Fault-related biogenic mounds in the Ceduna Sub-basin, Australia. Implications for hydrocarbon migration

• Eocene mounds are interpreted in the Central Ceduna Sub-basin.
• They display geometries and internal configurations typical of biogenic build ups.
• They show relationships with underlying reactivated faults intersecting the prospective Cretaceous petroleum systems.
• A sub-bottom profile anomaly suggests a relationship between hydrocarbons seep and mound development.
• We propose that natural hydrocarbon seep along faults can supply nutrients for localised biogenic mound growth.

The Early Cenozoic in the Bight Basin represents a transitional period from the slow to fast oceanic crust spreading of the Southern Ocean between Australia and Antarctica. This period is characterised by widespread igneous activity and the occurrence of biogenic mound complexes. In the proximal part of the Ceduna Sub-basin these mound complexes thrive on local topographic highs, exploiting the shut-down in siliciclastic sediment supply during the Eocene. New interpretation of 2D and 3D seismic data from the central Ceduna Sub-basin shows additional, undrilled, Paleocene-Eocene biogenic mound complexes. They display rugose morphology, progradational and aggradational internal organisation and high impedance contrast with the surrounding sediments. These mounds typically show a clear relationship with underlying reactivated fault segments. These faults intersect potential oil mature Cretaceous source rock intervals and therefore one plausible mechanism for the development of fault-related mound complexes in the central Ceduna Sub-basin relies on natural hydrocarbon seeps along faults supplying nutrients for a benthic chemosynthetic organisms and localised biogenic mound growth. The presence of a sub-bottom profile anomaly overlaid by a vertical water column acoustic contact in close vicinity of a mound complex support this model. The occurrence of seep related biogenic mounds would validate the presence of mature source rock(s) and hydrocarbon charge in the central Ceduna Sub-basin. If confirmed the biogenic mounds would imply fluid migration occurrence until at least the Eocene.