Abstract

The Yampi Shelf comprises part of a Palaeozoic to Mesozoic flexural ramp margin, which dips to the north-west away from the flanking cratonic (Proterozoic) Kimberley Block. The basement itself has a rugose topography, with some basement blocks being elevated by as much as 500 m above the surrounding basement. Progressive onlap of the Early Cretaceous post–rift seals onto the ramp means that the regional seal both thins and becomes sandier margin-ward. In margin-ward locations, some prominent basement highs are ‘bald’ of seal, whereas in others, the seal thins dramatically onto the back of topographically prominent, landward-dipping tilt blocks.

Acquisition of three geochemical remote sensing technologies, namely RadarSat Synthetic Aperture Radar (SAR), water column geochemical sniffer (WaSi) and Airborne Laser Fluoresensor (ALF) data, and their integration with regional seismic and high resolution bathymetry data over part of the inboard Yampi Shelf, has revealed the following. Hydrocarbon seepage was confirmed independently by SAR, WaSi and ALF, though the relative response and sensitivity of each technology to seepage of varying rates and compositions appears to be quite different. Overall, the principal seeping hydrocarbon is dry (<1% wet) thermogenic gas (δ¹³C = −42.45). The first-order control on both the distribution (and perhaps the composition) of the detected seepage is the thickness of the regional Early Cretaceous seal. Small, localised (<4-5 km across) seeps tend to be related to seismically prominent gas chimneys associated with topographically conspicuous tilt blocks. These chimneys, whilst spectacular, result only in relatively small amounts of seepage, typically 2-4 times background; this seepage appears to be related to a thinning of the regional seal across the tops of the tilt blocks. All of the major seepage, which in some areas reached >300 ppm in the bottom waters (~100 times background), is closely associated with prominent basements horsts which are bald of seal. These intense zones of seepage tend to be large (10-30 km across), though their seismic expression is muted, and is often restricted to diffuse zones of relatively poor coherency, or the presence of a very prominent amplitude anomaly at the seafloor.

SAR anomalies are located ~10 km inboard from the intense WaSi anomalies, right over a prominent basement ‘headland’ which defines the regional ‘zero-edge-of-seal’. The SAR anomalies appear to be due to the leakage of relatively heavy, biodegraded oil, of a type similar to that found in the Cornea oil discovery nearby. This offset between the location of seeping dry gas and heavy oil (if truly causal and not due to drift associated with currents and wind etc), may be reflecting the differences in the relative permeability of gas and oil. The oil, being heavier, may have migrated further inboard, and only leaked when the seal is virtually gone; in contrast, the more mobile gas ‘breaks through’ the seal sooner, and as a consequence, dry gas seeps are developed in more basin-ward locations.

These observations suggest that where the regional seal thins over inboard basement highs, or where the seal itself pinches-out regionally, are prime areas in which to capture a ‘snap-shot’ of the present-day migration across a margin.