Analysing Ancient Focused Fluid Flow Systems in Offshore Sedimentary Basins of Australia

Abstract

There has been growing recognition of the value in studying focused fluid flow systems, such as the formation of fluid escape pipes or paleo-pockmarks, within the subsurface of sedimentary basins to better define petroleum system processes. While focused fluid flow features such as Hydrocarbon Related Diagenetic Zones (HRDZs) have been documented in offshore basins of Australia, there is little systematic assessment of their genesis nor their impact on petroleum systems. This study utilizes 3D seismic data to pioneer documentation of focused fluid flow features in offshore basins of Australia and develop geological models for the genesis of these focused fluid flow systems. Fault associated focused fluid flows were identified in the Exmouth Plateau of the Northern Carnarvon Basin and the Ceduna Sub-basin of the Great Australian Bight. In the Exmouth Plateau, 315 crater-like depressions along a paleo-surface, referred to as paleo-pockmarks, with underlying fluid escape pipes were identified within Jurassic and Triassic sequences using three 3D seismic surveys. These focused fluid flow features occur in linear trends parallel to but laterally offset from normal faults cutting these sequences. Amongst the identified linear trends of focused fluid flow features is the longest known pockmark chain yet documented (~72 km). The normal faults are interpreted to have intersected overpressured units in the Triassic Mungaroo Formation which then triggered vertical fluid migration. The nature of the fluids themselves however (i.e. whether they were hydrocarbons), could not be determined. In the Ceduna Sub-Basin of the Great Australian Bight along Australia’s southern margin, fluid escape pipes were identified on the Ceduna 3D seismic survey within Cenozoic sedimentary rocks and the shallowest 400 m of Upper Cretaceous Hammerhead Supersequence. Each fluid escape pipe overlies the upper tip of a normal fault that displaces strata within the Hammerhead Supersequence. The spatial distribution of fluid escape pipes across the study area varies with predominant fault type; occurring in random distributions with polygonal faults to the north-west, and in linear clusters with listric faults to the south-east. The spatial link between the fluid escape pipes and faults suggest that these faults have acted as migration pathways forfluids that were subsequently vented through the pipes. Regional thermal maturation studies suggests the migrating fluids are likely to be hydrocarbons sourced from deeper Cretaceous organic rich sequences. A series of fluid escape features and complex faulting structures (radial concentric and conjugate faults) apparently induced by deeper intrusions were identified within Mid Cenozoic sedimentary sequences of the Bass Basin imaged by the Labatt 3D seismic survey. A total of 101 fluid escape features were documented along a horizon interpreted within the Oligocene to Miocene Torquay Group sequence, manifesting as craterlike-depressions overlying fluid escape pipes. These craters are interpreted to be maar structures or hydrothermal vents, 42 of which have overlying volcanic vents. Three areas of radial faults and 6 areas of concentric faults were identified within the Oligocene to Miocene Torquay sequences whereas 13 pairs of conjugate faults were identified within the Eocene to Miocene Demon’s Bluff and Torquay sequences. The conjugate and radial fault structures are interpreted to have formed from emplacement of underlying igneous intrusions whereas the concentric faults are interpreted to have formed as a result of subsurface sediment evacuation. The varying mechanisms that generate focused fluid flow systems may impact petroleum systems in different ways. In the Exmouth Plateau, normal faults within the Jurassic and Triassic sequences are inferred to be poor fluid migration pathways due to focused fluid flow features forming laterally offset from the fault upper tips. Fluid escape features themselves, once established, can act as migration pathways that may compromise sealing unit integrity by providing secondary migration pathways for hydrocarbons within potential reservoirs, and thus increase exploration risk by reducing the probability of long-lived hydrocarbon accumulation. In a frontier exploration basin such as the Ceduna Sub-Basin, the presence of fluid escape features that may have formed as a result of hydrocarbon migration can improve the exploration potential. Though their occurrence in the region highlights potential trap breaches, the formation of focused fluid flow features in the region proves the presence of hydrocarbon producing source rocks and viable migration pathways. Though not directly associated with hydrocarbon generation, the fluid escape features and overburden deformation structures identified in the Bass Basin can influence petroleum exploration. They provide fluid migration pathways from deeper to shallower sequences, while the associated igneous intrusions may act to impede fluid migration. This study highlights the importance of studying ancient focused fluid flow systems (in addition to the better documented contemporary ones) to aid in hydrocarbon exploration. Improved understanding of the impact of paleo-focused fluid flows on subsurface fluid dynamics and petroleum system elements can be utilized to de-risk exploration.