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Rock Properties Of The Upper Jurassic And Lower Cretaceous, Northern Bonaparte Basin, Timor Sea.

Dolensky, Ingrid

Honours Degree, 1996

University of Adelaide


The magnitude of a seismic P wave reflection is dependent upon the acoustic impedance contrast between two rocks at the interface where the reflection is generated. A general indication of the magnitude and sign of reflection coefficients can be determined if the properties of the rocks can be estimated. For identification of hydrocarbon deposits, variations in the properties of the source, seal and reservoir rocks with depth are needed. This study attempts to determine the variations in impedance and reflection coefficients with depth for these units of the Upper Jurassic/Lower Cretaceous interval in the Northern Bonaparte Basin, Timor Sea, Australia.

Wireline log data from eleven wells was used to distinguish five lithological classes. Sonic and density wireline logs was used to calculate impedance with depth for each lithoclass within each well. The impedance data from all wells was combined to calculate impedance trends with depth for each lithoclass. These trends were then used to calculate variations in reflection coefficients with depth for adjacent lithoclasses.

Hydrocarbon bearing intervals were excluded from calculations for the case when the pore fluid is brine. There was not enough data from hydrocarbon intervals available to produce meaningful trends for the case when the pore fluid is gas or oil.

Variations in impedance between the lithoclasses were small. It was found that the soft shales lithoclass, corresponding to the Echuca Shoals Formation, was possibly overpressured in at least three wells. No combination of contrasting lithoclasses produced reflection coefficients of magnitude greater than 0.2 at depths greater than 200m. When the turbidite sands lithoclass was discounted, no lithoclass contrast produced a reflection coefficient magnitude larger than 0.1.

High amplitude seismic anomalies were not seen to be the result of variations between the lithoclasses examined in this study when the pore fluid is brine. Such anomalies are more likely to be due to variations in the pore fluid, and may be considered as possible direct hydrocarbon indicators (DHIs). Polarity reversals in seismic reflections occur in most cases of contrasting lithoclass at depths from 1000 to 3000m, and therefore may be indicative of a contrast between two of the lithoclasses used in this study, and should thus not be assumed to be DHIs in the Timor Sea region without further evidence.

Australian School of Petroleum



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