Eleanor C. Willoughby

Geophysical characterization of gas hydrates

Field study of natural gas hydrates is new in geoscience, as it is in commercial interest. It is only some 50 years since the early recognition of hydrate in hydrocarbon exploration wells in permafrost areas of northern Russia and the detection of hydrate-related marine bottom-simulating reflectors (BSRs) off eastern United States. Early work was driven mainly by scientific interest, including the role of hydrate in climate change, but most of the recent substantial expenditures have come from the energy potential. Detection, mapping, and characterization of natural hydrate occurrences by seismic and other methods have come a long way but still do not have the refinement of the techniques used in the hydrocarbon exploration industry. The early field surveys and studies tended to be of the type: “lets try everything and see what we learn.” Some were surprisingly useful, like electrical resistivity; others not so. Integration of the results of several types of surveys has been especially valuable. Much early effort was directed at “direct detection” of hydrate based on the substantial difference in physical properties of hydrate compared to sediment pore fluid, especially the high seismic velocity. This approach has had some success in field surveys but has been of most value in the analysis of downhole logs, especially where several parameters are available. The associated downhole logs provide a critical complement to field surveys through calibration (“ground truth”) by way of core physical property and composition analyses. Laboratory studies have been important for determining the changes in physical properties of hydrate under different conditions and of sediments containing varying amounts of pore-filling hydrate. The effects on field data of larger scale hydrate in nodules, veins, and more massive occurrences are not yet well determined. Along with the indicators based on hydrate physical properties, there has been increasing understanding, based mainly on field geophysical surveys, of the processes that form gas hydrate and of the structures that host hydrate. These include large structures in permafrost sedimentary basins, regional marine hydrate just above BSRs, hydrate associated with marine “vent” or “plume” structures, and hydrate contained in fracture networks. We are now beginning to understand the geophysical characteristics of each of these. Also, initial production proposals and testing have suggested that sand-hosted hydrate may be most amenable to gas extraction, so there is increasing focus on sand detection by geophysical methods as well as through structural indicators. Some of us had a simplistic view of hydrate occurrences and their detection, but we now recognize that they are at least as complex as for conventional hydrocarbons. We still have a long way to go; much is yet to be learned. However, this volume represents a major achievement in consolidating the considerable current geophysical knowledge of what is required for hydrate detection and mapping. If not a complete recipe, we at least have a clear description of the most valuable survey and study tools and their uses, along with the basic data processing methods and interpretations. Congratulations to the authors and to the editors for this important milestone.

Bottom Simulating Reflector and Gas Seepage in Okinawa Trough: Evidence of Gas Hydrate in an Active Back-Arc Basin

To look for gas hydrate, 22 multi-channel and 3 single-channel seismic lines on the East China Sea (ECS) shelf slope and at the bottom of the Okinawa Trough were examined. It was found that there was indeed bottom simulating reflector (BSR) occurrence, but it is very rare. Besides several BSRs, a gas seepage was also found. As shown by the data, both the BSR and gas seepage are all related with local geological structures, such as mud diapir, anticline, and fault-controlled graben-like structure. However, similar structural anomalies are quite common in the tectonically very active Okinawa Trough region, but very few of them have developed BSR or gas seepage. The article points out that the main reason is probably the low concentration of organic carbon of the sediment in this area. It was speculated that the rare occurrence of gas hydrates in this region is governed by structure-controlled fluid flow. Numerous faults and fractures form a network of high-permeability channels in the sediment and highly fractured igneous basement to allow fluid circulation and ventilation. Fluid flow in this tectonic environment is driven primarily by thermal buoyancy and takes place on a wide range of spatial scales. The fluid flow may play two roles to facilitate hydrate formation: to help gather enough methane into a small area and to modulate the thermal regime.