Recent estimates of the volume of gas that may be contained in the gas hydrate and free gas beneath the gas hydrate on the Blake Ridge range from about 70 trillion m3 of gas over an area of 26,000 km2 (Dickens et al., 1997) to about 80 trillion m3 of gas for an area of 100,000 km2 (Holbrook et al., 1996). The difference between these two estimates has been attributed to the observation that the amount of free gas directly measured within pressure-core samples (Dickens et al., 1997) from beneath the gas hydrate is significantly larger than that estimated from borehole vertical seismic profile data (Holbrook et al., 1996). Other published studies indicate that the gas hydrate at the crest of the Blake Ridge alone (area of about 3,000 km) may contain more than 18 trillion m3 of gas (Dillon et al., 1993). The broad range of these estimates demonstrates the need for high-resolution measurements of the gas-hydrate and associated free-gas volumes on the Blake Ridge. The log-interpreted gas-hydrate and free-gas saturations from Sites 994, 995, and 997 in this report provide several of the critical parameters needed to accurately calculate the volume of gas on the Blake Ridge. The volume of gas that may be contained in a gas hydrate accumulation depends on five "reservoir" parameters (modified from Collett, 1993): (1) areal extent of the gas hydrate occurrence, (2) "reservoir" thickness, (3) sediment porosity, (4) degree of gas-hydrate saturation, and (5) the hydrate gas yield volumetric parameter that defines how much free gas (at standard temperature and pressure [STP]) is stored within a gas hydrate (also known as the hydrate number). In the following section, the five "reservoir" parameters (Table 4) needed to calculate the volume of gas associated with the gas hydrate on the Blake Ridge area are assessed. In addition, the volume of free gas trapped beneath the gas hydrate on the Blake Ridge is also assessed (Table 5).
The region in which seismic reflection profiling indicates the occurrence of gas hydrate on the Blake Ridge (shaded area on the map in Fig. 1) extends over an area of ~26,000 km2 (Dillon and Paull, 1983; Dickens et al., 1997). Despite the fact that the reflection seismic characteristics of the bottom-simulating reflector (BSR) and inferred gas hydrate occurrences are similar throughout the Blake Ridge, it may be inappropriate to extrapolate gas hydrate and other geologic data from Sites 994, 995, and 997 to the entire 26,000 km2 gas hydrate accumulation. Most certainly there are local variations in the distribution of gas hydrate on the Blake Ridge that need to be further evaluated; however, this work is beyond the scope of this study. The following volumetric assessment of the amount of gas hydrate, therefore, has been conducted on a site-by-site basis; that is, for each site drilled on the Blake Ridge (Sites 994, 995, and 997) we have individually calculated the volume of gas hydrate and associated free gas within a 1 km2 area surrounding each drill site (Table 4, Table 5). For this "resource" assessment, we have defined the thickness of the gas hydrate-bearing sedimentary section at all three drill sites to be the total thickness of logging Unit 2, which ranges from about 217 to 265 m thick (Table 4). The core-derived sediment porosities in logging Unit 2 range from about 50% to 80%, and average ~58% (Table 4). Gas-hydrate saturations in logging Unit 2 at all three drill sites, calculated from the standard Archie relation (Fig. 9A-C), range from an average value of about 3% to 6% (Table 4). The hydrate gas yield parameter or hydrate number is a factor that describes how much of the clathrate-cage structure is filled with gas. In this assessment, we have assumed a hydrate number of 6.325 (90% gas filled clathrate), which corresponds to a gas yield of 164 m3 of methane (at STP) for every cubic meter of gas hydrate (Collett, 1993). Our calculations indicate that the potential volume of gas within the log-inferred gas hydrate at each drill site (Sites 994, 995, and 997) on the Blake Ridge ranges from about 670,000,000 to 1,450,000,000 m3 gas per km2 (Table 4).
For comparison purposes only, if we assume the geologic conditions and gas hydrate distribution at Site 997 (Table 4) is representative of the entire seismic delineated gas hydrate accumulation (26,000 km2) on the Blake Ridge, it can be assumed that there is about 37.7 trillion m3 of gas within the Blake Ridge gas hydrate accumulation. One of the reasons that this estimate differs from that of Dickens et al. (1997) and Holbrook et al. (1996) is because both of these previous studies have included the volume of free gas trapped beneath the gas hydrate in their total gas "resource" estimate. Therefore, we have also used the available log data from Sites 994, 995, and 997 to calculate the volume of free gas (Table 5) below the gas hydrate on the Blake Ridge. The available acoustic and resistivity downhole log data have been used to identify free gas-bearing zones within logging Unit 3 of Holes 994D, 995B, and 997B (Table 5). At Site 994, one 64-m-thick free-gas zone has been identified, while at Sites 995 and 997, two laterally continuous free gas-bearing zones have been delineated with combined total thicknesses of 72 and 194 m, respectively (Table 5). The average porosity of the sediments within the free gas-bearing zones at all three drill sites on the Blake Ridge range from about 53% to 55% (Table 5). The free gas-bearing zones at Sites 994 and 995 are characterized by free-gas saturations, calculated from the standard Archie relation of about 1% (Fig. 9A-C; Table 5). However, the free-gas saturations within the zone immediately below the base of the gas hydrate at Site 997 average about 5% (Table 5). Our calculations indicate that the potential volume of gas within the log inferred free gas-bearing zones at each drill-site (Sites 994, 995, and 997) on the Blake Ridge ranges from about 743,000,000 m3 of gas (at STP) per km2 at Site 997 to about 55,000,000 m3 of gas (at STP) per km2 at Site 995 (Table 5). If we follow Dickens et al.'s (1997) suggestion and assume that the volume of free gas below the gas hydrate at Site 997 is representative of the free gas volumes trapped below the gas hydrate throughout the Blake Ridge area (26,000 km2), then the total volume of gas occurring as free gas on the Blake Ridge is about 19.3 trillion m3 (at STP). Thus, the combined volume of gas within the gas hydrate and underlying free-gas accumulations on the Blake Ridge would be about 57 trillion m3 of gas, which is near the volume (70 trillion m3 of gas) estimated by Dickens et al. (1997). However, it is unlikely that the Site 997 downhole log or pressure core-calculated gas-hydrate concentrations are representative for the entire Blake Ridge. We know, for example, that the volume of free gas below the gas hydrate at Site 995 is much less than the volume of free gas at Site 997 (Table 5). More work is needed to assess the regional distribution and variability of both the gas hydrate and associated free-gas accumulations on the Blake Ridge before accurate resource estimates can be made.