IODP Proceedings    Volume contents     Search
iodp logo

doi:10.2204/iodp.proc.308.203.2008

Results

Sites U1319, U1320, and U1321 were drilled in the Brazos-Trinity Basin IV (Figs. F1, F2). Sites U1319 and U1320 were cored and penetrometer measurements were made. Sites U1322, U1323, and U1324 were drilled in the Ursa Basin (Figs. F1, F3). Sites U1322 and U1324 were cored and penetrometer measurements were made.

We present the temperature and pressure data from the penetrometer deployments at the four sites in this report. In the main text, we present our best estimate of the in situ temperature and pressure. In “Appendix A,” we describe how the DVTPP was calibrated and present a detailed description of each DVTPP deployment. In “Appendix B,” we describe how the T2P was calibrated and we describe each T2P deployment. In “Appendix C,” we present a discussion of the pressure state within the drill pipe based on the DVTPP pressure measurements.

The temperature and pressure data are available in Microsoft Excel format in the “APP_A” and “APP_B” folders in “Supplementary material.” These data have been recalibrated and consequently are different and improved relative to the data discussed in the Expedition Reports section of this volume. The original raw data can be found in the “DOWNHOLE” folder in “Supplementary material.” The penetrometer data are integrated with the rig instrumentation system data (“TruView data”) in order to better understand and assess the quality of each measurement.

Summary of deployments

Twenty DVTPP deployments and twenty-eight T2P deployments were completed during Expedition 308 (Table T1). The deployment number (Table T1) reflects the deployment sequence of each tool during Expedition 308 (Expedition 308 Scientists, 2005). Deployments are divided into three types: Type I, Type II, and Type III (Table T1; Fig. F6).

Figure F7A illustrates ideal penetrometer deployments for the DVTPP and the T2P (Type I; Table T1). The tip pressure is at maximum during insertion and, subsequently, pressure declines with time. At the end of the deployment, the shaft pressure of the T2P is much greater than that of the tip pressure. This is because the shaft has a much larger diameter. As a result it disturbs a greater region around the penetrometer and this takes a greater amount of time to subside to the in situ pressure. A detailed comparison of the DVTPP and the T2P geometries and their consequent behavior during insertion and dissipation is presented by Long et al. (2007a).

Figure F7B presents deployments for both the DVTPP and the T2P that were slightly dislodged when the drill string was raised subsequent to penetration to decouple itself from the penetrometer through the CDS (Types IIA and IIB). In this situation, the tool pressure dropped abruptly when the bit was raised. Analysis of the temperature record from both tools and the accelerometer record from the DVTPP shows that coincident with the abrupt drop in pressure there was frictional heating and movement of the tool (Flemings et al., 2006; Long et al., 2007b). The pressure either decayed toward the formation pressure after it rebounded to a certain level (Type IIA; Table T1) or kept building during the dissipation phase (Type IIB; Table T1; Fig. F7B).

Type III includes all the unsuccessful deployments that failed to yield useful information about the in situ conditions. Type III deployment problems are three-fold. First, in early cases there was an internal hydraulic leak in the DVTPP and the tip pressure of the T2P. The leaks resulted in abrupt and erratic drops in pressure during the dissipation phase (Fig. F7C). Eventually, the internal hydraulic leak was repaired. Second, in the worst case the tool dislodgement weakened the seal around the probe and created communications with the borehole fluid, ruining the pressure and temperature measurements (see “Appendix A,” “Appendix B”). Third, the tool did not record any reliable data because of electronic and/or mechanical failure. The latter was especially true for the T2P, as it was prone to bending because of its very narrow diameter tip.

During several deployments, the DVTPP was not fully decoupled from the BHA because of friction in the CDS (in “Appendix A,” see DVTPP Deployments 1, 2, 3, 8, 12, and 13). In these cases, the tool moved during the dissipation phase. Frictional heating caused by these tool movements may have compromised the temperature measurement. Continuous movement of the tool may also have affected the pressure measurements. In several T2P deployments, circulation of the drilling fluid resumed during the dissipation phase. In these cases, it is often possible to see a slight pressure and temperature increase at the onset of circulation. (in “Appendix B,” see T2P Deployments 2, 3, 4, 5, and 12). In some cases, the onset of circulation resulted in further tool insertion (in “Appendix B,” see T2P Deployments 6 and 7). The tool disturbance caused by pumping fluid may affect the accuracy of the pressure and temperature measurements.

Data extrapolation

During Expedition 308, T2P temperatures equilibrated to formation temperatures (see “Appendix B”). In contrast, temperatures measured with the DVTPP did not equilibrate to in situ temperatures (see “Appendix A”). The reason for this is that the DVTPP has a significantly larger geometry. We use inverse time (1/t) extrapolation to estimate the in situ temperature for the DVTPP deployments (Davis et al., 1997; Villinger and Davis, 1987).

Pressures measured by both the T2P and the DVTPP penetrometers did not reach in situ pressures during the dissipation phase (see “Appendix A,” “Appendix B”). In the absence of detailed soil properties, we used two empirical approaches to infer the in situ pressure from the partial dissipation records: 1/t extrapolation and 1/√t extrapolation. Accuracy of the extrapolated in situ pressures depends on the tool that was used, pressure port, type of deployment, depth of deployment, and the pressure decay time (Long et al., 2007b). Long et al. (2007b) showed that 1/t extrapolation more closely matches theoretical modeling results than the extrapolation does when pressure decays <80% of the penetration-induced pressure. The error of a good (Type I) deployment with long dissipation time (e.g., 90 min) should be within 0.1 MPa, whereas the error of a deep deployment with short decay time could be more than 0.5 MPa.

Table T2 presents the interpreted in situ pressure and temperature for the T2P and the DVTPP deployments during Expedition 308.

In situ temperature

Brazos-Trinity Basin IV

Figure F8 presents the in situ temperatures taken at Sites U1319 and U1320. The geothermal gradient at Site U1320 is 23.1°C/km. The only measurement at Site U1319 suggests a higher geothermal gradient than that at Site U1320.

Ursa Basin

Figure F9 presents the in situ temperatures taken at Sites U1322 and U1324. The geothermal gradient at Site U1324 is bilinear. The thermal gradient is 18.6°C/km in the sediments above 360 meters below seafloor (mbsf), corresponding to lithostratigraphic Unit I, which is predominantly composed of terrigenous clay and mud with a marked paucity of silt and sand (see the “Site U1324” chapter). The geothermal gradient is 16.7°C/km in lithostratigraphic Unit II, which extends from 360 to 600.8 mbsf and includes interbedded silt and very fine sand with beds and laminae of mud and clay (see the “Site U1324” chapter). Sediments are predominantly clay and mud at Site U1322. The geothermal gradient is 21.9°C/km, which is significantly higher than that at Site U1324.

In situ pressure

We present our pressure results with respect to hydrostatic pressure and overburden stress. The hydrostatic pressure is calculated starting from the seafloor and assuming a seawater density of 1.024 g/cm3. Bulk density data from shipboard moisture and density (MAD) measurements were integrated to calculate the overburden stress. The static pressure of the water column above seafloor was subtracted from the pressure results.

Brazos-Trinity Basin IV

We have only one pressure measurement at Site U1319. The T2P penetration was completed at 80.5 mbsf. The last recorded pressure of the T2P tip equals the overburden stress, whereas that of the shaft exceeds the overburden stress. This clearly shows that pressures had not dissipated to in situ pressure (Fig. F10B). The pressure dissipation time was only 35 min for this deployment. The 1/√t extrapolation of the tip pressure, which should give a better estimation of in situ pressure (Long et al., 2007b), suggests that the formation pressure is 0.37 MPa higher than hydrostatic (Fig. F10D). The 1/√t extrapolated in situ pressure suggests that the formation pressure at 80.5 mbsf is 0.37 MPa higher than the hydrostatic pressure (Fig. F10D). The shaft pressure was still higher than the overburden stress after 1/t and 1/√t extrapolation (Fig. F10C, F10D).

We made two T2P and two DVTPP deployments at Site U1320, but only one deployment can be used to estimate the in situ pressure. The last recorded pressure was slightly greater than the hydrostatic pressure (Fig. F11B). The estimated in situ pressure by both 1/t and 1/√t extrapolation suggests that formation pressure at 126.3 mbsf is close to hydrostatic pressure (Fig. F11C, F11D).

Ursa Basin

Figure F12 presents the pore pressure measurements at Site U1322, where sediments are predominantly clay and mud. The last recorded pressures are scattered, with some of them equal to or exceeding the overburden stress (σv) (Fig. F12B). This indicates that pressures had not dissipated to the in situ pressure at the end of the deployments.

The 1/t extrapolation predicts consistently higher pressure at the shaft sensor of the T2P than that at the tip sensor (Fig. F12C). Some shaft pressures are still equal to or even higher than the overburden stress (Fig. F12C). These indicate that 1/t extrapolation of the shaft pressure overestimates the in situ pressure, consistent with theoretical modeling presented by Long et al. (2007b).

Application of 1/√t extrapolation drives the shaft pressure closer to the tip pressure (Fig. F12D). The results make more physical sense because ultimately the shaft pressure and tip pressure converge at the in situ pressure. We believe the 1/√t extrapolation provides more accurate in situ pressure estimate than the 1/t extrapolation does for the shaft pressure.

Nevertheless, both extrapolation approaches predict significant overpressure and similar trends. The overpressure ratio (λ* = [u0uh]/[σvuh]) is as high as 0.75. Overpressure starts to drop from ~200 mbsf.

Figure F13 presents the pore pressure measurements at Site U1324. Both extrapolation approaches predict significant overpressure in the sediments above ~200 mbsf (Fig. F13C, F13D) that correspond to hemipelagic silty claystone. Within this section, the magnitude and trend of the overpressure are similar to those at Site U1322 (Fig. F14). The sediments below 300 mbsf have less overpressure (Fig. F13). The overpressure seems to be constant within lithostratigraphic Unit II, in which sediments are composed of silty claystone interbedded with beds of silt and very fine sand. The transition occurs at the section from 200 to 300 mbsf.