Subsurface occurrences of natural gas hydrate can be classified into six types: (1) pore-space hydrate, (2) platy hydrate, (3) layered/massive hydrate, (4) disseminated hydrate, (5) nodule hydrate, and (6) vein/dyke hydrate. The anomalies of chloride contents in pore water, core temperature depression, core observation, as well as visible gas hydrates confirmed well-interconnected and highly saturated pore-space hydrates as an intergranular pore filling within sand layers within the methane hydrate stability zone. Hydrate saturations are higher than 60% throughout most hydrate-dominant sand layers and in some parts there is close to 100% pore saturation. Muddy sediments such as silts and clays are free of hydrate or contain low concentrations.
Figure 5.4 shows the sites where natural gas hydrate has been recovered or where its presence is inferred (Milkov 2004; Kvenvolden and Rogers 2005; Klauda and Sandler 2005). Subsea gas hydrates have been thought to comprise the preponderance of hydrate to be found in the geosphere. Moreover, they are to be found much closer to markets than are Arctic hydrates (Fig. 5.4). Promising accumulations are thought to exist off the east, west, and Gulf of Mexico coasts of the USA, as well as offshore Japan, India, China, and other important energy-consuming nations. Originally, the gas endowment was estimated to be 10,000 times larger than the global conventional gas endowment; however, most recent estimates are smaller, though large uncertainties persist.
Ocean Drilling Program investigations at a number of sites, including Blake Ridge offshore South Carolina, Hydrate Ridge offshore Oregon, and the Cascadia Margin offshore Vancouver, were funded in part by the National Science Foundation. With its abundant gas seeps in deep water, the Gulf of Mexico is probably the most promising marine gas hydrate province in US waters (Milkov and Sassen 2002). However, there has been no hydrate exploration activity there. A small number of hydrate-related wells were drilled in Keathley Canyon and Atwater Valley lease blocks in 2005, primarily to investigate drilling safety. This work was
sponsored by US Department of Energy, and Chevron was the lead contractor. To achieve an assessment of marine gas hydrate resources, the US government would need to adopt the model used by the governments of Japan and India: an exploration and drilling campaign encompassing, for example, 20 sites dispersed over a significant geographic area.