Authors / CoAuthors
Mitchell, C. | Totterdell, J.M.
Abstract
The Ceduna Sub-basin of the deep-water frontier Bight Basin contains a Middle Jurassic-Late Cretaceous sedimentary succession in excess of 15 km thick. Nine offshore exploration wells have been drilled in the basin, mostly clustered around the inboard edge of the Ceduna Sub-basin. As a result, the distal mid-Late Cretaceous strata predicted to contain potential source rock facies, had previously not been sampled. The presence of high quality source rocks in the basin was therefore an open question. 2D seismic data was used to delineate targets for sampling of the pre-Campanian section of the basin. Identified targets included potential source intervals of Albian-Santonian age at locations on the seaward edge of the Ceduna and Eyre Terrace where canyon formation, slumping and faulting have exposed the section. Also, a series of sites were selected to test for potential hydrocarbon seepage in the basin. These sites include areas where recently reactivated deep-seated faults were exposed at the seafloor, basin margin areas where facies thin, and areas where possible seepage was identified from Synthetic Aperture Radar (SAR) data. In February and March 2007, a 3-week marine acquisition programme was carried out on the RV Southern Surveyor. Potential dredge targets were first surveyed with 30 kHz EM300 swath bathymetry and observed with Topaz 3.5 kHz sub-bottom profiler. Near-live swath processing and slope analysis techniques enabled site specific dredge sampling of seafloor terrains where Cretaceous section outcropped or slopes were sufficient to ensure only a thin cover of overlying sediments. Targets include fault scarps and eroded sides of canyons. A better-than-expected number of successful dredges were collected (total of 37) from water depths ranging from 1600-4500m. Geochemical analysis of 259 dredge samples for total organic carbon (TOC) and pyrolysis yields (Rock Eval) identified good to very good organic richness in 13 samples, with TOC values between 2.1% and 6.2%. Of these, seven show liquids potential with Hydrocarbon Index (HI) values ranging between 274 and 479 (mgHC/TOC). The rocks with the best source rock characteristics came from high priority sampling sites in the westernmost Ceduna Sub-basin. Organic geochemical analysis has provided evidence for preservation of organic matter under anoxic conditions close to or at the sediment-water interface. Biostratigraphic analysis of these organic-rich rocks has yielded an age around the Cenomanian-Turonian boundary. Although the dredged rocks are immature for hydrocarbon generation, interpretation of an extensive seismic grid across the basin and petroleum system modelling have shown that this succession occurs with the oil window in the central Ceduna Sub-basin. The results of this study provide the best evidence to date for the presence of good quality liquids-prone source rocks in the basin, successfully addressing a key industry concern in this petroleum exploration frontier.
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nonGeographicDataset
eCat Id
65696
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
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2601
Australia
Keywords
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- External Publication
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- petroleum exploration
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- petroleum geology
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- geochemistry
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- geology
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- sedimentary basins
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- marine
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- AU-SA
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
Publication Date
2008-01-01T00:00:00
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