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

Sediment traps were deployed on the Northwest Shelf (NWS) of Australia in N ovember 1996, to determine fluxes of organic matter and inorganic elements from the photic zone to deeper waters transecting the Exmouth Shelf to Exmo uth 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 hydrocarbo ns consisted of biomarkers indicative of marine zooplankton, phytoplankton and bacteria. GC analyses also revealed a significant component of petroleu m-derived hydrocarbons through the presence of an unresolved complex mixtur e (UCM) that was approximately 4 times the biogenic hydrocarbon flux at sha llow 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 pl us hopane and sterane biomarkers indicating a mature and moderately degrade d 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 compositi on are mostly not known on the NWS. However, there are indications that the re is an active petroleum system with some similar characteristics in the s outhern Carnarvon Basin as shown by oils of anomalous composition to the Ch ervil, North Herald and South Pepper fields. These results indicate that ex ploration strategies in the region should not be confined to oil reserves b ased on the Upper Jurassic Dingo claystone source. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.