Geophysical delineation of volcanics and intrusives offshore NW Australia using global analogues

Abstract

The northwestern margin of Australia is a prolific hydrocarbon province, with large areas affected by volcanism and igneous intrusions, such as the outer Exmouth and Browse basins as well as parts of the Canning Basin. Volcanic features can easily be mistaken for potential hydrocarbon traps. Some have subsequently been drilled with disappointing and costly results. Proper delineation of volcanic features is therefore crucial in exploration efforts especially with current NW shelf exploration progressing in ever deeper water closer to the continent-ocean boundary. Most igneous rocks drilled on the northwest shelf consist of doleritic intrusions, basalt flows and volcaniclastics. Normally doleritic intrusions show a sharp contrast on seismic data due to their high acoustic impedance resulting in bright amplitudes on the seismic stack as seen on spectacular sill complexes on the Exmouth plateau as well as in the Canning basin. The high impedance contrast of volcanics also contributes to seismic imaging problems below the volcanic flows or intrusions, this becomes more evident when basalt flows thicken as evidenced on the North Atlantic margin. Similar potential thickening of basalt and volcaniclastic sequences is encountered on the NW edge of the Browse basin. This has important consequences for prospectivity in this region. Another common imaging problem is interbed multiple generation due to the high impedance of the volcanics. Other problems arise when volcanics have been exposed to significant weathering and subsequent burial resulting in lower impedance values, making it more difficult to distinguish them from clastic rocks. Large volcanic features such as igneous complexes and volcanoes, can mimic antiformal structures on seismic data, but can be easily de-risked by integrating gravity and magnetic data, especially by using data filtering techniques. More subtle volcanic features can be more difficult to distinguish and could be mistaken for reefs or potential hydrocarbon traps. Submerged volcanoes might also be prospective due to reefs growing on top of them as evidenced by examples from western India.