Abstract Outcrops of mixed carbonate/siliciclastic strata comprise the Neoproterozoic Wonoka Formation and Patsy Hill Member of the Bonney Sandstone at Patawarta diapir in the Flinders Ranges, South Australia, which is a ramping allochthonous salt sheet flanked by suprasalt and subsalt minibasin strata. Lithofacies distribution, thicknesses, and stratal geometries are analyzed to correlate suprasalt and subsalt Wonoka and Patsy Hill strata within a depositional and halokinetic sequence stratigraphic framework. Wonoka and Patsy Hill strata represent regional deposition in a storm-dominated carbonate shelf environment. The subsalt minibasin at Patawarta diapir contains upward-shallowing outer wave-dominated shelf to coastal plain facies in the Wonoka Formation and tidally dominated barrier bar facies in the Patsy Hill Member. The Patsy Hill Member records a major shift in depositional systems from the underlying carbonate-dominated Wonoka Formation to the overlying silici-clastic-dominated Bonney Sandstone. Wonoka strata correlate laterally between the suprasalt and subsalt minibasins in terms of facies distribution and depositional setting. In contrast, Patsy Hill facies show significant lateral variation between supra-salt and subsalt positions, suggesting strong control on deposition by a diapiric high. This is especially evident from the lateral distribution of quartzite pebble conglomerates, which record different periods of exposure and erosion of the allochthonous salt sheet during deposition. Subsalt Wonoka and Patsy Hill strata compose a tapered composite halokinetic sequence that displays upturn over a distance of 560 m and stratal thinning (1100 to 110 m) toward the diapir. Suprasalt Wonoka and Patsy Hill strata display significant but gradual thinning (1,882 to 19 m) over a distance of 3-5 km forming a broad, open fold with local upturn at the salt-sediment interface. The suprasalt and subsalt minibasins at Patawarta diapir have been evaluated to generate a depositional model that accounts for lateral and vertical salt migration and salt-modified bathymetry in a shallow marine environment characterized by alloch-thonous salt.

Abstract An important question in the exploration of salt basins is whether or not salt welds can seal hydrocarbons in subweld traps. Any answer must start with the observation that many supraweld traps throughout the world are charged with hydrocarbons from subweld source rocks, requiring migration through welds. However, not all welds are the same, and this conceptual paper examines various factors that may influence the sealing capacity of salt welds. The probability of weld seal is enhanced by: (1) the presence of remnant evaporite along the weld; (2) relatively impermeable lithologies across the weld; (3) subweld reservoirs that are encased in shales rather than in contact with the weld; (4) the presence of clay gouge or smear generated during faulting; and (5) an original base-salt geometry that creates divergent subsalt hydrocarbon migration pathways. Another factor that must be considered is the timing of overburden deformation with respect to that of hydrocarbon generation and migration. Although weld seal is certainly a risk, traps that invoke weld seal should not be summarily discarded. Instead, each prospect should be evaluated separately in light of the factors presented here in order to derive a better assessment of the inherent risk. In addition, the ideas presented here need to be tested with observations obtained from surface exposures and a combination of subsurface well and seismic data.