Electromagnetic modeling and in situ measurement of gas hydrate in natural marine environments

During Ocean Drilling Program Leg 170, electromagnetic logs were acquired at three sites on the Costa Pica continental margin, Gas hydrate was observed in sediment cores from nearby holes. These data provide the first opportunity to study and quantify hydrate concentration using logging-while-drilling, a technique used to record logs before in situ conditions change significantly, We develop a mixture model that allows for separation of the sediment, pore fluid, and hydrate volume fractions from dielectric permittivity and conductivity measurements. However, the relatively low frequency (2 MHz) of these measurements and the high formation conductivity in this setting do not allow for quantitative estimates of hydrate concentration using the dielectric effect. Pseudoporosity logs can nevertheless be derived from the mixture model and compare well with core measurements. We recommend that future in situ electromagnetic measurements be made at frequencies greater than about 20 MHz in order to increase the influence of the dielectric permittivity on such measurements.