Introduction

Over 70 locations with gas hydrate samples and indirect indications of hydrate have been identified in the sea floor by the beginning of 2002 (Fig. 1, also see Kvenvolden, this volume). In 23 of them, gas hydrates have been recovered from the sea floor sediments by drilling or gravity coring. All submarine gas hydrates are 1. related to infiltration of gas-containing fluids in and through the temperature-pressure field of the gas hydrate stability zone (HSZ); 2. distributed mainly as accumulations; and 3. can be subdivided into two groups: Accumulations at or just below the sea floor and accumulations situated from tens to hundred meters below the sea floor. Most accumulations of the first group are related to focused fluid discharge at the sea floor. The second group of accumulations is controlled by the general migration of gas-rich pore water or by gas diffusion in pore fluids.

Our principal approach is to study in detail naturally occurring gas hydrate accumulations in-situ. We define gas hydrate accumulations as comprising a volume of sediments in which some water in the pore spaces has been displaced by a significant amount of gas hydrate. Thus, we identify such accumulations as definite geological bodies. Within fluid discharge areas the boundaries of hydrate accumulations are defined by a diffusion halo within which the pore water is highly saturated by gas. Therefore, the external boundaries and the shape of gas-saturated halos can be used to identify the accumulations related to the fluid discharge areas.

Although the role of gas hydrate formation in the fluid discharge areas is not yet widely understood, we argue that it should be considered of importance to both gas hydrates as a potential hydrocarbon source and as a factor influencing global climate change.

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