The Derim Derim Dolerite, greater McArthur Basin, Australia: Using subsurface data to characterise a mesoproterozoic magma plumbing system

The application of high-resolution seismic reflection data has spurred major advances in knowledge of the emplacement of sub-volcanic mafic magma plumbing systems in sedimentary basins, highlighting the importance of interconnected sheet intrusions in facilitating lateral magma transport, the links between host rock mechanical properties and emplacement processes, and providing insights into how intrusive activity in basins impacts their resource potential. However, most studies have focused on Mesozoic-Cenozoic mafic magma plumbing systems situated along offshore rifted margins characterised by extensive subsurface datasets. The extensive Mesoproterozoic ( c. 1300 Ma) Derim Derim Dolerite, which intrude the greater McArthur Basin in northern Australia, provide a unique opportunity to study the emplacement of a Proterozoic magma plumbing system due to its penetration by numerous hydrocarbon and mineral drillholes, in addition to seismic reflection coverage. Understanding the emplacement of this system is important because of its interactions with prospective unconventional shale reservoirs in the Velkerri and Kyalla Formations, which represent one of the world's oldest known petroleum systems. This paper focuses on characterising the intrusion emplacement and magma plumbing system using an array of subsurface data, to constrain the distribution, morphology, and emplacement mechanisms of the Derim Derim Dolerite. The morphology of the large, strata-concordant intrusions encountered at shallow present-day depths (<2 km) in the greater McArthur Basin is indicative of greater original emplacement depths of >3–4 km, suggesting a significant extent of uplift and erosion. Density-derived porosity values for the Velkerri and Kyalla Formations are anomalously low for their present-day depths, corroborating a greater palaeo-depth at time of emplacement. The extent of alteration of the host rock surrounding the Derim Derim Dolerite is highly variable, with a small number of occurrences of graphitisation of the organic matter adjacent to intrusions. However, this appears to be highly localised and the detrimental impact of the Derim Derim Dolerite on potential reservoir and/or source rocks appears generally minimal. • Multidisciplinary characterisation of Mesoproterozoic magma plumbing system. • Hosted within one of the world's oldest known petroleum systems. • Seismic, petrophysical and lithological study of Derim Derim Dolerite intrusive complex. • Intrusion morphology suggests greater emplacement depths than present-day depths.