The facies architecture of submarine basaltic volcanoes and their effects on fluid flow

Volcanic-affected hydrocarbon basins commonly contain a variety of intrusive and extrusive volcanic rocks. All extrusive facies are ultimately sourced from volcanoes. Importantly, volcanoes link the extrusive components to the underlying magmatic plumbing system; features which may act as subsurface conduits and baffles for fluid flow. Volcanoes also provide insights into the timing of both intrusive and extrusive activity, thus helping constrain hydrothermal and contact metamorphic processes associated with magma intrusion. However, in comparison to the intrusive components of volcanic systems, the criteria for recognising these important features in seismic data are less well known. In addition, the facies of which volcanoes are commonly composed are poorly characterised from well and seismic data.In this study we use a combination of 3D seismic data, well data and field analogues to detail the architecture of submarine basaltic volcanoes constructed in the Bass Basin, offshore southern Australia. These volcanoes are Miocene in age and were emplaced in a thermally subsiding rift basin. Our studies indicate that the volcanoes are composed of volcaniclastic rock such as hyaloclastite and pyroclasts, produced during effusive activity and magma-water interaction. These facies present a range of drilling complications, and may act as either seals, reservoirs or migration pathways. After their eruption, the volcanoes were encased in a sequence of claystones, and continued to focus subsurface fluid flow and sediment recycling for 20 Myr after their extinction. We conclude that basaltic volcanoes are important components of volcanic-affected basins. This study can be used to help recognise basaltic volcanoes in other data sets, and provide insights into their impacts on petroleum system.