Large-scale conical sandstone intrusions and polygonal fault systems in Tranche 6, Faroe-Shetland Basin
Introduction
Sandstone intrusions are a common class of soft-sediment deformation structure with a long history of field-based research (e.g., Murchison, 1827; Newsom, 1903; Jenkins, 1930; Taylor, 1982; Parize, 1988; Surlyk and Noe-Nygaard, 2001; Boehm and Moore, 2002). Recently, research has expanded to include a number of subsurface analyses of large-scale intrusions, by using seismic imaging techniques (e.g., MacLeod et al., 1999; Lonergan et al., 2000; Molyneux et al., 1999; Huuse and Mickelson, 2004; Huuse et al., 2004; Shoulders and Cartwright, 2004) integrated with core data where available (Duranti et al., 2002; Duranti and Hurst, 2004). This use of three-dimensional (3D) seismic data has facilitated the identification of a class of sandstone intrusion that displays a downward-tapering conical geometry (Molyneux et al., 1999).
Understanding the factors controlling the geometry of large-scale sandstone intrusions is important because widespread sand intrusion may complicate the depositional geometry of hydrocarbon reservoirs (e.g., Dixon et al., 1995; MacLeod et al., 1998; Purvis et al., 2002; Bergslien, 2002) and conical sandstone intrusions may themselves form hydrocarbon reservoirs (Huuse et al., 2005; Huuse and Mickelson, 2004). The presence of high-permeability intruded sands within otherwise impermeable sequences may have important implications for vertical connectivity. In the absence of constraints it is possible that such intrusions may form high-permeability fluid conduits long after their emplacement (Hurst et al., 2003; Schwartz et al., 2003). This may affect the hydrocarbon prospectivity of a basin, improving it by providing effective fluid conduits to feed shallow reservoirs, or degrading prospectivity by acting as a seal breach and possibly acting as a drilling hazard when targeting deeper prospects.
All the published examples of large-scale conical sandstone intrusions are found within thick polygonally faulted claystone sequences (e.g., Lonergan and Cartwright, 1999; Lonergan et al., 2000; Molyneux et al., 1999; Huuse et al., 2004). Observations that conical sandstone intrusions and polygonal faults may have similar dimensions and dips have been made and interpreted as sand intrusion occurring along polygonal fault planes (Lonergan et al., 2000; Gras and Cartwright, 2002; Molyneux et al., 1999). In contrast, crosscutting relationships between conical intrusions and polygonal faults have also been observed (Huuse et al., 2001, Huuse et al., 2004; Shoulders and Cartwright, 2004). Hence, the level of control exerted by polygonal faulting on the geometry of conical sandstone intrusions is still poorly constrained.
This paper aims to assess the relationship between polygonal faults and large-scale conical sand intrusions in Tranche 6 of the Faroe-Shetland Basin. We present a large dataset of dip measurements taken from both polygonal fault planes and discordant high-amplitude reflections within an Eocene–Oligocene claystone sequence. We will argue that these high-amplitude reflections originate from large-scale conical sandstone intrusions. This is the first time that 3D seismic data have been interrogated specifically to explore the relationship between polygonal faults and conical sandstone intrusions. We will show that the data collected are consistent with conical sandstone intrusions forming independently from pre-existing polygonal faults in the Faroe-Shetland Basin.
Section snippets
Geological setting
Uplift of the Scottish massif resulted in increased clastic input to the Faeroe-Shetland Basin during the Paleocene (Hall and Bishop, 2002; Fig. 1) and the deposition of large, sand-rich deep-water fans during the early Middle Eocene (Mitchell et al., 1993). This was followed the deposition of predominantly muddy sediments during the later Eocene and Oligocene. Inversion structures formed through the reactivation of extensional faults occurred during the Eocene in the southwest of the basin (
Data and methods
Our case study is focused on the Eocene to Recent succession in the basin centre and is based on 2600 km2 of 3D seismic data, tied to several exploration wells. The 3D seismic cube used during this study consists of zero-phase P-wave data with a line spacing of 12.5 m. The dominant frequency is in the order of 80 Hz and vertical resolution (λ/4) of 6–7 m within the Eocene–Oligocene succession. The 3D seismic data is approximately zero-phase at the seabed, and the positive (red) peak amplitude
Characteristics of polygonal faults within the study area
A complex network of extensional polygonal faults extends throughout the Eocene–Oligocene claystone succession but is not present within the Middle Eocene sand-dominated fans (Fig. 3). The polygonal faults are gently listric, with decreasing dip with increasing depth (Fig. 4A). The average fault plane dip is 58° (±2°); however, fault plane dip ranges from 55° and 85° at the level of the INU but shallows to between 30° and 50° at the level of the Middle Eocene fans (Fig. 4A). Only a single
Discussion
The values for polygonal fault dips in the study area (average dip of 58° with individual measurements ranging between 23° and 85°) are consistent with previously published studies (see Table 1). The polygonal faults in the study area are arranged in a single tier and commonly display a subtle listric geometry becoming shallower dipping with increased depth. In contrast, there is no relationship between the depth of conical sandstone intrusion and the angles of their discordant limbs, which
Conclusions
In Tranche 6 of the Faroe-Shetland basin, polygonal faults and conical sandstone intrusions within the Eocene–Oligocene succession have distinct dip populations with little overlap. There is a strong depth dependency on the dip of polygonal faults, but no such relationship is seen in conical sandstone intrusions. The conical sandstone intrusions are unlikely to have undergone significant post-emplacement compaction, and the distribution of intrusion dips is uniform throughout the
Acknowledgements
We are grateful to ExxonMobil and Schlumberger GeoQuest for providing seismic and well-data interpretation software. We would like to thank David Roberts, Andrew Hirst and an anonymous reviewer for their constructive views and feedback. We also thank Mairi Nelson, Catherine Boudon and Richard Davies for their discussion of the topics presented in this paper.
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