| Co-Chiefs: Ryo Matsumoto and Charles Paull | Staff Scientist: Paul Wallace |
| Cruise Dates: 1 November-19 December 1995 | Operations Superintendent: Eugene Pollard |
ODP Leg 164 was devoted to investigating the in
situ characteristics and amounts of natural gas hydrates stored in marine
sediments. The program involved drilling three sites to 750 m depths on the Blake
Ridge off the southeastern coast of the United States. The holes drilled at each of
these sites extended down through the zone where gas hydrates are stable and into
the sedimentary section below. Short holes (50 m) were drilled at four sites on
the crests of two diapirs on the Carolina Rise, where gas hydrate-bearing
sedimentary sections have been disturbed by the intrusion of diapirs.
An unprecedented level of success was achieved using the pressure core sampler (PCS), a device that returns a short core to the surface at formation pressures so that gases are not lost. Gas volumes captured by the PCS indicate gas concentrations grossly in excess of gas saturation, demonstrating that free gas exists beneath the BSR and intermittently throughout the sedimentary section below.
Vertical seismic profiles were used to determine the precise depth of the BSR. The results indicate normal sediment velocities above the BSR. However, velocities as low as 1400 m/s were measured beneath the BSR at Site 997.
More than nine in situ temperature measurements at each site were made to establish temperature gradients. Extrapolation of these thermal gradients to depth makes it possible to estimate the maximum depth at which gas hydrate is stable, based on experimentally determined phase boundaries. The results indicate that the base of gas hydrate stability is ~30 m (Site 997) to ~100 m (Site 994) below the observed BSR depth.
Objective: To assess the amount of gas trapped in hydrate-bearing sediments.
Conclusion: Preliminary analyses from Leg 164 drill sites on the Blake Ridge indicate that gas hydrate occupies 1% to 2% of the sediment volume in a zone that is 200-250 m thick. In fact, the site without a BSR also contained ~1% gas hydrate through a similarly thick zone. If the rest of the ~26,000-km2 region around the Blake Ridge where BSRs are present contains as much gas hydrate, rough estimates indicate that about 10 GT of methane carbon is stored in this region. Given the number of localities worldwide in which gas hydrate occurs, the results of ODP Leg 164 provide further evidence that methane stored as gas hydrate in marine sediments represents a significant component of the global fossil fuel carbon reservoir.
Objective: To understand lateral variability in gas hydrate abundances.
Conclusion: Interstitial water chemistry is very similar at the three deeply drilled (~750 mbsf) sites on the transect along the Blake Ridge (Sites 994, 995, and 997). Downhole profiles of all dissolved species show the same general trends with the same approximate depths for maxima and minima. Chloride profiles were used as a proxy indicator for the amount of in situ gas hydrate. When gas hydrate forms in sediment, water and gas are removed from the pore space, causing the residual pore waters to become saltier. Over long periods of time, the local salinity anomalies produced by gas hydrate formation diffuse away. However, many of the cores that were recovered contained surprisingly fresh interstitial waters, indicating that gas hydrate had decomposed within these sediments during drilling and core recovery. A zone with anomalously low chloride values occurs at approximately the same depths (~200 to 440 mbsf) over the 10-km transect. Furthermore, the two depth intervals that contain the largest chloride excursions (at approximately 250 and 400 mbsf) are also depth correlative. Thus, the similarity of the chloride profiles at the ridge sites strongly suggests that gas hydrate is correlative over the expanse of the Blake Ridge, including Site 994, where no BSR is present in the seismic profile.
Objective: To understand the relationship between BSRs and gas hydrate development.
Conclusion: Downhole sonic log and vertical seismic profile data indicate decreasing interval velocities near the depth of the BSR. The change in sediment velocity may be related either to changes in the amount of hydrate above the BSR or to the presence of gas bubbles below. Both types of sonic data indicate that gas bubbles are present at greater depths, but simple shipboard analyses of the absolute velocities do not require that the gas be present immediately below the BSR at all these sites. This issue can be resolved by combining pressure core sampling data, temperatures of cores measured after recovery, and interstitial-water chloride concentrations. Whereas the pressure core sampler data indicate that either hydrate or gas bubbles must be present, the temperature and interstitial-water chloride data demonstrate that gas hydrate is not present below the BSR. Thus, gas bubbles are present beneath the BSR at these sites.
Objective: To investigate the distribution and in situ fabric of gas hydrate within sediments.
Conclusion: As anticipated, the recovery of gas hydrate proved to be difficult. The cores were very gassy, causing sediment to extrude from the liners as the cores arrived on deck. Some large pieces of gas hydrate were recovered. Gas hydrate recovered at Site 996 occurred as massive pieces, as veins filling vertical fractures, and as rod-shaped nodules. Fine-grained hydrate was not directly observed in sediments from any of the sites, but proxy measurements, such as the chloride concentration of interstitial pore waters, indicated that fine-grained hydrates had been present prior to core recovery. Calculations indicate that sediments in the zone from 200 to 450 mbsf had a minimum average gas hydrate content of 1% and that some individual samples contained more than 8% gas hydrate.
Objective: To establish changes in physical properties (porosity, permeability, P-wave velocity, thermal conductivity, etc.) associated with gas hydrate formation and decomposition in continental margin sediments.
Conclusion: Well-log measurements show that the gas hydrate-bearing and free gas-bearing zones are associated with distinct characteristics, whereas shipboard lithologic and physical properties measurements did not indicate any differences in these sediments. The distinctions between shipboard and downhole measurements, thought to result from the presence of gas hydrate in situ, will be the subject of shore-based research.
Objective: To determine whether gas contained in gas hydrate is produced locally or has migrated from elsewhere.
Conclusion: The interstitial-water chemical data at Site 996 suggest that methane is transported upward along faults from underlying gas hydrate-bearing sediments. This is consistent with the occurrence of hydrate as vein fillings. At Sites 994, 995, and 997, constraints on the sources of gas contained in gas hydrate must await shore-based isotopic and organic geochemical studies, as well as detailed analysis of the PCS data that will delineate the distribution of hydrate, free gas, and methane dissolved in interstitial waters.
Objective: To investigate the role of gas hydrate in the formation of authigenic carbonates.
Conclusion: Fine-grained, disseminated authigenic carbonate was present in sediments at all sites. Indurated diagenetic carbonate was recovered primarily at Site 996 but also from one horizon at Site 993. At Site 996, interstitial-water chemical data suggest that carbonate precipitation is caused by intense microbial oxidation of methane, which results in high alkalinity and high bicarbonate concentrations. The methane arriving at the seafloor at this site is largely biogenic and has a composition similar to that of the gases from Sites 994, 995, and 997. Fluids and gases venting at Site 996 have probably been transported upward from the underlying gas hydrate-bearing sediment section that slopes up around the diapir.
Objective: To determine the chemical and isotopic compositions, hydration number, and crystal structure of natural gas hydrate.
Conclusion: Gas hydrate decomposition experiments indicate that volumetric ratios of gas:water range from 130 to 160 and show that gas in the hydrate is ~99% methane. Numerous hydrates sampled for storage in pressure vessels will be used for shore-based isotopic and crystallographic studies.
Objective: To determine the role of gas hydrate in stimulating or modifying fluid circulation.
Conclusion: An intriguing association of pore waters was documented. Pore waters are systematically fresher than seawater within the gas hydrate-bearing sections. One of the potential explanations of such a pattern is that salinity changes associated with gas hydrate decomposition may be stimulating fluid circulation in these sedimentary sections. Other anomalies await shore-based isotopic and modeling studies, including variations in the amounts of free gas and gas hydrate and the sedimentary section, the occurrence of both thermogenic and biogenic hydrocarbon gases, and unexplained patterns for major and minor elements in dissolved pore waters.
Objective: To investigate the potential connection between large-scale sediment failures and gas hydrate decomposition.
Conclusion: Drilling along the top of the Cape Fear Diapir, which breeches the seafloor within the scar of the giant Cape Fear Slide, revealed soft-sediment deformation features and several biostratigraphic gaps. Both are the result of mass-transport processes associated with sediment failure and were caused by the Cape Fear Slide or emplacement of the diapir. The results of drilling did not provide any direct evidence as to whether large-scale sediment failures are related to gas hydrate decomposition.
Objective: To establish the origin of the Carolina Rise Diapirs and their influence on associated sedimentary gas hydrate.
Conclusion: Chloride concentrations in interstitial waters from sediments overlying the Cape Fear Diapir (Sites 991, 992, 993) and the Blake Ridge Diapir (Site 996) are high; the ratio of chloride to other ions suggests nearby sources of evaporitic salts. Thus, it seems likely that both diapirs are salt cored. Presumably, the salt has risen from deep within sediments of the Carolina Trough, the base of which is approximately 8 km below the seafloor at these sites.