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Downhole logging

Hole M0015A

Open borehole logging was carried out in two stages in Hole M0015A (72.15 mbsl) because of borehole instability. By positioning an open shoe casing at 20.0 mbsf, the bottom part of Hole M0015A could be logged. Borehole conditions were hostile, indicated by the caliper and image logs (Fig. F71); primary cavities up to 90 cm can be observed in the lower part of the section (below 22.5 mbsf). The optical imaging tool got stuck in the cavity at ~27.5 mbsf while logging up. Tool recovery resulted in cable head damage, and a complete changeover of the entire logging setup was necessary to continue downhole logging. It was therefore decided to stay above this interval in the second logging attempt; the basal part of the last deglacial sequence (lithologic Unit I) was imaged only with the acoustic borehole televiewer (ABI40). Its top part was logged by placing the open shoe casing at 7.5 mbsf.

The lowermost interval of the last deglacial sequence (Unit I) (base to 29.03 mbsf; Cores 310-M0015A-31R through 41R; Fig. F72) is characterized by an extremely large borehole diameter, isolated high-reflectivity fragments (coral fragments), and overall low reflectivity values, which can be classified as a rubbly lithofacies. Lithologies (~25.13–29.03 mbsf) consist of very open frameworks with branching coral colonies. Intervals displaying extensive development of microbialites are characterized by a more intense response in acoustic reflectivity. In Hole M0015A, the uppermost lithologies (23.0–25.13 mbsf; Cores 310-M0015A-23R through 26R) consist of coralgal-microbialite frameworks dominated by tabular and branching coral colonies. Striking images show cavities up to 90 cm in size and less intense (compared to Tiarei) microbialite encrusting (Fig. F72). Within these primary cavities, thrombolitic fabrics can often be observed “hanging” down from cavity ceilings. Formation electrical resistivity values gradually decrease from 2.6 to 0.9 m, where the lowest values can easily be correlated with cavities and the highest values can be correlated with intense microbialite encrustation. Sonic velocities (VP) were very difficult to obtain; however, a range in VP from ~1550 m/s (seawater in cavities) to 3531 m/s was recorded.

In the upper part of the last deglacial sequence (15.50-22.60 mbsf; Cores 310-M0015A-13R through 22R), formation resistivity slightly increases toward the top. VP ranges from 1805 to 3718 m/s, and sonic Stoneley wave velocities are, on average, 1197 m/s. An optical image of a medium-sized cavity shows that this unit consists of thick branching coral assemblages (22.60–19.50 mbsf). The top 4 m shows an increase in thickness of the branches along with increasing intensity of microbialite encrusting.

From 15.50 mbsf upsection, a change in coral morphology can be observed. Large-sized cavities are absent, and microbialite lining of mostly tabular and locally encrusting corals can clearly be distinguished in the optical and acoustic images (e.g., 9.75–10.50 mbsf; Core 310-M0015A-9R). Formation resistivity values slightly decrease toward the top, where the lowest values can be correlated with medium- to small-sized primary cavities.

Hole M0017A

Borehole conditions in Hole M0017A (56.45 mbsl) were extremely harsh, and it was difficult to deploy logging probes because the borehole was highly unstable and the relatively soft lithologies penetrated (overall low acoustic amplitude values in the ABI40 image log) resulted in very murky borehole fluids. Calipers show a large increase in borehole diameter at various intervals. Optical images are seriously affected by murky borehole waters; only at the top, just below the casing, is the quality reasonable. Acoustic images are not affected by this, and they are a high-quality visual representation (millimeter scale) of the lithologies cored (Fig. F73).

In Hole M0017A (Fig. F74), the last deglacial sequence can be divided into three intervals based on the downhole logging data:

  • Interval 1 (31.55–29.75 mbsf; Cores 310-M0017A-18R through 19R) comprises assemblages of branching coral colonies with large-sized cavities. Formation electrical resistivity is low in this interval.
  • Interval 2 (25.34–29.75 mbsf; Cores 310-M0017A-16R through 17R) has formation electrical resistivity values that gradually decrease from 3.0 (~29.45 mbsf) to 1.63 Ωm, where lowest values can easily be correlated with the cavities. Sonic velocities were very difficult to obtain; however, a clear decrease in VP from 3603 to ~2173 m/s shows that analogs to the formation resistivity trend can be observed in this interval. Stoneley wave velocities could not be measured because at the end of the VP run the sonic tool got stuck below the casing. During tool recovery, the wireline cable was badly damaged. At the time of logging operations, the temperature of the borehole fluid was ~27.3°C, pH values were ~7.82, and electrical conductivity was ~56.65 mS/cm (0.177 m). Comparing the acoustic reflectivity response in this interval with an ABI40 image of the same interval in Hole M0015A shows that this interval consists of a branching coral interval as well. Resistivity values increased from ~1.63 to 3.46 Ωm in the interval from 24.72 to 23.40 mbsf, marking the transition from the middle to the upper part of the interval.
  • Interval 3 (13.50–23.40 mbsf; Cores 310-M0017A-10R through 15R) shows a gradual decrease in resistivity values from 3.46 to 1.92 Ωm. The temperature of the borehole fluid decreased from ~27.13°C, pH values were ~7.82, and electrical conductivity was ~56.65 mS/cm (0.177 m). VP decreased at first from 3521 to 2032 m/s (24.03–21.64 mbsf). From 21.20 mbsf uphole, VP ranged from 2024 to 3000 m/s over relatively short intervals. The upper part of Hole M0017B shows a more homogeneous acoustic reflectivity response, indicating that pronounced differences in lithologic components in the last deglacial sequence and large-scale cavities are absent.

Synthesis of geophysical downhole logging at Maraa

Wireline logging operations at Maraa sites produced nearly complete downhole coverage of the last deglacial sequence from 41.65 to 102 mbsl. Tools have different data coverage in various holes (see the “Downhole logging” sections in the individual site chapters). In Figure F75, borehole televiewer images, natural radioactivity logs (total counts), and electrical resistivity logs are plotted in meters below sea level. In each of the logged boreholes, the boundary between the last deglacial sequence and the older Pleistocene sequence is indicated. The depth below present-day sea level of the top of the older Pleistocene sequence is highly variable and therefore indicates a rugged morphology prior to the development of the last deglacial sequence. Resistivity does not show any direct signal, but spectral gamma ray logs indicate a significant increase of counts in the older Pleistocene sequence (Holes M0005D and M0007A). Caliper, borehole fluid characterization, and acoustic tools all contain particular information on parameters for specific intervals and lithologies.