The characterisation and provenance of crude oils stranded on the South Australian coastline. Part II: Potential parent petroleum systems.

In 2014-2016 more than 600 specimens of semi-solid crude oil were recovered from 30 ocean beaches along the coastline of South Australia, as part of the recently completed Great Australian Bight Research Program. All are believed to be products of submarine oil seepage. Their source-specific biomarker signatures provide the basis for their assignment to sixteen oil families, some previously unrecognised. Two of these families (asphaltite and asphaltic tar) likely originated from Cretaceous marine source rocks in the offshore Bight Basin. The others comprise waxy oils of lacustrine, fluvio-deltaic and marine source affinity. Their biomarker characteristics do not match those of any Australian crude oil. However, they are strikingly similar to those of oils found in Cenozoic and Mesozoic basins throughout the Indonesian Archipelago and elsewhere in Southeast Asia.

The characterisation and provenance of crude oils stranded on the South Australian coastline. Part I: Oil types and their weathering.

Semi-solid crude oil has been known to wash ashore along the South Australian coastline for over 120 years. The early reports pre-date offshore petroleum exploration and tanker shipping activities in Australian waters, suggesting that this stranded oil originates from natural offshore seepage. Three physically distinct varieties are represented: waxy bitumen, asphaltite and tar. In order to distinguish this natural "background" contamination of the coastline from any potential anthropogenic sources of petroleum, such as oil spills, whole-oil GC-MS analysis was employed to identify at least seven geochemically different types of stranded oil, based on a suite of 633 specimens collected from the coastline during three annual surveys of 30 ocean beaches between 2014 and 2016. The waxy bitumens, which in terms of their biomarker alkanes display an atypical pattern of alteration due to weathering in the marine environment, are more severely altered than similar specimens collected 25 years ago.

The natural hydrocarbon loading of the South Australian coastline.

Crude oil released from natural offshore seeps may strand in coastal environments. Understanding the different types of oil which accumulate on a given coastline, in addition to their spatial distribution and abundance, may be used to establish an environmental baseline for natural "background" petroleum contamination. Here we summarise the hydrocarbon loading of thirty beaches on Australia's southern margin based on three annual surveys in 2014-2016. Comparison with the results of surveys conducted in 1990 and 1991 reveals a marked reduction in hydrocarbon loading. Furthermore, modern samples of the most commonly encountered oil, attributed to a lacustrine petroleum system in the Indonesian Archipelago, are significantly more degraded than those of prior studies. We attribute this reduction in hydrocarbon loading to prolonged oil production in Southeast Asia, which in turn results in reduced reservoir pressures and the eventual cessation of formerly active offshore seepage.

Remotely constraining the temporal evolution of offshore oil systems.

An understanding of the temporal evolution of a petroleum system is fundamental to interpreting where hydrocarbons may be trapped in the subsurface. However, traditional exploration methods provide few absolute constraints on the timing of petroleum generation. Here we show that 187Re/187Os geochronology may be applied to natural crude oil seepage to determine when petroleum generation occurred in offshore sedimentary basins. Using asphaltites collected from the South Australian coastline, our determined Re-Os age (68 ± 15 million years ago) is consistent with their derivation from a Late Cretaceous source rock in the nearby Bight Basin, an interpretation similarly favoured by source-specific biomarker constraints. Furthermore, the calculated initial 187Os/188Os composition of the asphaltites, a value inherited from the source rock at the time of oil generation, suggests that the source rock represents the later stage of Oceanic Anoxic Event 2. Our results demonstrate a new approach to identifying the origin of crude oils encountered in coastal environments by providing direct constraints on the timing of petroleum generation and potential source rock intervals in poorly characterised offshore sedimentary basins prior to exploratory drilling.