Appendix 10: Non-Potential of Natural Gas Hydrate Occurrence in Queen Charlotte Basin8

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1 British Columbia Offshore Hydrocarbon Development Appendix 10: Non-Potential of Natural Gas Hydrate Occurrence in Queen Charlotte Basin8 Natural gases such as methane, ethane, propane typically occur as a gas phase on the Earth (Figure 1, Kvenvolden and Lorenson, 2001). However under quite special conditions these gases can combine with water to form a solid form called Gas Hydrate, or clathrate. Globally these gas hydrates represent a tremendous reserve of natural gas, especially methane, in the Earth. The global amount of natural gas tied up in hydrates is estimated to be 10,000 Gt or 2 x 1016 m3 (= 6 x 105 tcf), e.g., Kvenvolden (1993). For comparison Figure 2 shows the amounts of natural gas contained in conventional reservoirs is about onehalf of this amount. The west coast of North America, including the west coast of Vancouver Island is well known to have large accumulations of gas hydrate in the specific shallower (ca m) offshore sediments. Because of the potential economic significance of these hydrates, there is considerable interest in their formation and occurrence. Figure 1. Global distribution of natural gas hydrates (after Kvenvolden and Lorenson, 2001). Organic Carbon Reservoirs (excluding carbon in rocks and sediments) Oceans 980 Atmosphere 3.6 Land 2,800 Gas Hydrates 10,000 Fossil Fuels 5,000 Gigatons (1015 tons) of carbon Figure 2. Major organic carbon pools (after Whiticar, 1990) 8 Submission to Dr. D. S. Strong, Chair, Scientific Review Panel, BC Offshore Hydrocarbon Development prepared by Dr. Michael J. Whiticar, School of Earth and Ocean Sciences, University of Victoria, BC December, Report of the Scientific Review Panel Volume Two Appendices 55

2 In addition to their economic potential, gas hydrates are of interest because they: 1. Pose geotechnical concerns, such as large-scale submarine slumping, 2. Can cause difficulties during drilling operation due the possibility of overpressured gases beneath the gas hydrate stability zone, 3. Are a major factor in greenhouse gas storage and climate change. There are three primary conditions that must be satisfied in order that methane hydrates are naturally able to form and be preserved. These are: 1. Sediment porewaters (or rarely water column) is saturated with CH 4 (free gas) 2. Sufficient pressure is available (hydrostatic pressure, P) 3. The temperature (T) of the water and sediment is suitably cold. The first condition, i.e., that the waters are saturated with respect to methane is frequently met in shallow coastal waters, such as found in the Queen Charlotte Basin. There are numerous sources for this methane, but most commonly in this setting the gas is of bacterial or thermogenic origin, e.g., Whiticar (1990). The combination of temperature and pressure (water depth) necessary for methane hydrate formation and stability are shown in Figure 3. In the lightly shaded region, the pressure is either too low (too shallow) or the temperature too high for hydrate to exist (e.g., Sloan, 1998). The darker region in Figure 3 shows the depth-temperature (P, T) region in which gas hydrate is stable. For illustration, the range in temperature and water depths in the Hecate Strait are given as the white box in the figure. The specific situation in the Hecate Strait is more clearly indicated in Figure 5. This figure incorporates the usual maximum and minimum temperatures measured at different water depths in the summer and winter (Crawford, 2001). The maximum temperature of ca. 16 C is recorded in the summer surface waters (Figure 4, after J. Gower and J. Wallace, Institute of Ocean Sciences, DFO, Sidney). The coldest water is 5 6 C in the deeper waters, e.g., 200 m. Figure 3. Phase Stability of methane and methane hydrate (after Sloan, 1998) 56 January 15, 2002 Report of the Scientific Review Panel

3 Figure 4. Summer surface water temperature (after Gower and Wallace, 2001) Cool water (10 C appears blue, warmer waters are in red, ca. 16 C). Report of the Scientific Review Panel Volume Two Appendices 57

4 Figure 5. Hydrate non-occurrence in the Hecate Strait Figure 5 shows this temperature distribution with depth together with the known P,T methane hydrate stability zone. This figure clearly illustrates that even if the waters were saturated with methane the typical waters of the Hecate Strait are not within the hydrate zone. The form of any methane in these waters would be either dissolved or free gas. The situation does not change in the sediments, i.e., any hydrocarbons gases would be in gas form and not hydrate. This is because the temperature will continue to increase with sediment depth due to the geothermal gradient. It should be noted that offshore, where the water depths (pressures) are greater and the bottom waters are colder (2 4 C), hydrates are stable and can accumulate. However, these hydrates only form if sufficient gas is present. Such is the case in the areas such as the Cascadia Margin, e.g, off Vancouver Island and Oregon coast. 58 January 15, 2002 Report of the Scientific Review Panel

5 References Crawford, William R., 2001 Oceans of the Queen Charlotte Islands and Gwaii Haanas National Marine Conservation Area Reserve. Canadian Hydrographic Service, Fisheries and Oceans Canada Kvenvolden, K.A Gas hydrates as a potential energy resource--a review of their methane content, in Howerll, D.G., ed., The future of energy gases: U.S. Geological Survey Professional Paper 1570, p Kvenvolden K.A. Lorenson, T.D., A global inventory of natural gas hydrate occurrence. USGS Special Map, walrus.wr.usgs.gov/globalhydrate/browse.pdf Sloan, E. D., Clathrate Hydrates of Natural Gases, 2nd ed., Marcel Dekker Inc., New York. Whiticar, M.J., A geochemical perspective of natural gas and atmospheric methane. 14th EAOG Mtg. Paris, In: B. Durand et al. (eds.), Advances in Organic Geochemistry 1989, Org. Geochem., 16: Report of the Scientific Review Panel Volume Two Appendices 59