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.