Three recent EIS-funded biostratigraphy projects have revolutionized our understanding of Jurassic plays in the northern Perth Basin, and with the next round of EIS funding opening in early 2025, there’s never been a better time to leverage this co-funded initiative.

Jurassic and Triassic exploration in the Perth Basin had previously been held back by issues with tight reservoirs and concerns over critical seals (relating to intense post-Early Triassic faulting and high sand-to-shale ratios). This all changed following Strike Energy and Talon’s successes in drilling the Walyering-5 and -6 wells, which were shown to host considerable and accessible gas reserves in the Cattamarra Coal Measures and Cadda Formation.

And if these Jurassic formations could host economic reserves, it made sense to have another look at the thick fluvial plays of the Yarragadee Formation too. Suddenly it wasn’t all about the Early Permian!  

Need for Higher Resolution Biostratigraphy in the Perth Basin

With the success at Walyering and a renewed interest in both stand-alone Jurassic targets and potential secondary targets when drilling down to Early Permian Kingia and High Cliff Sandstone reservoirs, it was clear that a better understanding of the stratigraphy was required.

Traditionally, correlations between Perth Basin wells have proven challenging using lithostratigraphy, wireline logging or seismic mapping techniques alone, while the existing biostratigraphic data hasn’t been particularly helpful either — it was of low-resolution and mostly qualitative (lacking quantitative data / fossil assemblage counts).

Furthermore, the Jurassic spore-pollen zones in the Perth Basin are very long-ranging (e.g., the Murospora florida Zone extends for over 15 million years) and are difficult to consistently subdivide with just standard first and last appearance datums (FADs and LADs).

It was immediately evident that the existing palynological schemes would not provide the resolution required by modern hydrocarbon explorers.

What was needed was the development of higher resolution “bioevent” stratigraphies utilising both traditional FADs and LADs along with quantitative events (increases, abundance spikes, and acmes of common spores and bisaccate pollen). The distribution of these short-lived abundance events may be environmentally or climate-driven and are thus often not useful as broader regional or national zonal marker events, but they are excellent for subdivision at the field scale and sub-basinal levels.

However, recognition of these bioevents requires much finer biostratigraphic sampling programs and fully quantitative biostratigraphic data collection than have been applied before in the Perth Basin.

Key marine dinocyst species from the Cadda Formation in the northern Perth Basin. A: Escharisphaeridia sp. (new Cadda Fm species); B: Dissiliodinium sp.; C-D: Phallocysta erregulensis. Scale bar = 10 microns.

Exploration Incentive Scheme (EIS) – a fantastic funding initiative!

Whilst the recent Walyering successes have been the spark for developing higher resolution palynological breakdowns for the Jurassic of the Perth Basin, it is the Department of Energy, Mines and Industry Regulation’s (DEMIRS) incredible Exploration Incentive Scheme (EIS) that has provided the means to get this work started.

MGPalaeo has completed three major biostratigraphic development projects through this co-funded initiative:

  1. Walyering Biostratigraphy Review – with Strike South West Pty Ltd
  2. Jurassic biostratigraphy of the Gingin area – with Mineral Resources Ltd, Energy
  3. Biostratigraphy of the Dandaragan area – with Mineral Resources Ltd, Energy

These are all available for download from the WAPIMS website.

New “Event” Stratigraphies for the Cattamarra Coal Measures, Cadda and Yarragadee formations

The first of these EIS studies concentrated on the fluvial to marine claystones, coals and sandstones of the Early to Middle Jurassic Cattamarra Coal Measures and Cadda Formation, with new and review samples from the Walyering and Ocean Hill wells.

Thirteen new bio-events within the Early to Middle Jurassic succession were identified, including the first subdivisions of the major Cadda marine incursion. Surprisingly, below the Cadda Formation, no definitive evidence for brackish or marine conditions was observed within the Cattamarra Coal Measures. A sharp contact between the stacked, fluvial, coal measures deposits and the marine depositional settings of the Cadda Formation is now envisaged.

This first study provided a robust biostratigraphic framework for the Walyering and Ocean Hill wells with significant implications for exploration, appraisal, and development activities in the region.

Two further studies with Mineral Resources (Energy) Ltd focussed on the Gingin and Dandaragan areas, aiming to improve the biostratigraphic resolution of central Perth Basin Jurassic successions. These projects again targeted the upper Cattamarra Coal Measures and Cadda Formation but also had a significant focus on the younger, fluvial Yarragadee Formation.

Multiple new correlative bioevents were recorded in both areas (12 bioevents in the Gingin area; 11 bioevents in the Dandaragan area), and encouragingly we noted the same dinocyst subdivisions of the marine Cadda Formation as those recorded from the Walyering Field study. This threefold division of the Cadda Formation comprises an initial influx of Phallocysta erregulensis (likely an opportunistic taxon that quickly filled newly available shallow marine niches) followed by a major increase in two larger marine dinocysts, Escharisphaeria and Dissiliodinium, and finally a short interval with only rare marine dinocysts remaining, marking the cessation of marine deposition prior to the fully fluvial settings that characterise the lower Yarradagee Formation.

Threefold subdivision of the marine Cadda Formation from Gingin-1. Full well-to-well correlation panels for all three EIS projects are available from the WAPIMS website.

Future Opportunities for EIS-Funded Projects

The new palynological data has assisted the development of a more robust chronostratigraphic framework of the Jurassic in part of the central and northern Perth Basin. In particular, the new bioevent data allows confident correlations of the stacked Cattamarra gas reservoirs and subdivision of the Cadda Formation (it is not just the main regional seal, but also has reservoir potential). This new framework will reduce uncertainty in the regional mapping of reservoir packages in the basin and aid in the identification of faulted-out or eroded intervals.

These three EIS projects are just the beginning of improving our biostratigraphic knowledge of the Perth Basin. There is still much to learn about the Jurassic stratigraphy, let alone the Triassic depositional successions, or even looking for better preserved Permian intervals north of the current core exploration areas.

With the next round of EIS funding opening to submissions in February 2025, we would love to hear from your company if you are interested in furthering the industry stratigraphic knowledge of this or other Western Australian onshore basins. So pick up the phone or shoot us an email ([email protected]) and let’s discuss project ideas.