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An extensive volume of 3D seismic data over a number of oil and gas fields in Australia's North West Shelf and Gippsland Basin has been examined for evidence of the effects of hydrocarbon migration and/or leakage. For comparative purposes, 2D and 3D data have also been studied over a number of adjacent traps, including dry traps and partially to completely breached accumulations. Fields and traps investigated include Bayu-Undan, Jabiru, Skua, Swift and Tahbilk in the Bonaparte Basin, Cornea in the Browse Basin, North Rankin, Chinook, Macedon, Enfield and Zeewulf in the Carnarvon Basin, and Kingfish in the Gippsland Basin. The principal goal of the study is to provide representative case studies from known fields so that, in undrilled regions, the exploration uncertainties associated with the issues of hydrocarbon charge and trap integrity might be reduced.Direct indicators of hydrocarbon migration and/or leakage are relatively rare throughout the basins studied, though the discoveries themselves characteristically show seismic anomalies attributable to hydrocarbon leakage. The nature and intensity of these hydrocarbon-related seismic effects do, however, vary dramatically between the fields. Over traps such as Skua, Swift, Tahbilk and Macedon, they are intense, whereas over others, for example Chinook and North Rankin, they are quite subtle. Hydrocarbon-related diagenetic zones (HRDZs), which had been identified previously above the reservoir zones of leaky traps within the Bonaparte Basin, have also been recognised within the Browse, Carnarvon, Otway and Gippsland Basins. HRDZs are the most common leakage indicators found and are identified easily via a combination of high seismic amplitudes through the affected zone, time pull-up and degraded stack response of underlying reflectors. In some cases (the Skua and Macedon Fields), the HRDZs actually define the extent of the accumulations at depth.Anomalous, subtle to strong, seismic amplitude anomalies are associated with the majority of the major fields within the Carnarvon Basin. The strength and location of the anomalies are related to a complex interplay between trap type (in particular four-way dip-closed versus fault dependent), top seal capacity, fault seal integrity, and charge history. In some areas within the Carnarvon, Browse and Bonaparte Basins, shallow amplitude anomalies can be related directly to gas chimneys emanating from the reservoir zone itself. In other instances, the continuous migration of gas from the reservoir has produced an assortment of pockmarks, mounds and amplitude anomalies on the present day sea floor, which all provide evidence of hydrocarbon seepage. In the Browse Basin, strong evidence has been found that many of the modern carbonate banks and reefs in the region were initially located over hydrocarbon seeps on the palaeo-seafloor.The examples and processes presented demonstrate that the analysis of hydrocarbon leakage indicators on seismic data can help to better understand exploration risk and locate subtle hydrocarbon accumulations. In mature exploration provinces, this methodology may lead to the identification of subtle accumulations previously left undetected by more conventional methods. In frontier regions, it can help to identify the presence of a viable petroleum system, typically the principal exploration uncertainty in undrilled regions.

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The Ashmore Platform–Timor Sea region of Australia’s North West Shelf is an area of significant petroleum exploration potential, with several large commercial oil fields present. Moreover, exploration activity seems likely to continue at current levels for the foreseeable future, and may also extend into deeper water, given high oil prices and improved drilling technologies. The area is also one of high conservation value, with both the Cartier Marine reserve and Ashmore Reef (a Category 1 marine park), as well as numerous other genetically-rich carbonate seed bank systems, closely associated spatially with exploration activities. Balancing the conservation and resource values in this area will present a key challenge into the future.The magnitude of this challenge has been highlighted by recent work undertaken by AGSO, which involved the acquisition and interpretation of assorted remote sensing data, such as high-resolution bathymetry (including sidescan sonar), satellite synthetic aperture radar (SAR), Landsat, water column geochemical sniffer, airborne laser fluorosensor, seismic data and seafloor sediment sampling. These studies have shown that, at both a regional and local scale, the development of these important carbonate systems appears to directly relate to the geological development of the area.At a regional scale, the collision between the Australian and Eurasian crustal plates in the Pliocene (At a local scale, new data also strongly suggest that the locations of the majority of reefs and carbonate banks and build-ups in the area are associated with active and palaeo-hydrocarbon seeps. These seeps are localised over either fault systems which tap the reservoir, along migration fairways, or at the pinch-out of the regional Cretaceous top seal. Our interpretations suggest that the reefs and the build-ups formed by a sequential process. Firstly, hydrocarbon seepage (induced by collisionrelated faulting) localised small seafloor (chemolithotrophic) biological carbonate communities, which ultimately formed topographically positive features. These higher relief features were subsequently preferentially colonised by an assortment of reef-building biota, whose rapid growth progressively kept up with rising sea-level (which was driven principally by collisionrelated subsidence). The most favourable conditions for initial reef colonisation probably occurred during periods of relatively low sea-level, when the areas around the reefs were located at much shallower water depths (Clearly, the fact that the genetically rich carbonate communities in this area are probably causally related to natural hydrocarbon seepage (and the attendant processes which drove that seepage) will present a series of almost unique exploration, development (especially engineering) and conservation challenges.

RadarSat and ERS Synthetic Aperture Radar (SAR) satellite data have been used for oil slick mapping as part of a systematic interpretative study of the offshore Canning Basin, as well as part of the northern Carnarvon Basin, extending from the inner shelf to the abyssal plain. These seepage data have been integrated with regional geological data, more than 12,000 km of reprocessed Airborne Laser Fluorosensor (ALF) survey data, seismic DHI indicators, water column geochemical sniffer data, potential field data, earthquake data and 2D Petromod basin modelling, to provide new insights into the region’s petroleum prospectivity and key exploration risk factors.From a prospectivity viewpoint, this study has highlighted several areas and processes. Firstly, it is clear that overpressure in the region is principally controlled by the thickness of the Tertiary carbonate wedge and we predict that overpressure may be present in parts of the deeper water Canning Basin. Secondly, the offshore Canning Basin contains a relatively low density of SAR-mapped oil slicks, though this appears to be due to a combination of factors, namely a paucity of vertical conduits for leakage, a predominantly condensate-prone charge and a small slick size.Significantly, several as-yet untested areas emerge from our observations. In the offshore Canning Basin, a 'window' exists in about 1,500–2,500 m of water, where the Triassic source rocks are particularly well placed for liquids generation. Morever, a large area in a radius some 20–80 km outboard of the Bedout High, also appears to have significant untested liquids potential, with respect to sourcing from the Triassic. The shallow section through this region contains a vast area with abundant seismically mapped gas chimneys and other seepage indicators, supporting the conclusions from the remote sensing and basin modelling of significant hydrocarbon charge in this region. Finally, the study indicates that liquids have been generated within the Palaeozoic section of the Bedout Sub-basin.