Occurrence Of Bsrs And Hydrate Localities 31 Norwegian Margin

Two provinces on the mid-Norwegian Margin show BSR. Whereas the reflectors on the outer Voring Plateau are related to diagenesis (Skogseid and Eldholm, 1989), the BSR in the vicinity of the Storegga slide are related to gas hydrates (Bugge, 1983; Mienert and Bryn, 1997, Mienert, et al, 1998, Bouriak et al., 2000). These BSRs are located at the seaward termination of a thick wedge of Plio-/Pleistocene sediments, and it is likely that fast sedimentation caused burial of large amounts of organic material. It has been suggested that gas hydrates have destabilized the slope in this area, and that this is one reason for catastrophic slope failure (Bugge et al., 1987, Mienert et al., 1998). Generation of this slide has been related to tsunamis on the facing Norwegian coast (Dawson et al., 1988, Bondevik et al., 1997).

The Storegga Slide is one of the world's largest submarine landslides having moved a total of 5600 km3 of sediment with an original thickness of the slumped layer of up to 450 m from an area with a size comparable to that of mainland Scotland (Jansen et al., 1987; Bugge et al., 1988, Evans et al., 1996). A coincidence of dissociation of gas hydrates and slope failures exists at the Mid-Norwegian continental margin (Bugge et al, 1987; Jansen et al., 1987; Mienert et al., 1998). High-resolution seismic data allowed to identify two parallel-occuring BSR and associated velocity anomalies in this area (Posewang & Mienert, 1999a) indicating a complex gas and gas hydrate system. Low levels of methane and minor propane from three ODP drill sites on the Voring Plateau are dominated by biogenic gas (Whiticar and Faber, 1989). This situation suggests that not all hydrocarbons derived from more deeply buried sediments have yet reached the HSZ

North of the northern sidewall of the Storegga Slide, the lower boundary of the HSZ is determined in high-resolution reflection seismic data by a strong BSR which occurs at a depth of approximately 0.35 s two-way travel time (TWT) (Fig. 2). According to the velocity analysis, the corrected depth range of the BSR is 250-285 mbsf (Posewang, 1997, Mienert and Bryn, 1997). The BSR is easily traceable throughout a grid of seismic profiles, indicating the large extent of gas hydrates in this area. The BSR cuts reflections from the dominant strata, mimics the shape of the sea floor and is characterized by high amplitudes. The blanking above the BSR possibly indicates an increase of the hydrate concentration in the sediments. Parts of the BSR along this section are disturbed and exhibit amplitude variations (marked with a,b in Fig. 2). Anomalous high amplitudes of crossed horizons and vertical wipe-out zones are typical indicators for the existence of free gas below the BSR.

Figure 2. Section of seismic reflection profile acquired with a 2-liter airgun and a 6-channel streamer north of the northern Storegga escarpment. A strong BSR occurs at a depth of 0.35 s TWT bsf. Along this section, parts of the BSR are disturbed and exhibit amplitude variations (marked with a, b). The appearance of a second strong horizon in a depth of 0.125 s TWT is interpreted as the top of gas hydrates (from Posewang and Mienert, 1999a).

Figure 2. Section of seismic reflection profile acquired with a 2-liter airgun and a 6-channel streamer north of the northern Storegga escarpment. A strong BSR occurs at a depth of 0.35 s TWT bsf. Along this section, parts of the BSR are disturbed and exhibit amplitude variations (marked with a, b). The appearance of a second strong horizon in a depth of 0.125 s TWT is interpreted as the top of gas hydrates (from Posewang and Mienert, 1999a).

The velocity information from the HF-OBH data shows zones of alternating high and low velocity (Posewang and Mienert, 1999a). High velocities with a maximum of 1850 m/s indicate the existence of gas hydrates in a 180 m thick layer. Below these gas hydrate cemented sediments the velocity drops down to a minimum of 1400 m/s, a value lower than the SVS caused by gas-bearing sediments. The thickness of the low-velocity layer is not clearly identified. The transition between gas hydrate cemented and gas-bearing sediments corresponds with the occurrence of the BSR in seismic sections (Posewang and Mienert, 1999a).

A low-velocity zone within the HSZ reflects the complicated hydrate formation mechanism in this area (Posewang and Mienert, 1999a). At a depth of approx. 0.125 s TWT, the velocity drops from 1580 m/s to 1350 m/s, indicating free gas in the sediments. Above the free gas layer, a lithological boundary possibly acts as a seal for rising gas. The significant impedance contrast between the gas-bearing sediments above and gas-hydrated sediments below produces a strong reflection on seismic records in a depth of 0.125 s TWT bsf interpreted as the top of gas hydrates (Fig. 2).

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