Acquisition of long-offset (8–10 km), long-record length (12–18 sec), 2D reflection seismic and ship-borne potential fields data (WestraliaSpan by Ion/GXT and New Dawn by PGS) on the North West Shelf of Australia provide the opportunity to study rift processes in the context of modern models for rifted margins (Manatschal, 2004). Basement and Moho surfaces were interpreted on seismic reflection data. Refraction models from Geoscience Australia constrain Moho depth and initial densities for gravity modelling through standard velocity-density transformation. 2D joint inversion of seismic reflection and gravity data for Moho depth and basement density constrain depth to basement on seismic. 2D gravity and magnetic intensity forward modelling of key seismic lines constrain basement thickness, type and density. Late Permian and Jurassic-Early Cretaceous rift zones were mapped on seismic reflection data and constrained further by inversion and forward modelling of potential fields data. The Westralian Superbasin formed as a marginal basin in Eastern Gondwana during the Late Permian rifting of the Sibumasu terrane. Crustal necking was localised along mechanically-weak Proterozoic suture belts or Early Paleozoic sedimentary basins (such as Paterson and Canning). Mechanically-strong cratons (such as Pilbara and Kimberley) remained intact, resulting in necking and hyper-extension at their edges. Late Permian hyper-extended areas (such as Exmouth Plateau) behaved as mechanically-strong blocks during the Jurassic to Early Cretaceous continental break-up. Late Permian necking zones were reactivated as failed-rift basins and localised the deposition of the Jurassic oil-prone source rocks that have generated much of the oil discovered on the North West Shelf.

Late Paleozoic rifting of the NW Shelf of Australia formed a wide basin that fundamentally controlled the Mesozoic continental rifting and passive margin development; however, structural style associated with this extensional process remains poorly constrained. Here, we integrate high‐resolution seismic data with well data from the proximal domain of the NW Shelf to show that the Mermaid fault system in the eastern Dampier Sub‐basin is a large‐scale (∼80 km long), low‐angle (5–20°) normal fault with up to 10 km offset formed during the Late Paleozoic rifting. In its northern and southern domains, the Mermaid fault has a planar geometry with characteristic extensional structures. In its central domain, seaward‐dipping, domino‐style normal faults in the upper crust extend downward into a highly reflective seismic zone that arches upward to form an elliptical dome. Growth strata in the hangingwall half‐graben and footwall basement reflections indicate that the Mermaid fault initiated as a high‐angle normal fault and was rotated by footwall exhumation during extension. Intrabasement deformation along and below the Mermaid fault is characterized by moderate‐ to high‐amplitude, semi‐continuous seismic reflectors that are distinct from transparent reflection character in the overlying basement unit. Comparison of the Mermaid fault system to well‐studied detachment faults in metamorphic core complex settings reveals striking similarities. We therefore suggest that the Mermaid fault developed as part of a nascent metamorphic core complex with a warm to hot footwall that enabled middle‐lower crustal flow during the Late Paleozoic continental rifting, likely induced by southward subduction of the Paleo‐Tethys Ocean.

The greater Westralian Superbasin comprises multiple petroleum systems ranging in age from the Early Paleozoic to the Paleogene (Bradshaw et al, 1994). A subset of these systems is typified by marine incursions with a deposition of liquids-prone source rocks. Variability in Westralian sediment fill and source rock stratigraphic position can be demonstrated on a continuous mega-regional 2D deep reflection seismic line that extends from Carnarvon through Browse and into the Bonaparte Basin. Beginning in 2013, BHP Billiton initiated a comprehensive regional study of the Westralian margin to better risk existing and new play fairways. From this work, a hydrocarbon systems analysis from the Dampier Sub-basin and its application for exploration as a regional analogue is described. From a compilation of both open-file and proprietary data, a subset of Dampier well penetrations was chosen, based on the quality of available source rock data. 1D models were constructed and thermally calibrated to BHP Billiton’s recent re-interpretation of the sub-regional crustal architecture. The ultimate expelled petroleum (UEP) was calculated at each well and then extrapolated regionally to determine the total basin hydrocarbon potential. Maturity of the source rock is described using the state of thermal stress (STS) parameter (Pepper and Corvi, 1995). Compared with more data- and labour-intensive 3D basin modelling, integration of 1D basin models, UEP and STS parameters allow for a rapid quantitative and regional-scale basin analysis. Using this workflow in data-constrained basins like the Dampier Sub-basin serves as an important analogue for assessing and risking petroleum systems in both of the established and frontier portions of the Australian margin.

Real-time well site biostratigraphy monitors the stratigraphic progress of drilling wells and can show if the well is conforming or deviating from predicted stratigraphy; it can also show whether total depth criteria have been met. New technology implemented by BHP Billiton Petroleum on wells in the Gulf of Mexico make it possible to undertake well site biostratigraphy remotely. An automated microscope has been developed that can scan and photograph nannofossil slides at a quality almost indistinguishable from conventional microscopy. A technician present at the well makes a slide from ditch cuttings and places it on the microscope, where software scans the slide at several focus levels, compresses the files and transfers the images to the office. A biostratigrapher in the office looks at the images and makes taxonomic identifications. When implemented on the first well, the technology provided immediate results by being the first data acquisition process to recognise that a section was faulted out. Remote well site biostratigraphy also creates a verifiable record, making it possible to quality-control biostratigraphic interpretations; it also frees up space on the rig and improves health and safety for biostratigraphers. Delays in file transfers from the rig to the office are being addressed by reducing the size of the image files by discriminating the fossils from the background.