Migration, Leakage and Seepage Characteristics of the Offshore Canning Basin and Northern Carnarvon Basin: Implications for Hydrocarbon Prospectivity

Abstract

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.

Yampi Shelf, Browse Basin, North-West Shelf, Australia: a test-bed for constraining hydrocarbon migration and seepage rates using combinations of 2D and 3D seismic data and multiple, independent remote sensing technologies

“…This clustering may be related to minor fault reactivation and seal failure along this major fault system. Another clustering of slicks is located along the Bonaparte-Browse Transition Zone, though the significance of these slicks has been discussed elsewhere by O'Brien et al (2003).…”

“…Finally, coral spawning might be expected to produce an abundance of slicks over, as well as around the edges of, the banks, but this is not observed. Other supporting factors for a seepage origin include the fact that many of the banks/reefs that have slicks around them have been drilled and they typically had strong shows/residual columns at reservoir level, as well as abundant gas chimneys on seismic data; some even had abundant thermogenic hydrocarbons in the seafloor sediments located directly under the slicks (O'Brien et al, 2003). If these slicks are not seepage-related, then it would seem much more likely that they are due to laminar flow or other topography-related flow processes around the reef and banks, rather than coral spawning.…”

Australian offshore natural hydrocarbon seepage studies, a review and re-evaluation

“…5). A bathymetric/current flow interpretation is also favoured for SAR slicks on the southern edge of the Bedout Sub-basin in the central North West Shelf (Jones et al, 2007), which were previously suggested to be related to hydrocarbon seepage at the edge of the regional seal (O'Brien et al, 2003c). In contrast, a coral spawning event appears to have contributed to the formation of annular to crescent-shaped SAR slicks associated with submerged reefs and shoals over the Browse-Bonaparte basin transition zone (Jones et al, 2005b.…”

“…Assessments of natural hydrocarbon seepage that have included interpreted SAR slicks have been documented for the Timor Sea (O'Brien et al, , 2003a(O'Brien et al, , 2003bJones et al, 2005bJones et al, , 2006Rollet et al, 2006), the central North West Shelf (O'Brien et al, 1998b(O'Brien et al, , 2003cJones et al, 2007), the Great Australian Bight (Struckmeyer et al, 2002) and the Arafura Sea .…”

“…3b). ALF fluors identified over the western Roebuck and northern Carnarvon Basins in 1998 were interpreted to have a spectral signature that was generally consistent with a condensate origin, with preferential clustering of fluors with a heavier or more oil-prone spectral signature (O'Brien et al, 2003c).…”