FROZEN HEAT | Volume 2

Geophysical methods hold great promise for the remote detection and quantification of gas hydrate deposits because of the strong changes in the physical properties that are induced by the presence of gas hydrates. Replacing water or gas in the sediment pore space with solid gas hydrates results in marked increases in the both the electrical resistivity and the acoustic wave velocity of the sediment. These physical changes can be detected with electromagnetic and conventional reflection seismic technologies deployed from ships and used to evaluate large regions. The data, when integrated with models that correlate physical properties to gas hydrate occurrence, make it possible to make general estimates of the location, extent, and concentration of gas hydrate deposits prior to drilling (Shelander et al. 2010). Box 2.3 Changing approaches to gas hydrate exploration

Direct detection of gas hydrate deposits, particularly those that are widespread and highly concentrated, is a relatively new development in gas hydrate science. Previously, the presence of gas hydrates in marine sediments had been deduced seismically from the presence of a bottom- simulating reflector (BSR; Shipley et al. 1979), which commonly marks the base of the gas hydrate stability zone (GHSZ). Physically, the BSR is the transition from gas-hydrate-bearing sediments to gas-charged (or at least gas-hydrate-free) sediments below. While early research focused on how to exploit the seismic character of a BSR and infer gas hydrate concentrations above and free gas concentrations below the reflection (Hyndman and Spence 1992; Yuan et al. 1996), more recent analyses show that the seismic characteristics of BSRs cannot easily be related to the concentration of the pore-filling material (Chapman et al. 2002), a conclusion confirmed by drilling results (Tsuji et al. 2009). A complementary technique, controlled-source electromagnetic imaging (CSEM; Edwards 1997), attempts to exploit the increased electrical resistivity of gas-hydrate-bearing sediments. However, the physical nature of electromagnetic wave propagation through marine sediments results in a reduced lateral and vertical resolution, compared to seismic imaging. As a result, CSEM may be more suitable for imaging chimney structures and other fracture- dominated systems (Schwalenberg et al. 2005). Much prior gas hydrate exploration used sea-floor phenomena, such as seabed hydratemounds, pockmarks, mud volcanoes, and depth of sulphate penetration, as general indicators of the nature of historical or current gas seepage. However, while these are interesting physical features for understanding natural systems, they have not yet been shown to be useful in prospecting for deeper reservoirs. Recently, approaches to exploring for gas hydrate deposits have shifted towards a more integrated evaluation of the full petroleum system (Collett et al. 2009). This approach incorporates geologic information (such as the availability of gas sources, fluid migration pathways, and suitable reservoirs) with direct geophysical indicators (such as anomalous strong reflectors or high calculated velocities) in a way regularly applied in the oil and gas industry (Saeki et al. 2008 Boswell and Saeki 2010). The approach acknowledges that all exploration has great uncertainty, and that no single tool or piece of evidence will be definitive and reliable. Instead, exploration uncertainty is best managed by a comprehensive evaluation of all relevant data to provide confidence in the occurrence of each necessary part of the system.

Approx. 1,500 m

Bounding Fault

Bounding Fault

Approx. 3,000 m

Thickness and gas hydrate concentration increasing

Below resolution

Figure TB-2.3: A gas hydrate prospect delineated on the Alaska North Slope. The image shows geophysically-inferred gas hydrate trapped within a sand layer at the intersection of two fault planes (green). (Courtesy US Geological Survey).

FROZEN HEAT 48