DOWNHOLE MEASUREMENTS

Logging Operations

Downhole logging was performed in Hole 1119C. The drill string was placed at 100 mbsf as the logging tools were lowered to the bottom of the hole. Before logging, the drill string was raised to 80 mbsf. The drill string had to be maintained at 80 mbsf to keep the upper hole wall from collapsing.

Three tool string configurations were run in Hole 1119C, in the following order: the triple combination, the FMS-sonic, and the GHMT (see "Downhole Measurements" in the "Explanatory Notes" chapter). A repeat interval was measured with the triple combination, and two full passes were made with the FMS-sonic. Logging speeds are shown in Figure F31. Logging operations began at 2300 hr on 25 August 1998 and finished at 1300 hr on 26 August 1998. Weather conditions were excellent, with less than 1 m of heave throughout. The wireline heave compensator was used during all measurements.

The hole conditions, however, were poor, with an uneven borehole wall, many breakouts, and occasional ledges. The borehole diameter frequently exceeded 18 in and in places (e.g., 364 mbsf) was as narrow as 5 in (Fig. F32A).

The bottom of the borehole was particularly narrow, and became progressively more constricted with each logging run. This was probably because of the influence of swelling clays and cave ins. Deteriorating conditions at the bottom of the hole restricted logging to a maximum depth of 486 mbsf for the triple combination, 470 mbsf for the first pass of the FMS-sonic, 465 mbsf for second pass of the FMS-sonic, and 461 mbsf for the GHMT (Fig. F31). The NMRS tool on the GHMT tool string failed to work.

Data Quality

The quality of some of the data was substantially reduced by poor borehole conditions (Figs. F32A, F32B). The lithodensity (HLDS) and neutron porosity (APS) tools on the triple combination are particularly sensitive to hole conditions, as both tools must be pressed up against the hole wall in order to make a reading. When the hole is of a constantly varying diameter, it is impossible for these tools to maintain wall contact. The other tools on the triple combination, the hostile environment natural gamma (HNGS), resistivity (DIT), and temperature tools (TLT), are less affected by borehole conditions.

Lithodensity and neutron porosity data were edited against the caliper log to remove features that were artifacts of the borehole conditions. Comparison of the lithodensity log data with index properties (see "Physical Properties") shows that the log data correlate well with results from the core (Fig. F33). Log-based lithodensity values increase at a greater rate with depth than index bulk density values obtained from the core (Fig. F33). This can be attributed to expansion of the index samples after retrieval of the cores. The log data represent in situ conditions.

The FMS data were also affected by poor borehole conditions. The pads of this tool have a maximum expansion of 15 in. The hole diameter frequently exceeds 15 in Hole 1119C, and where this occurs the FMS cannot image the borehole wall.

The magnetic susceptibility data from the GHMT tool string is of good quality. There is not much variation in the susceptibility signal, but it correlates well with core-based susceptibility (Fig. F34).

Correlation between the resistivity, susceptibility, and gamma-ray logs and core lithology (see "Lithostratigraphy") indicates that lower gamma-ray values, higher resistivities, and lower susceptibilities are indicative of the sandy intervals. It is difficult, however, to assess the significance of the edited bulk density and neutron porosity logs, although a sharp increase in both resistivity and bulk density at ~435 mbsf correlates with an interval of high core-based bulk density values (Fig. F33). This horizon contains a siltstone bed (see "Lithostratigraphy").

Logging Units

The poor hole conditions and the associated decrease in data quality in the HLDS and APS measurements make it difficult to assign distinct logging units to this hole. In addition, the amplitudes of fluctuations in the data are relatively minor. For example, over the entire section logged, deep resistivity (IDPH) values only vary between 0.8 and 1.5 m, and magnetic susceptibility values vary between 300 and 500 (arbitrary units). Typically, both of these parameters can vary downhole by an order of magnitude. Nevertheless, logging units can be defined in Hole 1119C, mainly on the basis of fluctuations in the resistivity, magnetic susceptibility, and natural gamma-ray values (Fig. F35). Highs in the resistivity values can generally be correlated with lows in the magnetic susceptibility and gamma data. Downhole variations in porosity, calculated from the edited bulk density values (see "Downhole Measurements" in the "Explanatory Notes" chapter), are also useful as a guide to changes in lithology.

The logging units seem to correlate quite well with major seismic reflectors and the lithostratigraphic units (Fig. F35). Differences in the depths between the logging units and the lithostratigraphic units are relatively small (Fig. F35), and can possibly be attributed to inconsistencies between logging-based depths and core-based depths that arise because of core expansion.

Log Unit 1: Base of Pipe to 200 mbsf

This unit is characterized by low-amplitude, rhythmic variations in the resistivity, magnetic susceptibility, and gamma-ray values (Fig. F35). Porosities are typically 43% to 50%.

Log Unit 2: 200-300 mbsf

Fluctuations in the resistivity, magnetic susceptibility, and gamma-ray values in this section are comparable in amplitude to log Unit 1, but have an increased frequency (Fig. F35). Porosity values again range between 43% and 50%.

Log Unit 3: 300-420 mbsf

Fluctuations in the resistivity, magnetic susceptibility, and gamma-ray values in this unit have a greater amplitude than those recorded above, although their frequency is similar to that seen in log Unit 2 (Fig. F35). Porosity values are more variable, ranging between 28% and 57%.

Log Unit 4: 420-485 mbsf

Within this unit less variability can be seen in the resistivity, magnetic susceptibility, and gamma-ray values (Fig. F35). Porosities tend to be relatively low, with an overall range of 27% to 46%.

FMS Results

Two FMS images are shown for pass 2 (Fig. F36): one with dynamic normalization and one with static normalization. Static normalization takes the white of the most resistive bed and the black of the most conductive bed (see "Downhole Measurements" in the "Explanatory Notes" chapter and Schlumberger, 1989) in the entire pass and normalizes the entire image to those extremes. It is a better representation of the absolute range of resistivities in the FMS and allows correlations to be made between horizons. Dynamic normalization does the same thing, but only within a sliding 2-m window. This enhances the visual resolution of the image but does not allow accurate matching of resistivity values over the entire image because each level is independently normalized.

The FMS images are affected by regular washouts of the borehole wall. The images were, however, repeatable between both runs. Where the FMS data are reliable, the sedimentary structure is characterized by horizontal planar bedding and lamination. Also, the images often appear speckled, which is indicative of bioturbation (Fig. F36A). Toward the base of the section (415-426 mbsf), thin (0.1-0.2 m) resistive sand beds can be seen, with a spacing of ~2 m (Fig. F36B).

Downhole Temperature

The Temperature Logging Tool (TLT; see "Downhole Measurements"  in the "Explanatory Notes" chapter) recorded the temperature of the fluid in Hole 1119C. These data can be used to estimate the downhole thermal gradient (Fig. F37). The temperature results suggest a thermal gradient of 16.5° C/km for the slow thermistor. The uphole measurements are between 0.5 and 2° C greater than the downhole measurements, showing that the borehole was still equilibrating during data acquisition. These temperatures do not represent in situ formation temperatures.