In this document we
present a summary of the wireline logging and logging while drilling (LWD)
results from ODP Leg 193. This was the first drilling leg to investigate the
geological, geophysical, geochemical, hydrological, and biological processes of
a convergent margin hydrothermal system in the Manus Basin in the Bismarck Sea (Figure
1).
Island arc settings and their associated felsic volcanic sequences have long been recognized as ideal places for a variety of valuable hydrothermal ore deposits. These range from massive sulfide deposits rich in both base and precious metals to deep-seated porphyry copper gold deposits. Understanding how such ore bodies were created in the past, by deciphering the interplay between igneous, structural, hydrothermal, and hydrologic processes in a close modern analog of such setting, will improve the capability of future exploration to recognize favorable signals of economic potential in ancient sequences.
Figure 1. Major active hydrothermal sites at convergent margins of the western Pacific Ocean.
The western margin of the Pacific plate displays numerous convergent segments or subduction zones, most of which, over the past two decades, have been shown to exhibit seafloor hydrothermal activity at one or more sites in their vicinity (Figure 1). The first place where hydrothermal "chimney" deposits and associated vent fauna were discovered, other than on a mid ocean spreading axis, was in the Manus Basin in the Bismarck Sea north of Papua New Guinea (Both et al., 1986). This was at a site now called Vienna Woods on the basaltic Manus spreading center, near the apex of a wedge of backarc oceanic crust (Figure 2). By contrast, the Eastern Manus Basin has a more complex geological construction involving the creation of continental-type crust, and the Manus Basin accordingly shows closer affinities to ancient ore body settings. It contains the PACMANUS (Papua New Guinea-Australia-Canada-Manus) hydrothermal field, discovered in 1991 (Binns and Scott, 1993), where the host volcanic sequence is conspicuously siliceous. Now thoroughly surveyed at the seafloor surface, PACMANUS is the site where the first subsurface study of an active felsic-hosted convergent margin hydrothermal system was conducted during Leg 193. As anticipated, we found significant differences between this site and hydrothermal activity hosted by mafic volcanic rocks and sediments on divergent margins (seafloor spreading axes) previously drilled during Ocean Drilling Program (ODP) Leg 158 to the TAG hydrothermal area on the Mid-Atlantic Ridge and Legs 139 and 169 in the northeast Pacific, respectively. The differences are profoundly important in understanding chemical and energy fluxes in the global ocean, as well as for understanding mineral deposit geology.
Figure 2. Regional tectonic map setting of the PACMANUS site drilled during Leg 193.
The Manus Basin is a rapidly opening (~10 cm/yr) backarc basin set between fossil and active subduction zones (Manus Trench and New Britain Trench, respectively) within a complex zone of oblique convergence between the major Indo-Australian and Pacific plates (Figure 2). The present-day configuration of spreading segments, extensional faults, and obliquely oriented transform faults in the Manus Basin is well established by bathymetric, sidescan sonar, seismic reflection, gravity, and magnetic surveys (Taylor, 1979; Taylor et al., 1991), and by microseismicity (Eguchi et al., 1989), which defines left-lateral movement on the transform faults.
In contrast to the wedge-shaped Manus spreading center, where new backarc oceanic crust has been forming, the rift zone of the Eastern Manus Basin lying between the islands of New Ireland and New Britain, and between two major transform faults (Djaul and Weitin Faults), is a pull-apart zone of distributed extension on mostly low-angle faults approximately normal to the transforms. Bathymetry, gravity modeling, and reverse magnetization indicate that basement of the Eastern Manus Basin (called the Southeastern Rifts by Martinez and Taylor, 1996) is arc crust equivalent to the Eocene–Oligocene exposures on New Britain and New Ireland. Reflection seismic traverses across the Eastern Manus Basin (Taylor and Crook, unpubl. data) show essentially undeformed graben and half-graben fills equivalent to an age of ~1 m.y. at current sedimentation rates. This is consistent with rifting in the Eastern Manus Basin covering a similar duration to spreading on the central Manus spreading center. The sediment fill is commonly tilted, denoting block rotation on listric master faults. The dredging of fault scarps, where seismic profiles indicate exposure of lower and more deformed sequences, has yielded fossiliferous calcareous mudstones and volcaniclastic sandstones ranging in age from early Miocene to the Pliocene/Pleistocene boundary. Although mainly of deeper marine origin, these are contemporaneous with the Miocene Lelet Limestone and Pliocene Rataman Formation, which overlie the Eocene–Oligocene Jaulu Volcanics of New Ireland (Stewart and Sandy, 1988), and with equivalent sequences on New Britain. Undated, mildly metamorphosed basalts dredged from inner nodal scarps near the active ends of the Djaul and Weitin transform faults may represent the presumed arc volcanic basement.
The overall aims of Leg 193 were to delineate the subsurface volcanic architecture, the structural and hydrologic characteristics, the deep-seated mineralization and alteration patterns, and the microbial activity of the PACMANUS hydrothermal field. From these data and subsequent laboratory analyses of samples and structural data, we planned to pursue the following specific scientific objectives.
1) Quantify the manner in which fluids and metals derived from underlying magmatic sources, and from leaching of wallrocks by circulated seawater, respectively, have combined within the PACMANUS hydrothermal system. This would be approached by applying geochemical and isotopic modeling to the vertical and lateral variations in hydrothermal alteration styles and sulfide mineral occurrences established by the drilling.
2) Evaluate the mechanisms of subsurface mineral precipitation, including comparison of exhalative and subhalative mineralizing processes, assessing the consequences of fluid phase separation, and seeking explanations for the elevated contents of copper, zinc, silver, and gold in massive sulfide chimneys at the PACMANUS seafloor.
3) Delineate probable fluid pathways within the system and establish a hydrologic model by measuring and interpreting variations in physical properties and fracture patterns of fresh and altered bedrocks.
4) Test whether the volcanic construction of Pual Ridge is a simple "layer cake," with potential older exhalative or subhalative massive sulfide horizons concealed by younger lavas or, alternatively, whether inflation of the volcanic edifice by lava domes or shallow intrusions is the predominant process in this submarine felsic volcanic environment.
5) Develop a petrogenetic model for Pual Ridge igneous rocks and seek evidence pertaining to the nature of the possible underlying source for magmatic components in the hydrothermal fluids.
6) Develop an integrated understanding of coupling between volcanological, structural, and hydrothermal phenomena in the PACMANUS system for comparison with equivalent hydrothermal phenomena at mid-ocean ridges and to provide a new basis for interpreting ancient ore environments.
7) Establish the nature, extent, and habitat controls of microbial activity within the hydrothermal system and interpret the differences encountered in diversity and biomass in terms of nutrient supplies and environmental habitats in the context of the geochemical and hydrologic understanding of the total hydrothermal system.
To achieve these goals, a total of 13 holes were drilled in four different sites (Figure 3), Site 1188 (Snowcap hydrothermal field), Site 1189 (Roman Ruins hydrothermal field), Site 1190 (Reference site), and Site 1191 (Satanic Mills hydrothermal field). In the following sections, a summary of the logging results and operations is given. Details of the overall results of the leg can be found in the Leg 193 scientific prospectus or in the preliminary report.
Figure 3. Distribution of hydrothermal deposits within the PACMANUS field with names assigned to active sites containing massive sulfide chimneys and drilling locations of Leg 193.
Knowing the downhole temperature conditions while drilling and logging proved to be essential for avoiding damage to the LWD and wireline tools. Before LWD operations began, a core barrel temperature tool (CBTT) was deployed on a core barrel during the initial drilling stages of the leg. The core barrel temperature tool (CBTT) was developed by the Borehole Research Group of the Lamont-Doherty Earth Observatory (LDEO) for assessing temperature conditions while drilling and determining if the conditions were favorable for subsequent LWD operations in hydrothermal environments. These temperature measurements could then be correlated to pump rates used during coring operations in order to determine the feasibility of performing LWD operations in the high temperature conditions that were encountered in the Manus Basin.
The temperature vs. time record from Core 193-1188A-4R shows profiles for both the internal (Tint) and external (Text) temperature sensors. The time of initialization, a period of ~6 min where Text was disconnected, a heating up period when the tool was at the rigfloor, a cooling trend as the core barrel was released from the rig shows an average temperature of 28.2 ± 0.8ºC. These measurements imply either effective cooling of the borehole through pumping an average of 50 strokes per minute (spm), which is ~250 gal/min, or very cool shallow conditions for this area.
Figure 4. Core barrel temperature tool measurements obtained while drilling Core 193-1188-4R in the Snowcap hydrothermal field.
Maximum temperature
thermometers were also deployed in Sites 1189 and 1191 to provide an idea of
downhole temperature conditions. The maximum temperature measurements recorded
with the thermometers showed that the conditions were favorable for attempting a
75 m LWD hole in Site 1189. The same reasoning was used for the Site 1191.
Unfortunately, the bit, core barrel, and CBTT pressure case were lost at this
site effectively ending any other potential deployment of the CBTT during Leg
193. Overall, the temperature data collected in these sites during drilling
operations indicated that the conditions were favorable for attempting LWD
measurements without risking damaging the tools because of high temperature
conditions.
The wireline tool strings also had an
external sensor that allowed for real time temperature measurements of the
borehole fluids. This sensor was located on the cablehead as part of the high
temperature/pressure gamma ray cartridge (HTGC) that was used on top of both the
triple combo and the FMS/Sonic tools strings (Figure
5). Two thermocouples were used to
calibrate the cable head temperature measurements.
Figure 5. Schematic diagram of the triple combo and FMS/sonic tool strings with real time temperature capabilities.
After assessing temperature conditions, both wireline and LWD tools were deployed in several holes and the results from the wireline and LWD operations are described below.
The Snowcap hydrothermal field, which is characterized by extensive diffuse venting of low temperature hydrothermal fluids, was explored by drilling a total of 6 holes and obtaining downhole measurements in three of them, Holes 1188A, 1188B, and 1188F.
Temperature measurements made with the CBTT in Hole 1188A provided the information needed for attempting and LWD resistivity at the bit (RAB) deployment in Hole 1188B. As shown in Figure 6, the first ever use of the RAB tool in ODP provided a continuous record of electrical resistivity at three depths of investigation (deep, medium, and shallow) and of natural radioactivity from seafloor to 72 meters below seafloor (mbsf). The RAB also provided electrical images of the borehole wall with the same three different levels of investigation. The RAB records the total gamma radiation but not its spectrum. Therefore, the contribution of the main radioactive elements such as potassium, thorium, and uranium cannot be individually separated from the total spectrum measurements. The log curves of electrical resistivity and gamma ray are displayed together with the rate of penetration and bit rotation in Figure 6.
Figure 6. Log curves obtained from the resistivity at the bit tool in Hole 1188B.
The average resistivity and gamma ray log curves from the RAB were used for characterizing the lithostratigraphy of Hole 1188B. Figure 7 shows deep resistivity and gamma ray curves, the logging units identified from the logs, and the core lithostratigraphic units found in the upper 75 m of neighboring Hole 1188A. Overall, eleven logging units and 31 logging subunits were identified from 5 different relationships found in the electrical resistivity and gamma ray measurements (Figure 7). These relationships are high gamma ray - high resistivity, high gamma ray - low resistivity, low gamma ray - low resistivity, transitional responses, and unclear relations that are labeled unclassified.
Figure 7. Preliminary interpretation of the logging curves obtained in Hole 1188B with the resistivity at the bit tool.
Based on log responses and core observations, the high gamma ray - high resistivity values of logging Units 2, 5, and 8 may reflect less fractured and/or less altered sections of the borehole that may be representative of rhyodacite flows. In many cases, this is supported by a decreasing rate of penetration (Figure 6) in logging units with high resistivity which may be indicative of harder layers. Logging units characterized by low resistivities are interpreted to represent altered or fractured layers (logging Units 3, 7, and 9). The presence of both, seawater or conductive clay minerals causes the resistivity to decrease. The higher gamma ray values of logging Unit 9 may be explained by the presence of K- or U-bearing minerals as seen in logs from Holes 1188F, 1189B, and 1189C. Logging Unit 7 shows a sharp decrease in resistivity and low gamma ray values that may represent a fractured zone, but it may also correspond to a high porosity zone with abundant vesicles.
The RAB images show results that might be suitable for formation evaluation and structural analyses. As an example, the depth interval between 18.7 and 32 mbsf is shown as 2 dimensional and 3 dimensional image representations (Figure 8). The light colored features correspond to the high resistivity layers of logging Units 3, 4 and 5. Although the resistivities for the entire logged section tend to be low, the high resistivity contrast and patchy nature of this subvertical feature may imply higher concentrations of anhydrite as the reported resistivity values for this calcium sulfate are in the range of 10,000 Wm or higher (Serra, 1972a, 1972b; Rider, 1996). The low resistivity and gamma ray values found in logging Subunits 3B and 3D also appear as darker colors or conductive features in Figure 8.
Figure 8. Detail image of deep resistivity measurements between 18.7 and 32 mbsf
Wireline logging operations in Hole 1188F began after setting two casing strings to depths of 58.9 and 190.4 mbsf. The water depth was estimated from pipe measurements at ~1653 mbsf and the drill pipe was placed at ~185 mbsf. Drilling operations achieved a TD of 386.7 mbsf in Hole 1188F and all wireline tool deployments reached a logging TD of 356 mbsf. An obstruction was encountered 30 m above the hole's TD and all attempts to get past this were unsuccessful.
Figure 9. Overview of the wireline logs from Hole 1188F.
Although caliper measurements show that Hole 1188F is oversized (Figure 9), eleven logging units were defined. The enlarged borehole affects several of the measurements, specially the porosity, density and velocity curves. The FMS produced only several short intervals with high-resolution images and most of the borehole has intervals where only a range of one to three pads were in direct contact with the borehole walls. In most instances, the neutron porosity data is high and density readings are low with values approaching 1 g/cm3 between 210 and 240 mbsf, which can also be explained by an enlarged borehole (Figure 9). All gamma ray curves show high values (> 500 gAPI) for the interval between 197.3 and 208.9 mbsf (Figure 9). The spectral gamma ray measurements show a significant increase in uranium and potassium within this interval. FMS images show that this unit corresponds to the part of the borehole that was cemented during casing operations (Figure 10). FMS images also show that the interval below the cemented part corresponds to a highly fractured interval followed by a zone showing breccias composed of high resistivity clasts and horizontal to subhorizontal fractures (Figure 10). Another interval with increased gamma ray and uranium values is present between 238.7 and 245.0 mbsf. The bottom of the hole is marked by increases in the photoelectric factor, total gamma ray counts, electrical resistivity and density values as well as intermediate to high P-wave velocities. FMS images of the bottom of the hole show that although there is a high fracture density, there is a small amount of brecciation when compared to shallower sections (Figure 11).
Figure 10. FMS images showing a 19-m interval immediately below the 10.75-in casing shoe that was inserted inside Hole 1188F. The cement extends for 3 m below the casing shoe. Below this depth, high fracture density and brecciated zones are displayed as well as an enlarged section of the borehole.
Figure 11. The bottom most part of the Hole 1188F showing the best FMS images of the entire logged interval.
Temperature measurements were made during wireline operations as well as 5 and 7 days later with the UHT-MSM temperature probe (Figure 12). Wireline cable head temperature measurements were obtained during three tool deployments. The profiles show an average steady increase in temperature starting from inside the casing at ~30 mbsf to a depth of ~234 mbsf. The interval from 234 to 289 mbsf shows that all temperature profiles have a decreasing trend followed by increasing temperatures until reaching the logging TD at 357 mbsf. The maximum-recorded temperature was 313ºC. This temperature was recorded with the second string at the bottom of the hole during the repeat section. The second pass of with the FMS showed a maximum temperature of 98.4ºC at ~15 m shallower than the previous high temperature reading of 99.6ºC.
Figure 12. Temperatures recorded in Hole 1188F during wireline logging operations and with the UHT-MSM temperature probe several days later.
The temperature profile recorded 5 days later using the UHT-MSM probe shows a much smoother profile than the temperature measurements from the wireline operations especially because the drill pipe was placed at 20 mbsf (Figure 12). In the upper part of Hole 1188F, temperatures are lower than those obtained with the wireline temperature sensor down to 250 mbsf. However, a sharp increase in temperature is observed starting from 250 mbsf to the bottom of the hole. The maximum-recorded temperature is 304ºC, which is an increase of 204ºC over the previous wireline measurements. Additional temperature measurements were planned 3days later for determining the amount of thermal rebound in Hole 1188F and the tool recorded a maximum temperature of 313ºC lack">Wireline logging operations in Hole 1189B began with the deployment of the high temperature/pressure telemetry gamma ray cartridge with real time cable head temperature capabilities (MTEM). The water depth was estimated from pipe measurements at ~1693 mbrf, and the bottom of the hole was estimated at 206 mbsf. A casing string was set to a depth of 31 mbsf, and the drill pipe was placed at ~28 mbsf. The MTEM sensor recorded a high temperature of ~48ºC. The average dimensions of Hole 1189B are good for the acquisition of high resolution logging data as the hole diameter decreases from 14.7-in at the top to 10-in toward the bottom (Figure 13).
Figure 13. Downhole wireline logs of Hole 1189B. A total of 8 logging units were identified from the changes observed with depth.
Figure 14. FMS images of the topmost logged section of Hole 1189B showing the presence of conductive minerals that resemble disseminated sulfide or oxide deposits.
Figure 15. A change in alteration as seen in the FMS images from Hole 1189B. Several fractures are also visible.
A total of eight logging units were identified in Hole 1189B (Figure 13). Similar to the results from Holes 1188B and 1188F, total gamma ray counts vary greatly with uranium measurements making the larger contribution to the total gamma ray spectrum. FMS images show significant changes with depth in the styles of alteration and fracture density. Below casing, the images show that the upper part of the borehole contains higher concentration of disseminated conductive minerals than the lower sections (Figure 14). These conductive minerals seem to correlate to higher concentrations of sulfides found in the recovered cores. Changes in alteration are also prominent features in the FMS images (Figure 15) as well as changes in resistivity of from conductive layers that are interbedded with more resistive and foliated units (Figure 16). The bottom of the logged interval shows completely different features as the images show a more resistive and fractured unit that tends to correspond to the less altered cores and a series of steep dipping fractures (Figure 17).
Figure 16. FMS image showing alternating conductive and resistive thin and shallow dipping units from Hole 1189B
Figure 17. Steep fractures in a potentially faulted zone found in Hole 1189B. This zone corresponds to a potential fluid conduit for hydrothermal circulation
LWD/RAB operations in Hole 1189C reached a depth of 166 mbsf and the water depth was estimated at 1700 mbrf.
After drilling Hole 1189C, additional logging operations were planned to establish a direct comparison between LWD and wireline measurements and for facilitating the interpretation of the subsurface stratigraphy. This marked the first direct comparison between LWD and wireline data in ODP.
The upper 70-m of Hole 1189C are irregular and measurements from the FMS calipers show an average diameter of 13.5-in (Figure 18). Below 50 mbsf there are several small isolated zones where the FMS calipers were close to being fully extended. Borehole sections between 44 and 49 mbsf as well as between 22 and 29 mbsf are also close to reaching the maximum aperture of FMS calipers. A section between 17.5 and 21 mbsf shows a divergence in caliper readings because an obstruction that was encountered in every logging run. Good agreement exists between the total gamma ray measurements from wireline and LWD measurements in the overlapping interval. As in previous holes, there is a good correlation between the total gamma ray and the uranium logs suggesting that at least between 20 and 55 mbsf, uranium makes the largest contribution to the total gamma ray spectrum.
Borehole images show that most of Hole 1189C is characterized by subhorizontal and subvertical fracturing as well as alternating numerous resistive and conductive feature. RAB images from 50 to ~60 mbsf show a series of conductive subhorizontal features that seem to represent fractures (Figure 18). This section also displays a significant vertical to subvertical resistive features and a dipping resistive unit resembling a vein at 51 mbsf. A comparison between RAB and FMS images shows good agreement with some of the larger features (Fig. 1189-K-15). Although the FMS displays higher resolution and definition of structural features, fractures are clearly identified in both sets of images. However, the resistive units are not as clear in the FMS logs perhaps because of the lower coverage of the borehole provided by the wireline tool.
Drilling in active hydrothermal environments is inherently difficult mainly because high temperatures and poor hole conditions lead to hole instabilities and poor core recovery. These conditions also lead to difficulties in acquiring logging data. During Leg 193, similar circumstances were encountered however, the accurate determination of downhole temperatures, the usage of casing strings, and the variation of drilling parameters during the cruise allowed for the safe deployment and downhole data acquisition using both LWD and wireline tools. Because of poor core recovery, the LWD data provided continuous downhole records that will allow for the reconstruction of the volcanic architecture of this hole as well as a representation of structural features, lithostratigraphic sequences, and alteration patterns. The temperature-time records obtained during this leg will also allow the construction of thermal rebound models for determining the role that magmatic fluids and heat fluxes associated hydrothermal systems play on the genesis of major sulfide deposits and biological communities.
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Anne C. M. Bartezko,Logging Scientist, Angewandte Geophysik, RW Technische Hochschule Lochnerstrasse 4-20, Aachen 52064, Federal Republic of Germany.