A SEISMIC SURVEY IN THE COOPER BASIN IN QUEENSLAND

Abstract

Seismic reflection and refraction surveys in the Queensland sector of the Cooper Basin have commonly been impaired by strongly reflecting Permian coal measures and irregular near-surface duricrust. Penetration problems have consequently frustrated attempts to delineate the thickness of the assumed-prospective Permian section. The problem is well illustrated in and around Authority To Prospect No. 219P, where extensive seismic exploration has been unable to establish whether the thin Permian section, intersected in the various wells on structural highs, thickens substantially on the associated flanks. The Clifton seismic survey commissioned by the University of Sydney has exploited multifold CDP profiling using an array of shallow explosive sources, in combination with high fidelity digital recording and processing, to provide effective penetration in an area to the west of Alliance Oil Development N.L. Thunda I well.

The Clifton data reveal an unconformity beneath the Permian "P" horizon near the Thunda well: the unconformity dies out off-structure It is probable that thickening of a Devonian-Lower Carboniferous section is chiefly responsible for the accumulation of sediments evident in both seismic and gravity data between the Thunda and Galway structures. A combined interpretation of gravity and seismic data indicates a complex pattern of faulting not previously delineated. Two dimensional modelling indicates that the gravity field comprises significant components from structure within the sediments, from the sediment-basement interface, and from isostatic compensation at the Moho.

Deep crustal reflections were successfully recorded in the Clifton survey using source charges of only 5 x 4.5 k. An event at 11-12 secs is identified as a reflection from a laminated-type Moho transition zone near a depth of 33 km.