Subsurface fluid flow focused by buried volcanoes in sedimentary basins: Evidence from 3D seismic data, Bass Basin, offshore southeastern Australia

There is growing evidence that intrusive magmatic bodies such as sills and dikes can influence the migration of fluids in the deep subsurface. This influence is largely due to permeability contrasts with surrounding sedimentary rocks or because of interconnected open fractures within and around intrusions acting as conduits for migrating fluids. The role of buried volcanoes in influencing crossstratal fluid migration in sedimentary basins is less well-established. However, several studies have highlighted spatial linkages between extinct hydrothermal vent complexes and fluid seepage, suggesting that buried extrusive features can also influence subsurface fluidflow pathways, potentially leading to migration of hydrocarbon fluids between the source and reservoir. We have developed 3D seismic reflection data from the Bass Basin in offshore southeastern Australia that image an early Miocene volcanic complex with exceptional clarity. This volcanic complex is now buried by <1.3 km of younger sediments. The largest volcano within this complex is directly overlain by a vertical feature interpreted to be a fluid escape pipe, which extends vertically for approximately 700 m across the late Miocene-Pliocene succession. We suggest that the buried volcanic complex was able to focus vertical fluid migration to the base of the pipe because its bulk permeability was higher than that of the overlying claystone sequence. The fluid escape pipe may have initiated through either (1) hydraulic fracturing following fluid expulsion from a deep, overpressured subvolcanic source region, (2) differential compaction and doming of the overlying claystones, or (3) a combination of these processes. Our results suggest a hitherto unrecognized role for buried volcanoes in influencing dynamic subsurface processes in sedimentary basins. In particular, our study highlights that buried volcanoes may facilitate cross-stratal migration of hydrocarbons from source to reservoir, or through sealing horizons.