Deep-marine reservoirs form some of the most attractive targets in many mature and frontier basins. Determining the distribution and geometry of these reservoirs, especially within tectonically-active settings, remains a major challenge, however, principally due to the complex interaction of a variety of extra- and intra-basinal controls (i.e. tectonics, climate, sea-level, etc). In this talk I use 3D seismic reflection, well and core data to provide a regional synthesis of the subsurface expression of a series of tectonically-controlled, deep-marine reservoirs that are developed along the western Norwegian margin. I will also outline the key controls on the deposition of these reservoirs and illustrate the key trapping styles.

Turbidite sandstones represent the best reservoirs; in core, individual beds are up to a few metres thick, but amalgamated units up to several tens of metres thick are common. Sandy-mudstone debrites are observed, but they are of poor reservoir quality and may form barriers or baffles to fluid flow. Synthetic seismograms indicate that sandstone-dominated deposits are expressed on seismic data as packages of high-amplitude reflections. Amplitude mapping indicates that 11 slope fans are recognised and that these were fed by sediment routed through upper slope canyons incised into the eastern basin margin. These fans are either ponded behind or overstep intra-basin highs; the key trapping styles are: (i) stratigraphic, and related to up-dip pinch-out of the fans into slope mudstones; or (ii) structural, and related to differential compaction-related drape of fans across underlying fault blocks.

The areal extent of the onshore drainage catchments that supplied sediment to the fans has been estimated based on scaling relationships derived from modern source-to-sink systems. The results of our study suggest that the Turonian fans were sourced by drainage catchments that were up to 2200 km2 and which extended up to 140 km from the shoreline. The estimated inboard extent of the catchments correlates to the innermost structures of a large fault complex, which is thought to have defined the position of the regional drainage divide in this region since the Devonian. I suggest that increased sediment supply to the Turonian fan systems reflect tectonic rejuvenation of the landscape, rather than eustatic sea-level or climate fluctuations. The duration of fan deposition is thus interpreted to reflect the “relaxation time” of the landscape following tectonic perturbation, and fan system retrogradation and final abandonment is interpreted to reflect the eventual depletion of the onshore sediment source. Future exploration success in tectonically-controlled deep-marine reservoirs relies on a robust understanding of the seismic expression, sedimentology, stratigraphic architecture and trapping styles associated with turbidite systems deposited on bathymetrically-complex slopes. Furthermore, important insights into reservoir size and location can be gained by considering the complete “sediment routing system”.

The emplacement of shallow-level igneous intrusions in sedimentary basins may impact significantly on the development of petroleum systems. For example, the circulation of related hydrothermal fluids, which may reduce the porosity and permeability of host rock reservoirs, and associated host rock deformation may result in the formation of “forced fold” traps. Understanding the geometry and evolution of sub-volcanic intrusive networks in volcanogenic basins is thus of interest to the petroleum industry. Whilst field-based studies permit a detailed investigation of magma properties and localised host rock relationships, outcrops are often too small to fully characterise the three-dimensional geometry and size of large igneous complexes. Furthermore, ancient volcanic edifices, and their relation to the sub-volcanic “plumbing system”, are typically obscured at outcrop due to post-emplacement erosion or caldera collapse. In contrast, seismic reflection data, although typically limited in terms of their vertical resolution, can provide spectacular images of the intrusive and extrusive components of igneous networks.

In this study we use 2D and 3D seismic reflection and borehole data from the offshore Bight Basin (southern Australia) and Exmouth sub-basin (north-western Australia), to illustrate the seismic expression and range of geometries associated with sill-dominated, intrusive igneous networks connected to submarine volcanoes and vents. Three main types of sill are documented: (i) tabular sills; (ii) saucer-shaped sills; and (iii) transgressive sills. Seismic data resolution restricts a detailed analysis of sill volume, but our analysis indicates that the sills are up to 150 m thick, 16 km wide and 208 km2 in map-view area. In both basins, forced folds, which may represent hydrocarbon traps, are developed above a range of sills. In the Bight Basin, the fold amplitudes are consistently less than the thickness of the underlying intrusions. We interpret that this discrepancy reflects fluidisation and ductile flow of coal or carbonaceous claystones during sill emplacement at relatively shallow depths. In both study areas the sill-dominated networks are overlain by large (13 km wide by 800 m high), sub-circular mounds, the majority of which occur above the tips of sills; these mounds are interpreted as extrusive volcanic vents, adjacent to which pinch-out traps, which are related to stratigraphic onlap, may be developed

From an applied perspective, the sill-dominated networks, although areally quite extensive, are not anticipated to impact the vertical migration of hydrocarbons, due to the presence of pervasive normal fault networks that may allow shallow level reservoirs to access deeply-buried source rocks. Although the sills may locally impact the reservoir quality of the host rock successions, forced folding, which is associated with sill emplacement in the shallow sub-surface, can result in the formation of viable hydrocarbon traps.