Magnetic data modelling to support seismic exploration in distinguishing volcanics at the reservoir level in the Outer Browse Basin NW Australia.

Slakeska, Beti

Abstract:

The Browse Basin, located off the NW coast of Australia contains a number of major gas fields. The Icthyus field alone is estimated to contain 530 million barrels of condensate and 12.8 trillion cubic feet of natural gas with an estimated life of 40 years. The Outer Browse Basin is also considered to be highly prospective but technically more challenging. Water depths exceed 2000m and mafic volcanics of varying thickness can occur at the main reservoir level.

In the Browse Basin it is well documented that seismic exploration often encounters problems in determining the thickness of the volcanics and also in distinguishing volcanics from carbonates.A number of prospective leads have been identified by previous explorers but the interpretations of some of these leads remains ambiguous as problems exist with seismic methods alone .

The Browse and Outer Browse Basins have been surveyed by quite detailed aeromagnetic surveys (500m line spacing) and the magnetic data has the potential to assist in mapping the volcanics where problems are encountered with seismic interpretation. Whilst the magnetic method cannot produce unique solutions and is faced with challenges such as varying magnetic susceptibility within the volcanics which are at a depth of 4km the magnetic data can still play a significant role in that it can confirm the presence of volcanics and can be gainfully used in assessing different geologic models

An example of one such lead is discussed in this paper. The Coolabah lead has a significant magnetic response which has been attributed by previous magnetic modelling to a thick 2km mafic volcanic horizon within the prospective reservoir. This has had a significant negative impact on the prospectivity of the lead as seismic profiling cannot confirm or refute the estimated thickness of the volcanics. 2D and 3D modelling3D modelling of the magnetic data associated with the Coolabah anomaly suggests that an alternative interpretation is plausible which involves both a deeper possibly intrusive source and a potentially thinner sequence of volcanics at the reservoir level.