Fault controlled focused fluid flow in the Ceduna Sub-Basin, offshore South Australia; evidence from 3D seismic reflection data

Abstract

This paper investigates the spatial link between fluid escape pipes and underlying faults in the Ceduna Sub-basin. The Ceduna Sub-basin is the largest depocentre of the Mesozoic Bight Basin that formed as a result of rifting between Australia and Antarctica beginning in the Middle Jurassic. The study area is in the northwest of the Ceduna Sub-basin, within which sets of fluid escape pipes were identified in 3D seismic data along the study area's south-western edge. Set 1 pipes are observed within the siliciclastic Wobbegong and carbonate Dugong Supersequences of Paleocene to Pleistocene age, while Set 2 pipes are observed within the top 400 ms (~485 m) of Santonian to Maastrichtian deltaic Hammerhead Supersequence. Dimensions of Set 1 and Set 2 pipes range from 50 ms (60 m) to 400 ms (485 m) in height and 70–180 m in width. Roots of the fluid escape features overlie the upper tips of normal faults within the Hammerhead Supersequence. The spatial distribution of fluid escape features and predominant underlying faulting style within the Hammerhead Supersequence varies within the study area; randomly distributed fluid escape features overlie polygonal faults to the northeast, whilst linear clusters of fluid escape features overlie normal faults in the southwest. Spatial statistics analysis of fluid escape features and underlying faults support the interpretation that the faults have acted as fluid migration pathways. The polygonal and normal faults developed in the Late Cretaceous and are interpreted to have transmitted fluids probably sourced from the Lower-Upper Cretaceous Blue Whale Supersequence, and the Late Cretaceous White Pointer and Tiger Supersequences. These supersequences have been identified as containing potential source rocks, and geochemical analysis them suggest that hydrocarbon generation initiated as early as the mid-Cretaceous (~95 Ma) and continued through to the Cenozoic (~15 Ma).