What can deep seismic reflection data tell us about hydrocarbon prospectivity in onshore

Abstract

Now in its third year, Geoscience Australia’s Onshore Energy Security Program has acquired several suites of regional geological and geophysical data. The data includes several deep seismic reflection surveys that have been designed to image: basement provinces with high geothermal gradients that may contain uranium enrichments and are potential candidates for geothermal energy; geological terrane boundaries; and, sedimentary basins that are known to host petroleum system elements but are under-explored. Seismic signals are recorded down to 20 seconds two-way-time (TWT) which corresponds to 25–35 km depth depending on dominant lithologies. Basinal sections normally extend down to 6–8 sec TWT and the data is of such high quality that any section of the seismic profile can be enlarged without significant loss of resolution. Deep reflection surveys are able to image the relationship between crystalline basement and overlying basin sequences very clearly and also allow interpretations of structural styles as well as impacts of deformational processes on the basin-fill. A new basinal section was discovered beneath the Eromanga Basin suite of sediments. Named the Mullangera Basin,its structural style and basement relationship seem to indicate some affinity with the Georgina Basin, located further west. The succession is clearly composed of several sequences that contain both fine and coarse-grained sediments. If a geological relationship with the Georgina Basin can be ascertained, a new hydrocarbon prospective area could be delineated. Another new section was discovered beneath the Devonian section of the Darling Basin. Judging by the fast acoustic velocities, the entire basin-fill sequence appears to be very dense and therefore largely non-porous and of low permeability. While low poro-perm characteristics would normally be translated to low prospectivity for oil and gas, the potential for unconventional hydrocarbons such a tight-gas and shale-gas warrants further attention.