Leg 194 Logging Summary
Shipboard Scientific Party
The Marion Plateau carbonate platform, offshore northeast Australia, is seen as a natural laboratory to study the causes, magnitudes, and effects of sea-level change on continental margin sediments. The carbonate production is sensitive to sea-level variation as the production is generally very high during sea level highstands and shutdown during sea-level lowstands. Two transects of platform sites across to adjacent slope sites give insight into the different record at certain times of platform development.
During Leg 194 a series of eight sites were drilled in the Coral Sea (Fig. 1). Scientific objectives of Leg 194 are e.g. the determination of:
Oligocene to Holocene mixed carbonate and siliciclastic sediments were recovered. Three of the sites reached volcanic basement of yet unknown age. The magnitude of sea-level change was much less than originally predicted and it was discovered, that the platform development of the SMP, which was thought to be only upper Miocene, succeeded in several stages and started earlier than anticipated from seismic interpretation. The SMP grew during at least three phases, one of them simultaneously with the NMP (middle Miocene).
Preliminary results are found at http://www-odp.tamu.edu/publications/prelim/194_prel/194toc.html
In total five sites were logged with various success (Tab. 1). Hole conditions turned out to be critical as the poorly cemented coarse-grained clastic carbonates were very instable. Site 1197 was attempted to be logged but hole condition deteriorate while lowering the tools and the operation had to be abandoned. However, the achieved logging data are generally of high quality and of great use for the leg objectives.
Table 1: Summary of the holes logged during ODP Leg 194.
Four logging runs were completed; a first run included the triple combo (natural gamma-ray, density, porosity, resistivity tool) with the Lamont temperature tool on the bottom and the Lamont multi-sensor gamma-ray tool (MGT) on top and a second run with the formation micro-scanner and dipole shear sonic imager combination (FMS-DSI). For the third run the malfunctioning DSI tool was replaced with the long spaced sonic tool (LSS). The last run was the check shot survey with the well seismic tool (WST), using an 80 in3 water gun as source. The WST tool could not pass a tight spot at 158 mbsf, thus only three stations were measured at shallow depths. The water gun signal was not clean since a low-frequency precursor disturbed the main peak, producing erroneous results. All other logs are of good to excellent quality (Fig. 2). The Lamont MGT tool also proved to record natural gamma-ray with high resolution and accuracy.
The dominant lithologies in Hole 1194B are carbonates with small admixtures of clastics that show overall low natural gamma radiation. Downhole measurements from Hole 1194B retrieved a continuous geophysical record from the basement (425 mbsf) to sediments at 84 mbsf. Density, porosity and velocity display general downhole trends of increasing or decreasing values, respectively, as a result of compaction but large excursions and inversions interrupt these trends. Logs in Hole 1194B can be grouped into three log facies (log Units 1-3) that correspond to the lithologic and seismic units off the NMP.
The top interval (log Unit 1, 84-114.5 mbsf) with low log values correlates to the onlapping sediments of Megasequence B. The bottom of log Unit 1 is marked by a dolomitized hardground, which shows high natural gamma radiation (Fig. 3).
Log Unit 2 (114.5-260 mbsf) that is characterized by increased variability accompanied by distinct peaks in all log data roughly coincides with the high-amplitude inclined slope reflections of NMP. Lithologically this interval is characterized of neritic outer-ramp, neritic upper slope deposits above shallowing-upward deeper slope deposits. Thus, the variability and especially the peaks in the logs are most likely the combined result of variations in sedimentation rates, clastic content and cementation, as it is expected in pulsed, proximal slope sedimentation. For example, two of the peaks in uranium and velocity correlate with hardground surfaces, one of which was not recovered in the core.
Log Unit 3 from 260 mbsf down to the bottom of the sedimentary strata shows little log signatures but with regular variability suggesting to be the record of distal cyclic shelf sedimentation. Changes in log signatures in Hole 1194B correlate well with the lithologic unit boundaries indicating that facies changes across unit boundaries produce a distinct petrophysical signal.
Logging operation in Hole 1195B was short due to deteriorating hole conditions. We achieved only one run with the triple combo to a depth of 517.5 mbsf, which was only about 2 m above coring depth. Three check shots in shallow depths with the well seismic tool (WST) were completed. The successful first pass retrieved excellent logging data (Fig. 4). By integration of check shot data in the upper part, logging data and core data a synthetic seismogram could be calculated (Fig. 5).
The logged interval of Hole 1195B is divided in 5 log units (Fig. 4). These units correspond roughly with the major lithological units but the logs allow for a further subdivision. Log Unit 1 has low gamma-ray, density, and resistivity values and display little log variations, which is in concert with the rather homogeneous lithology of unconsolidated skeletal wacke-to packstone of lithologic Unit II. The boundary to log Unit 2 at 240 mbsf coincides with the seismic sequence boundary c-b. In the logs it is marked by an increase in density, resistivity, and natural gamma-ray values in conjunction with regular amplitude. The transition from lithologic Unit II to Unit III is also one of the major changes in sedimentation with the onset of cyclic alternations of light gray and greenish-gray intervals. They probably correspond to changes in composition that cause the cyclic appearance in all the logs. With the beginning of log Unit 3 at 418 mbsf, density and resistivity values slightly increase and the porosity and the natural gamma-ray decrease. Towards the bottom of this unit occur again higher amplitudes in the gamma-ray and resistivity logs. This pattern of high variation in the lower part and less variation in the upper part is similar to log Unit 2, indicating a repetition of the sedimentation pattern, although with a reduced thickness. At 451 mbsf the log curves become smooth again, with slightly lower densities, resistivity and gamma-ray values. The base of this log Unit 4, however, shows a dramatic gamma-ray peak that correlates to a glauconite-rich layer. The last significant change in log character was detected at 468.5 mbsf with a significant drop in the resistivity and an increase of gamma-ray values, in particular uranium with 3-9 ppm. The lithologies for log Unit 5 observed in the cores comprise glauconite sands and packstone with glauconite. In summary, the small but cyclic variations of most of the logged section accurately reflect the rather uniform depositional sedimentation in this distal portion of the Marion Plateau.
Logging operation at Site 1196 included a run of the triple combo with the Lamont MGT on the top and the Lamont temperature tool on the bottom. Because of electrical malfunctioning during deployment the MGT was removed and a shortened string was lowered into the hole. Logging started uphole from a tight spot at ~ 835 mbrf. During the run strong vibration on the tool string loosened several joints and unscrewed the end piece (~ 4" length) of the temperature tool. In the second run the FMS-sonic combination reached 524 mbsf. The hole was wide open (> 17") for long distances so that the HLDT and APS detectors, as well as the FMS pads did not have always the required contact to the borehole wall. Overall, high quality logging data were achieved (Fig. 6). During the check shot survey, travel time at 13 stations between 94.4 and 523.5 mbsf were measured with the well seismic tool (WST). The WST produced clear first arrivals for accurate time-depth conversion. How important the check shot survey was is seen in Figure 7. Based on core data the time-depth conversion would have been wrong as only solid rock samples were recovered, suggesting a fast formation. The check shot time-depth relation proved the velocity log to reflect true formation velocities.
Based on the logs and the FMS images 3 log units can be distinguished (Fig. 6). Each of these units can be interpreted as a phase of SMP growth, whereby log Unit 1 seem to correlate with the late Miocene, Unit 2 to the middle Miocene, and Unit 3 to an early Miocene platform growth phase. Within the log units, subunits are recognized which might be related to development of individual platforms.
Log Unit 1 (76-128.4 mbsf) is characterized by the uranium and resistivity logs and consists of two intervals with different log facies.
Log Unit 2 (128.4-163) shows overall low resistivity values and generally low HSGR values with individual peaks. It correlates to lithologic Subunit IC, which is a reefal facies composed of skeletal rudstone/boundstone alternating with dolomitic. The transition to the overlying lithologic Subunit IB is sharp and reminiscent of a flooding surface based on the FMS image and the high peaks in HSGR and resistivity. The upper log interval coincides with the dolomitic floatstone of lithologic Subunit IB and the dolomitic floatstone/rudstone of lithologic Subunit IA.
The top of log Unit 3 is placed at 163 mbsf, where both resistivity and velocity increase drastically downhole and HSGR values start to fluctuate around a higher median. Three intervals based on the variation in the uranium log are distinguished in this unit. In the lowermost interval (309.3-345.5 mbsf) increasing uranium content from values of 4 ppm up to 15 ppm might indicate a regressive facies trend (Fig. 8). The overlying log interval with increased uranium content correlates to the skeletal floatstone with grainstone matrix of lithologic Subunit IIA. The high resistivity and velocity values towards the top of log Unit 3 are interpreted to record exposure events towards the end of this platform growth phase. Moldic porosity observed in the cores supports the interpretation of exposure.
The change down to log Unit 4 is taken at 345.5 mbsf where both, HSGR and velocity, decrease markedly. Log Unit 4 provides the rather uniform log signature of a pervasively dolomitized platform. FMS images document a highly fractured platform at the bottom of the interval that corresponds to well-lithified dolostones of lithologic Subunit IIIC.
Logging operation in Hole 1198B comprises only the triple combo because of bad hole conditions. The tool reached only 235.5 mbsf.
The natural gamma-ray data are the most useful logs as they are corrected for the borehole size. In the 135 m interval covered by the natural gamma-ray log (HSGR) we can distinguish two major changes in log responses (Fig. 9). Between 74 - 110.5 mbsf (log Unit 1) the HSGR is very low with ~20 gAPI. It can roughly be correlated to carbonate-rich pack- to grainstone. In cores a gradual downhole increase in clay content can be observed. In the logs a change to higher HSGR values occurs at 110.5 mbsf, that might reflect this increase. The lower part of log Unit 1 (110.5-199 mbsf) shows an increase in the HSGR log up to 30 gAPI with small-scaled variations.
Below 199 mbsf (log Unit 2) the HSGR drops to ~12 gAPI indicating the transition from Megasequence D to Megasequence C, the latter consisting of coarse grained carbonate-rich rud- and floatstones.
The triple combo tool string reached the depth of 418 mbsf, which was 1.5 m above coring TD of 419.5 mbsf. The good hole conditions resulted in very good log quality. The second run with FMS-sonic tools could not pass the tight spot at 129.2 mbsf and logging started above this horizon. The tool measured a hole deviation of 7.5˚, which was likely the cause for preventing the tools passing the bottom of a caved interval.
The logging data in Hole 1199A provide important information on the lithology and architecture of the SMP edifice, especially in the lower part of the hole where core recovery was low (Fig. 10). Two karst holes are inferred from the logs, each of approximately 10 m thickness at 118 - 129.2 mbsf and 155 - 162 mbsf, respectively. The logs show significant differences between this site and Site 1196. However, the measured section can again be subdivided into 3 log Units that are related to the three growth stages of the platform edifice.
Log Units 1 and 2 show a similar pattern in log responses in both holes with zone of low resistivity and natural gamma-ray (HSGR) values that is overlain by an interval of high resistivity and HSGR values. The low value zone corresponds to the karstified reefal unit (lithologic Subunit IC). The first peak of resistivity and HSGR at 115 mbsf correlates to the onset of the overlaying dolomitic floatstone of lithologic Subunit IB.
Figure 10: Overview of logging data in Hole 1199A.
Log Unit 3 (130.5 - 275 mbsf) corresponds to lithologic subunits ID and IIA, and the middle Miocene platform growth phase. As at Site 1196, the top of the log Unit 3 is a high resistivity interval, corresponding to a dolomitic floatstone (lithologic Subunit ID). A karst cave, indicated by 100% porosity, separates this top from the skeletal floatstones and grainstones below, which makes up most of this platform interval. While the porosity, density and resistivity log responses are similar as in Hole 1196A, the HSGR signature differs. Extremely high and strongly fluctuating HSGR values in the top 50 m are not present in Hole 1196A.
Log Unit 4 (275 - 418 mbsf) corresponds to lithologic Subunit IIB that is composed of skeletal grainstone and floatstone. The logs display a low variability and a near linear trend of slightly decreasing resistivity and density and increasing porosity. HSGR has increasing values and variability from 386.2 to 363 mbsf above which an abrupt decrease to lower values and variability occurs. This log character is similar to the corresponding interval in Hole 1196A, with the exception of the HSGR that has higher values in Hole 1199A. Assuming that the abrupt change of the HSGR values is a correlative surface, a thickness variation of about 50 m exists between equivalent facies in the two holes.
The observed thickness variations in log Units 3 and 4 between Sites 1196 and 1199 strongly suggest large lateral facies heterogeneities and a complicated architecture of the early to middle Miocene platform stages.
After Leg 191, Leg 194 was the second deployment of the Lamont Multi-Sensor Gamma Ray Tool (MGT) in ODP. In Hole 1194B a full logging run was performed and excellent data recorded. Figure 11 shows the comparison of the Schlumberger Hostile Environment Natural Gamma Ray Sonde (HNGS) and the MGT. The two curves show a good correlation, with the MGT gamma ray data showing a higher resolution than the HNGS data. The correlation of multi sensor track (MST) total gamma counts and MGT total gamma counts shows that the two measurements have approximately the same vertical resolution (Fig. 12). Hence, in sections with good core recovery a detailed core-log integration can be performed.
The much better vertical resolution of the MGT compared to the Schlumberger HNGS gives the opportunity for a more detailed analysis of continuous sections for high-resolution cyclic pattern. This is especially important on low recovery legs with paleoceanographic objectives.
Heike Delius, Logging Staff Scientist, University of Leicester, Dept of Geology, Leicester LE1 7RH, UK
Gregor Eberli, JOIDES Logging Scientist, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami FL 33149-1098, USA
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