--> Drilling a Volcanic Complex at Toro-1 Provides Insights Into Jurassic Rifting in the Exmouth Sub-Basin, Western Australia

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Drilling a Volcanic Complex at Toro-1 Provides Insights Into Jurassic Rifting in the Exmouth Sub-Basin, Western Australia

Abstract

The recent Triassic gas discovery at Toro-1, Exmouth Sub-basin, Western Australia, drilled through an amorphous seismic feature within Jurassic-aged sedimentary rock. This feature covers ~150km2, is over 500m thick, and has clear top and base reflectors, with minimal internal reflectivity and slower seismic velocities than surrounding units. From morphology and seismic character the feature was interpreted as a volcanic complex that has partially altered to clay. An analogue study, undertaken to investigate potential drilling hazards in extrusive volcanics, assessed the main risks as losses into fracture networks and swelling clays. Furthermore, the slow seismic velocities indicated potentially high pore pressure within the feature. The feature was normally pressured and no drilling incidents occurred. However, casing had to be washed through tight spots, shortly after which total losses persisted until the hole section was isolated. The soft blue-green cuttings were distinct from background sediments. On-site XRF results showed a clear Ti/Th ratio change at the feature's boundaries, confirming volcanic rock of basaltic origin. Early Jurassic to Cretaceous basalts are common along the Western Australian margin and are associated with the rifting of Greater India and other micro-plates from Australia. Previous authors have attributed the Exmouth Sub-basin, Tithonian-aged strata to post-rift sag phase of Rift I (Oxfordian). However, the presence of Tithonian-aged extrusive volcanism suggests these strata are more likely associated with the initiation of the later Rift II (Valanginian) that formed the Cuvier Abyssal Plain and the initiation of this second break-up may be more widely expressed than previously assumed.