Methane Hydrate in Marine Sediment - Defining the Characteristics of an Unclassic Reservoir

James S. Booth, Kathleen M. Fischer, Mary M. Rowe

Unlike classic oil and gas reservoirs, the location of methane hydrate accumulations is determined by ambient physical conditions: a hydrate will form wherever the requisite pressure and temperature conditions exist, if adequate supplies of methane and water are available. Sediment properties may control key reservoir features, however, such as total storage capacity, concentration or habit of the hydrate and extraction capability, all of which may have economic, environmental, or geohazard significance.

The purpose of this study was to determine what role the properties of the host sediment play in the accumulation of methane hydrate in the marine environment. The following assumptions were made: sediments are normally consolidated; methane is generated in sediments below the reservoir and transported upward; and no hydraulic gradient exists to cause advective flow, i.e., no external geologic forces such as tectonic compression are present. Through examination of the literature and modeling, five sediment properties were tentatively identified that may affect reservoir characteristics: total porosity (sets reservoir storage capacity), thermal conductivity (lower values depress local isotherms and thus increase the absolute thickness of the hydrate zone), proportion of surface-active inerals such as montmorillonite (the higher the percentage, the greater the P/T range of the stability zone after crystallization), bulk sediment diffusion coefficient (higher values indicate increasing methane flux into the host sediment) and segregation potential (a concept established in ice lensing research -- higher values promote thicker, more concentrated hydrate deposits).

Experiments involving genesis of methane hydrates under simulated subseabed conditions are in progress to verify the effect of these properties and to quantify their relative and absolute importance.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995