Estimating the amount of hydrate and free gas from surface seismic

Abstract

Marine seismic data and well log measurements at the Blake Ridge offshore South Carolina show that prominent seismic Bottom Simulating Reflectors (BSRs) are caused by sediment layers with gas hydrate overlying sediments with free gas. We apply a theoretical rock physics model to 2D Blake Ridge marine seismic data to determine gas hydrate and free gas saturation. High-porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity; effective pressure; mineralogy; elastic properties of the pore-filling material; and water, gas and gas hydrate saturation of the pore space. To apply this model to seismic data, we first obtain interval velocity using stacking velocity analysis. Next, all input parameters to the rock physics model, except porosity and water, gas and gas hydrate saturation, are estimated from geological information. To estimate porosity and saturation from interval velocity, we first assume that the entire sediment does not contain gas hydrate or free gas. Then we use the rock physics model to directly calculate porosity from the interval velocity. Such porosity profiles appear to have anomalies where gas hydrate and free gas are present (as compared to typical profiles expected and obtained in sediment without gas hydrate or gas). Porosity is underestimated in the hydrate region and is overestimated in the free-gas region. We calculate the porosity residuals by subtracting a typical (without gas hydrate and gas) porosity profile from that with anomalies. Next we use the rock physics model to eliminate these anomalies by introducing gas hydrate or gas saturation. As a result, we obtain the desired 2D saturation map. The maximum gas hydrate saturation thus obtained is between 15% and 20% of the pore space (depending on the version of the model used). These saturation values are consistent with those measured in the Blake Ridge wells (away from the seismic line) which are about 12%. Free gas saturation varies between 1% and 2%. The saturation estimates are extremely sensitive to the input velocity values. Therefore, accurate velocity determination is crucial for correct reservoir characterization.