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‘River of Sand’: An Alternative Modern Source to Sink Analogue for the Highstand Supply of Reservoir Quality Siliciclastic Sands to a Deepwater Environment, Offshore Fraser Island, Australia.

Sandstrom, Marianne L.

Honours Degree, 2005

University of Adelaide

Abstract

Petroleum exploration of deepwater environments is conducted using sequence stratigraphic models. Modern analogues highlight new mechanisms for the supply of sediments to the deep marine, redefining the sequence stratigraphic models applied to petroleum exploration.

The ‘River of Sand’ provides an alternative modern source to sink analogue for the Posamentier model for the highstand supply of reservoir quality siliciclastic sands to a deepwater environment. Differences between the ‘River of Sand’ analogue and the Posamentier model are potentially significant refinements that may prove important during the petroleum exploration of a similarly formed deepwater environment target.

Grain size, sequential loss on ignition, x-ray fluorescence, U-Pb dating and Hf analysis of detrital zircons analyses were conducted using the 2005 voyage of the RV Southern Surveyor grab samples. The results were integrated with all available data from all the previous surveys to produce a single coherent fully integrated dataset for interpretation.

Mathematically rigorous textural, compositional and statistical distribution maps were created to ‘fingerprint’ the surficial sediments, establishing their spatial distribution and relationship to the ‘River of Sand’. The surficial sediments in the study area are predominantly medium, fine and very fine sands, silt and clay (<500 microns). The pattern of mean () grain size distribution indicates that the continental shelf has surficial sediments dominated by medium (1.0 to 2.0 ) and fine sands (2.0 to 3.0 ) whereas the continental slope is characterised by very fine sands (3.0 to 4.0 ), silts and clays (>4.0 ). The pattern of standard deviation () distribution across the study area indicates an approximate trend of decreased sorting to the north and in an offshore direction.

Four component analysis and grain size distribution curves suggest that the surficial sediments of the study area can be divided into 12 textural provinces. These textural provinces correspond with similar variations in texture, colour, mineralogy and biota suggested by photography. The spatial arrangement of the textural province indicates that there is a mixing trend with increasing distance offshore, from simple/unmixed, with a single sediment component, to complex/mixed, with multiple sediment inputs.

Statistically valid multivariate cluster analyses were performed to identify significant groupings amongst the grab samples and develop a provenance related facies interpretation. Five significant texture and composition based facies, GSA1 to GSA5, were identified by the Ward’s cluster analysis. The five facies provide a spatial distribution model for the surficial sediments within the study area. Hervey Bay (GSA1), ‘River of Sand’ (GSA2) and Modern Tropical Reef and Temperate Carbonates (GSA3) facies meet seawards of the Breaksea Spit and Breaksea Shoal System. These three sediment groups intermix as they are transported across the continental shelf break and onto the continental slope. A component of pelagic mud is also introduced, which, when dominant, produces the Mixed Muddy Continental Slope Facies (GSA4), or, when minor, produces the Mixed Sandy Continental Slope Facies (GSA5). The degree of mixing is a function of the transport pathway utilised. The GSA2 facies appears to persist unmixed along the Central Valley which is the hypothesised ‘River of Sand’ transport pathway.

High percentage (>80%) sand and quartz and low percentage (>20%) carbonate values are spatially associated with the ‘River of Sand’ transport pathway, even appearing to ‘turn back’ towards the southeast in response to the structural control exerted by the Central and Trunk valleys. The ‘River of Sand’ (GSA2) is characterised by moderately sorted (2.42standard deviationfine sands (0.87mean grain size, 94.79% sand) with a small silt component (4.23%) and a low percentage carbonate (3.64%).

The Pacific Gondwana Orogen (500 to 700 Ma), a distinctive zircon age group tracer for the Middle Triassic Hawkesbury Sandstone of the Sydney Basin, was identified in the two ‘River of Sand’ grab samples analysed, GR002 and GR014. An unanticipated result was the identification of the Pacific Gondwana Orogen (500 to 700 Ma) zircon age group in grab sample GR015, which was sampled to the north and outside of the hypothesised ‘River of Sand’. This result indicates a new level of complexity as it implies that the Northern Valley may be or have been an additional transport pathway when the system was less extensive.

The ‘River of Sand’ is a proven and constrained modern source to sink analogue for the highstand supply of reservoir quality siliciclastic sands to a deepwater environment. The ‘River of Sand’ can now be used to further refine the existing Posamentier model prior its application to petroleum exploration of similarly formed deepwater environment targets.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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