AAPG/GSTT HEDBERG CONFERENCE
“Mobile Shale Basins – Genesis, Evolution and
Hydrocarbon Systems”
Growth and
dewatering of polygonal fault systems in the
STEFAN BÜNZ*, STEINAR
HUSTOFT AND JÜRGEN MIENERT
Department
of Geology,
* Tel: +47 77646266, Fax: +47 77645600, Email: [email protected], (Corresponding author)
Polygonal
fault systems have been identified in more than 50 sedimentary successions on
passive continental margins and cratonic basins(Cartwright et al., 2003;
Cartwright and Dewhurst, 1998). Such fault systems are composed of small normal
faults that are organized into polygonal networks best observed in plan view.
This non-tectonic class of fault systems is layer-bound and occurs in
fine-grained sediments. The development of these faults commences at an early
stage of burial. The formation of polygonal fault systems results from sediment
contraction and subsequent pore fluid expulsion (Cartwright and Lonergan,
1996). The processes leading to contraction and water expulsion are still
debated (Cartwright et al., 2003; Goulty, 2002). Possible processes involved in
their development include syneresis of colloidal sediments (Cartwright and
Lonergan, 1996) and compaction due to gravitational loading (exceptionally low
coefficients of friction) (Goulty and Swarbrick, 2005). Understanding of such
fault systems is important as they might interact with adjacent reservoirs
(Stuevold et al., 2003), and because they might control fluid flow on a
regional scale (Henriet et al., 1991).
Figure 1: Distribution of polygonal faults
on the Norwegian Margin after Hjelstuen et al. (1997) and Berndt et al. (2003).
The 3D seismic data used for this study is located in the southeastern
Polygonal
faults systems on the Norwegian Margin are widespread (Figure 1) (Berndt et
al., 2003; Hjelstuen et al., 1997). They occur in the two dominant sedimentary
basins, the Vøring and the Møre basins, within the Brygge and Kai formations,
but have also been discovered in Late Cretaceous sediments in the vicinity of
the Ormen Lange dome (Stuevold et al., 2003). Berndt et al. (2003) suggested
that polygonal faulting and fluid expulsion in the upper Brygge and Kai
formation sediments is an ongoing process since Miocene times. Here we use 3D
seismic imaging techniques to map fault throws throughout a whole tier in order
to investigate the growth and compaction history of the faults and its
dewatering system. The 3D seismic data used in this study is from the
southeastern
Figure 2: Fault throw map of a horizon from approximately the middle of
the tier. The polygonal nature of the slide can be clearly recognized. This map
shows quite some variation of fault throw throughout the whole area. The
highest throws (up to 90 – 100 ms) occur on WSW – ENE striking faults in the
southeastern half of the interpreted area. The throw of faults striking about N
– S to NW – SE is about half as high (50 – 60 ms). In general, fault throws in
the northeastern half are smaller and fault density appears to be lower.
The
polygonal faults above the Helland Hansen dome span over a depth interval of
about 1500 m. They cover an area that is clearly larger than that of the 3D
seismic coverage. However, data quality is rather poor in the northern half;
therefore main mapping was concentrated in the southern half of the 3D block.
The polygonal fault pattern is clearly observed in plan view (Figure 2) showing
some curved to rectangular shape. The first results show that in general, our
maps illustrate some significant variation in fault throw throughout the mapped
area. The polygonal fault system can be described by two characteristic fault
systems. The “major” faults dominantly strike in WSW – ENE direction (Figure
2), and have a maximum throw of about 90 to 100 ms with an average of about 50
– 60 ms. These faults are particularly developed in the southwestern half of
the interpreted area where fault density also appears to be denser. The primary
strike of the intersecting “minor” faults is approximately between N – S and
NW- SE (Figure 2). These faults have a maximum throw of about 50 – 60 ms with
an average of about 20 – 30 ms and are generally shorter than the “major”
faults.
Variation
in fault throws and fault density across the mapped area may be attributed to
variation in grain size (Dewhurst et al., 1999). However, the WSW – ENE strike
of the “major” faults coincides with the strike of large normal faults that are
situated underneath the tier suggesting that the growth of the polygonal faults
in this area may be linked to tectonic stresses. Sediments above the polygonal
fault system show evidence for fluid flow. There appears to be a link between
the density of such fluid flow features and the polygonal faulting. Both, the
growth of the polygonal faults and its link to fluid flow have to be further
investigated, but the quantitative study of fault throws throughout the whole
tier shows great potential in advancing our understanding of polygonal fault
systems.
References
Berndt, C., S. Bünz, and J. Mienert,
2003, Polygonal fault systems on the mid-Norwegian margin: A long term source
for fluid flow, in P. Van Rensbergen, R. Hillis, A. Maltman, and C. Morley,
eds., Subsurface Sediment Mobilization: Geological Society Special Publication,
v. 216: London, UK, Geological Society of London, p. 283-290.
Cartwright, J., D. James, and A.
Bolton, 2003, The genesis of polygonal fault systems; a review, in P. van
Rensbergen, R. Hillis, A.-J. Maltman, and C.-K. Morley, eds., Subsurface
sediment mobilization: Geological Society Special Publications, v. 216:
Cartwright, J. A., and D. N. Dewhurst,
1998, Layer-bound compaction faults in fine-grained sediments: Geological
Society of America Bulletin, v. 110, no. 10, p. 1242-1257.
Cartwright, J. A., and L. Lonergan,
1996, Volumetric contraction during the compaction of mudrocks: A mechanism for
the development of regional-scale polygonal fault systems: Basin Research, v.
8, no. 2, p. 183-193.
Dewhurst, D. N., J. A. Cartwright, and
L. Lonergan, 1999, The development of polygonal fault systems by syneresis of
colloidal sediments: Marine and Petroleum Geology, v. 16, no. 8, p. 793-810.
Goulty, N. R., 2002, Mechanics of
layer-bound polygonal faulting in fine-grained sediments: Journal of the
Geological Society, v. 159, p. 239-246.
Goulty, N. R., and R. E. Swarbrick,
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Stuevold, L. M., R. B. Faerseth, L.
Arnesen, J. Cartwright, and N. Moller, 2003, Polygonal faults in the Ormen
Lange Field,
AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago