At A Glance
This study showcased the power of integrating sedimentology and palynology to interpret palaeoenvironments with a level of detail not discernible through lithology alone.
The combination of these disciplines enabled a high-resolution reconstruction of deltaic processes in the Caley Member, translating directly to better subsurface models, improved prediction of reservoir geometry, and more informed exploration decisions in the Bedout Sub-basin.
It’s a reminder that in stratigraphy, the story lies not just in the rocks—but in what once lived among them.
In offshore Western Australia’s Bedout Sub-basin, the Middle Triassic Caley Member offers a compelling record of ancient deltaic environments within a mixed river-, wave-, and tide-influenced system.
Nearly 120 m of continuous core from the Roc 2 well provided an exceptional opportunity to analyse sedimentary architecture and ecological signals across this dynamic depositional setting.
By integrating high-resolution sedimentological and palynological datasets, the study moved beyond traditional facies analysis to reconstruct a nuanced, process-driven model of palaeoenvironmental evolution.
This integrated approach revealed subtle but critical variations in depositional energy, salinity, and ecological stress that would have otherwise gone undetected.
Mixed-influence deltas are notoriously complex, with depositional environments changing rapidly over short distances due to the interplay of multiple coastal processes.
In such systems, predicting reservoir quality and geometry requires more than sedimentary facies alone—it demands understanding of water salinity, vegetation sources, and environmental stress.
The challenge in Roc 2 was to differentiate mouth-bar deposits that appeared lithologically similar, but were shaped by very different combinations of wave, river, and tidal energy.
Without independent environmental proxies, these distinctions would be ambiguous, limiting the predictive power of the stratigraphic model and potentially obscuring key reservoir heterogeneities.
The synergistic use of sedimentology and palynology allowed for confident interpretation of depositional environments across a single deltaic complex.
Sedimentology
Detailed core logging identified multiple coarsening-upward successions indicative of progradational distributary mouth-bars. Sedimentary structures such as wave ripples, hummocky cross-stratification, mud drapes, and rare synaeresis cracks provided insights into hydrodynamic energy and depositional processes.
Palynology
Palynomorphs—including spores, pollen, freshwater algae (Botryococcus), and marine acritarchs (Micrhystridium, Veryhachium)—were used as environmental proxies. Changes in abundance, preservation, and species composition revealed variations in salinity, terrestrial input, and ecological stress.
Facies Association A reflected a classic prograding mouth-bar, beginning in a fluvially dominated prodelta setting and transitioning to wave-modified sands. The sedimentology showed rhythmic bedding and hummocky cross-stratification, while palynology revealed fluctuating freshwater input, evidenced by abundant Botryococcus and declining Micrhystridium. The trace fossil record mirrored these shifts, with bioturbation intensity dropping as energy and sedimentation rates increased. These findings confirmed a shift from distal prodelta mudstones to proximal sandy mouth-bar deposits.
Facies Association B revealed a starkly different setting. Here, sedimentological features suggested lower energy and limited wave influence, which was matched by palynological evidence: Micrhystridium was extremely rare, Botryococcus was common, and stressed ichnofacies (notably Glossifungites) dominated. This suite of indicators pointed to a restricted, stressed lagoonal setting with episodic isolation from open marine waters—important for modelling baffles or flow barriers in potential reservoir intervals.
Facies Association C captured the most dynamic environmental shifts. Alternating sandstone and mudstone beds, paired with mixed trace fossil assemblages and very rare and sporadic spinose acritarchs, indicated repeated shifts in salinity and energy—likely representing a periodically restricted interdistributary bay system affected by both fluvial influx and tidal backflooding.
The figure below shows a schematic model of an asymmetric, mixed-influence delta, highlighting the depositional settings for each of the facies associations:
- a) Approximate location of the distributary mouth-bars in Facies Association A situated within the delta front environment.
- b) Approximate location of distributary mouth-bars in Facies Association B building into a restricted lagoonal setting.
- c) Approximate location of distributary mouth-bars in Facies Association C within a partially open lagoon/embayment.
- d) Alternative location of distributary mouth-bars in Facies Association C within an interdistributary bay.
Please Contact us to discuss how our integrated palynological and sedimentological services can help your next campaign.
Or to find out more about this case study:
Allgöwer, A.M. & Lignum, J.S., 2019. Fine-scale variations in distributary mouth-bar morphology in the Middle Triassic Caley Member of the Bedout Sub-basin, Western Australia, in KEEP, M. & MOSS, S.J. (Eds), The Sedimentary Basins of Western Australia V: Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, WA, 2019, 23 pp.