Field trials of non-seismic geophysical techniques for petroleum exploration in New South Wales

Abstract

The Wilga Park 1 gas discovery well, drilled in 1985, was the first valid structural test for hydrocarbons in the Gunnedah Basin in northern NSW. A further untested structure (Nyora) is located some 30 km to the west. Both structures were located and delineated solely by the use of seismic reflection techniques. Both structures are relatively small (7?10 sq km) and to assist in evaluation of optimal survey procedures the Department of Mineral Resources conducted trial surveys with alternative geophysical methods. The shallow Wilga Park structure (600 m to gas trap) was delineated by resistivity profiling due to the resistive Pilliga Sandstone (70 to 100 ohmm) contrasting with the deeper Purlawaugh Formation (7 to 10 ohmm). The resistivity data were duplicated using the dipole?dipole array (galvanic) and coincident loop SIROTEM (inductive) method. An induced polarization anomaly was observed at depth over strong faulting, which may have been a source of hydrocarbon gas seepage. The gravity method at Wilga Park was only partially successful as a regional gradient masked the anomaly. The untested Nyora structure was well delineated by gravity and magnetic methods. The magnetic anomaly was interpreted to be due to a fault bounded teschenite sill at 400 m depth, which was also defined by strong seismic reflections. The gravity anomaly, however, was modelled to be due to a basement high. Induced polarization data indicated anomalous responses over the fault margins of the structure and a depleted zone over the sill, which conforms with the model for an alteration/sulphide plume over gas and oil pools. It is concluded that potential field techniques could be cost effectively applied to exploration for these small structures, to supplement seismic data. Electric methods provide a useful adjunct data set that has positive exploration incentives if drilling proves the 'plume' to be a hydrocarbon indicator.