Estimation of Gas Hydrate and Free Gas Saturation, Concentration, and Distribution from Seismic Data
McMechan, George A.
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Gas hydrates contain a major untapped source of energy and are of potential economic importance. The theoretical models to estimate gas hydrate saturation from seismic data predict significantly different acoustic/ elastic properties of sediments containing gas hydrate; we do not know which to use. Thus, we develop a new approach based on empirical relations. The water-filled porosity is calibrated (using well-log data) to acoustic impedance twice: one calibration where gas hydrate is present and the other where free gas is present. The water-filled porosity is used in a combination of Archie equations (with corresponding parameters for either gas hydrate or free gas) to estimate gas hydrate or free gas saturations. The method is applied to single-channel seismic data and well logs from Ocean Drilling Program leg 164 from the Blake Ridge area off the east coast of North America. The gas hydrate above the bottom simulating reflector (BSR) is estimated to occupy Ο3-8% of the pore space (Ο2-6% by volume). Free gas is interpreted to be present in three main layers beneath the BSR, with average gas saturations of 11-14%, 7-11%, and 1-5% of the pore space (6-8%, 4-6%, and 1-3% by volume), respectively. The estimated saturations of gas hydrate are very similar to those estimated from vertical seismic profile data and generally agree with those from independent, indirect estimates obtained from resistivity and chloride measurements. The estimated free gas saturations agree with measurements from a pressure core sampler. These results suggest that locally derived empirical relations between porosity and acoustic impedance can provide cost-effective estimates of the saturations, concentration, and distribution of gas hydrate and free gas away from control wells.