DOWNHOLE LOGGING

Operations

LWD operations at Site 1249 consisted of drilling two dedicated LWD boreholes: Holes 1249A and 1249B. LWD operations at Site 1249 began at 1700 hr Universal Time Coordinated (UTC) on 21 July 2002 by spudding Hole 1249A at a water depth (drillers depth) of 788.50 meters below rig floor (mbrf) on the crest of southern Hydrate Ridge. The LWD tools deployed in Hole 1249A included the GeoVision Resistivity (GVR; RAB), MWD (Powerpulse), Nuclear Magnetic Resonance (NMR-MRP) tool, and Vision Neutron Density (VND) tool. Drilling proceeded at reduced ROP of 15 m/hr and lower fluid circulation rates of 15 spm to minimize formation washout in the unconsolidated sediments below seafloor. No real-time MWD or NMR-MRP data were recorded over this interval, as the pump rate was insufficient to activate the turbines in the downhole tools. The ROP was increased to ~25 m/hr and fluid circulation was returned to more normal levels at a bit depth of 30 mbsf, and real-time MWD and NMR data were recorded to TD (90 mbsf). The LWD tools were pulled to ~60 m clear of the seafloor at 2400 hr UTC on 21 July for the dynamic positioning move to Site 1250. The total bit run took ~7 hr.

On 24 July, we returned to Site 1249 to deploy and test the RAB-8 tool and coring system. LWD operations for Hole 1249B began with the initialization of the RAB-8 tool at 2115 hr UTC (24 July). The RAB-8 bottom-hole assembly (BHA) used for this test is described in "Downhole Logging" in the "Explanatory Notes" chapter. Though the RAB-8 BHA was extensively tested before sea deployment, the inner mandrel in the RAB-8 BHA was too high to connect with the 6⁵/₈-in full-hole pin above. The internal diameter of the pin was ground down, and the clear passage of the MDCB was tested before downhole deployment. Hole 1249B was spudded at 0400 hr UTC on 25 July in 788.50 mbrf water depth (drillers depth) on the summit of southern Hydrate Ridge. Drilling proceeded ahead to 30 mbsf, where RAB coring operations began with sequential 4.5-m-long (with liners) and 9-m-long (without liners) core recoveries through the shallow gas hydrate-bearing sedimentary sequence to 90 mbsf. Bit rotation varied from 15 to 45 rpm (increasing with depth), and the average ROP using this system was ~8 m/hr. The RAB-8 was recovered at the rig floor at 2000 hr on 25 July, and the downhole log data recorded in computer memory were downloaded. Total testing time was ~22 hr.

Logging Quality

Figure F34 shows the quality control logs for GVR (RAB), MWD, NMR-MRP, and VND LWD tool string deployed in Hole 1249A. Other than about a 3-m-thick zone immediately below the seafloor, the upper ~32 m of Hole 1249A was drilled at an ROP of 15 m/hr (±3 m/hr). From a depth of ~32 mbsf to the bottom of the hole, the ROP was more variable, but it generally averaged ~25 m/hr (±3 m/hr). The ROP of 25 m/hr, which was sufficient to record one sample per 4-cm interval (~25 samples per meter), was obtained over 85% 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 in Hole 1249A. The increased pump rates below 30 mbsf and a ROP of 25 m/hr yielded enhanced NMR-MRP porosity data, with a data sampling resolution of ~1 sample per 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 differential caliper values are <1 in over 90% of the total drilled and cored sections in Hole 1249A. Only the uppermost ~10 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 0.1 g/cm³ (Fig. F34), 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. In Hole 1249A, the time-after-bit (TAB) measurements were 10 ± 5 min for ring resistivity and gamma ray logs and 80 to 120 min for the density and neutron porosity logs (Fig. F34). The TAB values remained relatively constant over most of the hole, coinciding with the steady ROP.

The LWD RAB-8 tool in Hole 1249B yielded high-quality data over the drilled and cored interval of the hole to a TD of 90 mbsf. Eight rotary cores were recovered from Hole 1249B with 32.9% recovery, on average, through the 45-m cored interval (Table T18). The RAB-8 test cores were normally processed and archived and will be correlated to the RAB-8 logs over the same depth interval. RAB-8 images and logs from Hole 1249B are of high quality but require additional postcruise depth corrections and analysis to account for the coring process. Preliminary observations suggest, however, that the recorded RAB data from Hole 1249B correlates well with the LWD logging curves in Hole 1249A. In the future, when drilling in harder formations with faster rotary coring, core recovery and logging data quality are expected to improve using the RAB-8 coring system. This successful test marks the first ever logging-while-coring experiment, which is new technology that allows for precise core-log depth calibration and core orientation within a single borehole and without a pipe trip.

The depths relative to seafloor for the LWD logs from Hole 1249A 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 1249A, it was determined that the gamma ray log pick for the seafloor was at a depth of 787.0 mbrf. The rig floor logging datum was located 11.0 m above sea level.

The RAB-8 log data from Hole 1249B were not available on the ship for immediate analysis. Therefore, only the downhole LWD logs from Hole 1249A were used to assess the geology of Site 1249 (Figs. F35, F36). As discussed above, the LWD logs from Hole 1249A are of excellent quality. Lower pump rates through the shallow subsurface section greatly reduced the effect of borehole washouts on the logs in the near-surface unconsolidated sediments. At Site 1249, the downhole LWD logs dramatically reveal a thick, almost-continuous, gas hydrate-bearing sedimentary section throughout the entire drilled interval in Hole 1249A (Fig. F37).

The logged section in Hole 1249A consists of one "logging unit" based on the analysis of the acquired downhole LWD log data (Fig. F35).

Logging Unit 1 (0-90 mbsf; TD of Hole 1249A) is characterized by numerous intervals of very high electrical resistivity, with measured average peak values exceeding 160 m. The gamma ray log generally increases with depth from ~40 American Petroleum Institute gamma ray units (gAPI) near the top to >70 gAPI near the bottom of the hole. Logging Unit 1 at Site 1249 is included within lithostratigraphic Unit I-II (0-89.50 mbsf), which was described by the shipboard sedimentologists as diatom-bearing clay to silty clay stratigraphic sequences (see "Lithostratigraphy"). The downhole logging-measured density increases with depth in the hole (1.3 at the top to near 1.8 g/cm³ at the bottom). Near the top of Hole 1429A, the density log reveals several conspicuous low-density intervals (ranging from ~1.1 to 1.5 g/cm³). Because of drilling safety concerns, none of the holes at Site 1249 were cored or drilled through the predicted depth of the BSR (estimated at ~115 mbsf).

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 hydrate in the borehole, thus yielding information about the nature and texture of gas hydrate in sediments. The RAB image in Figure F38 has a mottled appearance and is characterized by light (high resistivity) bands, which in many cases can be traced across the display. These light continuous high-resistivity bands likely represent gas hydrate occupying high-angle (40°-80°) fractures in Hole 1249A. In addition, the bright mottled appearance of the RAB image in Figure F38 may also reflect the presence of disseminated gas hydrate in the sediments.

Sediment porosity can be determined from analysis 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 logs have been used to calculate porosity for Hole 1249A. 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 bulk density in Hole 1249A (Fig. F35) increase with depth and are relatively consistent (as discussed above); however, the density log measurements are much more variable near the top of the hole (0-15 mbsf). The LWD log-derived density measurements from Hole 1249A were used to calculate sediment porosity () 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) in Hole 1249B range from an average value at the seafloor of 2.69 to ~2.71 g/cm³ at the bottom of the hole (see "Physical Properties"). The density logging-derived porosities in Hole 1249A generally range from ~60% to 75% (Fig. F36). However, the density log porosities in the upper part of the hole (0-15 mbsf) are more variable, ranging from 70% to near 100%. The zones near the top of the hole that exhibit very high density log porosities (i.e., very low formation densities) are also characterized by very high resistivities (in one case exceeding 500 m), which suggests the presence of massive gas hydrate layers.

The LWD neutron porosity log from Hole 1249A (Fig. F36) yielded sediment porosities ranging from an average value at the top of the logged section of ~70% to ~60% near the bottom of the hole. The "total" sediment porosities derived by the LWD NMR tool in Hole 1249A (Fig. F36) ranged from ~30% at a depth of 16 mbsf to ~55% near the bottom of the hole (56 mbsf).

The comparison of core- and log-derived porosities in Figure F36 reveals that the density and neutron log-derived porosities are generally higher than the core-derived porosities in Hole 1249A (16-56 mbsf). However, the NMR-MRP porosities are lower than the core-derived porosities throughout the entire hole. The higher value of density- and neutron log-derived porosities compared to NMR-MRP porosities can be attributed to the presence of gas hydrate. Porosities calculated from NMR-MRP tool, density, or neutron logs in gas hydrate-bearing reservoirs are subject to error because most downhole porosity devices are calibrated to the physical properties of water-bearing sediments (as reviewed by Collett and Ladd, 2000). Therefore, downhole log-derived porosities need to be corrected for the presence of gas hydrate. The required correction for density- and neutron-derived porosities is relatively small, but NMR-MRP porosities are more significantly affected by the presence of gas hydrate. The effect of gas hydrate on the downhole logging-derived porosities from Site 1249 will be further examined after the cruise.

The presence of gas hydrate at Site 1249 was documented by direct sampling, with numerous specimens of gas hydrate recovered from near the seafloor to a depth of 75.07 mbsf in Holes 1249B-1249F. From these occurrences of gas hydrate, 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 massive and disseminated gas hydrate is present throughout the logged and cored interval at Site 1249. 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. Hole 1249A is characterized by intervals of high electrical resistivities, but we have no acoustic data (because no wireline logging was conducted at Site 1249) to further evaluate the occurrence of gas hydrate or free gas at this site.

Resistivity logging data were used to quantify the amount of gas hydrate at Site 1249. For the purpose of discussion, it is assumed that the high resistivities measured in Hole 1249A are due to the presence of gas hydrate. Archie's Relation,

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

(see "Downhole Logging" in the "Explanatory Notes" chapter), was used with resistivity data (R_t) from the LWD RAB tool and porosity data () 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 Archie-derived S_w , with

S_h = 1 - S_w .

For Archie's 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 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 1249 were a = 1, m = 2.8, and n = 1.9386. The Archie Relation yielded water saturations (Fig. F37) ranging from a minimum average value of only ~8% in Hole 1249A to a maximum of 90% near the seafloor, which implies gas hydrate saturations at Site 1249 ranging from 10% to 92%.