(1) Lamont-Doherty Earth Observatory, Palisades, NY
ABSTRACT: Hydrate Concentration Estimates from Sonic Logs: Quantifying Amplitude Losses in Frozen Porous Media
Hydrate concentration estimates from sonic logs: quantifying amplitude losses in frozen porous media
Sonic logs show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed in the Mallik 2L-38 research well, MacKenzie Delta, Canada. This well penetrated permafrost to 1150 m depth where hydrate occupies up to 80% of the pore space. A suite of high quality well logs allows us to extend published methods and quantify the amount of hydrate present in the host sediment by measuring sonic amplitude loss. We use the technique developed by Frazer et al. to calculate Qp-1 and Qs-1 and compare these results with conventional logs. Resistivity, Vp and Vs logs increase with higher hydrate concentration in the Mallik well, similar to observations in other hydrate deposits such as those on the Blake Ridge. The logs in both locations indicate stiffening of the sediment framework and electrical insulation of the pore space due to the presence of hydrate. Using estimates of hydrate concentration in the Mallik well derived from the resistivity log, we observe a linear increase in both Qp-1 and Qs-1 with higher hydrate concentration, which is not intuitive for a stiffer sediment framework. Numerical modeling allows us to simulate the recorded waveforms and confirm the results. Leclaire et al. predict a loss of amplitude in frozen porous media and their model compares well with our observations. We suggest that this model can be used effectively to quantify hydrate concentration in similar environments and provide new insight into how hydrate and its sediment host interact.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado