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The Leading Edge; February 2001; v. 20; no. 2; p. 200-203; DOI: 10.1190/1.1438914
© 2001 Society of Exploration Geophysicists
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Estimation of gas-hydrate concentration and free-gas saturation from log and seismic data

Umberta Tinivella and Jose M. Carcione

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy

Corresponding author: utinivella@ogs.trieste.it

The first 20% of the full text of this article appears below.

When no direct measurements are available, detailed knowledge of the compressional- and/or shear-wave velocity fields is essential for quantitative estimation of gas hydrate and free gas in bottom-simulating reflectors (BSR). Discrepancies between experimental velocity profiles and the velocity for water-filled sediments reveal the presence of gas hydrate (positive anomalies) and free gas (negative anomalies).

We use the model developed by Carcione and Tinivella (2000), based on a three-phase Biot-type theory, to obtain wave velocities of sediments saturated with water and gas hydrate, and the theory of Santos et al. (1990) to calculate the wave velocities of a porous medium saturated with a mixture of water and free gas. This model assumes that the free gas is uniformly distributed in the pore space. We use a second approach to model patchy saturation of free gas (Dvorkin et al., 1999).


   ODP Leg 164, site 995
 
This site is in the Blake Ridge area offshore South Carolina (Figure 1). Our analysis is focused on borehole 995 because data acquired at sites 994 and 997 were severely affected by enlarging hole conditions. The velocity profile is derived from in situ measurements obtained by VSP.


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  		Figure 1. Location of ODP Leg 164, site 995.

 
To obtain the theoretical velocity, we used porosity and density measured in a laboratory. The shear modulus versus depth was obtained by a linear fit of sparse S-wave log data. Other parameters, not available from the CDP Leg 164 data set, are obtained from Hamilton's data set for marine sediments (Hamilton, 1979).

Figure 2 compares the theoretical P-wave velocity of water-saturated sediments (broken line) and the VSP wave velocity (solid line). The curves indicate that it is possible to estimate the presence of gas hydrate and free gas from the velocity anomaly. As observed by Guerin et al. (1999), compressional- and shear-wave velocity gradients . . . [Full Text of this Article]






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