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Bureau Seminar, October 4, 2013

Insights into the development of major rift-related unconformities
from geologically constrained subsidence modelling

Link to streaming video: available 10.04.2013 at 8:55am

Chris Jackson
Visiting Research Fellow
Bureau of Economic Geology

Palaeobathymetry and the palaeo-structure of rift basins is a crucial parameter in the modelling of hydrocarbon systems, as it controls sediment sourcing, sediment distribution, source rock maturation and hydrocarbon migration. Despite the importance of palaeobathymetry it is often difficult to determine with accuracy across a basin, as sedimentological and biostratigraphic analysis of well data only provide an estimate of palaeo-water depth at a single location and it is not possible to use seismic stratigraphic techniques to determine water depths in deep water. Reconstructing palaeobathymetry using reverse basin modelling techniques, accounting for sediment loading and tectonics, provides an estimate of water depth across a basin and can offer insights into lithospheric extension mechanisms.

Seismic reflection data from the southern Halten Terrace, offshore mid-Norway, indicate the presence of potential Late Jurassic palaeo-structural highs in the form of now buried and tilted peneplains (the Sklinna Ridge and Frøya High). Whether these structures were subaerial in the Late Jurassic, and if they could have been important sources of reservoir quality sands, is unknown. In this study we restore the Late Jurassic palaeobathymetry of the southern Halten Terrace along 2D seismic reflection profiles using a combination of sedimentology, seismic stratigraphy and reverse basin modelling, to investigate the likely magnitude and distribution of eroded sands from these potential palaeo-highs. This integrated geological and geophysical approach allows crustal input parameters involved in flexural backstripping and thermal basin modelling to be constrained. Values of stretching factor (β) and elastic thickness (Te) that can best restore a palaeobathymetry that complies with water depth constraints from geology are Te = 16 km, and β = 1 - 1.2 from east to west across the Halten Terrace. These β values are the same as those determined from upper-crustal fault heave, with the addition of 40% extension to account for sub-seismic scale faulting.

Our study reveals that the Sklinna Ridge was subaerial in the Late Jurassic and was an important source of potential reservoir quality sediments. As well as providing insights for exploration, this study has also revealed that upper crustal stretching in the Halten Terrace is adequate to explain the degree of thermal subsidence that has occurred in the area since the Late Jurassic. We suggest a uniform lithospheric extension mechanism is appropriate here, similar to other locations worldwide, where stretching is mild (β = <2). A uniform stretching mechanism in the Halten Terrace contrasts with depth-depending stretching that is often reported from other areas of the mid-Norwegian margin (e.g. Vøring Basin).


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