Azimuthal Variability in Gas Hydrate Concentration using LWD Resistivity and Density Images

We estimate the concentration of gas hydrate in several boreholes drilled in the vicinity of Hydrate Ridge during ODP Leg 204. Logging-while-drilling (LWD) tools were used, which measure formation resistivity and density at azimuthal positions around each borehole as a function of depth. We utilize the density data to compute porosity and the resistivity data to compute water (Sw) and hydrate (1-Sw) saturation at each azimuthal position in these holes. The results allows for delineation of the shape, geometrical distribution, and azimuthal orientation of the porous sediment structures that are saturated with gas hydrate as a function of depth. This 360-degree approach contrasts with conventional methods that use wireline logs or core data and produce a single saturation value at each measurement depth. In addition, LWD data are acquired only minutes after the formation is drilled, limiting the extent of hydrate dissociation on the measured in situ properties. From these results on Hydrate Ridge, the clay-bearing sediments within the gas hydrate stability zone (GHSZ) appear to contain patchy zones with high concentrations of hydrate that are locally distributed around the borehole. Patchy zones occur throughout the GHSZ and at all of the Sites (1244-1251), where LWD data were acquired. LWD neutron and density data indicate that there is no significant free gas occurrence within the GHSZ, confirming that the images represent gas hydrate features, not free gas, in these sediments. Within the limitations of the method, which include uncertainties in the Archie parameters, the accuracy of porosity measurements, and the effects of ship heave, among others, our estimates of azimuthal hydrate concentration compare closely, when averaged around the borehole, to estimates from other conventional log and core measurements. However, the computation of this information from LWD images adds significantly to our understanding of the spatial distribution of hydrate in these formations.