DOWNHOLE LOGGING

Operations

LWD at Site 1246 began at 0830 hr Universal Time Coordinated (UTC) on 19 July 2002 by spudding Hole 1246A at 861.50 meters below rig floor (mbrf) water depth (drillers depth) on the northeastern flank of Hydrate Ridge. The LWD tools (6-in collars) included the Geo Vision Resistivity (GVR) resistivity-at-the-bit (RAB) tool with 9¹/₈-in button sleeve, the MWD (Powerpulse), the NMR-MRP tool, and the Vision Neutron Density (VND) tool. Drilling proceeded at ~25 m/hr to TD at 180 mbsf without difficulty, and real-time data were transmitted to the surface to evaluate formation properties. Two stands of drill pipe were pulled up from the bottom of the hole without rotating (sliding test) to evaluate the effects of drilling on the NMR-MRP tool. Tools were pulled to the rig floor at 0300 hr UTC on 20 July, and the data from Holes 1245A and 1246A were downloaded. The total bit run was ~19 hr.

Logging Quality

Figure F28 shows the quality control logs for Hole 1246A. The target rate of penetration (ROP) of 25 m/hr (±5 m/hr) was generally achieved, although some large variations in ROP were observed just below the seafloor. The measured ROP was sufficient to record one sample per 4-cm interval (~25 samples per meter) and was obtained over 94% of the total section of the hole. The quality of RAB images is thus quite high, and no significant resolution loss is observed with variation in ROP at Hole 1246A. However, the quality of the RAB images in the upper 50 mbsf of Hole 1246A (Fig. F29) is degraded by an apparent problem associated with low rates of bit rotation. The NMR-MRP tool porosity data were enhanced by using a slow drilling rate, resulting in a sampling resolution of approximately one sample per every 15-cm interval.

The differential caliper log (DCAL), 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 DCAL values are >1 in over the uppermost 50 mbsf of the hole, showing washouts that have degraded the near-surface LWD data. The density correction (DRHO) (calculated from the difference between the short- and long-spaced density measurements) varies from 0 to 0.14 g/cm³ (Fig. F28), which shows the generally good quality of the density measurements. A standoff of <1 in between the tool and the borehole wall indicates high-quality density measurements with an accuracy of ±0.018 g/cm³.

Time-after-bit (TAB) measurements are 10 ± 2 min for ring resistivity and gamma ray logs and 89 ± 5 min for density and neutron porosity logs (Fig. F28). TAB values remain relatively constant, coinciding with steady ROP while drilling.

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 logging data to the appropriate depth as determined by the drillers pipe tallies. For Hole 1246A, it was determined that the gamma ray log pick for the seafloor was at a depth of 859.0 mbrf. The rig floor logging datum was located 10.9 m above sea level for this hole.

Data from Hole 1246A show excellent quality LWD logs. The presence of gas hydrate was identified from ~55 to 109 mbsf by high resistivities and RAB image anomalies, allowing quantitative estimates of gas hydrate saturations. Within the zone of expected gas hydrate stability (above a depth of ~114 mbsf), the RAB image reveals several conspicuous high-resistivity zones that appear to be gas hydrate-bearing intervals. Low to high gamma ray and density logging measured interbedding throughout Hole 1246A that may indicate lithologic changes associated with turbidites and/or the presence of free gas below the GHSZ.

Logging Units

The logged section in Hole 1246A is divided into three "logging units" on the basis of obvious changes in the LWD gamma ray, density, and electrical resistivity measurements (Fig. F29).

Logging Unit 1 (0-55 mbsf) is characterized by increasing resistivities and densities with depth as measured by the LWD tools. However, this trend in the downhole logging data is probably in part a result of degraded logging measurements within the enlarged portion of the borehole near the surface as shown in Figure F28. Logging Unit 1 includes all of lithostratigraphic Unit I (0-21 mbsf) and the upper portion of lithostratigraphic Unit II (21-138 mbsf), which are composed of clay to silty clay sediments (see "Lithostratigraphic Unit II" in "Lithostratigraphic Unit" in "Lithostratigraphy"). The transition from logging Unit 1 to logging Unit 2 is defined by a significant increase in electrical resistivity from ~0.8 to >1.6 m. The density and gamma ray logs also reveal a pronounced change at the contact between logging Unit 1 and 2, with an increase in the LWD recorded densities and gamma ray values of >0.3 g/cm³ and 10 American Petroleum Institute gamma ray units (gAPI), respectively.

Logging Unit 2 (55-109 mbsf) is characterized by zones of distinct high resistivities, with peak resistivity values >2 m. The gamma ray log in this unit shows a characteristic cyclicity of values that may reflect the interbedded sand and clay turbidite sequences as described by the shipboard sedimentologists for lithostratigraphic Unit II (21-137 mbsf) (see "Lithostratigraphic Unit II" in "Lithostratigraphic Unit" in "Lithostratigraphy"). The downhole logging-measured densities in Unit 2 reveal several conspicuous high-density intervals within the depth interval from 55 to 68 mbsf and from 103 to 109 mbsf, with peak values exceeding 2.0 g/cm³. These two anomalous intervals likely represent the regionally described seismic Horizon B (~60 mbsf) and Horizon B´ (~100 mbsf) (see "Lithostratigraphy"). Because of the lack of acoustic transit-time logging data from Site 1246, the RAB resistivity logging image has been used to select the depth of the boundary between logging Units 2 and 3, which is marked by a drop in the recorded borehole resistivity with depth. Also noted on the density log is a subtle drop in density at the contact between Unit 2 and 3, which roughly corresponds to the depth of the BSR at this site.

Logging Unit 3 (109-180 mbsf; TD of Hole 1246A) corresponds to the lower part of lithostratigraphic Unit II (21-138 mbsf), which is described as an interbedded sand, silt, and silty-clay turbidite sequence. Logging Unit 3 is generally characterized by lower and more uniform resistivities compared to Unit 2.

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 NMR-MRP logs have been used to calculate sediment porosities for Hole 1246A. 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-derived measurements of density in Hole 1246A (Fig. F29) generally increase with depth, with values averaging ~1.4 g/cm³ near the seafloor to >1.8 g/cm³ at the bottom of logging Unit 3. The density logging measurements are degraded in logging Unit 1 as discussed above. The LWD log-derived density measurements from Hole 1246A were used to calculate sediment porosities () using the standard density-porosity relation:

= (_m - _b)/(_m - _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) from Hole 1246B ranged from an average value at the seafloor of 2.71 to ~2.69 g/cm³ at the bottom of the hole (see "Physical Properties"). The density log-derived porosities in logging Units 1-3 (0-180 mbsf) of Hole 1246A range from ~40% to 80% (Fig. F30).

The LWD neutron porosity log from Hole 1246A (Fig. F30) yielded sediment porosities ranging from an average value at the top of the logged section of ~70% to ~60% in logging Unit 3. The "total" sediment porosities derived by the NMR-MRP tool in Hole 1224A (Fig. F30) ranged from ~80% near the seafloor to ~50% near the bottom of the hole.

In studies of downhole logging data, it is common to combine and compare porosity data from different sources to evaluate results and assess the accuracy of a particular measurement. The comparison of core-derived and log-derived porosities in Figure F30 reveals that the density-derived porosities are generally similar to the core porosities in logging Units 2 and 3 from Hole 1246A. However, the LWD density-derived porosities are generally higher than the core-derived porosities in Unit 1. At the top of logging Unit 2, the LWD density log also appears to be ~3 m deeper than the core-derived densities. This offset is best explained by local variability in the geology of this site, with Hole 1246A (LWD hole) being drilled more than 80 m west of Hole 1246B (core hole). The neutron porosities are slightly higher than the core-derived porosities throughout most of the hole. The NMR-MRP tool porosities are generally lower than the core-derived porosities in logging Units 2 through 3. The NMR-MRP tool porosities in logging Unit 1 (above ~25 mbsf) appear to have been significantly degraded by washouts in the upper part of the hole. The NMR-MRP tool porosity log also exhibits numerous low-porosity zones throughout the entire hole, which will be further evaluated after the cruise.

Gas Hydrate

The presence of gas hydrate at Site 1246 was documented by several strong IR anomalies in Hole 1246B at depths of 66.45, 96.64, 104.93, 105.08, and 109.20 mbsf. Despite the limited presence of gas hydrates, it was inferred, based on geochemical pore water 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 Unit 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 Unit 2 at Site 1246 is characterized by a distinct stepwise increase in electrical resistivities, but we have no acoustic data (because no wireline logging was conducted at Site 1246) to further evaluate the presence of gas hydrate or free gas at this site. One of the most notable appearances of gas hydrate at Site 1246 was imaged by the RAB tool in Hole 1246A at a depth of 96-97 mbsf. The presence of gas hydrate was also inferred in Hole 1246B at a depth of 96.64 mbsf based on IR anomaly data. The shipboard sedimentologists have described this interval as a volcanic glass-bearing horizon in the cores from Hole 1246B (Horizon B´ at a depth ~100 mbsf) (see "Lithostratigraphy"). The LWD resistivity tool also reveals several thin high-resistivity zones within logging Unit 1 (0-55 mbsf), which may suggest the possible presence of gas hydrate.

Resistivity logging data have been used to quantify the amount of gas hydrate at Site 1246. For the purpose of discussion, it is assumed that the high electrical resistivities measured in logging Unit 2 are due to the presence of gas hydrate. The Archie Relation,

S_w = (aR_w/^mR_t)^1/n

(see "Electrical Resistivity" in "Downhole Logging" in the "Explanatory Notes" chapter), was used with resistivity data (R_t) from the LWD RAB tool and porosity data (^m) from the LWD density tool to calculate water saturations in Hole 1246A. It should be noted that gas hydrate saturation (S_h) is the measurement of the percentage of pore space in sediment occupied by gas hydrate, which is the mathematical complement of the Archie-derived S_w , with

S_h = 1 - S_w .

For the Archie Relation, the formation water resistivity (R_w) was calculated from recovered core water samples and the Archie a and m variables were calculated by a crossplot technique that compares the downhole logging-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 1246 were a = 1, m = 2.8, and n = 1.9386.

The Archie Relation yielded water saturations (Fig. F31) ranging from an average minimum value of ~70% to a maximum of 100% in logging Unit 2 (55-109 mbsf) of Hole 1246A, which implies the gas hydrate saturations in logging Unit 2 range from 0% to 30%. It also appears that logging Unit 1 may contain several thin gas hydrate-bearing intervals. The low water saturations shown in logging Unit 3 within the depth interval from ~138 to 153 mbsf, which are below the zone of potential gas hydrate stability, may indicate the presence of free gas-bearing sediments.