin.

Fig. 3.  Well correlation for key wells in the southern Bonaparte Basin scaled to age using updated biostratigraphic data and parasequence interpretation (PS1 through PS9). Depositional environment for each parasequence is also shown: CARB, carbonate facies; DSF, distal shoreface; LSF, lower shoreface; USF, upper shoreface; FSB, foreshore/beach; DC/SF, distributary channel/sandflat; EST, estuarine; TMF, tidal mudflat; FTD, flood tide delta; DLA, delta-lagoonal; RMB, restricted mud basin; OMD, offshore muds. Geologic Time Scale after Gradstein et al. (2020).

Post drill analysis of the P50 play interval across the SBB highlights that while several gas discoveries have been made, reservoir effectiveness is a key exploration risk. New and revised biostratigraphic and palynofacies data for the upper Permian and lower Triassic succession in the SBB has enabled refinements of the depositional models for the P50 play interval. The refined depositional model advances understanding of the spatiotemporal distribution of depositional environments during the late Permian and early Triassic, helping to predict the spatial distribution of effective reservoir fairways. In particular, the diachronous nature of the latest Permian depositional system should be considered when assessing the potential distribution and character of reservoir intervals across the SBB.

The data that support this study will be shared upon reasonable request to the corresponding author.

All authors confirm there are no conflicts of interest.

No funding from external organisations was received for this research.