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Logging Summary

IODP Expedition 335:

Superfast Spreading Rate Crust 4

Expedition 335 Scientific Party

    Figure 1. Location map of IODP Expedition 335.

    In the spring of 2011, IODP Expedition 335 returned to ODP/IODP Hole 1256D, in the Eastern Equatorial Pacific Ocean (see Figure 1), for the first time since december 2005 (See IODP Expedition 309/312 Proceedings). The objectives were to deepen this basement reference hole into the gabbroic rocks of lower oceanic crust and address fundamental questions on the formation of the lower crust and on the seismic layer 2/3 transition in a superfast spreading environment. A more complete overview of the expedition can be found in the Expedition 335 Preliminary report.

    Initially, a temperature log was planned to record the formation equilibrium temperature before drilling, and a complete logging program was designed to fully characterize the newly drilled formation at the end of the expedition. Because of hole trouble, technical failures and time constraints, the triple combo was the only logging tool string deployed during the expedition. It recorded the resistivity, density, porosity and natural radioactivity of the formation, as well as the hole size and the temperature of the borehole fluid, reaching the maximum depth of 1520 mbsf, 80 m below the deepest logs recorded at the end of Expedition 312. Despite its limited scope, the logging program confirmed the beneficial results of the various hole remediation operations during the expedition, showing that Hole 1256D was left in optimal conditions for future deepening into the lower oceanic crust.

Logging Operations

    Figure 2. A) Logging tool string used during Expedition 335. B) Hole trajectory and size, derived from the inclinometry data of the General Purpose Inclinometry Tool (GPIT) and the caliper log recorded during Expedition 335.

    The triple combo tool string deployed at the end of Expedition 335 is shown in Figure 2A. Several of the tools included were used for the first time in IODP: the HRLA (High Resolution Laterolog Array) was used to measure resistivity beyond the range of the Dual Laterolog that was used in previous expeditions; the Logging Equipment Head with Mud Temperature (LEH-MT) was used to connect the tool string to the wireline and included a temperature sensor to measure the borehole fluid temperature; the Enhanced Digital Telemetry Cartridge (EDTC-B) was used to transmit the data recorded by the tool string, and included a scintillation gamma ray sensor to measure natural formation radioactivity; finally, the Modular Temperature Tool (MTT) had been used only once before and never in the temperature range encountered in Hole 1256D. All tools performed reliably and provided high quality data over the entire open hole.

    The several weeks spent clearing and stabilizing the interval between 920 and 970 mbsf and cleaning the deepest section of the hole required more than twenty lowerings of the drill string, and multiple instances of reaming tight intervals. After such operations, one of the goals of the triple-combo run was to record a caliper log over the entire hole, to determine its condition, assess the results of the cementing operations and to help plan the final cementing to stabilize Hole 1256D for future expeditions. Figure 2B is a rendering of the hole size and trajectory derived from the caliper log and from the inclinometry data of the General Purpose Inclinometry tool (GPIT). It indicates that the hole progressively deviates to the West and contains several enlarged intervals, in particular below 1400 mbsf where most of the efforts were focused during the final weeks of operations.

Logging Results

    Figure 3. Summary of the logs recorded during Expedition 335 in the deeper section of Hole 1256D.

    Figure 3 shows the main logs recorded in the deepest section of Hole 1256D, some of which was not reached by logging tools during Expedition 312. The hole size in the left track illustrates how irregular the hole is below ~1410 mbsf. The anomalously low density and high porosity readings below this depth, as indicated by the comparison with the measurements on core samples, are a direct consequence of the hole size and and are not reliable. The decoupling between the shallow and resistivity logs is also a consequence of the hole size. However the deepest resistivity measurement were not affected by the hole size.

    One of the most significant observations in the newly recorded data is a decrease in resistivity with depth, starting within Gabbro 1 (~1440 mbsf) and more apparent by Gabbro 2 (below ~1480 mbsf). This contrast with the expected resistivity increase in the plutonic section suggests that the deepest section might be fractured, possibly part of a fault, which could explain some of the difficulties encountered while coring. However, these resistivity values are in the same range as shallower in the sheeted dikes complex and could be representative of the actual electric properties of the dike screens and various levels of alteration.

Hole Size

Figure 4. Comparison between the main logs recorded by the Triple Combo tool strings during Expeditions 312 (blue lines) and 335 (red lines).
Figure 5. Comparison between the hole size during Expeditions 312 and 335 around the problematic zone 910-970 mbsf

The comparison betwen the hole sizes during Expeditions 312 and 335 in Figure 4 shows that the hole has changed little after 5 years and despite working the hole for several weeks before logging. Hole enlargements are indicated in the same intervals, and have similar extents. This is confirmed by the very good repeatibility between the different logs displayed for both expeditions in Figure 4.

The only significant difference between the two sets of logs is in the gamma ray log between ~920 and ~960 mbsf, suggesting that the cement used includes radioactive nuclides detected by the gamma ray sensor. This is shown more clearly in Figure 5, where the differences between the two holes are illustrated by showing intervals of hole enlargement and hole reduction between the two expeditions.

Intervals where hole size has decreased, presumably because of the cement emplaced when the bit was at ~960 mbsf, mostly coincide with higher gamma ray reading during Expedition 335. The largest difference in the gamma ray logs is between 925 and 930 mbsf, where the hole was the largest and presumably the largest volume of cement was deposited. The increase in hole size above 920 mbsf is probably a consequence of the several days spent trying to pass this depth. The cement reduced the hole size and its roughness between 930 mbsf and 970 mbsf, eliminating asperities and allowing the 15 smooth reentries after the cementing operations.

Temperature Logs

Figure 6. Temperature log recorded during Expedition 335 and comparison with previous temperature logs in Hole 1256D.

The comparison in Figure 6 between the temperature logs recorded by the two temperature tools during Expedition 335 and the temperatures measured during previous expeditions in Hole 1256D shows similar trends as the borehole fluid recovers from the disturbance of the drilling operations.

Several excursions to lower temperatures, in particular around 925 mbsf and at 1060 mbsf, at the top of the sheeted dike complex, coincide with intervals with lower resistivity, indicating more permeable intervals where the formation might have been invaded by the drilling fluid and is consequently recovering more slowly from the drilling process. The larger hole diameter in these intervals also implies larger borehole fluid volumes that could also contribute to slower thermal rebound. A kick at ~1300 mbsf, that was also observed during Expedition 312, coincides with lower resistivity and is probably also associated with fluid exchanges with the formation. These anomalies will be the object of numerical modeling, which in combination with other logs should provide estimates of the permeability in these intervals.

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

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