Figure 1: Location map of the sites drilled during Leg 201 in the equatorial pacific and on the Peru margin. Sites where logs were recorded are in red.
Leg 201 was the first ODP leg dedicated primarely to the study of life in deep marine sediments. The seven sites drilled (Figure 1) had been previously visited by ODP/DSDP legs and were chosen to be representative of the general range of bilogocal activity and diversity in marine sediments.
Figure 1: Location map of the sites drilled during Leg 201 in the equatorial pacific and on the Peru margin. Sites where logs were recorded are in red.
In addition to the unprecedented variety of chemical and biological analyses that were performed to describe the microbiological activity, an extensive characterization of the subseafloor environment was completed by core description and physical property measurements. The logging program was intended to define in-situ the physical and hydrological constraints on the downhole microbial community. A complete account of the Leg 201 initial findings can be found in the Leg 201 Initial Report .
Five sites were logged during Leg 201 (see Figure 1): Sites 1225 and 1226, in the Equatorial Pacific, were composed mostly of biogenic sediments and presented a biological activity intermediate between the organic-rich continental margin and the life-barren open ocean. Sites 1228 and 1229 were drilled on the Peru continental shelf in terrigeneous sediments extremely rich in organic material. Site 1230 on the lower slope of the Peru trench was at the transition between the accreted sediments and the continental shelf, and offered the opportunity to determine the mutual influence of gas hydrate and microbial communities. The Triple Combo tool stringwas run at the five sites. The FMS/Sonic tool string was added to the logging program at Hole 1230A in order to use the sonic log and the FMS images to better characterize the hydrate deposit. Logging operations are summarized in the following table:
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Sites 1225 and 1226 are both located on the eastern Equatorial pacific, and represented an intermediate level of bilogical activity between the high end on the Peru continental margin (sites 1227-1230) and the low- activity open ocean site (1231). The mostly biogenic sediments at these two sites were described during Leg 138 in adjacent Sites 851 and 846 (Mayer, L., N. Pisias, T. Janecek, et al., 1992). The main logs recorded with the Triple Combo tool string are shown in Figures 2 and 3.
Figure 2: Logs recorded in Hole 1225A
Figure 3: Logs recorded in Hole 1226B
These logs can be generally described by low natural radioactivity and physical attributes (density, resistivity) typical of high porosity biogenic sediments. The high porosity and low density are confirmed by the good agreement between measurements on core samples (grey circles) and the density and porosity logs.
The general lithology at these two sites, in brief carbonate and siliceous ooze and chalk, is common in the eastern equatorial Pacific. In this fairly uniform setting our characterization of the formation from the logs was based on the physical measurements with the clearest variations: the resistivity and the density logs. Because of the limited variations in all data, we identify only one logging unit in each site, made of several subunits. This classification is in agreement with the identification from core observation of only one lithologic unit in each site (see the lithostratigraphic summary). Similar descriptions were made during Leg 138 in Sites 851 and 846 (Mayer, L., N. Pisias, T. Janecek, et al., 1992). The logging subunits are defined more by trends in the downhole variations of the logs than by abrupt changes in any of the parameters. These trends could reflect changes in organic accumulation rates. Above 150 mbsf in Hole 1226B, the steady increase in gamma ray and in uranium concentration uphole, also observed during Leg 138 (Mayer, L., N. Pisias, T. Janecek, et al., 1992), indicates a high organic carbon content in reducing sediments that collect insoluble U(IV) from the sea water.
Peru continental shelf: Holes 1228A and 1229A
Sites 1228 and 1229 are both located on the Peru continental shelf, and were selected because of their high organic content. The mostly terrigeneous sediments at these two sites were described during Leg 112 in adjacent Sites 680 and 681 (Suess, E., von Huene, R., et al., 1988). The main logs recorded with the Triple Combo tool string are shown in Figures 4 and 5.
Figure 4: Logs recorded in Hole 1228A.
Figure 5: Logs recorded in Hole 1229A
The terrigeneous nature of the sediments at these sites is clearly shown by the gamma ray logs with significantly higher values than in Sites 1225 and 1226. The presence of sands is also partially responsible for the incomplete recovery (see recovery columns to the left of each figure), which made the logs in these sites particularly usefull for the characterization of the formation. The logs in the two sites are characteristic of sedimentation on a transgressive margin. Most logs, particularly the gamma ray, resistivity, and density, show a succession of sedimentation cycles indicating the fluctuations of sea level over the deposition history. Most cycles are 10 to 20 meters thick and are characterized by an increase upwards in gamma ray, density, and resistivity. This is the signature of fining upwards facies typical of transgressive margins: the finer, clay-rich sediments settle on top of the sequence as sea level rises or the basin sinks (Rider, 1996). Based on variations in the trends of these cycles, we have identified three logging units in each site. Each logging unit is divided into subunits corresponding mostly to individual cycles.
While overall the sediments recovered at the two sites are mostly terrigenous, the relative importance of the types of sedimentation can be seen in the ratio of thorium to uranium concentrations at Site 1229 (Figure 5d). This ratio can be used as an indicator of the relative strength of marine and terrestrial deposition (Rider, 1996). High values indicate more terrestrial input. Low values indicate higher uranium, and sediments of marine origin. The three logging units at this site were partially defined by the trends in this ratio.
Among the most striking features in the logs at these two sites are the very strong peaks in gamma radiation, in uranium content in logging unit 1, and a series of strong spikes in the resistivity logs. The gamma ray peaks in unit 1 are similar to features observed in the logs recorded in Hole 679E during Leg 112, that were attributed to the presence of uranium-bearing phosphorite layers created by the reworking of phosphate nodules (Suess, E., von Huene, R., et al., 1988). Similar phosphorite layers can be seen also in logging units 1 and 2 at Site 1229. The resistivity spikes in logging unit 3 at Site 1228 are possibly created by layers of hard sandstone cemented by calcitic dolomite that were recovered during Leg 112 (Suess, E., von Huene, R., et al., 1988). These dolomitic layers are the main reasons for the particularly low core recovery in this unit.
Site 1230 on the lower slope of the Peru trench was chosen to determine the mutual influence of gas hydrate and microbial communities. Because sonic velocity and electric resistivity are the logs the most sensitive to the presence of gas hydrate, the FMS/Sonic tool string was run at this site in addition of the Triple Combo. The main logs at this site are shown in Figure 6.
Figure 6: Logs recorded in Hole 1230A
Despite the good hole conditions indicated by the caliper log in Figure 6a, and while most data are of good quality, the porosity values are consistently ~20% higher than the core measurements. The presence of bound water with clays generally increases the porosity measured with neutron logs, but there was no similar discrepancy in the Sites 1228 and 1229 where the clay content was generally higher than at this site. Because the most dominant component of these sediments is diatom ooze, the core measurements might be underestimating the voids in the diatom skeletons, which are sensed by the porosity log. However, this should not account for the extent of the difference between the core and log measurements. We used the grain density measured on core samples to derive a porosity curve from the density log, by interpolating the grain density measurements at the logging sampling interval (0.1524 m) and assuming a uniform water density of 1.05 g/cc. The resulting profile (red line in Figure 6g) is in much better agreement with core measurements.
The general trend of the logs is controlled by the nature of the sediments, and by the location of the site on the lower slope of the Peru Trench, at the transition between the continental crust and the accretionary complex. The sediment composition is intermediate between the equatorial Pacific sites (Sites 1225 and 1226), which were dominated by biogenic sediments with extremely low gamma-ray counts, and the shallow margin sites (Sites 1228 and 1229), which contained mostly terrigenous sediments. The thorium concentrations consistently higher than the uranium values suggest that the clastic sediments are of continental origin (Rider,1996).
We distinguish two logging units, which correspond to the two lithologic units identified from the cores (see the lithostratigraphic summary). Logging unit 1, above 216 mbsf is made mostly of undisturbed biogenic sediments with varying amounts of siliclastics that define the various subunits. Unit 2 is composed of sediments under intense deformation belonging to the accreted complex. The top of this unit is marked by a sharp increase in resistivity and density. It is also clearly defined in the FMS electric images (Figure 7d) by a bright massive feature that dips ~69° to the north east at 216 mbsf.
Figure 7: Formation Microscanner (FMS) images recorded in Hole 1230B.
Identification of gas hydrate from logging data: Because of the instability of hydrate at surface conditions, logs are critical to evaluate in situ the extent of hydrate deposits. The measurements most sensitive to hydrate presence are resistivity, which reacts to the insulating properties of ice and to the drilling-induced freshening of pore water, and sonic velocity, which responds to the solidifying effect of solid ice in the pore space (Collett, 1998). In addition to an increase in sonic velocity, the presence of hydrates has been shown to increase sonic attenuation and decrease the amplitude of sonic logging waveforms (Guerin et al., 1999). In Figure 8 we show the different possible indicators of hydrate, and identify four broad intervals (in blue) where logging data could indicate the presence of gas hydrate surrounding Hole 1230A.
Figure 8: Identification of intervals where logging data show that gas hydrate could be present.
The extent of these intervals is mostly defined by areas where dipole waveforms, and to a lesser extent monopole waveforms, have lower amplitudes. Guerin et al. (1999) observe that at low hydrate concentrations, dipole waveforms are more sensitive to the presence of hydrate, which could explain the stronger amplitude contrasts in the dipole waveforms in Hole 1230A. The FMS image in this figure indicates that if hydrate is present in these intervals, it is disseminated in fine features, such as shown in Figures 7b and 7c, rather than in massive accumulations. The density log in this figure helps determine intervals where increase in resistivity cannot be attributed to hydrate. Because hydrate has a slightly lower density than water, its presence has little effect or a negative effect on density. Such a comparison to density indicates that resistivity spikes at 224 mbsf, 232 mbsf or between 216 and 220 mbsf are likely not created by hydrate, but by cemented layers. In comparison, resistivity spikes at 128 mbsf, 140 mbsf and 241 mbsf could be due to gas hydrate. The 1-meter-thick bright steep layer at 241 mbsf in Figure 7e could be particularly rich in hydrate.
Collett, T.S., 1998. Well log evaluation of gas hydrate saturations, Trans. SPWLA 39th Logging Symposium, paper MM.
Guerin, G., D. Goldberg, and A. Meltser, 1999. Characterization of in situ elastic properties of gas hydrate-bearing sediments on the Blake Ridge, J. Geophys. Res. ,104 (8), 17,781-17,796.
Mayer, L., N. Pisias, T. Janecek, et al., 1992. Proc ODP Init Repts, 138: College Station, TX (Ocean Drilling Program).
Rider, M. 1996. The geological interpretation of well logs (2nd ed.): Caithness (Whittles Publishing).
Suess, E., von Huene, R., et al., 1988. Proc ODP Init Repts, 112: College Station, TX (Ocean Drilling Program).