ON THE RESOURCE EVALUATION OF MARINE GAS HYDRATE DEPOSITS USING SEA-FLOOR TRANSIENT ELECTRIC DIPOLE-DIPOLE METHODS

Methane hydrates are solid, nonstoichiometric mixtures of water and th e gas methane. They occur worldwide in sediment beneath the sea floor, and estimates of the total mass available there exceed 10(16) Kg. Sin ce each volume of hydrate can yield up to 164 volumes of gas, offshore methane hydrate is recognized as a very important natural energy reso urce. The depth extent and stability of the hydrate zone is governed b y the phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures. In sea depths greater than abo ut 300 m, the pressure is high enough and the temperature low enough f or hydrate to occur at the seafloor. The fraction of hydrate in the se diment usually increases with increasing depth. The base of the hydrat e zone is a phase boundary between solid hydrate and free gas and wate r. Its depth is determined principally by the value of the qeothermal gradient. It stands out on seismic sections as a bright reflection. Th e diffuse upper boundary is not as well marked so that the total mass of hydrate is not determined easily by seismic alone. The addition of electrical data, collected with a seafloor transient electric dipole-d ipole system, can aid in the evaluation of the resource. Methane hydra te, like ice, is electrically insulating. Deposits of hydrate in porou s sediment cause an increase in the formation resistivity. The data co nsist of measurements of the time taken for an electrical disturbance to diffuse from the transmitting dipole to the receiving dipole. The t raveltime is related simply to the resistivity: the higher the resisti vity, the shorter the traveltime. A sounding curve may be obtained by measuring traveltimes as a function of the separation between the dipo les and interpreted in terms of the variation of porosity with depth. Two exploration scenarios are investigated through numerical modeling. In the first, a very simple example illustrating some of the fundamen tal characteristics of the electrical response, most of the properties of the section including the probable, regional thickness of the hydr ate zone (200 m) are assumed known from seismic and spot drilling. The amount of hydrate in the available pore space is the only free parame ter. Hydrate content expressed as a percentage may be determined to ab out +/-epsilon given a measurement of traveltime at just one separatio n (800 m) to epsilon%. The rule holds over the complete range of antic ipated hydrate content values. In the second example, less information is assumed available a priori and the complementary electrical survey is required to find both the thickness and the hydrate content in a h ydrate zone about 200 m thick beneath the sea floor containing 20 and 40% hydrate in the available pore space, respectively. A linear eigenf unction analysis reveals that for these two models, the total mass of hydrate, the product of hydrate content and thickness, may be estimate d to an accuracy of about 3 epsilon% given measurements of traveltime to an accuracy of epsilon% over a range of separations from 100 to 130 0 m. The value of the electrical information depends directly on the a ccuracy to which transient arrivals can be measured on the sea floor i n water depths exceeding 300 m over a separation of the order of a kil ometer, the error parameter epsilon. While results of appropriate surv eys, or even noise measurements, have not been published in the open l iterature, surveys on a smaller 100 m scale have been conducted by my group. Based on these data, I suggest that the value of epsilon may be of the order of 3%.