Chemostratigraphy of the Sherbrook Supersequence – 13 wells from the offshore Otway Basin

Chemostratigraphic analysis

Elemental chemostratigraphy is the study of rock units whereby chemical compositions are used to distinguish between chemically differentiable zones and to correlate between these zones (Riley et al. 2024). Chemostrat Australia Pty Ltd was engaged by Geoscience Australia to undertake Inductively Coupled Plasma-Optical Emission Spectrometry and Mass Spectrometry (ICP-OES and MS) analysis (Jarvis and Jarvis 1992), to establish a regional elemental chemostratigraphic framework for the Sherbrook Supersequence, offshore Otway Basin. For each of the 1185 samples submitted for analysis, 50 elements were quantified. These data were then analysed and interpreted to identify chemically distinct chemostratigraphic sequences, packages and units that can be correlated between wells.

Sampling of cuttings

1185 cuttings samples from the Sherbrook Supersequence (and overlying stratigraphy) in 13 wells, located offshore Otway Basin, were sampled at the National Offshore Petroleum Data and Core Repository facility in Carlisle, Western Australia.

Initially wells to be sampled were chosen based on their ability to provide regional representation of the top and base of the Sherbrook Supersequence (Fig. 1 wells in purple) and a focus on the less well explored parts of the basin, however as the study progressed it was necessary to adapt due to many of the selected wells having limited cuttings sample available for destructive analysis. As a result, the geographic distribution of data (Fig. 1 wells in yellow) is driven by sample availability rather than geology.

Chemostratigraphic interpretation

A chemostratigraphic framework was developed, with nomenclature adapted from Forbes et al. (2020). The framework for this study (Riley et al. 2024) comprises two chemostratigraphic sequences, seven chemostratigraphic packages and eighteen chemostratigraphic units. A chemical gamma-ray curve (the ChemGR) has been calculated and compared to the measured gamma-ray (GR), to demonstrate the representativeness of the samples.

Application of chemostratigraphy at a basin and sub-basin scale

Despite the sparse distribution of the wells (Fig. 1) and structural complexity between wells (Nicholson et al. 2024), the application of chemostratigraphy, and specifically the resultant chemostratigraphic interpretation from Riley et al. (2024), in a regional context is valuable. The analysis of the results is best examined using the two key transects shown in Fig. 1. Correlations between wells based on the chemostratigraphic interpretation are evident across Transect A (Fig. 2) in the western part of the offshore basin; and, across Transect B (Fig. 3) within the Shipwreck Trough. These transects were chosen based on geological setting and the correlation of the chemostratigraphic interpretation with supersequence boundaries.

Transect A (Fig. 2) from the western part of the offshore basin includes a number of wells from the study (Riley et al. 2024) which recommended the base Sherbrook Supersequence marker be reviewed in four wells; Crayfish-1A, Copa 1, Breaksea Reef-1-ST3, and Callister-1. Following the recommended review, the latter three wells resulted in changes to both the top and base of the Sherbrook Supersequence (Fig. 2), albeit predominantly due to reinterpretation with available biostratigraphy. While Crayfish-1A has such a compressed Sherbrook succession that no changes were warranted.

In the Shipwreck Trough wells (Fig. 3), the base of the Sherbrook Supersequence is commonly interpreted within chemostratigraphic unit 5.6.2. And, with the exception of La Bella-1, the shipwreck chemostratigraphic package 5.6 has good correlation with the lower Sherbrook Supersequence. In this part of the basin a more targeted chemostratigraphic interpretation of the units within Package 5.6 could benefit correlation, as could extending sampling and interpretation deeper into the Shipwreck Supersequence.

When considering the chemostratigraphic interpretation at a basin scale we can see that, unsurprisingly, due to the significant unconformity near the Paleogene–Cretaceous boundary, the chemostratigraphic analysis shows a basin wide and consistent change between chemostratigraphic sequences 5 and 6, commonly near the base of the Wangerrip Supersequence, except for Thylacine-2, Somerset-1 and Crayfish-1A, where it is in the upper Sherbrook.

Sherbrook–Shipwreck supersequence boundary

The objective to better constrain the supersequence boundary between the Shipwreck and Sherbrook supersequences, in the offshore Otway Basin, is not achieved through chemostratigraphy. Forbes et al. (2020) differentiated the Skull Creek (Sherbrook Supersequence) and Belfast (Shipwreck Supersequence) mudstones into S5-P6 and S5-P4 (or P5) respectively. The use of this previous work (Forbes et al. 2020) to develop the framework for Riley et al. (2024) enabled differences and gaps to be highlighted. With the offshore and onshore chemostratigraphic data and interpretation for the Sherbrook Supersequence, a regional integrated study to examine the Skull Creek and Belfast Mudstones, and their progression from the onshore to the offshore is a logical next step.

The consistency of the offshore depositional environment during the Cretaceous, in contrast to the onshore, is evident in those key ratios provided in the data that represent changes in sedimentary processes (Riley et al. 2024). With biostratigraphic certainty providing constraints in many of the wells, we can clearly see that from the deposition of the Shipwreck Supersequence through the Sherbrook Supersequence, in the offshore Otway, there are no indications of change in sorting, composition, sediment recycling, weathering or provenance.

Although chemostratigraphy does not clearly distinguish the K90 (Charles et al. 2019) boundary, opportunities to better correlate between wells in the offshore Otway Basin, within the Sherbrook Supersequence and upper Shipwreck Supersequence, come from the sequences, packages and units of the chemostratigraphic interpretation. This approach is particularly valuable in the offshore Otway context because it is independent of lithology, which has proven a challenge in differentiation of sequences in the Otway for many years.