Logging Summary

IODP Expedition 301:

Juan de Fuca Hydrogeology

Introduction

Figure 1. Bathymetric map showing the sites drilled during IODP Expedition 301 (black stars) and ODP Leg 168 (red dots).

Figure 2. Two-way travel time seismic reflection profile showing the location of Site 1301 that was drilled during IODP Expedition 301 (Expedition 301 Preliminary Report, 2004). This seismic data was acquired with the 2000 ImageFlux survey (Sonne, SO149). Vertical lines on this figure represent approximate total depth of Holes 1301A (shallower hole) and 1301B (deeper hole).

During Integrated Ocean Drilling Program (IODP) Expedition 301, sediments and oceanic crust were drilled at Site 1301,which is located on the Endeavour segment of the Juan de Fuca Ridge (Figure 1). The primary objectives of the IODP Expedition 301 were to evaluate the formation-scale hydrogeologic properties within oceanic crust; determine how fluid pathways are distributed within an active hydrothermal system; establish linkages between fluid circulation, alteration, and microbiological processes; and determine relations between seismic and hydrologic anisotropy (Expedition 301 Preliminary Report, 2004).

Site 1301 is located above a buried basement ridge (Second Ridge), where sediment thins to 250-265 m (Figure 2). Site 1301 was positioned ~1 km SSW of Site 1026, which was drilled during ODP Leg 168 (Figure 1). Three holes were drilled at Site 1301 (Hole 1301A, 1301B, and 1301C) and logging operations were carried out along the deeper oceanic crust interval penetrated in Hole 1301B. For further information and geological setting, please refer the Expedition 301 Preliminary Report.

Logging Operations

Figure 3. Schematic showing the configuration of the wireline logging tool strings that were used during IODP Expedition 301.

Figure 4. Results of triple combo measurements and sonic tool string measurements from Hole 1301B. Black dots in the porosity, density, and P-wave velocity panels represent shipboard measurements on core samples.

The wireline logging operations consisted of four tool string deployments (Figure 3): (1) the triple combo, (2) Ultrasonic Borehole Imager (UBI), (3) Formation MicroScanner (FMS)/sonic, and (4) the Well Seismic Tool (WST). Brief descriptions of the operations and tool string configurations are as follows:

(1) The triple combo tool string consisted of the logging equipment head - mud temperature (LEH-MT) cable head with sensors for measuring spontaneous potential (SP), temperature and tension, the Hostile Environment Natural Gamma-Ray Sonde (HNGS), the Hostile Environment Lithodensity Sonde (HLDS), the Accelerator Porosity Sonde (APS), and the SlimXtreme Array Induction Imager Tool (QAIT). This tool string measured the basic physical properties from 350 mbsf to 578.2 mbsf, without problems (Figure 4). The caliper arm shows that the borehole is almost in gauge below ~ 464 mbsf but very irregular and oversized between 352 and 464 mbsf, reaching >18 inches between 395-405 mbsf.

(2) The UBI tool string consisted of the LEH-MT with sensors for measuring SP, temperature and tension, Scintillation Gamma Ray Tool (SGT), the General-Purpose Inclinometry Tool (GPIT), and the UBI. An obstruction was found in the hole at 428.2 mbsf. Several attempts to get past the obstruction failed thus, UBI data were only acquired from 350 mbsf to 428.2 mbsf.

(3) The FMS/sonic tool string consisted of the LEH-MT with sensors for measuring SP, temperature and tension, the SGT, the Dipole Sonic Imager (DSI), the GPIT, and the FMS. The obstruction was found at the same depth of 428.2 mbsf resulting in a shortened logged interval. The sonic velocity logs contain several isolated intervals of with noisy data, but most of the logged interval seems to have reliable results (Figure 4).

(4) Rigging up procedures began with the deployment of a generator injector seismic source (GI-gun) consisting of a 45 in3 generator chamber volume and a 105 in3 injector chamber volume and by placing several observers around the ship for compliance with the IODP marine mammal policy. The procedure included a 1-hr observation period prior to the use of the seismic source where the Mate on watch and the marine mammal observers on the aft end of the ship began observations. Observations continued throughout the duration of the seismic experiment and no marine mammals were sighted within the 700 m safety zone. After the initial observation period, the “soft start” procedure began with the seismic source being fired at 30-sec intervals starting at a pressure of 500-psi and gradually increasing the pressure to the “operational” pressure 2000 psi over a 30-minute period. During the WST experiment, the GI-seismic source was operated at 2000 psi air pressure with a time delay between the generator and injector shots of 40 ms. As in the case of UBI and FMS/sonic deployments, the tool could not pass the obstruction located at 428.2 mbsf and the WST stations were spaced at 20-m from the top of the obstruction to the bottom of the bottom hole assembly.

Logging Results

Figure 5. (A) WST stacked waveforms and (B) determination of interval velocities for the 100-m section of open hole that was logged.

Resistivity, Porosity, Density, Gamma ray, and Spontaneous potential

Most of the resistivity curves show values within the basement ranging between 0.27 and 146 Wm. Among the set of resistivity curves, the 10-in depth of investigation curves show lower electrical resistivity values than the other resistivity curves of the same vertical resolution. In general, all curves follow similar trends.

Values of neutron porosity show a large range from 4 to 100%. Neutron porosity values are particularly high above 462 mbsf where the borehole is enlarged and lower in the bottommost part of the hole where values are mostly between 5 and 20%, which represent values that are closer to porosities measured on core samples (~2 to 9%).

Density values range from 1.23 to slightly over 3.00 g/cm³ over the entire logged section of the borehole. Below 462 mbsf, density values are between 2.5 and 3.0 g/cm³ and close to the range of values measured on core samples, which average 2.78 ± 0.08 g/cm³. Above 460 mbsf density values are lower due to the irregular and enlarged shape of the borehole.

Total gamma ray values (HSGR) range from 5.3 to 13.2 gAPI. Potassium values are low with values between 0 and 0.48 wt.%. Thorium and uranium values are mostly between 0 and 1 ppm. The slight increase in gamma ray values below 515 mbsf may be caused by slightly higher alteration, which is the only depth interval with highly altered rocks identified in the cores samples. Typical secondary minerals in this interval are saponite, iron hydroxides, and celadonite. Celadonite may contain potassium and thus could increase the gamma ray values.

SP values vary between –170 and 24 mV and tend to increase within the enlarged borehole intervals while decreasing within more massive intervals. Particularly, low-values are observed above 378 mbsf.

Sonic Velocity

P-wave velocities range from 4000 m/s to 6000 m/s, and correlate well with the average laboratory velocity measurements of ~5300 m/s that were obtained from core samples (Figure 4). S-wave velocities range from 2000 m/s to 3000 m/s. However, several sections have anomalous velocities, especially below 385 mbsf where both P- and S-wave velocities are low because the borehole is enlarged and irregular. Although the tool worked well, the processing of the waveforms was not straightforward and further processing had to be done onshore to improve the quality of the results.

Logging Units

A preliminary interpretation of the geophysical logs yielded the identification of 21 logging units (Figure 4). Most of the logging units seem to be characterized by massive sections bounded by fractured intervals. In Figure 4, the yellow shading parts represent the massive intervals and the white shading parts represent mostly fractured intervals. A few of the massive flow units (purple shading on the Figure 4) can also be identified in the downhole logs as slight increases in electrical resistivity, low neutron porosity, and high density values. Above 462 mbsf, pillow basalt units are characterized by the enlarged borehole intervals however, a pillow basalt unit below 474 mbsf is characterized by low-porosity and high-density values with small spikes of high-porosity and low-density that may be related to thin fractured intervals.

Vertical Seismic Profile (VSP)

The waveforms at the each WST station were stacked and a travel times were determined from the first breaks of the waveforms acquired at four stations (Figure 5). In some instances it was difficult to determine the first break therefore, the median of the first break for each stacked trace were also used to determine interval velocities. The gradient of the travel time first break used to estimate an interval velocity produced a result of ~5220 m/s whereas the median yielded an interval velocity of 4990 m/s. Core sample and sonic log measurements show a slightly higher range of velocities and the difference may reflect the different scales of core measurements, sonic logs, and seismic experiments.

Borehole Images (FMS and UBI)

The quality of the FMS and UBI images were poor because of two main reasons. The section of the borehole that was imaged is characterized by washouts and irregularities that hinder the acquisition of high-resolution images. In addition, the new heave compensating system used during Expedition 301 may have not been working properly.

Summary

Overall, the logging data expand upon core-based observations and provide in situ measurements at Site 1301. The triple combo was the only tool string that provided reliable data of the entire borehole revealing ideal places for Packer experiments and allowing for the interpretation of several logging units that correlated to physical and lithological changes identified from core-based observations. The downhole coverage obtained with the other tool string deployments consisted of only 1/4 of the borehole’s total depth because of a borehole obstruction. The VSP experiment obtained the best results of any subsequent tool deployment allowing for the estimation of the shallow basement velocity profile

Gerardo J Iturrino: Logging Staff Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964, USA.