GAS HYDRATE RESOURCE POTENTIAL IN THE NORTHWESTERN GULF OF MEXICO

Alexei V. Milkov¹ and Roger Sassen^2
1 BP America, Exploration and Production Technology Group, Houston, TX
² Geochemical and Environmental Research Group, Texas A&M University, College Station, TX

It is commonly believed that gas hydrate is a potential energy source because the amount of methane carbon in gas hydrates (10,000 Gt) is twice as large as the amount of carbon present in other fossil fuel deposits (coal, oil, natural gas) (Kvenvolden, 1993, Collett, 2002). However, recent direct measurements of gas hydrate concentration at the Blake Ridge (Dickens et al., 1997) and Hydrate Ridge (Milkov et al., 2003) suggest that the global submarine gas hydrate reservoir contains (1-5)x10¹⁵ m³ of methane (500-2,500 Gt of methane carbon) (Milkov, 2004). Therefore, the statement that there is more carbon in gas hydrates than in all other fossil fuels may not be defendable.

Even so the updated estimate suggests that the global gas hydrate inventory may contain a very significant volume of gas. The global volume of hydrate-bound gas may be 2-10 times greater than the global conventional gas endowment that includes undiscovered gas resources, reserve growth, remaining reserves and cumulative production (0.436x10¹⁵ m³, USGS World Energy Assessment Team, 2000). However, the knowledge of the global volume of hydrate-bound gas is not greatly important in the considerations of gas hydrate as a future energy resource. On the global scale, gas hydrate reserves are likely to represent only a small fraction of the gas hydrate resource because the largest volume of hydrate-bound gas is in sub-economic stratigraphic accumulations (Milkov and Sassen, 2002; Hovland et al., 1997). However, some concentrated gas hydrate accumulations may be exploited profitably, and those should be subjected to detailed economic analysis.
The northwestern Gulf of Mexico (NW GOM) continental slope is one of the best-studied gas hydrate provinces. The volume of hydrate-bound gas in this area has been estimated based on the relationship between geologic setting, water depth and the gas hydrate stability zone (Milkov and Sassen, 2001). A conceptual model of gas hydrate occurrence defines two types of gas hydrate accumulations in the NW GOM: (1) structurally-focused thermogenic and bacterial gas hydrate on the rims of minibasins, and (2) disseminated bacterial methane hydrate within stratigraphic accumulations of minibasins. Structurally-focused gas hydrate is estimated to contain ~8-11×10¹² m³ of C₁-C₅ hydrocarbon gas at standard temperature and pressure (STP), representing an important target for future economic exploitation. The estimated volume of bacterial methane hydrate of minibasins is ~2-3×10¹² m³ of methane at STP, but appears to be too widely disseminated to represent a viable exploration target.

The resource potential of individual gas hydrate accumulations has been estimated at GC (Green Canyon) blocks 184/185, GC 234/235, GB (Garden Banks) 388, MC (Mississippi Canyon) 798/842, GC 204, MC 852/853, and AT (Atwater Valley) 425/426 sites in the Gulf of Mexico at water depths ~500-2000 m (Milkov and Sassen, 2003). These structural accumulations may contain from 4.7x10⁸ m³ to 1.3x10¹¹ m³ of gas at STP conditions. The in-place resources in individual gas hydrate accumulations are comparable (by volume) with the reserves in very small to major conventional gas fields. Various geologic, technologic, and economic factors affect the economic potential of studied accumulations. The MC 852/853 appears to be characterized by the most favorable combination of these factors, and thus is suggested to have the highest economic potential. The economic potential of gas hydrate accumulations at GC 204, GB 388, and AT 425/426 sites is ranked as “average”. Gas hydrate accumulations at GC 234/235, GC 184/185, and MC 798/842 sites contain only small volumes of hydrate-bound gas, and likely have no economic potential. Future gas hydrate research should focus on the detailed study of large structural gas hydrate accumulations from which gas may be profitably recovered (e.g., the MC 852/853 site). However, the production of hydrate-bound gases from marine sediments faces major technological challenges and remains uncertain.

References

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