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doi:10.2204/iodp.proc.311.106.2006

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, gamma ray 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 U1328 is situated at the Bullseye cold vent, where seismic records show a blank zone beneath the seep and a weak BSR (219 mbsf). Massive gas hydrate had previously been found at the surface of this site (Riedel et al., 2002, 2006). The presence of massive forms of gas hydrate was also inferred from LWD data in Hole U1328A, where very high resistivity layers associated with low densities occur from 0 to 46 mbsf and near 95 mbsf (see "Gas hydrate and free gas occurrence"). LWD resistivities did not show evidence of free gas below the BSR, but there was indication of gas below the BSR in the sonic waveform coherence (see "Gas monitoring with real time logging-while-drilling/measurement-while-drilling data"). Specific objectives at this site were to determine the concentration of gas hydrate in the sediment column, determine the concentration of free gas below the BSR, and retrieve samples of massive gas hydrate.

Operation of pressure coring systems

Pressure coring tools were deployed 11 times at Site U1328 (Table T20): two PCS cores in Hole U1328B, with the top core targeting massive gas hydrate; one PCS core in Hole U1328C, which targeted the high-resistivity layer near 95 mbsf; one FPC core in Hole U1328D, which targeted massive gas hydrate near the seafloor; and two deployments each of the HRC and the FPC along with three deployments of the PCS in Hole U1328E, with the top HRC and FPC cores targeting massive gas hydrate, the middle PCS core targeting the ~95 mbsf high-resistivity layer, and the bottom PCS core targeting free gas below the BSR. Figures F49 and F50 show the pressure history of the cores during deployment, coring, recovery, and chilling in the ice shuck. The ice shuck had no ice in it when Core 311-U1328-10P was immersed.

Five of the six PCS runs recovered full cores at some pressure (Table T20). The first two deployments (Cores 311-U1328B-4P and 7P) had particularly unusual pressure profiles (Fig. F49). Despite immersion of the autoclave in the ice shuck, the pressures rose rapidly and peaked sharply at pressures well above in situ pressures (20 and 24 MPa, respectively), before rapidly falling again prior to the tool being removed from the ice. Pressures were still decreasing rapidly when the data loggers were removed. The other PCS deployments recovered cores with only ~30%–80% of in situ pressure (Table T20). After the PCS cores were degassed and X-rayed (see "Degassing experiments"), they were extruded using a hydraulic pump, curated, and samples for IW, porosity and headspace gas analyses were taken.

The first FPC (Core 311-U1328D-3Y) and HRC (Core 311-U1328E-3E) deployments targeted massive gas hydrate. Core 311-U1328D-3Y returned empty and showed no evidence of penetrating the formation. Core 311-U1328E-3E penetrated massive gas hydrate that was not retained as an intact core. The depth of Core 311-U1328E-3E was determined by the previous partial XCB core, which had been drilled until a hard formation was reached. The recovered pressure in Core 311-U1328E-3E (4 MPa) was below in situ pressure (12.8 MPa) because the side valve on the HRC had failed; the core was returned to full in situ pressure and quickly X-rayed, but the core liner was nearly empty. When the autoclave was depressurized and inspected, gas was released and small pieces of gas hydrate were found in the autoclave, though there was very little associated sediment.

The remaining FPC and HRC deployments (Cores 311-U1328E-7Y, 11Y, and 12E) all failed because of adverse heave and weather conditions. The FPC deployments that recovered Cores 311-U1328E-7Y and 11Y suffered from large tensions on the sand line during coring (as confirmed subsequently from the rig data), when during normal operation the sand line should be slack. The HRC deployment was terminated by an ill-timed 4 m heave that lifted the drill string and tool off the bottom during the crucial coring stroke, returning the corer with an empty and shattered liner.

Degassing experiments

At Site U1328, the five PCS cores that were successfully recovered under pressure were investigated by controlled shipboard degassing experiments (Table T21). Two PCS cores were taken within the near-surface gas hydrate–bearing section from 0 to 46 mbsf (Cores 311-U1328B-4P at 14.5 mbsf and 7P at 26.0 mbsf). PCS Cores 311-U1328C-5P and 311-U1328E-10P were both recovered from 92.0 mbsf where very high resistivity layers associated with low densities were seen in the LWD data. The deepest PCS core was taken at a depth of 233.0 mbsf, very close to the seismically inferred BSR depth (Core 311-U1238E-13P).

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 van (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 T22) as described in "Pressure coring" in the "Methods" chapter. 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 the five PCS cores from this site yielded 2.7–61.6 L of gas and showed variable methane concentrations with depth (Table T22). Mass balance calculations yield pore space methane concentrations of 123–3775 mM. They indicate 2%–15% of gas hydrate in the pore space of shallow Cores 311-U1328B-4P (14.5 mbsf) and 7P (26.0 mbsf), gas hydrate pore space concentrations of 0.7%–38% for Cores 311-U1328C-5P and 311-U1328E-10P (92.0 mbsf), and a free gas concentration of 58% in the pore space of Core 311-U1328E-13P (233.0 mbsf) from below the depth of the BSR (Tables T23, T24; Fig. F51).

Methane was the major component of the released gas for all PCS cores (Table T22). In Cores 311-U1328C-5P and 311-U1328B-7P with low gas hydrate contents, methane accounted on average for 70% ± 8% and 76% ± 4% of the emitted gas, respectively. In all other cores, methane concentrations ranged from 91% ± 5% to 98% ± 1%. Nitrogen was the second most abundant gas, contributing 0.7% ± 0.3% to 23% ± 9% to the gas released. 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 337–1052 ppmv ethane.

During four 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 gas hydrate dissociation could be expected and a steady decrease of core pressure vs. removed gas volume was observed (Fig. F52). In contrast, during the degassing of Core 311-U1328E-13P from below the depth of the BSR, the pressure inside the PCS remained at a constant high level of 5 MPa while the initial 25 L of gas was released. This pressure indicates gas hydrate stability, and the observed pressure plateau and rebound of pressure are typical for gas hydrate dissociation. However, X-ray images and repeated gamma ray density profiles of Core 311-U1328E-13P show no evidence for gas evolution within the sediment, and IW analysis does not indicate pore water freshening caused by decomposition of gas hydrate. Therefore, we conclude that the observed gas hydrate was artificially formed from free gas when the PCS was brought into the gas hydrate stability field upon entering the temperature-controlled laboratory.

X-ray scans before and after degassing, as well as gamma ray density scans during degassing experiments (Fig. F53), confirmed the presence of gas hydrate within some of the cores. The initial X-ray scan of Core 311-U1328B-4P shows low-density layers that correspond to large gas cracks in the final X-ray image, which also shows a large void in the bottom of the core barrel (Fig. F53). The time series of density profiles (Fig. F54) shows the formation of this void in the core barrel as gas and sediment were forced down and out of the bottom of the inner core barrel. IW chlorinities from this core are extremely high, indicating current gas hydrate formation (see "Interstitial water geochemistry"). It is not possible to quantify gas hydrate through pore water freshening in this core because no background chlorinity can be assumed.

Differential density profiles (i.e., gamma ray density profiles from which the initial profile has been subtracted) of Core 311-U1328B-7P show expansion throughout the core, with movement of pieces of sediment out of the bottom of the barrel (Fig. F53B). The final X-ray scan shows a low-density, highly expanded core. Based on the gamma ray density measurements and the X-ray image, this core was initially 95 ± 3 cm long. An IW sample taken in a less expansive portion of the core had chlorinities that agreed with the general background value for Hole U1328B.

Core 311-U1328C-5P, in contrast to most of the other pressure cores from Site U1328, showed only a small amount of expansion, all of which occurred between the first and second gamma ray density scans (Fig. F53C). This core also had chlorinities that agreed with the background value for Hole U1328B.

Core 311-U1328E-10P, collected in a steel barrel, originally contained ~35 ± 5 cm of core, based on the gamma ray density scans (Fig. F55). However, this core was completely homogenized during the degassing process as sediment and gas forced their way down and out of the inner core barrel. No IW sample was taken from this destroyed core.

The X-ray images and repeated gamma ray density profiles of Core 311-U1328E-13P (Fig. F53E), which yielded 62 L of gas, show little evidence for gas evolution from the sediments during degassing. Although some expansion took place in the lowermost 10 cm of the core, the overall density decrease in the core was less than that seen in Core 311-U1328C-5P, which only released 2.74 L of gas. None of the three IW samples taken showed evidence of pore water freshening. It is concluded that little of the gas released from Core 311-U1328E-13P during the degassing experiment actually comes from the sediments in the core.

Gas hydrate concentration, nature, and distribution

Based on mass balance calculations from pressure coring, Site U1328 contained highly variable amounts (0.7%–38.0%) of methane hydrate in the sediment column (Table T24; Fig. F51). This cold vent site was expected to contain large amounts of gas hydrate but also to be laterally heterogeneous. Both of these expectations were confirmed from the pressure coring results. The single core (Core 311-U1328B-13P) from below the BSR, which may or may not have been below the base of the GHSZ given the uncertainty of the temperature data (see "In situ temperature profile"), contained evidence for free gas.

The highest concentrations of gas hydrate were found not near the seafloor, as expected, but in a core from 92 mbsf (Core 311-U1328E-10P), which corresponds to a zone of very high resistivity in the LWD data from Hole U1328A. A pressure core taken at the same depth in Hole U1328C (Core 311-U1328C-5P) contained almost no gas hydrate, and the IR images (Fig. F38) and salinity and chlorinity profiles (Fig. F25) show no anomalies near 90–100 mbsf. The targeted resistivity feature might be a steeply dipping fracture in Hole U1328A (see "Logging-while-drilling borehole images"), in which case any attempt to correlate this feature with other holes would be futile. There is also evidence for general lateral heterogeneity from LWD and wireline logs from different holes at Site U1328 (see "Logging-while-drilling and wireline logging comparison").

On two occasions during Expedition 311, the PCS returned a significant amount of natural gas that could not be attributed to the recovered core (Cores 311-U1328E-13P and 311-U1329C-23P). We suggest that during these deployments, free gas from the formation was collected directly in the outer core barrel of the PCS. During the coring operation, the outer barrel of the PCS is unlikely to collect gas because drilling fluids are flowing down the outer core barrel. However, when coring is completed, this flow is stopped and the actuator raises the inner barrel, enlarging the opening from the formation to the outer core barrel. At this stage in the recovery of the PCS core, gas in the formation could freely bubble into the outer core barrel prior to the ball valve being closed. If cuttings were to block the main hole, the actuator would "swab" any gases trapped in the bottom of the hole into the outer core barrel. In this way the outer barrel may end up as a "gas sampler" for free gas that has either been released from the formation during drilling or released by the dissociation of gas hydrate during the heating caused by drilling.

The abrupt pressure rise that Cores 311-U1328B-4P and 7P experienced in the ice shuck (Fig. F49) might be associated with gas hydrate dissociation, assuming a thermal lag between the core and the recording thermistor above the autoclave. The equally abrupt pressure decrease would then correspond to the reformation of gas hydrate. However, such a pressure increase (or decrease) was not shown by similar cores that contained large amounts of gas hydrate (e.g., Cores 311-U1327D-10P and 311-U1328E-10P) and were near the gas hydrate phase boundary. Pressure increases of this type were seen in other gas hydrate–bearing pressure cores during Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003) and might contain kinetic information about the distribution and surface area of the gas hydrate.

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