Publication Name: The Sedimentary Basins of WA
Authors: G. W. O'Brien, P. Quaife, R. Cowley, M. Morse, D. Wilson, M. Fellows and M. Lisk
Publication Volume: 2
Date Published: December 1998
Number of Pages: 29
Reference Type: Book Section
Abstract:
Poor trap integrity, which relates to Mio-Piiocene continental convergence and fault reactivation, is the principal exploration uncertainty within Australia's Timor Sea. In this area, the style of the Neogene structural reactivation is a direct function of the firstorder, margin-scale architecture. Flexural extension, associated with localised proto-foreland formation of the Timor Trough, has resulted in the extensional reactivation of pre-existing Jurassic rift faults. At a sub-basin scale, the nature (throw and orientation) of the Neogene faulting is due to a complex interplay between the orientation and displacement of the underlying Jurassic fault segments, the total amount of Neogene flexuring (particularly the proximity to Timor Trough), the manner in which fault displacement is transferred along strike, and the nature and orientation of the under-pinning Proterozoic structures.In spite of this complexity, empirical data show that the faults which define the moderate integrity traps (ie. the traps that host the majority of the oil in the Timor Sea) have only leaked along very limited parts of their length. Typically, the 'leaky' fault segments in these traps extend for no more than 1500 m, with 500-1000 m being more typical. These leaky segments are effectively 'point sources' for hydrocarbon leakage: they produce intense, but areally restricted, hydrocarbon enrichments, which can be detected via Airborne Laser Fluorescence (ALF) and geochemical sniffer techniques.
In contrast, breached traps have 'leaky' fault segments which extend for 3000 to 6000 m along strike. The hydrocarbons were lost from these traps several million ( -5.5 Ma) years ago. In spite of this, areally extensive ALF anomalies are commonly present over these traps, probably because these traps are still receiving low level charge from the mature source depocentres. Consequently, geochemical sniffer anomalies associated with these traps are typically very weak, because insufficient volumes of hydrocarbons are presently migrating through the trap to substantially affect the concentratron within the water column.
Very close and consistent agreement has been obtained between the distribution of seepage defined by ALF and sniffer data (specifically the methane concentration in the bottom waters), which provides powerful evidence that the ALF anomalies at the sea surface are transported there from the seafloor by the methane, either as a dissolved phase or as a thin 'skin' on the surface of the methane gas bubbles.
This study has shown that the acquisition of very high resolution remote sensing geochemical data, when interpreted within a regional geological framework, can significantly reduce exploration uncertainty, even in a well-explored but complex geological province such as the southern Timor Sea.