A regional chronostratigraphic framework for play-based resource assessments in the Eromanga Basin

Keywords: carbon capture and storage, chronostratigraphy, conventional hydrocarbons, groundwater, low-carbon energy resources, play mapping, regional play intervals, unconventional hydrocarbons.

Australia has abundant energy resources that provide energy security and economic benefits for all Australians. Global demand is now shifting to ‘net-zero’ energy resources to abate the impacts of global climate change associated with greenhouse gas emissions. Continuing our nation’s energy-enabled prosperity requires maintaining an adequate clean energy supply chain into the future. The Australia’s Future Energy Resources (AFER) project is assessing the occurrence and producibility of potential low-carbon energy resources in underexplored regions of central Australia as part of Geoscience Australia’s Exploring for the Future (EFTF) Program.

Unlocking low-carbon energy resources in the central Australian region requires systematically evaluating and mapping the following: the presence; effectiveness; and interconnectivity of key geological elements. These control the distribution of conventional and unconventional petroleum resources. The same elements also enables the evaluation and mapping of potential geological storage intervals as well as deep unallocated groundwater resources for potential ‘green’ hydrogen fuel production. The play-based resource assessment approach used over several decades by the petroleum industry (e.g. Longley and Brown 2016) provides a workflow for assessing multiple sub-surface sediment-hosted resources to ensure future exploration and production provides carbon-neutral energy resources while ensuring minimal impact on allocated groundwater resources. A key requirement for undertaking play-based resource assessments is to develop a chronostratigraphically based geological framework to define the main play intervals. Regional play schemes have been developed and applied over several decades for petroleum explorers in Western Australia (Longley et al. 2002; Marshall and Lang 2013), but have not as yet been readily available for most onshore Australian basins.

This paper presents a chronostratigraphic play scheme that has been developed for low-carbon energy resource assessments in the Eromanga Basin as part of Geoscience Australia’s AFER Project. The regional play scheme has been adapted from a new chronostratigraphically based geological and hydrogeological framework developed as part of Geoscience Australia’s Great Artesian Basin Groundwater Project (Hannaford et al. 2022; Norton and Rollet 2022). Geoscience Australia’s assessment of the status of groundwater in the Great Artesian Basin (Eromanga, Surat and Carpentaria Basins) will continue under Geoscience Australia’s National Groundwater Systems Project (EFTF Program) aiming to support cohesive and sustainable intergenerational water management practices across borders, while equipping government, industry and the community with geoscience data and information to make informed decisions.

The Jurassic–Cretaceous western Eromanga Basin and underlying Permian–Triassic age Pedirka and Simpson Basins in central Australia are underexplored with only 42 petroleum wells and 5 stratigraphic wells drilled since exploration began in the 1950s (Fig. 1), and about 15 000 line kilometres of 2D seismic data acquired across the assessment area covering about 190 000 km². Exploration initially focussed on conventional petroleum resources during several exploration programs between the 1950s and 1980s. Exploration was rejuvenated with Central Petroleum’s conventional and unconventional drilling program between 2008 and 2010, and continues in the region with several companies focussing on identifying low-carbon energy resources and associated carbon capture and storage (CCS) prospects. The presence of an active petroleum system has been demonstrated by minor oil and condensate recovered from Triassic and Jurassic reservoirs at Poolowanna 1, and identification of residual oil saturations in several wells (Blamore 1, Colson 1 and Simpson 1; Ambrose et al. 2007; Central Petroleum 2008). A key uncertainty for conventional hydrocarbon exploration has been the structural integrity of traps, with most exploration targets defined on sparse, relatively low-quality seismic data, and several previously drilled traps appearing to have been breached during a Cenozoic phase of fault reactivation (Ambrose et al. 2002, 2007). The AFER Project aims to reduce the uncertainty in identifying valid traps for conventional hydrocarbons and geological storage prospects by reprocessing about 3750 line km of seismic data across the western Eromanga Basin (Fig. 1)

Fig. 1.  Location of the western Eromanga Basin resource assessment area showing the western boundary for the Eromanga Basin, and extents of older underlying Triassic, Permian and pre-Permian Basins. Also shown are the locations of petroleum exploration wells in the assessment area, and seismic lines that are being reprocessed by Geoscience Australia.

The western Eromanga Basin forms part of the Great Artesian Basin and hosts important groundwater resources for remote communities in central Australia. Previous groundwater assessments in the western Eromanga region have identified that a major uncertainty regarding the development of any unconventional hydrocarbon resources is the limited understanding of connectivity between aquifers within the stacked Permian, Triassic and Jurassic–Cretaceous Basins (Wohling et al. 2013). The Queensland portion of the Eromanga Basin has previously been evaluated as having high potential for geological storage of CO₂ based on the presence of regionally extensive and effective fluvial reservoirs that are effectively sealed by lacustrine and marine shales (Bradshaw et al. 2009). Understanding the CCS potential of the western Eromanga Basin requires the evaluation of the extent to which these high-potential reservoir–seal intervals occur in the assessment area.

The Eromanga Basin is a large Jurassic to Cretaceous age intracratonic basin that extends over an area of more than 1 000 000 km² across central and eastern Australia. Up to 3000 m of clastic sediments were deposited in three broad basin successions: an Early Jurassic to Early Cretaceous fluvial and lacustrine succession; an Early to mid-Cretaceous marine succession; and a Late Cretaceous fluvial–lacustrine succession (Passmore 1989; Gallagher et al. 2008). The stratigraphic architecture reflects an interplay between the creation of subsidence-controlled accommodation, fluctuations in sediment supply rates and changing sediment provenances (Lang et al. 2001; Cotton et al. 2006). These allocyclic depositional controls were linked to changes in intra-plate stress fields, eustatic sea-level fluctuations and dynamic (mantle-driven) topography during the breakup of Gondwana (Harrington et al. 2019; Young et al. 2021). The stratigraphic architecture of the western Eromanga Basin was initially influenced by regional uplift in the order of 500–800 m over the western Australian continent throughout the Jurassic and Early Cretaceous, which provided a continual influx of fluvial sandstones during deposition of the Algebuckina Sandstone (Struckmeyer and Totterdell 1990; Norvick 2003; Harrington et al. 2019). A rapid increase in subsidence rates, combined with globally high sea levels in the Early Cretaceous, resulted in a marine incursion across much of the Australian continent and deposition of marine sediments across the Eromanga Basin (Gallagher et al. 2008). Open marine and marginal marine sedimentation continued until the Late Cretaceous when the basin returned to non-marine sedimentation during deposition of the Winton Formation before basin closure during an E–W Late Cretaceous compressional event (Cotton et al. 2006; Kulikowski et al. 2021). Rapid deposition of the Late Cretaceous Winton Formation was the critical event for hydrocarbon generation and expulsion from many Permian, Triassic and Jurassic source intervals (Lowe-Young et al. 1997).

The Eromanga Basin is Australia’s most prolific onshore oil province where it overlies the Cooper Basin in central Australia, with about 215 million barrels of oil produced in South Australia – the majority of oil has been produced from fluvial reservoirs in the Hutton Sandstone and Namur Sandstone (South Australia Department of Energy and Mines 2021). Despite this, there is no published play framework for resource assessments in the Eromanga Basin. Geoscience Australia’s Great Artesian Basin Groundwater Project has recently completed a collaborative study with MGPalaeo and Catherine Jane Norton to refine the chronostratigraphic framework and biostratigraphy of the Great Artesian Basin (Hannaford et al. 2022; Norton and Rollet 2022). The Great Artesian Basin Groundwater Project has compiled a standardised chronostratigraphic framework for the Eromanga, Surat and Carpentaria Basins, which links the lithostratigraphic units from each basin to key regional chronostratigrapic surfaces enabling regional correlations and the mapping of aquifers and aquitards (Fig. 2). These chronostratigraphic surfaces follow the widely adopted hierarchical scheme developed by Santos for exploration in Mesozoic and Cenozoic successions in central Australia (Gallagher et al. 2008; Mackie 2015).

Fig. 2.  Chronostratigraphic chart for the Eromanga Basin modified from the recently published Geoscience Australia and MGPalaeo Great Artesian Basin chronostratigraphic chart (Hannaford et al. 2022) using the 2020 geological time scale (Gradstein et al. 2020). The chart shows the nine regional play intervals defined in this study for resource assessments in the Eromanga Basin. Also included are the North West Shelf dinocyst zones of Partridge (2006; after Helby et al. 1987), the central Australian palynological zones of Price (1997), the Santos Eromanga Basin chronostratigraphic surfaces (Gallagher et al. 2008), and the North West regional play intervals and sequence stratigraphic surfaces of Marshall and Lang (2013).

The Geoscience Australia and MGPalaeo revised chronostratigraphic framework is applied in this paper to develop a common Mesozoic play framework scheme for assessing sediment-hosted resources in the Eromanga Basin (Fig. 2). The basin succession is sub-divided into nine regional play intervals related to the tectonostratigraphic evolution of the Eromanga Basin. Each play interval represents a regionally significant reservoir and/or seal interval for hydrocarbon accumulations, groundwater systems or CCS intervals. Seven of the eleven play interval boundaries correspond to one of the Santos regional unconformities (designated by ‘U’ in the chronostratigraphic surface nomenclature of Gallagher et al. 2008; Fig. 2). Other play interval boundaries are associated with either regional transgressive or maximum flooding events.

The play interval nomenclature follows a simple system based on the name of the main component stratigraphic units. It was initially hoped that the alpha-numeric play interval scheme developed for the North West Shelf by Longley et al. (2002) and refined in Marshall and Lang (2013) could be applied to the Eromanga Basin, particularly as many of the chronostratigraphic surfaces delineating the Eromanga play intervals appear to correlate to major chronostratigraphic surfaces on the North West Shelf (Fig. 2). However, there is rarely a one-to-one correlation in the play intervals from these two areas. Applying the North West Shelf play interval nomenclature would therefore require both splitting and compounding these established names into a complex naming system that would have little meaning to explorers from either the North West Shelf or central Australia.

Nine play intervals have been interpreted and correlated in well logs across the western part of the Eromanga Basin using chronostratigraphic surfaces picked by Norton and Rollet (2022). The play interval correlations show the transition in Jurassic and Early Cretaceous strata from clearly differentiated fluvial reservoir and fluvial–lacustrine seal intervals in the east to an amalgamated fluvial sandstone succession corresponding to the Algebuckina Sandstone in the west (Fig. 3). A regional seal extends across the western basin area in the marine Wyandra–Wallumbilla play interval. Successful exploration in the western Eromanga Basin requires understanding the spatial extent and effectiveness of these reservoir and seal intervals, and identifying optimal locations where favourable conditions occur for producing sediment-hosted resources or injecting CO₂.

Fig. 3.  Well-log correlation across the western Eromanga Basin (flattened on the CC40 flooding surface) showing the relationships between the newly defined play intervals in this paper, the established lithostratigraphic units, and the ‘Santos’ chronostratigraphic surfaces interpreted in wells across the assessment area by Norton and Rollet (2022; see Fig. 1 for location). Jurassic and Early Cretaceous play intervals (Poolowanna to Namur-Murta) show a transition from distinct fluvial reservoir and lacustrine shale seal intervals in the east, to amalgamated fluvial sandstones in the west. Early Cretaceous marine shales and siltstones (Wyandra–Wallumbilla play interval) provide a regional seal across the assessment area.

The western Eromanga Basin is an underexplored area with potential to provide future low-carbon energy resources, CCS and potential hydrogen opportunities. Understanding the region’s low-carbon energy potential requires a play-based exploration approach for resource assessments. Such an approach involves systematically evaluating the presence, effectiveness and connectivity of regional reservoir and seal intervals, and fluid migration pathways for hydrocarbons, groundwater systems and injected CO₂ plumes. Geoscience Australia has commenced a play-based resource assessment of this area through integrating post-drill analysis with common risk segment mapping of nine newly defined and interpreted play intervals corresponding to the main reservoir and seal intervals. A Cretaceous and Jurassic chronostratigraphic framework has been developed to support this play-based resource assessment. The chronostratigraphic framework will be expanded in future work to include Permian and Triassic age play intervals for assessing the low-carbon energy resources of the underlying Pedirka and Simpson Basins.

Data sharing is not applicable as no new data were generated or analysed during this study.

All authors confirm there are no conflicts of interest.

No funding from external organisations was received for this research.