Measuring Ancient Rates of Weathered Petroleum Accumulation in the South China Sea

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IODP expeditions 367 and 368 are drilling sedimentary successions to basement at locations in the South China Sea. This is to investigate crustal-break up mechanisms during tectonic rifting. Rifting in this way creates a sedimentary basin which is subsequently filled by sediment. The sediment-fill within the South China Sea is derived from a number of hinterlands in Southern China and neighboring countries in which there are both productive and non-productive petroleum systems. The erosion of formations within these petroleum-bearing hinterlands, and the subsequent transport of the petroleum-bearing sediments to the South China Sea by river, will have lead to the accumulation of trace amounts of petroleum within sediments over geological time. Measuring residual petroleum is complicated because most of the components of petroleum degrade at the Earth's surface, leaving only the most chemically resistant components behind. Analysis of the organic content of sediment by GC-MS can detect a few of the compound-types present in weathered petroleum, but with increased weathering the amount that can be detected decreases. Eventually only the most asphaltic and heavy petroleum-components remain. This residual petroleum is difficult to analyse, and so for South China Sea samples will be analysed by a new surface enhanced Raman scattering-based method. This method is highly sensitive and capable of detecting sub parts per billion levels of residual petroleum. The use of newly developed methods for analysing trace amounts of weathered and residual petroleum in this context will greatly help in their trial and development. This development is needed to prepare these technologies for routine environmental base-line monitoring as well as more general petroleum and oil analysis. During this project measurements will be made during shipboard activities using a portable instrument, and then compared to laboratory-based measurements. This comparison of off-shore and laboratory methods is key for developing methods that can be applied at point of need. From a geoscience and natural science perspective, examining weathered petroleum accumulation over geological time in this way, at a fixed position, has a number of benefits and advantages. One of these is access to a pre-human baseline for petroleum within sediments; e.g. measurements are made on sediments deposited before human exploitation of petroleum began. Another benefit is access to data from the multiple climatic and depositional modes experienced by the site over the it's long geological history. This helps to understand how climate change in the modern era may affect the attenuation of petroleum in the natural environment.