GEOPHYSICS, VOL. 67. NO. 2 (MARCH-APRIL 2002): P. 582-S93. 12 FIGS. 10. I I L)0/1.14686 IY Estimation of gas hydrate and free gas saturation, concentration, and distribution from seismic data Shaoming Lu* and George A. McMechan* ABSTRACT Gas hydrates contain a major untapped source of en- ergy 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 -3-8% of the pore space (-24% 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 simi- lar to those estimated from vertical seismic profile data and generally agree with those from independent, in- direct 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 saturation, concentration, and distribution of gas hydrate and free gas away from control wells. INTRODUCTION Gas hydrates are icelike solids composed of natural gases (mainly methane) and water that occur under appropriate con- ditions of pressure and temperature (usually high pressure and low temperature). Because of their large volumes trapped in shallow sediments, gas hydrates are a potential source of en- ergy, submarine geologic hazards, and a factor in global climate change (Kvenvolden, 1998). However, the estimated amounts of in-situ gas hydrates worldwide are highly speculative and range widely from 3.1 x loL5 m3 to 7.6 x 10l8m3 (Kvenvolden, 1988).Although the figure has converged to about 2.0 x 10l6 m3 in recent years (Makogon, 1997; Kvenvolden, 1998), accurate estimates are difficult because knowledge of the distribution and saturation of gas hydrates in sediments is very incomplete. Most occurrences of gas hydrates are in deep oceanic regions and are generally recognized on the basis of bottom simulating reflectors (BSRs) on seismic sections. BSRs are strong reflec- tions that are approximately parallel to the sea-floor reflection and are believed to mark the base of gas hydrate stability zones. Beneath this stability zone, free gas may be present, which pro- duces a large impedance contrast. The amount of free gas below the BSRs can be large (Dickens et al., 1997) and therefore po- tentially economically important. We show that a reasonable estimate of the distribution and saturation of gas hydrate above the BSRs and free gas below the BSRs from seismic sections is technically feasible. Various theoretical, semiempirical, and ad hoc models have been proposed to relate gas hydrate saturation to seismic veloc- ity. Wood et al. (1994), Yuan et al. (1996), and Korenaga et al. (1997) use Wyllie et al.’s (1958) time average equation to esti- mate gas hydrate saturation after the velocity structure is ob- tained from seismic data. Lee et al. (1993,1996) use a weighted equation, and Ecker et al. (2000) use a theoretical rock model (Ecker et al., 1998) to estimate gas hydrate saturation. How gas hydrate modifies the acoustic/elastic properties of hydrated sediments is poorly known (Andreassen et al., 1995; Makogon, 1997), and the predictions obtained using different models vary Manuscript received by the Editor August 19,2000; revised manuscript received May 29,2001. *University of Texas at Dallas, Center for Lithospheric Studies, PO. Box 830688 (FA 31), Richardson, Texas 75083-0688. E-mail:mcmec@utdallas.edu. @ 2002 Society of Exploration Geophysicists. All rights reserved. 582