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The 3-D Geometry and Distribution of Sub-Seismic Scale Halokinetic Unconformities: Implications for Salt Minibasin Evolution and Associated Reservoir Architecture


Salt is recognised to laterally seal potential reservoir rocks adjacent to the flanks of salt walls. Halokinetic uplift and erosion of the flanks, coupled with salt withdrawal from, and subsidence of, the intervening areas, produces salt ‘minibasins’ with a sedimentary fill typically characterised by complex unconformities at sub-seismic scale. These relationships have received comparatively little attention despite their ability to control the extent and geometry of potential traps along the flanks of the salt walls, and their potential to strongly compartmentalise reservoir strata. We present an analysis of the 3D geometry and distribution of well-exposed, salt-related unconformities from the Salt Anticline Region of Utah, USA. The study uses extensive fieldwork and detailed terrestrial photogrammetry, from which we interpret the extent and geometry of unconformable strata between salt walls to produce 3D models of minibasin fill. We backstrip the 3D models to produce an analysis of the evolutionary history of the minibasins (and, by inference, the evolution of salt-wall growth) to produce generic relationships of likely geometries in a variety of salt-minibasin settings. Our models demonstrate that salt-induced unconformities are strongly 3D in geometry and restricted spatially, especially in relation to the ends of salt walls and zones of salt-wall overlap. As a result, substantial uplift, deformation and erosion of potential reservoir strata can occur. This produces a strongly heterogeneous minibasin fill, and complex localised reservoir geometries with the potential to develop highly compartmentalised traps below a seismically resolvable scale. Despite the inherent complexity we observe, our results can be distilled to produce generic models related to the distribution, geometry and the inter-relationships of salt walls in 3D. Consequently, the study has wider implications for the evolution of halokinetically controlled basins, and interpretation of their fill at sub-seismic scales. We apply our models to the Melville Basin of the Southwestern Approaches, UK, where a recent evaluation of newly reprocessed seismic data has recognised a sedimentary fill partly influenced by halokinesis. Our work provides insight into the likely sub-seismic scale geometry of the basin’s fill adjacent to salt walls as part of a reappraisal of the area’s hydrocarbon prospectivity.