Studies on the nature of crude oil strandings collected from the South Australian coastline

Abstract

The stranding of semi-solid to solid crude oil on the South Australian coastline has been known since the late 1800s. Previous studies have reported two main varieties, referred to as asphaltite and waxy bitumen. The former represents a single oil family thought to originate from the nearby Bight Basin, while the latter comprises multiple families, geochemically related to petroleum systems in the Indonesian Archipelago and transported to the Australian coastline by regional ocean currents. Annual surveys of thirty beaches across Australia’s southern margin were conducted in 2014– 2016 to collect examples of these stranded oils as part of the Great Australian Bight Research Program. This thesis, comprising four individual research articles, employs the geochemistry and spatial distribution of these oils to further our understanding of potential petroleum systems within the Bight Basin and construct an environmental baseline of the natural or “background” hydrocarbon contamination of the South Australian coastline. The first article documents the successful application of rhenium-osmium (Re-Os) radiometric dating to oil seepage, constraining the timing of asphaltite generation and comparing these results to what is known of the geology of the offshore Bight Basin. It demonstrates that the Re-Os geochronometer has the potential to be used as an exploration tool by dating oils of unknown origin encountered in coastal environments. The second article reports a series of highly asphaltic tar strandings recovered from the coastline in 2016 and compares them to the well-studied asphaltites. Similarities in their respective source-specific biomarker signatures suggest that they may share a common origin within the Bight Basin. However, notable points of variation preclude their direct correlation. The observed differences could be attributed to alteration of the asphaltic tar by thermochemical sulphate reduction, or their derivation from similar but separate source rocks. The third article summarises the key features which may be used to identify the 16 different oil families encountered over the course of the project and discusses their likely provenance. The results of this study demonstrate that the South Australian coastline collects a far greater variety of geochemically distinct crude oils than previously reported. With the exception of the aforementioned asphaltic tars, all of the newly identified oils likely originate from petroleum systems in Southeast Asia. The fourth article quantifies the hydrocarbon loading of the coastline, demonstrating which areas are most prone to accumulating these documented oils. Despite the greater variety of oil families identified during this study, the overall abundance of waxy bitumen on the coastline has significantly declined since 1990–1991. Furthermore, modern examples of the most abundant type of waxy bitumen are more weathered than historical examples, suggesting a lack of fresh oil input to the marine environment. Given the prolonged history of oil production across the Indonesian Archipelago, it’s likely the decline in waxy bitumen loading may be due to reduced reservoir pressures, causing the cessation of formerly active seeps. This decline is likely to continue until the remnants of this waxy oil are no longer recognised on the coastline.