Twelve measurements of in situ temperature were made at this site: ten with the APCT tool and three with the DVTP. A detailed sequence of four measurements was made in Hole 1245C at approximately the depth of Horizon A, although there is some uncertainty about hole-to-hole depth correlation. Measurements span the depth range of 38-350 mbsf (Table T20). Raw data are shown in Figure F42. Only the portion of the data from the immediate time period before, during, and after tool insertion is shown.
APCT data were modeled using the software program TFIT (as described in the "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter) using measured thermal conductivities (see "Physical Properties"). Uncertainty in the extrapolated value of in situ temperature as a result of subjective analyst picking of tp, ti, and tf is <0.05°C for these high-quality records. Uncertainty resulting from possible errors in measured values of thermal conductivity is estimated to also be ~0.05°C. Instrument calibration uncertainties are present but are poorly quantified (see "Downhole Tools and Pressure Coring" in the "Site 1244" chapter). No in situ temperatures were derived from DVTP data because the time series suggest that the probe did not penetrate the seafloor properly.
The subsurface temperature data from the APCT tool are shown in Figure F43 along with the best-fit temperature gradient. Solutions with and without inclusion of the mudline temperature are shown and are not significantly different. The dark gray horizontal line at 134 mbsf marks the position of the BSR as determined from seismic reflection data and confirmed by wireline acoustic logging data (see "Downhole Logging"). The dashed horizontal line shows the predicted depth to the base of the GHSZ, assuming the temperature gradient determined for Site 1245 and hydrostatic pressure for a pure methane and seawater system (Maekawa et al., 1995). The temperature at the BSR, indicated by the downhole measurements, is 0.5°C colder than predicted. Possible explanations for this mismatch will be discussed elsewhere.
No in situ pressure measurements were made at this site.
The ODP PCS was deployed five times at Site 1245. All deployments were successful (i.e., a core under pressure was recovered). The main objectives of the deployments were (1) to construct a detailed profile of concentration and composition of natural gases in the upper part of the section (0-300 mbsf) and (2) to identify the presence/absence and concentration of gas hydrate within the GHSZ.
Specific depth intervals were targeted for deployment of the PCS. Three cores (Cores 204-1245C-3P [17-18 mbsf], 8P [57-58 mbsf], and 16P [120-121 mbsf]) were recovered from above the BSR. The other two cores (Cores 204-1245B-17P [147.1-148.1 mbsf] and 33P [291.2-292.2 mbsf]) were recovered from below the BSR.
The PCS chambers were degassed for 334-4685 min after recovery (Table T21). Pressure was recorded during degassing experiments (Fig. F44). Gas was collected in a series of sample increments (splits), and most were analyzed for molecular composition (see "Organic Geochemistry"). In addition, gas splits were subsampled for shore-based analyses. After degassing, the PCS chambers were disassembled. The lengths of the cores were measured (Table T21), and samples were taken for analysis of physical properties (see "Physical Properties").
Gas was collected in 20- to 1070-mL increments. The measured incremental and cumulative volumes are plotted vs. time in Figure F44. The cumulative volume of released gas varies from 315 (Core 204-1245C-3P) to 19,025 mL (Core 16P) (Table T21). The volume of the last gas splits varies from 10 (Core 204-1245C-8P) to 20 mL (Cores 17P and 16P). This observation suggests that almost all gas present in the cores was collected.
Gases released from the PCS are mixtures of air (N2 and O2), CH4, CO2, and C2+ hydrocarbon gases (see "Gas Hydrate and Pressure Cores" in "Organic Geochemistry"). The abundance of air components in the PCS gas samples (2.6%-29.3% of gas mixtures) suggests that air was not always properly displaced from the PCS by seawater during deployments. Methane is the dominant natural gas present in collected gas splits. The molecular composition of gases from the PCS is similar to the composition of gas voids at adjacent depths (see Fig. F45).
Sediments in cores recovered with the PCS have lithologies that are similar to sediments recovered by the APC and XCB at adjacent depths (see "Physical Properties"). However, the porosity of sediments from the PCS is often different from the porosity of sediments at adjacent depths (Fig. F45) for reasons that are not yet understood. Porosity values measured in samples from APC and XCB cores taken near the PCS were used to estimate the in situ methane concentration (Table T21).
The concentration of in situ methane was estimated based on data from the degassing experiments (i.e., total volume of methane) and core examination (i.e., length of recovered core and the porosity of sediments). The calculation yields equivalent concentrations varying from 10.2 to 1,116.6 mM of methane in pore water (Table T21). These concentrations have been compared with the theoretical methane-solubility curve extrapolated from values calculated for higher pressures (greater depths) (Handa, 1990; Duan et al., 1992) (Fig. F46).
Preliminary analysis suggests that gas hydrates have been present in relatively high concentrations (12%-16% of pore volume) in Core 204-1245C-16P recovered from above the BSR. In addition to high gas concentrations, strong evidence of the presence of gas hydrate was found in the pressure record of core degassing (Fig. F44E). The estimate of gas hydrate saturation is consistent with those based on well logging data (see "Downhole Logging"). However, other cores retrieved from above the BSR (Cores 204-1245C-3P and 8P) suggest that only dissolved methane is present in many intervals within the GHSZ. No free gas seems to be present in intervals ~15 and ~160 m below the BSR (Cores 204-1245B-17P and 33P, respectively). Additional comparisons of measured methane concentrations with theoretical methane solubility, both above and below the BSR, will be performed on shore to better estimate if methane was present in situ in solution, in free phase, or as gas hydrate.
Four deployments of the HYACINTH pressure coring tools were made at Site 1245, two each with the HRC and FPC (see "Operations"). The HRC cores (Cores 204-1245B-46E [HRC 5] and 204-1245C-29E [HRC 6]) were at 407 and 201 mbsf, respectively. The FPC cores (Cores 204-1245C-18Y [FPC 7] and 27Y [FPC 8]) were at 129 and 195 mbsf, respectively. All the cores were located below the BSR (located at ~134 mbsf), apart from Core 204-1245C-18Y (FPC 7), which was ~5 m above the BSR. Technical and operational difficulties prevented any cores from being recovered under full pressure for further analysis. The FPC recovered a good core (90 cm) from 129 mbsf but apparently had difficulty penetrating the stiffer lithology at 195 mbsf and only recovered a 15-cm-long section. We recovered a 38-cm core (Core 204-1245B-46E; HRC 5) in indurated claystone, but recovery of Core 204-1245C-29E (HRC 6) was hampered by operational difficulties when the active heave compensator (AHC) failed and only a 20-cm-long core was recovered.
Two HRC deployments, Cores 204-1245B-46E (HRC 5) and Core 204-1245C-29E (HRC 6) were made at depths of 407 and 201 mbsf, respectively (see Table T22). The first attempt to run Core 204-1245B-46E (HRC 5) was aborted after difficult hole conditions required high pump rates, which were incompatible with the HRC operation.
On the second attempt, the tool was lowered at 50-70 m/min and stopped at 1245 m with the drill string already at TD to minimize the risk of borehole instability. It was lowered on the wireline to the landing position, and 5 m of slack wire was paid out before closing the blowout preventer (BOP). Pumping began at 80 gallons per minute (gpm), and a pressure peak was observed at 620 psi (first shear pin sheared). Pumping continued at 110 gpm, but a second pressure peak (which would have indicated full stroke) was not observed after 24 min. Pumping was stopped, and the BOP was opened. The drill string was picked up to 1285.3 m before pumping was continued for 1 min at 90 gpm, in an attempt to ensure a full stroke. After pumping stopped, the tool was raised on the wireline, at first slowly and then at 60-70 m/min, to the surface. When broken out of the drill string, it was observed that the central rod was not fully retracted until several jerks on the tugger line had occurred. The DSA tool was removed and was returned to the trestles on the pipe racker for disassembly.
We observed that the liner had broken above the core catcher, the piston cap had unscrewed, high loads had damaged a bearing, and the valve had not closed. Despite this, 38 cm (recovery = 38%) of stiff indurated claystone was recovered. We concluded that a variety of factors could have caused the problems encountered, including poor motor performance, but these problems were attributed to insufficient heave compensation and poor hole conditions. The HRC was completely overhauled and the motor replaced before the next deployment (Core 204-1245C-29E [HRC 6]).
The HRC was deployed in Hole 1245C as Core 204-1245C-29E (HRC 6) at 201 mbsf. The tool was run into the hole at 40-70 m/min and stopped at 1030 m. Pumping was stopped, the tool was lowered onto the landing shoulder, and 5 m of slack wire was paid out. The drill string was then lowered to TD and held with 15 klb (weight on bit). The BOP was closed, and pumping began slowly with both passive and active heave compensation activated. A pressure peak was observed at 650 psi (first shear pin sheared) and pumping continued at 85 gpm. However, at this stage a sudden problem with the AHC occurred (the string was bouncing with weight on bit changing rapidly between 5 and 45 klb), and it was switched off while the passive compensator remained on. Coring could not continue, and the drill string was picked up ~5 m while pumping at 100 gpm. After pumping was stopped, the tool was lifted on the wireline slowly for the first 20 m (7 m/min) and then at 110 m/min while circulating. The tool was broken out of the string, the DSA tool was removed, the strongbacks replaced, and the DSA tool was returned to the trestles on the piperacker. We found that a full stroke had been achieved and the core liner had retracted into the autoclave with a 20-cm-long core (Core 204-1245C-29E) at the top, but the autoclave flapper valve had not closed. The inner sleeve had become stuck at the valve, which may have been caused by the sudden motions when the AHC failed.
Two FPC deployments, Core 204-1245C-18Y (FPC 7) and Core 204-1245C-27Y (FPC 8), were made at depths of 129 and 195 mbsf, respectively. During the coring procedure for Core 204-1245C-18Y (FPC 7), the AHC was turned off because it appeared to increase the variability of the weight on bit. A full core (90 cm long) was recovered, but the lower autoclave valve had not fully closed. The fall path of the valve was modified before the next deployment. During recovery of Core 204-1245C-27Y (FPC 8), the AHC was used. However, the inner rod failed to stroke out completely, indicating that the formation was too stiff for the hammer mechanism. This resulted in the recovery of a short core (15 cm long) and an imploded liner with inverted catcher fingers. It should be noted that the APC cores taken on either side of Core 204-1245C-27Y (FPC 8), namely Cores 204-1245C-26H and 28H, only recovered 4.8 and 3.8 m, respectively. Consequently, the working hypothesis that the operational limit of the FPC hammer mechanism is similar to the working limit of the APC appears to have been confirmed in this type of silty clay formation.