The magma plumbing system of the Northern Carnarvon Basin, offshore Australia: Multi-scale controls on basinwide magma emplacement, and implications for petroleum exploration

Abstract

Abstract The Northern Carnarvon Basin (NCB) located on Australia's North West Shelf hosts an extensive (∼40,000 km 2 ) intrusive igneous complex related to Mesozoic rifting and breakup. Using an extensive suite of modern 3D seismic reflection surveys, we have mapped this intrusive system across the NCB. We identify three predominant intrusion morphologies: Stacked sheets of large interconnected sill intrusions (up to ∼170 km long) and smaller (8 to 30 km long) isolated, strata concordant intrusions, which often interact with normal faults emplaced into deltaic sedimentary rocks; and variably sized (10 to 40 km long) saucer-shaped intrusions emplaced into marine shales, spread across seven zones (geographically constrained groups of intrusions of a specific morphology). We consider the zones' margin-parallel orientation, suggesting control by sub-crustal extensional processes during rifting; and, variation in intrusion morphology between these zones, suggesting a dominant control by host rock mechanical properties. We integrate previous work with our observations, constraining emplacement to between the Kimmeridgian and Valanginian, coinciding with key phases of margin evolution. Finally, we assess the impact of this intrusive complex on local petroleum systems. There is likely little to no adverse impact on source rock maturation or reservoir contamination by CO 2 . But, there is a spatial dissociation between the location of groups of intrusions and the gas fields, particularly in the Exmouth Plateau; this suggests that migrating hydrocarbons may be blocked, baffled and/or redirected by emplaced igneous rocks.