Joint Oceanographic Institutions, United States

During Ocean Drilling Program Leg 204, sonic logs were recorded in six holes across the southern summit of Hydrate Ridge, to measure in situ the sonic velocities of the sediments in an active gas hydrate system. In addition, vertical seismic profiles were acquired in four of these holes to help the integration of the borehole data within the high-resolution three-dimensional seismic survey of the area. Synthetic seismograms generated from the velocity and the density logs confirm the depth and the nature of the main reflectors identified in the seismic survey. In particular, the bottom-simulating reflector (BSR) was reached in five of the wells and is adequately reproduced by the synthetic seismograms. The most prominent horizons under the crest of Hydrate Ridge, Horizons A, B, and B', are also reproduced by the synthetics for the wells where they were present. The lower amplitudes of the synthetic reflections suggest that part of the free gas carried by these horizons, which have been identified as conduits to the Hydrate Ridge reservoir, escaped during drilling operation. We use a model for the cementation of grains by hydrate formation to estimate gas hydrate saturations above the BSR and the Gassmann model with fluid substitution to estimate free gas saturation below the BSR. These elastic models suggest that gas hydrate occupies approximately 10%-20% of the pore space over the BSR in the holes closest to the crest (Holes 1245E, 1247B, and 1250F) and that gas hydrate saturations further on the flank are <10%. These results agree well overall with saturations derived from resistivity logs by Archie's method but indicate a less heterogeneous gas hydrate distribution than the resistivity logs. The Gassmann model identifies up to 2% of free gas in Horizons A, B, and B' but also indicates pervasive presence of free gas below the BSR in the holes closest to the crest, 1245E, 1247B, and 1250F. In all the holes, the presence of gas hydrate or free gas results in the reduction of sonic logging waveform amplitudes, most prominently the high-frequency dipole waveforms.