Vertical fluxes of hydrocarbons on the Northwest Shelf of Australia as estimated by a sediment trap study

Abstract

Sediment traps were deployed on the Northwest Shelf (NWS) of Australia in November 1996, to determine fluxes of organic matter and inorganic elements from the photic zone to deeper waters transecting the Exmouth Shelf to Exmouth Plateau. Dry weight fluxes from the traps ranged from 124 to 616 mg m −2 day −1 and POC fluxes ranged from 22.8 to 43.9. The biogenic hydrocarbons consisted of biomarkers indicative of marine zooplankton, phytoplankton and bacteria. GC analyses also revealed a significant component of petroleum-derived hydrocarbons through the presence of an unresolved complex mixture (UCM) that was approximately 4 times the biogenic hydrocarbon flux at shallow stations and up to 7 times at the most offshore station. The molecular characteristics of these hydrocarbons were investigated using GC-MS and GC-MS-MS and compared with local and regional petroleum accumulations. GC-MS analysis showed the hydrocarbons included n-alkanes, alkylated aromatics plus hopane and sterane biomarkers indicating a mature and moderately degraded crude oil. The sterane and triterpane signatures also indicated that the source rock of this oil was marine sediment with a calcareous lithology and of probable Mesozoic age. Commercially exploited oils of similar composition are mostly not known on the NWS. However, there are indications that there is an active petroleum system with some similar characteristics in the southern Carnarvon Basin as shown by oils of anomalous composition to the Chervil, North Herald and South Pepper fields. These results indicate that exploration strategies in the region should not be confined to oil reserves based on the Upper Jurassic Dingo claystone source.