Possible pitfalls

As the above discussion demonstrates, BSRs are more complex and diverse than previously thought. What matters in the context of gas hydrate quantification, however, is that the presence of BSRs is a reliable indicator that a study area contains gas hydrates. However, there are some pitfalls.

BSRs appear to be obvious features, but they may not be noticed if interpreters don't expect them. In particular, inappropriate seismic processing that focuses on deep sediment sections may destroy BSRs. Also, horizontal layering, by destroying their distinctive cross-strata geometry, may hide BSRs.

In addition, BSRs are absent in many locations where gas hydrates are known to occur. We still do not know what conditions, in addition to the presence of gas hydrates overlying free gas, are necessary to "maintain" BSRs. Two mechanism that may preclude BSR formation are tectonic suppression of BSRs (Fig. 7), proposed for the Lima Basin off Peru, and the escape of gas through faults, which may be responsible for the general lack of BSRs in the hydrate-rich Gulf of Mexico. Moreover, the presence and amplitude of BSRs do not allow any immediate conclusions about whether gas hydrate is present in the shallower sediment section. This is in particular true for shallow-water gas hydrates and gas hydrate outcrops in the Gulf of Mexico.

1 Ma present

PERU

LOWER SLOPE tectonic uplift sedimentation seat k>o r temperature

- effect

113m net effect temperature

- effect

350 m tectonic subsidence sedimentation

500 m

BGHS

pressure effect

5 new sediment section ! original gas-hydrate-bearing section | original gas-hydrate-free section

BGHS 1 Ma

BGHS at present c-28 m temperature effect net effect

LIMA BASIN

BGHS

pressure effect

5 new sediment section ! original gas-hydrate-bearing section | original gas-hydrate-free section

Fig. 7: Tectonic suppression of BSRs as a proposed mechanism to preclude BSR formation. The Lima Basin off Peru (further upslope from the section shown in Fig. 4) is a good candidate for gas hydrate occurrence because of the high organic carbon content in the sediments. However, most of the Lima Basin lacks BSRs. Lima Basin is rapidly subsiding. This leads to an increase of pressure, moving the phase boundary of gas hydrate toward higher temperatures, i.e., downward ("pressure effect"). Sedimentation leads to the opposite effect: Assuming a constant thermal gradient, the BGHS moves upward with respect to the sediment column ("temperature effect"). Both effects have been quantified off Peru (von Huene and Pecher, 1999). On the lower slope (Fig. 4), both uplift and sedimentation lead to an upward movement of the BGHS, causing dissociation of gas hydrates, generation of free gas, and a strong BSR. In Lima Basin, the net effect of subsidence and sedimentation is a downward movement of the BGHS with respect to the sediment section. Eventual gas at the BGHS is predicted to be absorbed to form gas hydrate.

Finally, silica diagenesis from opal-A to opal-CT or opal-CT to quartz also causes BSRs (Hammond and Gaither, 1983). Those phase transitions are associated with a velocity increase and BSRs therefore display a positive polarity. However, apart from that, they look deceptively similar to hydrate BSRs (Fig. 8 - (Scholl and Creager, 1973)).

SITE 1(4

UMNAK PLATEAU

CHANCE COURSE

SITE 1(4

UMNAK PLATEAU

CHANCE COURSE

Gas Hydrate
Fig. 8: A "silica BSR" from the Bering Sea. Note the very similar appearance to gas hydrate BSRs (from Scholl and Creager, 1973)

Despite these possible pitfalls, BSRs may be used successfully to constrain regional gas hydrate occurrence, and with proper calibration, concentrations. Calibration from drilling of ODP Leg 164 together with BSR distribution around the Blake Ridge, led to an estimate of gas hydrate amount along the East coast of the U.S (Dickens et al., 1997; Holbrook et al., 1996). Similar studies along the world's margins would be very valuable for global estimates of gas hydrate quantities.

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