From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, February 8, 2013
Link to streaming video: available 02.08.2013 at 8:55am
Dr. Christopher Jackson
Statoil Reader in Basin Analysis*
Basins Research Group
Department of Earth Science and Engineering
*Currently visiting Research Fellow in the Applied Geodynamics Laboratory (AGL) at BEG.
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 localized host rock relationships, outcrops are often too small to fully characterize 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 clay stones 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 really 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.
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