3D seismic reflection analysis of fault growth, rift basin geometry and the
role of pre-existing structural fabrics
Dr Christopher Jackson
Dr Rebecca Bell
The growth and linkage of fault segments to form continuous, basin-bounding
normal fault zones is recognised as a first-order control on the size and shape of
rift basins, their geomorphology and their sedimentology and stratigraphy (e.g.
Prosser 1993; Anders and Schlische 1994; Gawthorpe et al. 1994; Gawthorpe
and Leeder 2000; Cowie et al. 2000, 2006). Large normal fault systems develop
when strain localises in the lithosphere, and pre-existing basement structure
plays an important role in determining where and how fast strain becomes
localised because of its influence on lithospheric strength. Fault segmentation
and branching, as well as the distribution of displacement in 3D, are also strongly
determined by underlying basement and cover structure (e.g. Henza et al. 2010,
2011; Frankowicz and McClay 2010; Allken et al., 2011). These factors have a
major impact on the size, geometry and compartmentalisation of normal faultrelated structural traps and on the localisation of fluid migration pathways at
both geological and production timescales.
Basement structure also plays an important role in determining the
paleotopography/bathymetry and the paleogeography as well as the source
areas and sediment routing at the onset of extension. The pre-rift landscape
becomes modified as normal fault growth changes the distribution of
uplift/subsidence and hence the geomorphic rates and processes also change.
The way in which the landscape responds to this ‘tectonic perturbation’ has
important implications for understanding synrift stratigraphy and particularly
the location, geometry and heterogeneity of syn-rift reservoirs. Furthermore,
recent work (e.g. Maniatis et al. 2009) suggests that syn-rift erosion and
deposition, through transferring mass from one location to another, will
feedback to also control the structural style and activity on normal faults and rift
basin geometry.
Although many rift basins have undergone multiple phases of rifting, a detailed
understanding of the influence of pre-existing structure on fault growth and synrift stratigraphy is lacking. For example, the Jurassic North Sea rift system was
built on the Permo-Triassic rift, which in turn was influenced by earlier
basement heterogeneities, many of which were related to the Caledonian
orogeny and post-Caledonian Devonian transtension. Any in-depth attempt to
relate fault orientations and fault growth during the later rift events must
therefore consider the impact of structures existing prior to the particular
extension phase.
The main aim of this project is therefore to develop a fundamental
understanding of how pre-existing structures in both basement and cover
influence the evolution of normal faults and normal fault populations on rifted
margins. We will use this understanding to analyse the geomorphological
evolution of rifts and the impact this has on erosion, sediment transport
pathways and the overall evolution of syn-rift depositional systems. Our main
aim will be achieved through a series of specific objectives:
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To investigate how pre-existing basement and cover structures influence
the nucleation, growth, interaction and linkage of normal fault segments
and their organisation and evolution within multi-phase rift basins.
To establish how pre-existing structures influence the evolution of rift
basins and use this analysis to understand the controls on rift topography,
sediment sources, major sediment transport pathways and depositional
systems in multi-phase rifts.
To quantify the effects of erosion, deposition and mass redistribution at
the Earth’s surface on fault evolution and rift basin morphology where
pre-existing structures exert a strong control on fault development.
To achieve these objectives we will use a large (1000 km2), high-resolution (25
m line spacing), 3D seismic reflection dataset from offshore NW Australia. In
these data a number of fault populations, which display different geometric
relationships are clearly imaged. Detailed three-dimensional mapping of key
faults and seismic horizons will form the foundation for a detailed, quantitative
analysis of the present-day geometry of the fault network, and investigation of
the temporal evolution of the fault population.
The results of this study will be used by the petroleum industry to: a) understand
more clearly the controls on, and variability of, structural traps related to normal
faulting, and b) develop models for subtle syn-rift plays. Thus the project will
contribute to reducing exploration risk and increasing the sophistication of synrift reservoir models. Although we will use seismic reflection data from offshore
Australia, the generic understanding of normal fault growth and the associated
evolution of syn-rift sedimentary systems will be applicable to rifted margins
world-wide.