ON THE RESOURCE EVALUATION OF MARINE GAS-HYDRATE DEPOSITS USING SEA-FLOOR COMPLIANCE METHODS

Methane hydrates are solid, non-stochiometric mixtures of water and th e gas methane. They occur worldwide in sediment beneath the seafloor a nd estimates of the total mass available there exceed 10(16) kg. Since each volume of hydrate can yield up to 164 volumes of gas, off-shore methane hydrate is recognized as a very important natural energy resou rce. The depth extent and stability of the hydrate zone are governed b y the phase diagram for mixtures of methane and hydrate, determined by ambient pressures and temperatures. In sea depths greater than about 300 m, the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor. The fraction of hydrate in the sedim ent usually increases with depth. The base of the hydrate zone is a ph ase boundary between solid hydrate and free gas and water. Its depth i s determined principally by the value of the geothermal gradient, and stands out on seismic sections as a bright reflection. The diffuse upp er boundary is not as well marked so that the total mass of hydrate ca nnot be determined by seismic measurements alone. Ocean surface gravit y waves induce a low-frequency, horizontally propagating pressure held which deforms the seafloor. The displacement of the seafloor depends on the oceanic crustal density and elastic parameters, particularly th e shear properties. Seafloor compliance is the transfer function betwe en seafloor deformation and pressure as a function of frequency. Compl iance measurements made at specific frequencies are tuned to structure at specific depths. Methane hydrate, like ice in permafrost, changes the physical properties of the material in which it is found, decreasi ng the density while increasing the compressional and especially the s hear velocities. We apply the method of Crawford, Webb & Hildebrand (1 991) and show how the addition of compliance data, which is particular ly sensitive to changes in shear velocity, can aid in the evaluation o f the resource. Two exploration scenarios are investigated through num erical modelling. In the first, a very simple example illustrates some of the fundamental characteristics of the compliance response. Most o f the properties of the section including the probable regional thickn ess of the hydrate zone, 200 m, are assumed known from seismic surveys and spot drilling. The amount of hydrate in the available pore space is the only free parameter. Hydrate content expressed as a percentage may be determined to about +/-2 epsilon given compliance measurements with epsilon per cent error. The rule holds over the complete range of anticipated hydrate-content values. In the second, less information i s assumed available a priori. The complementary compliance survey is r equired to find both the thickness and the hydrate content in hydrate zones about 200 m thick beneath the seafloor, which contain up to 20 a nd 40 per cent hydrate in the available pore space, respectively. A li near eigenfunction analysis reveals that for these two models the tota l mass of hydrate, the product of hydrate content and thickness, may b e estimated to an accuracy of about 2.81 epsilon and 1.83 epsilon per cent, respectively, given compliance measurements with an accuracy of epsilon per cent.