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doi:10.2204/iodp.proc.311.105.2006

Pressure coring

The main objectives of pressure coring during Expedition 311 were to quantify natural gas composition and concentration in sediments and to determine the nature and distribution of gas hydrate and free gas within the sediment matrix. To achieve these objectives, we

• Measured the quantity and composition of gases released during controlled degassing experiments,
• Conducted nondestructive measurements (X-ray imaging, P-wave velocity, and GRA density) at in situ pressure and during degassing, and
• Preserved gas hydrate–bearing sediments at in situ pressure for more comprehensive shore-based investigations.

The nondestructive measurements not only provide a direct indication of the existence of gas hydrate, but the resulting data (acoustic impedance) can be used to help interpret regional seismic data. Site U1327 was a near-reoccupation (375–600 m) of Leg 146 Site 889. The gas hydrate occurrence at Site 889 was estimated at 20%–30% of pore space in the 100 m thick interval above the BSR (Westbrook, Carson, Musgrave, et al., 1994). The BSR at Site 889 is a strong reflection event that occurs at 225 mbsf, with evidence of free gas beneath (Westbrook, Carson, Musgrave, et al., 1994). LWD data from Hole U1327A show a thick high-resistivity layer at ~125–140 mbsf, which according to Archie's equation suggests that gas hydrate occupies 40%–70% of the pore space (see "Gas hydrate and free gas occurrence"). Specific objectives at Site U1327 were to confirm and quantify the presence of gas hydrate above the BSR, with special attention to the high-resistivity layer, and free gas below the BSR.

Operation of pressure coring systems

Pressure coring tools were deployed fourteen times at Site U1327 (Table T18): eight PCS cores (three in Hole U1327C, three in Hole U1327D, and two in Hole U1327E), four HRC cores (three in Hole U1327D and one in Hole U1327E), and two FPC cores (both in Hole U1327D). Figure F47 shows the pressure history of the cores during deployment, coring, recovery, and chilling in the ice shuck. Based on the temperature and pressure records from the data loggers, all successful pressure cores were stabilized in the gas hydrate stability field (Fig. F48), though some of the PCS cores experienced brief (5–10 min) excursions out of the gas hydrate stability field during the latter portion of core recovery.

The PCS recovered five cores under pressure (Table T18): one above all target zones (Core 311-U1327E-3P; 80 mbsf), two near the depth of the high-resistivity layer seen in Hole U1327A data (Cores 311-U1237C-15P and 311-U1237D-10P; 121.8 and 155.1 mbsf, respectively), one between the high-resistivity layer and the BSR (Core 311-U1237C-24P; 197.3 mbsf), and one below the BSR (Core 311-U1237D-17P; 246 mbsf). The recovered pressures as measured by the internal data loggers were approximately half of in situ pressures.

The HRC recovered three cores under pressure (Table T18): one at the depth of the high-resistivity layer (Core 311-U1327D-4E; 125.3 mbsf) and two between this layer and the BSR (Cores 311-U1327D-12E and 14E; 170.5 and 217.7 mbsf, respectively). These cores recovered ~80% of in situ pressure. A broken catcher ring in the "technical" portion of the HRC prevented the transfer of Core 311-U1237D-4E, and it had to be depressurized in the transfer chamber.

The FPC recovered one pressurized core (Core 311-U1327D-13Y). This core became jammed in the transfer system, likely as a result of expansion caused by partial depressurization, and was completely depressurized in the transfer chamber. The other FPC deployment at this site, which recovered Core 311-U1327D-6Y, suffered from a core liner implosion and corer over-retraction.

After all analyses were complete (see "Measurements on HYACINTH cores"), cores were archived as described in "Pressure coring" in the "Methods" chapter. The PCS cores were extruded, and HRC Cores 311-U1237D-12E and 14E were transferred to storage chambers for further shore-based analysis.

Degassing experiments

At Site U1327, the five PCS cores recovered successfully under pressure were investigated by controlled shipboard degassing experiments (Table T19). The deepest PCS core (Core 311-U1237D-17P) was taken at 246.0 mbsf, which is ~25 m deeper than the seismically inferred BSR depth of 223 mbsf. Three of the PCS cores were taken from within the predicted depth interval of the GHSZ (Cores 311-U1237C-15P, 121.8 mbsf; 311-U1237D-10P, 155.1 mbsf; and 311-U1237C-24P; 197.3 mbsf). Core 311-U1327E-3P, the shallowest PCS core at this site, was taken from 80.0 mbsf.

All degassing experiments included the following steps. First, the volume and density of sediment inside the inner core barrel of the PCS was monitored by X-ray analysis. Next, the PCS was slowly degassed in a temperature-controlled laboratory (7°C), and the volume and composition of released gas and water, the pressure inside the core, and the ambient air pressure and temperature were monitored (Table T20) as described in "Pressure coring" in the "Methods" chapter. However, initial pressure readings are not available for Core 311-1327C-15P because the analog pressure gauge used was not suitable for the recovered low pressures. During the degassing procedure, the vertical density distribution of the PCS cores was repeatedly determined by GRA scans to examine the evolution of gas voids within the sediment. After degassing was completed, we X-rayed the PCS cores again, collected the water remaining in the outer core barrel for mass balance considerations, curated the sediment that was extruded from the cores, and subsampled it for IW chemistry, dissolved gases, and physical property analyses.

Degassing of the five PCS cores yielded 1.2 to 10.3 L of gas and showed variable methane concentrations with depth (Table T20). Mass balance calculations yield pore space methane concentrations of 83–674 mM. They indicate <0.3% of gas hydrate in the pore space of shallow Cores 311-U1327E-3P (80.0 mbsf) and 311-U1327C-15P (121.8 mbsf), gas hydrate pore space concentrations of 7.9% and 1.8% for deeper Cores 311-U1327D-10P (155.1 mbsf) and 311-U1328C-24P (197.3 mbsf), and a free gas concentration of 1.0% in the pore space of Core 311-U1327D-17P (246.0 mbsf) from below the BSR (Tables T21, T22; Fig. F49).

For all PCS cores, the composition of the released gas did not change significantly in the course of degassing (Table T20). Methane was the major component, accounting on average for 85% ± 8% of gas emitted from Core 311-U1327C-15P and for 95% ± 3% to 98% ± 2% from all other cores. Nitrogen was the second most abundant gas, contributing 12% ± 5% to the gas released from Core 311-U1327C-15P and <2% to the gas obtained from all other cores. Carbon dioxide, ethane, and higher hydrocarbon concentrations were below the detection limit of the Agilent gas chromatograph used for continuous gas analysis during the degassing experiments. A subset of gas samples from each core was analyzed using methods described in "Organic geochemistry" in the "Methods" chapter and yielded 113–729 ppmv ethane.

In all degassing experiments, the pressure inside the PCS cores dropped below the predicted gas hydrate stability conditions when the port valve of the PCS was first opened and water expanded from the outer core barrel into the manifold system. Therefore, no pressure plateaus or rebounds from dissociation of gas hydrate could be expected and a steady decrease of core pressure versus removed gas volume was observed for all cores (Fig. F50).

X-ray scans of PCS cores before degassing showed no evidence of massive gas hydrate (e.g., veins or nodules). Clasts or rocks were evident in Cores 311-U1327C-24P and 311-U1327D-10P (Fig. F51). Repeated density scans during degassing experiments showed overall lowering of densities caused by gas exsolution and core expansion, with some isolated sediment cracking (Fig. F51). Gas voids preferentially developed near the bottom of the PCS cores because core expansion can only occur out of the bottom of the inner core barrel. The rapid evolution of large cracks near the bottom of Core 311-U1327D-10P indicates exsolution of gas rather than presence of massive gas hydrate. This interpretation is supported by the IW chloride concentration that does not show any freshening as a result of gas hydrate decomposition (Fig. F51). None of the measured IW chloride concentrations in the PCS cores differed significantly from the background chloride trend (Table T4; Fig. F22).

Measurements on HYACINTH cores

Simultaneous and automated GRA density, P-wave velocity, and X-ray measurements were made in the Geotek pressure multisensor core logger system on Cores 311-U1327D-12E and 14E. All measurements took place at 12 MPa (near recovery pressure; 80% of in situ pressure). The velocity and density profiles are shown alongside X-ray images in Figure F52. Unlike the velocity profiles obtained for Core 311-U1329E-9E, there are no distinctive velocity highs in the profiles, yet in both cores the velocities are relatively high compared with what would be expected for unconsolidated sediments. A plot of velocity versus density in Figure F53 illustrates how the Site U1327 high-velocity data cluster above the expected unconsolidated line and near the highest velocities from Core 311-U1329E-9E.

The X-ray images show horizontal layering in both cores, and the X-ray scan of Core 311-U1327D-14E shows subvertical, low-density structures with a wispy nature, which could be gas hydrate veins. However, the elevated velocities are not limited to any horizontal layer or wisp-containing zone, and our provisional interpretation of these cores is that they may contain small amounts of disseminated gas hydrate that has created a stiffer sediment matrix throughout the core, with a commensurate increase in the P-wave velocity. This interpretation is supported by degassing of Core 311-U1329E-9E, which contained two discrete zones of elevated velocities; both zones evolved gas during depressurization (see "Pressure coring" in the "Site U1329" chapter). However, these sediments could also be indurated with carbonate or other minerals, and these cores should be examined in detail to determine the extent of cementation.

The two HYACINTH pressure cores from Site U1327 were transported to the Pacific Geoscience Centre of the Canadian Geological Survey, Sidney, British Columbia, directly after Expedition 311. Core 311-U1327D-12E was subsampled under pressure for microbiological pressure studies; the remainder of the core was curated. Core 311-U1327D-14E was rapidly depressurized, subsampled, and repressurized. The pressure was released on the storage chamber through a small valve and the ball valve was then opened. The core was removed from the chamber and quickly cut into 20 cm subsections, which were placed in Parr vessels and repressurized (Table T23). All vessels were filled from the same manifold, so variations in initial pressure reflect variations in gauge accuracy. Gas hydrate "flakes" were observed in the brittle clay material that was at the top and bottom of the core. These flat angular flakes, typically 3–6 mm wide and 0.5 mm thick by the time they were observed (~15 min after depressurization), were oriented subvertically within the sediments. These gas hydrate flakes may correspond to the wispy subvertical X-ray features observed in this core under pressure (Fig. F52).

Gas hydrate concentration, nature, and distribution

Based on mass balance calculations, sediments at Site U1327 contained gas hydrate at levels of at least 0.2%–1.8% of pore space (Table T22; Fig. F49). Core 311-U1327D-10P contained much more methane than the other PCS cores, corresponding to a gas hydrate saturation of 7.9% of pore space. The presence of free gas beneath the BSR, seen in the VSP velocities (see "Vertical seismic profile"), was confirmed by Core 311-U1327D-17P.

The high-resistivity layer seen in the Hole U1327A LWD data proved to be a moving target. Lateral heterogeneity between holes is responsible for the mismatch of IR data, wireline resistivity logs, and LWD resistivity logs at this site (see Fig. F66). The depth of the pressure cores at this site cannot be used as a simple measure of their location relative to this layer; the lateral correlation of the core position between holes must be taken into account. Examining the pressure core depths with respect to chlorinity (Fig. F22), IR (Fig. F36), and logging data (Fig. F66), Cores 311-U1327C-15P and 311-1327E-3P were taken above the layer of increased gas hydrate concentration; Cores 311-U1327C-24P and 311-U1327D-12E and 14E were taken below this layer but within the GHSZ; and Core 311-U1327D-10P was taken within the layer. Thermal anomalies indicating the presence of gas hydrate were found in the XCB cores both above and below Core 311-U1327D-10P (Fig. F36).

There is evidence for both disseminated (pore filling) and vein (sediment displacing) gas hydrate at this site. The P-wave velocity and gamma ray density profiles of Cores 311-U1327D-12E and 14E suggest a relatively uniform distribution of gas hydrate throughout the sediment at this site (Fig. F52). The X-ray images of the PCS cores and the gamma ray density profiles taken during degassing of the PCS cores (Fig. F51) are both consistent with this interpretation. Observations of subvertical flakes of gas hydrate and wispy, low-density structures in the corresponding X-ray images provide evidence for subvertical gas hydrate veins.

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