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doi:10.2204/iodp.proc.311.107.2006

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 U1329 was expected to be at the eastern limit of gas hydrate occurrence on the northern Cascadia margin, based on seismic evidence. A specific objective at this site was to confirm or disprove the presence of gas hydrate in the gas hydrate stability zone (GHSZ) and the occurrence of free gas beneath the BSR. Site U1329 had a relatively weak BSR estimated at 126 mbsf, but high resistivity values in the LWD logs from Hole U1329A (Fig. F59) suggested the presence of free gas at 145–165 mbsf.

Operation of pressure coring systems

Pressure coring tools were deployed eight times at Site U1329: six PCS cores for shipboard degassing experiments and one HRC and one FPC core for both shipboard analyses and for archiving cores at in situ pressure (Table T15). Figure F43 shows the pressure history of the cores during deployment, coring, recovery, and chilling in the ice shuck. Pressure cores rarely retain full in situ pressure for various reasons (see "Pressure coring" in the "Methods" chapter) and can stray across the gas hydrate stability boundary when warming during tool recovery and handling (Fig. F44). The ice water–filled shuck kept this warming to a minimum, but cores recovered far below in situ pressure (e.g., Core 311-U1329C-7P) spend as much as 20 min outside the gas hydrate stability field before reentering gas hydrate stability conditions.

The PCS deployment that recovered Core 311-U1329C-23P was particularly unusual. This was the only pressure core deployment well below the depth of the BSR. The coring lasted over an hour. During the protracted coring operation, a low-pressure event was recorded that lasted >10 min (Fig. F43). When the corer was retrieved, we discovered that the bit had sheared off in the formation, damaging the aluminum outer barrel beyond repair and ending operations in Hole U1329C. In spite of this, a core was recovered, though it was at very low pressure (350 kPa).

The deployment of HRC Core 311-U1329E-9E (Fig. F44) shows the value of immediately cooling the tool in the ice shuck after recovery at the rig floor. The autoclave was not recovered under full in situ pressure. Water had filled the pressure accumulator, which is designed to buffer the pressure inside the core from changes in tool volume (see "Pressure coring" in the "Methods" chapter). As the core warmed on recovery, it approached the gas hydrate stability boundary but quickly moved away from this boundary as it chilled in the ice shuck. After shipboard analyses were complete, the HRC core was stored in seawater at 10 MPa in a pressurized storage chamber for further shore-based studies (see "Pressure coring" in the "Methods" chapter). The FPC deployment at this site recovered a good core (Core 311-U1329E-8Y), but the inner rod overretracted and the core was not recovered at pressure.

Degassing experiments

At Site U1329, the four PCS cores that were successfully recovered under pressure were investigated by controlled shipboard degassing experiments. These cores represent sediments both above (Cores 311-U1329C-7P, 311-U1329E-7P, and 10P) and below (Core 311-U1329C-23P) the BSR (Table T16). The main objectives were to

• Determine the concentration and composition of natural gases,
• Identify the presence or absence and concentration of gas hydrate within the GHSZ, and
• Constrain the presence of free gas below the BSR.

The pressure readings for the recovered PCS cores were all below predicted gas hydrate stability conditions at the temperature of the laboratory (7°C); therefore, no pressure plateaus or rebounds from dissociation of gas hydrate could be expected.

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 T17) 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. As a result of a failure of the digital pressure recorder, however, no pressure readings are available for the degassing of Core 311-U1329C-7P. After degassing was completed, PCS cores were X-rayed again. Starting with Core 311-1329C-23P, we collected the water remaining in the outer core barrel for mass balance considerations and subsampled the sediment that was extruded from the cores for IW chemistry, dissolved gases, and physical property analyses.

Degassing the four PCS cores from Site U1329 showed variable gas concentrations and compositions with depth (Table T17). Mass balance calculations indicate little, if any, gas hydrate within the GHSZ but yielded evidence of free gas beneath the depth of the BSR (Tables T18, T19; Fig. F45).

PCS Cores 311-U1329C-7P (55.6 mbsf) and 311-U1329E-7P (73.5 mbsf), taken within the predicted depth of the GHSZ, yielded 1.49 and 1.62 L of gas, respectively. The composition of the released gas did not change significantly in the course of degassing. Methane was the major component, accounting on average for 69% ± 4% of the gas emitted from Core 311-U1329C-7P and 78% ± 3% of the gas released from Core 311-U1329E-7P. In both cores, nitrogen was the second most abundant gas, contributing 19% ± 2% and 17% ± 3%, respectively. Carbon dioxide, ethane, and higher hydrocarbon gases were below the detection limit of the Agilent gas chromatograph (GC) used for continuous gas analysis during the degassing experiments. A subset of gas samples was analyzed using methods described in "Organic geochemistry" in the "Methods" chapter and yielded average ethane concentrations of 34 and 32 ppmv and C₁/C₂ ratios >15,500.

Based on the methane mass balance analysis (see "Pressure coring" in the "Methods" chapter), the methane concentration within the pore space of the sediment is 69 mM for Core 311-U1329C-7P and 76 mM for Core 311-U1329E-7P (Table T19). Under in situ pressure, temperature, and salinity conditions, methane was slightly oversaturated, but the nondissolved methane would have filled <1% of the pore space as gas hydrate. Repeated density scans showed a few gas cracks developing during the degassing and an overall decrease in sediment density (Fig. F46). The X-ray scans did not reveal heterogeneity in the sediment except for a zone of what appeared to be disturbed core at 27–47 cm core depth in Core 311-U1329E-7P. A large gas crack developed in this zone of presumed disturbance.

PCS Core 311-U1329E-10P (125.0 mbsf), from near the base of the predicted GHSZ, yielded 1.89 L of gas. Throughout the degassing procedure, methane accounted for 91% ± 7% of the released gas and nitrogen amounted to 8% ± 5%. Carbon dioxide, ethane, and higher hydrocarbon gases were below the detection limit of the Agilent GC (Table T17). The released amount of methane corresponds to a pore space concentration of 92 mM, which falls near the value for a methane-saturated pore water at this depth with no gas hydrate or free gas. The uniform expansion of the core in the density scans (Fig. F47) supports this conclusion. The X-ray scan before degassing showed some inhomogeneity in the core, but none of the lower density regions between 5 and 25 cm developed gas cracks during degassing; therefore, we do not consider these regions to be gas hydrate related.

The deepest PCS core (Core 311-U1329C-23P; 188.5 mbsf) was taken at a considerable depth below the base of the GHSZ. The 4.19 L of gas obtained represent by far the highest gas yield at this site. At the same time, the composition of gas differs distinctly from that obtained from the three cores taken above the BSR. High methane concentrations (88% ± 5%) were accompanied by carbon dioxide (6% ± 0.3%) and ethane (0.2% ± 0.01%) (Table T17). The resulting C₁/C₂ ratio of 463 indicates a thermogenic origin for the gas. Assuming that all of the methane originated from the captured core, the total amount of methane inside the core corresponds to 236 mM in the pore space, which equates to a free gas concentration of 2.5% of pore volume. Unlike the other cores from this site, the core pressure did not decrease steadily in the course of the degassing experiment but stayed at a constant level of 0.22 MPa while the first 1.75 L of gas was released (Fig. F46). Densities did not decrease much during degassing, but some minor shifting of the rocks and clasts seen in the X-ray scan (Fig. F47B) is evident in the lower half of the core. It is noteworthy that the large yield of gas coincided with a very large headspace volume in the outer core barrel (1.67 L).

Measurements on HYACINTH cores

Core 311-U1329E-9E was recovered with 8 MPa pressure and was returned to 10 MPa (near in situ pressure) during the transfer into the Geotek pressure multisensor core logger (MSCL-P). Simultaneous and automated gamma ray density, P-wave velocity, and X-ray measurements were made in the MSCL-P system. The velocity and density profiles are shown next to the X-ray image in Figure F48. Two high-velocity zones can be seen at 18–28 cm (velocities up to 1630 m/s) and 73–84 cm (velocities up to 1730 m/s). These zones were associated with small density lows, deviating from the norm by <0.05 g/cm³. These low-density layers were also clearly visible on the X-ray images but could not be distinguished by the X-rays alone from other low-density zones in the same core.

The distinctive nature of the two high-velocity zones is illustrated in a plot of gamma ray density versus P-wave velocity (Fig. F49). Gamma ray density and P-wave velocity in these two high-velocity zones are not correlated, whereas the other subtle variations in velocity and density throughout the core show a positive correlation of increasing velocity with increasing density. This trend is similar to that found in the measurements made on APC and XCB cores from this site (see "Physical properties"). The high-velocity zones fall well outside the normal trend for ocean sediments, and we interpret this anomaly as being indicative of the presence of gas hydrate. X-ray images and gamma ray density profiles show that these zones do not contain any veins, nodules, or other massive gas hydrate, so any gas hydrate in these zones is distributed at a small scale.

Core 311-U1329E-9E was transported to the Pacific Geoscience Center, Sidney, British Columbia, after Expedition 311 and was slowly depressurized under controlled conditions in a 4°C cold van. We followed the degassing procedures used during Expedition 311. Core 311-U1329E-9E released 7.9 L of gas (7.7 L of methane), equivalent to 1%–2% of gas hydrate in pore space. The gas released was ~99% methane, with a trace (~200 ppmv) of ethane.

A typical plot of gas released versus pressure shows no pressure recovery (see "Released gas" on Fig. F50). However, if the gas inside the storage chamber (see "Calculated gas inside SC" on Fig. F50) is estimated using the volume of expelled fluid and the pressure inside the storage chamber, a pressure recovery is seen at ~3.0 MPa (see "Total gas" on Fig. F50). This pressure is lower than the expected pressure of methane hydrate stability at 4°C (~4.4 MPa).

Multiple gamma ray density profiles were collected during depressurization of the core while it was in the HYACINTH storage chamber (Fig. F51). The bulk of the core showed no expansion. Gas was released from two zones in the sediment and collected at the top of the core over time, eventually forcing sediment out of the storage chamber. The two zones that evolved gas correspond to the two higher velocity zones (Fig. F48), confirming that these zones contained gas hydrate. If all the excess methane contained in the core had come from within these two zones (15 and 10 cm thick, respectively) the gas hydrate concentration within the zones would have been 5%–10%, depending on the porosity. The remaining core was subsampled for further study.

The small decrease in density associated with these zones, if solely attributed to replacement of pore water by gas hydrate, would have required 40% of the pore space to be filled with gas hydrate. The gas hydrate may have inhabited layers that are intrinsically low in density, and the gas hydrate itself may have contributed little to the density anomaly. Alternatively, the formation of gas hydrate in silty clays could cause the sediment matrix to expand and hence lower the overall matrix density. Such "gas hydrate microheave" could be a precursor to the formation of massive gas hydrate veins, which have been observed elsewhere in silty clays (ODP Leg 204) and which must have forced the sediments apart. The lithology of these zones should be carefully examined in an effort to understand the nature of these gas hydrate zones in fine-grained sediments.

Gas hydrate concentration, nature, and distribution

Based on mass balance calculations from degassing experiments, Site U1329 may have contained a very small amount (<0.1%) of methane hydrate in the sediment column (Table T19; Fig. F46), with an isolated example of thin sediment layers containing 5%–10% gas hydrate by pore volume. Near the base of the GHSZ, the methane concentration drops below methane solubility and a gas hydrate phase is not present. This result agrees with the seismic evidence that this site represents the eastern extent of gas hydrate occurrence on the northern Cascadia margin. However, the methane concentration measured well below the GHSZ indicates the presence of free gas.

The low estimates of gas hydrate concentration from degassing experiments at Site U1329 are consistent with the distributed nature of gas release from the PCS, with the resistivity-derived water saturation near 100% (see "Gas hydrate and free gas occurrence"), and with the lack of IR anomalies (see "Infrared images") or chlorinity anomalies (see "Salinity and chlorinity"). Gas hydrate concentrations equivalent to <0.5% of pore volume would likely be undetectable by IR imaging or IW chlorinity analyses.

Although these integrated measures of gas hydrate indicate concentrations near detection limits, Core 311-U1329E-9E (114 mbsf) contains evidence for disseminated gas hydrate at concentrations of 5%–10% within 10 cm thick, low-density, high-velocity layers. The depth of this core falls within a zone of potential gas hydrate as calculated from resistivity-derived water saturation (112–120 mbsf; see "Gas hydrate and free gas occurrence"). No similar layers of gas hydrate were observed in any of the IR images of APC and XCB cores.

The single PCS core from below the GHSZ (Core 311-U1329C-23P) had a distinctly complex history (see "Operation of pressure coring systems" and "Measurements on HYACINTH cores"), given the significance of the results from this core. The calculation of free gas percentage in Table T19 assumes that during degassing all methane is released from the sediment enclosed in the PCS core. The methane contained within the outer barrel of Core 311-U1329C-23P must have come from the formation but did not necessarily all derive from the recovered sediment core. An alternate explanation for the source of this gas comes from the low-pressure event recorded during the coring operation, which indicated to the drilling crew a gas release from the formation. If a mere 15 mL of free gas had been captured in the PCS outer barrel at core depth, this could account for the 1.67 L of gas found in the outer core barrel after depressurization. Although a gas release event does not change the conclusion that free gas exists below the BSR at Site U1329, such an event would render the Core 311-U1329C-23P results qualitative rather than quantitative.

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