AAPG/GSTT HEDBERG CONFERENCE
“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”
dewatering of polygonal fault systems in the
STEFAN BÜNZ*, STEINAR HUSTOFT AND JÜRGEN MIENERT
* 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
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
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.
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AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago