The influence of Cenozoic fluid expulsion on the seafloor morphology of the Vulcan Sub-basin, northwest Australia: Implications of gas chimneys and linear diapirs for hydrocarbon exploration

Abstract

The Vulcan Sub-basin on Australia's Northwest Shelf represents a Cenozoic tectonically active basin shaped by the collision of the Australian Plate with the Banda Arc. This study investigates the influence of fluid expulsion on this basin floor morphology, emphasizing gas chimneys and linear diapirs, through integrated analysis of 3D seismic data, seismic attributes (variance, RMS, and Sweetness), and stratigraphy. Results delineate two distinct gas chimney types: isolated structures in stable caprock regions and clustered features in tectonically weak zones, such as the Swan Graben. Chimney dimensions range from metres to kilometres, with origins traced to Eocene overpressure regimes in Palaeocene-Eocene carbonate reservoirs and vertical migration facilitated by fault reactivation. Also, five evolutionary stages are identified: Palaeocene-Eocene hydrocarbon accumulation, Oligocene overpressure generation, late Oligocene chimney initiation, Oligocene-Miocene sedimentary hiatus, and limited Miocene-Quaternary reactivation. Also, the absence of seafloor pockmarks implies rapid carbonate burial or ductile fluid migration without breaching, contrasting global analogues. Linear diapirs, aligned NE-SW, reflect stress regimes from the Australian-Eurasian collision, with spatial distributions controlled by tectonic segmentation and depositional heterogeneity. The Puffin Formation, a critical regional seal, exhibits spatially variable integrity, directly impacting hydrocarbon leakage and preservation. Notably, linear diapirs within clay-rich layers atop gas chimneys display unique seafloor morphologies shaped by tectonically influenced fluid venting, underscoring subseafloor morphogenetic processes. Clustered chimneys intersect Cenozoic strata, challenging conventional exploration models focused on pre-Cenozoic reservoirs. Seismic attributes reveal residual hydrocarbon potential in Paleogene sandstones and carbonates, particularly within structural traps in the Cartier Trough and Londonderry High. This study highlights the dynamic interplay of tectonics, overpressure, and sedimentation in governing fluid expulsion, offering a framework to de-risk Cenozoic prospects. The findings advance understanding of submarine fluid venting's role in seafloor evolution and provide insights applicable to convergent margin basins globally, while establishing a foundation for future studies on tectonic-seal-fluid interactions.