Logging Summary

IODP Expedition 303:

North Atlantic Climate 1

Introduction

Figure 1. Map of the North Atlantic showing the location of Expedition 303 sites.

The primary objective of Expedition 303 was to place late Neogene–Quaternary climate proxies in the North Atlantic into a PAC (Paleointensity Assisted Chronology), a chronology based on a combination of geomagnetic paleointensity, stable isotope, and detrital layer stratigraphies. Sites drilled during Expedition 303 are located off Orphan Knoll (Newfoundland), on the Eirik Drift (southeast Greenland), on the southern Gardar Drift, and in the central Atlantic “ice-rafted debris (IRD) belt” (Fig. 1).

The primary logging objective of Expedition 303 was to provide detailed core-log integration to allow assessment of core expansion and to provide a quality control check of the spliced core record. Given the high sedimentation rates at most of the Expedition 303 sites, a secondary objective was to examine cyclicity within the logging data. It was hoped that millennial scale changes would be identifiable in Formation MircoScanner (FMS) data. However, because of operational difficulties and deteriorating weather conditions it was only possible to deploy the “triple combination” tool string at one site, Site U1305. Unfortunately, this meant that the highest-resolution tools (the Lamont Multi-sensor Gamma ray Tool [MGT] and the FMS-sonic) were not deployed during Expedition 303.

Results from Site U1305

Figure 2. Caliper, main and repeat pass gamma ray and core recovery records for Hole U1305C. gAPI = American Petroleum Institute gamma ray units.

Figure 3. Caliper, density, porosity, electrical resistivity and photoelectric effect (PEF) data for the interval 95 to 250 mbsf in Hole U1305C.

Figure 4. Caliper, total gamma ray and spectral gamma ray data (K, Th, U) for the interval 95 to 250 mbsf in Hole U1305C. HCGR = computed gamma ray headspace, HSGR = total spectral gamma ray, gAPI = American Petroleum Institute gamma ray units, in. = inches.

The caliper data show that the diameter of the borehole ranged from ~13.6 to 18.0 in (Fig. 2), resulting in data of variable quality. Reproducibility of data is high between passes (see gamma ray example in Fig. 2).

The density and porosity tools require good borehole contact. Thus, intervals with a large borehole diameter are characterized by high porosities and low densities (Fig. 3). Density and porosity data are also less reliable when the caliper is not open (i.e., above ~107 mbsf during the main pass).

The downhole logging data suggest that the formation is fairly uniform in the open hole (Fig. 3). As expected, the density and porosity data are generally inversely related to each other and show downhole trends of increasing density and decreasing porosity. Resistivity values are low reflecting the generally moderate to high porosity sediments. Photoelectric effect (PEF) values range between 1.0 and 3.3 b/e-, consistent with the clay-rich lithologies. Extremely low PEF values (>1.8, the PEF value of pure quartz) may be the result of poor contact with the borehole wall or extremely porous intervals (seawater has a PEF value of 0.807).

Gamma-ray values increase slightly with depth, possibly as a result of increasing clay content. The low uranium content of the formation results in very similar HSGR (Total gamma ray) and HCGR (summation of Th and K gamma rays only) values (Fig. 4). The uranium data suggest that total organic carbon values in the logged interval are consistently very low, as shown by discrete samples. Potassium and thorium display very similar trends downhole, suggesting that there are no major downhole changes in mineralogy (Fig. 4).

Core-Log Comparisons

Figure 5. Comparison of core and log physical properties from Hole U1305C. A: Gamma ray activity for the interval 95 to 250 mbsf. B: Density for the interval 95 to 250 mbsf. C: Gamma ray activity for the interval 190 to 215 mbsf. cps = counts per second; gAPI = American Petroleum Institute gamma ray units.

Figure 6. Correlation of spliced core gamma-radiation data (corrected counts) in red with logging data in black, for the depth interval of 100 to 200 m from Site U1305. In the left-hand panel is the spliced data in mcd; the right hand panel shows the corrected spliced data (in meters equivalent logging depth or meld) and the logging data (mbsf). Note that spliced core record in the right-hand panel has been smoothed in Sagan to allow easier correlation. cps = counts per second; gAPI = American Petroleum Institute gamma ray units.

All the downhole data sets display meter to decimeter scale variability that are most likely the result of subtle changes in lithology. A comparison of log- and core-derived natural gamma radiation and density records shows close agreement in downhole trends and patterns (Fig 5). Measured density values are very similar in both core and log data. Closer inspection of the gamma ray data suggests that 5-meter scale patterns can be recognized in both the core and log records (Fig 5).

Using the downhole log records as a depth reference, and the software program Sagan, it was possible to correlate the core measurements to equivalent logging depths to more precisely determine the amount of core expansion.

Figure 6 shows some of the tie points used to integrate core and log data. By recognizing similar patterns in the composite core record and the logging data, it was possible to convert the depths for core data from mcd (meters composite depth) to meld (meters equivalent logging depth). Using this method of core-log integration it will be possible to compare various physical properties measured in core and downhole. This allows us to more fully utilize and integrate measurements that were only made either downhole (such as spectral gamma and resistivity) or on core (such as color).

Stuart Robinson: Logging Staff Scientist, School of Human & Environmental Sciences, University of Reading, Whiteknights, PO Box 227, Reading, RG6 6AB, UK.