Seismic Expression and Petroleum System Implications of Igneous Intrusions in Sedimentary Basins: Examples from Offshore Australia

Jackson, Christopher A.¹; Magee, Craig¹; Schofield, Nick²; Golenkov, Bogdan¹
¹Earth Science and Engineering, Imperial College, London, United Kingdom.
²School of Geography, Earth and Enviromental Science, University of Birmingham, Birmingham, United Kingdom.

The emplacement of shallow-level igneous intrusions in sedimentary basins may impact significantly on the development of petroleum systems. 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. 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. 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.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012