Logging Summary

IODP Expedition 329:

South Pacific Gyre Microbiology

Introduction

Figure 1. Location map showing the sites drilled during Expedition 329.

The main objectives of the downhole measurement program for Expedition 329 were to document crustal physical properties, define structural and lithologic boundaries as a function of depth, and identify alteration in the basaltic basement from the passage of fluids. Downhole logging measurements complement core measurements by determining the thickness of lithologic units in intervals where core recovery was poor. Borehole logging and core-log integration are invaluable for reconstructing recovery gaps and estimating bulk geochemical and structural characteristics of deep basement drill sites. Logging data from the South Pacific Gyre will help characterize current physical properties that may set important constraints on the downhole microbial community.
Expedition 329 logging operations were planned for four sites: the three deep basement sites (U1365, U1367 and U1368) and the deepest of the sediment sites (U1371) (Figure 1).

The purpose of this plan was to fully characterize the nature of the basaltic basement particularly looking at alteration, structure and fluid flow to study potential habitability of the basement for microbial life.
Unfortunately due to poor hole conditions and weather only one site of the four was able to be logged, Site U1368. Two tool-strings were deployed at the site: triple combo and FMS-gamma.

Operations

Site U1368 was drilled to a depth of 104.5 meters below sea floor, ~ 75 meters into basaltic basement. Prior to the start of logging operations the bit was released, the BHA was raised to a depth of 34 mbsf and the hole flushed to clear debris. No mud was used for logging operations. The triple combo was the first toolstring deployed and contained natural and spectral gamma ray (HNGS), density (HLDS), GPIT (magnetic orientation), and resistivity (DIT-E). One downlog and two complete uplogs were completed. The second deployment consisted of FMS (resistivity images) and spectral and natural gamma (HNGS). The sonic tool (DSI) was not run to reduce the length of the toolstring in the relatively shallow hole. This toolstring also completed one downlog and two uplogs without incident.

Logging Results

Figure 2. FMS images showing highly fractured rubbly pillows and less fractured more massive pillows.

Figure 3. Comparison of percent potassium natural gamma ray measurements from downhole logs (purple) and whole round cores (green).

Preliminary interpretation of the downhole log data subdivided Hole U1368F into two units: Unit I covering the sediment sequence and Unit II covering the underlying basaltic basement sequence. FMS images show the basement sequence consists of a thick sequence of rubbly pillow lavas, with one or two larger meter scale lava flows (Figure 2). Formation MicroScanner (FMS) images were obtained for the open-hole interval between 39 and 103 m WMSF. The diameter of hole from the FMS calipers varied between 18.8 and 35.1 cm. High quality FMS images were obtained in much of the borehole; only intervals that were washed out showed poor quality images.

Standard, computed, and individual spectral contributions from 40K, 238U, and 232Th were part of the gamma ray measurements obtained in Hole U1368A with the HNGS. Downhole gamma ray measurements covered a total of 95 m of the lithostratigraphic sequence in Hole U1368F both in open hole and through drill pipe. Natural gamma measurements made downhole compare well with results of measurements on whole round cores (Figure 3).

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