GAS HYDRATE OCCURRENCES

The presence of gas hydrate at Sites 994 and 997 was documented by direct sampling; however no gas hydrate was conclusively identified at Site 995 (Shipboard Scientific Party, 1996b). Although a BSR does not occur in the seismic reflection profiles that cross Site 994, several pieces of gas hydrate were recovered from 259.90 mbsf in Hole 994C and disseminated gas hydrate was observed at almost the same depth in Hole 994D. One large, solid piece (about 15 cm long) of gas hydrate was also recovered from about 331 mbsf at Site 997 (Hole 997A). Despite these limited occurrences of gas hydrate, it was inferred, based on geochemical core analyses and downhole logging data, that disseminated gas hydrate occur in logging Unit 2 (which extends from a depth of about 190 to 450 mbsf) of all the holes drilled on the Blake Ridge (Table 2). The presence of gas hydrate in logging Unit 2 at Sites 994, 995, and 997 was inferred on the basis of the following observations (Shipboard Scientific Party, 1996a, 1996b, 1996c). (1) Cores from all three sites were observed to evolve large amounts of gas, which is indicative of gas hydrate-bearing cores. It is also speculated that gas evolution from decomposing gas hydrate may have been a factor that contributed to the low core recovery at all the Blake Ridge drill sites. (2) Pressure-coring (PCS) data indicate that the sediments on the Blake Ridge between about 200 and 450 mbsf contain methane concentrations that exceed expected methane pore-water saturations. The only known source for this methane is the decomposition of gas hydrate; thus, it was concluded that gas hydrate must occur within this interval of over-saturated gas. (3) Both the general trend of the interstitial-water chloride concentrations and the inter-sample variation in chloride concentrations (chloride anomalies) between 190 and 450 mbsf suggest the presence of gas hydrate throughout logging Unit 2. Gas-hydrate decomposition during core recovery releases water and methane into the interstitial pores, resulting in a freshening of the pore waters. (4) Temperatures of cores recovered on the Blake Ridge transect were quite variable within logging Unit 2. Low-temperature anomalies are interpreted as indicating areas where gas hydrate decomposition has occurred during core recovery. (5) Data from downhole logs also were interpreted as indicating the presence of gas hydrate in logging Unit 2. The downhole log evidence for gas hydrate is discussed in more detail later in this report.

The depths to the top and the base of the zone of gas-hydrate occurrence at Sites 994, 995, and 997 were determined using interstitial-water chloride anomalies (Shipboard Scientific Party, 1996a, 1996b, 1996c) and downhole log data (Table 2). Interstitial-water chloride anomalies established whether gas hydrate occurred within a given core sample. The observed chloride anomalies also allow the amount of gas hydrate to be established by calculating the amount of interstitial-water freshening that can be attributed to gas hydrate disassociation. The estimated volume percentage of gas hydrate in the recovered cores had a skewed distribution, ranging from a maximum of about 7.0 and 8.4 vol% at Sites 994 and 995 to a maximum of about 13.6 vol% at Site 997 (Fig. 5). However, these are minimum estimates because the baseline (undisturbed interstitial-water chlorinities) used to calculate these values may be lower than the actual in situ interstitial-water salinities. For a more complete discussion on the chlorinity-calculated gas hydrate contents, see Paull, Matsumoto, Wallace, et al. (1996).

As previously discussed, natural gas hydrate occurrences are generally characterized by an increase in log-measured acoustic velocities and electrical resistivities. The comparison of logging Units 1, 2, and 3 in all three holes on the Blake Ridge (Holes 994D, 995B, and 997B), reveal that logging Unit 2 is characterized by a distinct stepwise increase in both electrical resistivity (increase of about 0.1-0.3 m) and acoustic velocity (increase of about 0.1-0.2 km/s) (Fig. 4A-C). In addition, the deep reading resistivity device (RILD) reveals several anomalous high resistivity zones within the upper 100 m of Unit 2 at all three sites on the Blake Ridge (anomalous resistivities ranging from 1.4 to 1.5 m). At Site 994, gas hydrate was recovered (depth 259.90 mbsf) from the same interval that exhibits anomalous high resistivities in the upper part of Unit 2. Further comparisons indicate that the anomalous high resistivity zones do not correlate to any apparent acoustic velocity anomalies at Sites 994 or 995. However, at Site 997 the anomalous high resistivity zones in the upper part of Unit 2 are characterized by an acoustic velocity increase of about 0.3 km/s. The zone from which gas hydrate was recovered at Site 997 (depth of about 331 mbsf in Hole 997A) is also characterized by anomalous high resistivities and acoustic velocities. At Site 994, below the anomalous high resistivity zones at 216 and 264 mbsf, the resistivity log values are almost constant throughout logging Unit 2, whereas the acoustic velocities increase with depth over the same interval. However, both electrical resistivities and acoustic velocities in Unit 2 increase with depth at Sites 995 and 997. Examination of the natural gamma-ray and bulk-density logs from all three sites (Fig. 4A-C) reveals no apparent lithologic causes for the observed velocity and resistivity increases in Unit 2. The above observations are consistent with a material of increased resistivity and acoustic velocity but similar density, partially replacing some of the pore water in Unit 2. The depth of the boundary between logging Units 2 and 3 on the Blake Ridge is in rough accord with the predicted base of the methane hydrate stability zone and it is near the lowest depth of the observed interstitial-water chlorinity anomaly (Fig. 5). It has been concluded that logging Unit 2 at Sites 994, 995, and 997 contains some amount of gas hydrate.