Logging While Drilling


LWD operations at Site 1248 began with the spudding of Hole 1248A at 0530 hr Universal Time Coordinated (UTC) on 21 July 2002 at a water depth of 843.00 meters below rig floor (mbrf) (drillers depth) near the crest of southern Hydrate Ridge. Drilling proceeded at reduced penetration rates of 15 m/hr and lower fluid circulation rates of 15 strokes per minute (spm) to minimize formation washout at shallow depths below seafloor. No real-time MWD or Nuclear Magnetic Resonance (NMR-MRP) tool data were recorded over this interval, as the pump rate was insufficient to activate the turbines in the downhole tools. The penetration rate was increased to ~25 m/hr and fluid circulation was returned to more normal levels at a bit depth of 30 mbsf to TD (194 mbsf), and real-time MWD and NMR-MRP data were recorded. The LWD tools were pulled to ~60 m clear of the seafloor at 1500 hr on 21 July for a dynamic positioning move to Site 1249. The total bit run took ~10 hr.

Logging Quality

Figure F32 shows the quality control logs for Hole 1248A. The planned reduced ROP in the near-surface interval (0-30 mbsf) was only partially successful, with the most notable discrepancy in the upper 5 mbsf of Hole 1248A. Below a depth of ~35 mbsf, higher, more normal ROP values of 25 m/hr (5 m/hr) were achieved. An ROP of 25 m/hr was sufficient to record one sample per 4-cm interval (~25 samples per meter), which was obtained over 70% of the total section of the hole. With this relatively high sample rate, the quality of RAB images for Hole 1248A is quite high and no significant resolution loss is observed with variation in the ROP (Fig. F33). The increased pump rates below 30 mbsf and an ROP of 25 m/hr yield enhanced NMR-MRP porosity data, with a data sampling resolution of ~1 sample per 15-cm interval.

The differential caliper log, which gives the distance between the tool sensor and the wall of the borehole as recorded by the LWD density tool, is the best indicator of borehole conditions. The differential caliper values are <1 in over 70% of the total section in Hole 1248A. Only the uppermost 78 mbsf of the hole contains washouts >1 in. The density correction, calculated from the difference between the short- and long-spaced density measurements, varies from 0 to 0.09 g/cm3 (Fig. F32), which suggests very high quality density measurements. A standoff of <1 in between the tool and the borehole wall also indicates high-quality density measurements, with an accuracy of 0.015 g/cm3.

Below ~30 mbsf in Hole 1248A, the time-after-bit (TAB) measurements were 10 5 min for ring resistivity and gamma ray logs and 85 5 min for density and neutron porosity logs (Fig. F32). TAB values remain relatively constant over the interval, coinciding with steady ROP while drilling over most of the hole.

The depths, relative to seafloor, for all of the logs were fixed by identifying the gamma ray signal associated with the seafloor and shifting the log data to the appropriate depth as determined by the drillers pipe tallies. For Hole 1248A, it was determined that the gamma ray log pick for the seafloor was at a depth of 839.0 mbrf. The rig floor logging datum was located 11.0 m above sea level.

Interpretation of Logging-While-Drilling Logs

LWD logs from Hole 1248A show excellent-quality data (Figs. F33, F34). Lower pump rates through the shallow subsurface section greatly reduced the effect of borehole washout on the logs in the near-surface unconsolidated sediments. No sliding tests were conducted to evaluate downhole tool motion at Site 1248. The downhole LWD logs dramatically highlight gas hydrate-bearing sediments as high resistivity anomalies (Fig. F35). Resistivity and density log variations below the hydrate zone may indicate lithologic changes, possibly carbonates, and the presence of free gas. Borehole breakouts, which result from subsurface horizontal stress differences, are observed in the lower portion of the hole.

Logging Units

The logged section in Hole 1248A is divided into four "logging units" on the basis of obvious changes in the LWD gamma ray, bulk density, and electrical resistivity measurements (Fig. F33).

Logging Unit 1 (0-26 mbsf) is characterized by a 16-m-thick high-resistivity zone (2-18 mbsf), with a measured peak value >8 m. Logging Unit 1 is also characterized by increasing densities with depth as measured by the LWD tools. However, this trend in the downhole recorded density data is probably due in part to degraded log measurements within the enlarged portion of the near-surface borehole as shown in Figure F32. Logging Unit 1 is within lithostratigraphic Unit I-II (0-39 mbsf), which is described as a dark greenish gray silty clay sequence with abundant sulfide minerals (see "Lithostratigraphic Unit I-II" in "Lithostratigraphic Units" in "Lithostratigraphy"). The transition from logging Unit 1 to 2 is defined by a sharp increase in LWD-derived density (from ~1.63 to ~1.75 g/cm3) and a relatively subtle increase in resistivity (from ~1.1 to ~1.3 m)

Logging Unit 2 (26-116 mbsf) is characterized by zones of distinct high resistivities, with measured peak values >1.4 m. The gamma ray log in this unit shows a characteristic cyclicity of values that may reflect interbedded silty clay with silt and sand turbidite sequences as described by the shipboard sedimentologists for lithostratigraphic Unit III (39-149 mbsf) (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy"). The downhole log-measured density increases with depth in logging Unit 2 (1.65 g/cm3 at the top to near 1.8 g/cm3 at the bottom). Neither the RAB resistivity log nor the Vision Neutron Density (VND) tool density log reveals any distinct formation property changes at the expected depth of the BSR. The selection of the boundary between logging Units 2 and 3 at Site 1248 is also complicated by the lack of acoustic transit-time logging data. For Hole 1248A, the calculated Archie-derived water saturation log (Fig. F35), which integrates LWD log-derived density porosities and RAB resistivities, has been used to select the depth of the boundary between logging Units 2 and 3. Thus, the boundary between logging Units 2 and 3 was placed at the base of the deepest logging-inferred presence of gas hydrate (116 mbsf), which corresponds closely to the estimated depth of the BSR at this site.

Logging Unit 3 (116-137 mbsf) correlates with the lower part of lithostratigraphic Unit III (39-149 mbsf), which is described as an interbedded silty clay, silt, and sand turbidite sequence (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy"). Logging Unit 3 is generally characterized by more uniform resistivities compared to Unit 2. The transition from logging Unit 3 to Unit 4 is marked by an abrupt drop in gamma ray values (from ~ 65 to 55 American Petroleum Institute gamma ray units [gAPI]) and a more subtle drop in density (Fig. F33), which appears to mark the contact with the deformed sediments of the accretionary complex. A 2-m-thick anomalous interval, characterized by high resistivities (1.3 m) and low densities (<1.5 g/cm3) is present in logging Unit 3 within the depth interval from ~126 to ~128 mbsf, which suggests the presence of a free gas-saturated sand. This apparent free gas-bearing interval corresponds to seismic Horizon A, which has been mapped regionally (see "Introduction").

Logging Unit 4 (137-194 mbsf; TD of Hole 1248A), reflecting the upper portion of the deformed sediments of the accretionary complex, is characterized by almost constant gamma ray and density log measurements with depth.

Resistivity-at-the-Bit Images

The RAB tool produces high-resolution images of the electrical resistivity characteristics of the borehole wall that can be used for detailed sedimentological and structural interpretations. The RAB tool can also be used to make high-resolution electrical images of gas hydrates in the borehole, thus yielding information about the nature and texture of the gas hydrate in sediments. The RAB image in Figure F36 is characterized by light (high resistivity) to dark (low resistivity) bands, which in many cases can be traced across the display. The light continuous high-resistivity bands probably represent gas hydrate occupying low-angle fractures and nearly flat-lying stratigraphic horizons in Hole 1248A.

Logging Porosities

Sediment porosities can be determined from analyses of recovered cores and from numerous borehole measurements (see "Physical Properties" and "Downhole Logging," both in the "Explanatory Notes" chapter). Data from the LWD density, neutron, and LWD-NMR logs have been used to calculate sediment porosities for Hole 1248A. Core-derived physical property data, including porosities (see "Physical Properties"), have been used to both calibrate and evaluate the log-derived sediment porosities.

The VND LWD log-derived measurements of density in Hole 1248A (Fig. F33) increase with depth and are relatively consistent within logging Units 2 through 4, with values ranging from ~1.65 g/cm3 near the top of logging Unit 2 to >1.85 g/cm3 in logging Unit 4. The density log measurements are degraded in logging Unit 1 as discussed above. The LWD log-derived density measurements (b) from Hole 1248A were used to calculate sediment porosities () using the standard density-porosity relation,

= (m - b)/( - w).

Water density (w) was assumed to be constant and equal to 1.05 g/cm3; however, variable core-derived grain/matrix densities (m) were assumed for each logging density porosity calculation. The core-derived grain densities (m) in Hole 1248B ranged from an average value at the seafloor of 2.73 g/cm3 to ~2.68 g/cm3 at the bottom of the hole (see "Physical Properties"). The density log-derived porosities in logging Units 2-4 (26-194 mbsf) of Hole 1248A range from ~52% to 67% (Fig. F34). However, the density log porosities in logging Unit 1 (0-26 mbsf) are more variable, ranging from 57% to 72%, which is in part controlled by poor borehole conditions.

The LWD neutron porosity log from Hole 1248A (Fig. F34) yielded sediment porosities that range from an average value at the top of the logged section of ~65% to about 59% in logging Unit 4. The "total" sediment porosities derived by the NMR-MRP tool in Hole 1248A (Fig. F34) ranged from ~65% near the top of logging Unit 2 to about 50% near the bottom of the hole (BOH).

The comparison of core- and log-derived porosities in Figure F34 reveals that the density- and NMR-MRP-derived porosities are generally similar to the core porosities throughout most of the hole. However, the NMR-MRP porosity log also exhibits numerous low-porosity zones throughout the entire hole, which will be further evaluated after the cruise. The neutron porosities are slightly higher than the core-derived porosities throughout portions of logging Units 2-4 (26-194 mbsf).

Gas Hydrate

The presence of gas hydrate at Site 1248 was documented by direct sampling, with gas hydrate-bearing cores being recovered from both logging Units 1 and 2 (Fig. F35). It was inferred, based on geochemical core analyses (see "Interstitial Water Geochemistry"), IR image analysis of cores (see "Physical Properties"), and downhole logging data that disseminated gas hydrate is present in portions of logging Units 1 and 2. As previously discussed in "Downhole Logging" in the "Explanatory Notes" chapter, the presence of gas hydrate is generally characterized by increases in logging-measured electrical resistivities and acoustic velocities. Logging Units 1 and 2 at Site 1248 are characterized by intervals of high electrical resistivities, but we have no acoustic data (because no wireline logging was conducted at Site 1248) to further evaluate the presence of gas hydrate or free gas at this site.

Resistivity logging data were used to quantify the amount of gas hydrate at Site 1248. For the purpose of discussion, it is assumed that the high resistivities measured in logging Units 1 and 2 are due to the presence of gas hydrate. Archie's Relation,

Sw = (aRw/mRt)1/n,

(see "Downhole Logging" in the "Explanatory Notes" chapter) was used with resistivity data (Rt) from the LWD RAB tool and porosity data (m) from the LWD density tool to calculate water saturations in Hole 1248A. It should be noted that gas hydrate saturation (Sh) is the measurement of the percentage of pore space in a sediment occupied by gas hydrate, which is the mathematical complement of Archie-derived Sw , with

Sh = 1 - Sw.

For Archie's Relation, the formation water resistivity (Rw) was calculated from recovered core-water samples, and the Archie a and m variables were calculated by a crossplot technique that compares the downhole log-derived resistivities and density porosities. See Collett and Ladd (2000) for the details on how to calculate the required formation water resistivities and Archie variables. The values used for Site 1248 were a = 1, m = 2.8, and n = 1.9386.

Archie's Relation yielded water saturations (Fig. F35) ranging from a minimum value of only ~42% in logging Unit 1 to a maximum of 100% in portions of logging Units 1 and 2 (0-116 mbsf), so calculated gas hydrate saturations in Hole 1248A range from 0% to 58%. The low water saturations shown in logging Unit 3 (Fig. F35) probably indicate the presence of free gas-bearing sediments (as discussed above).