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Logging and core-log-seismic integration

Logging tools are used to determine physical, chemical, and structural properties of formations penetrated by drilling. Data are collected rapidly, continuously with depth, and, most importantly, measured in situ. Logs may be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is good, logging and core data are complementary and should be integrated and interpreted jointly, with logging data providing the in situ ground truth for core data. Where core recovery is incomplete or disturbed, logging data may provide a reliable means to characterize the borehole section.

Downhole logs record formation properties on a scale that is intermediate between those obtained from core samples and from geophysical surveys. Logging data are therefore complementary to centimeter-scale core studies, as well as seismic reflection images with a resolution of tens of meters scale. Logs provide measurements of in situ properties at a depth of investigation into the formation that varies depending on the tool. The logs are critical for calibrating geophysical survey data (e.g., through synthetic seismograms), providing the necessary link for integration between the core depth domain and the seismic time domain.

Two types of logging methods were attempted in support of Expedition 322: LWD and wireline logging. LWD data were acquired in Hole C0011A during Expedition 319, whereas wireline logging was planned for Expedition 322 at primary Site C0011 and contingency Site C0012. For the core-log-seismic integration (CLSI), we used VCDs, discrete sample measurements (e.g., P-wave velocity and density; see "Physical properties"), continuous multisensor core logger (MSCL) measurements, logging data, and prestack depth–migrated seismic data.

Logging while drilling

During Expedition 319, LWD and measurement while drilling (MWD) were conducted in riserless Hole C0011A (Expedition 319 Scientists, 2010). Figure F19A shows the configuration of the LWD/MWD BHA. MWD and selected LWD real-time measurements were transmitted through the drilling fluid by means of a modulated pressure wave (mud pulsing or fluid pulse telemetry) at a rate of 6 bits per second and monitored in real time. All LWD data were recorded into downhole memory and retrieved when the tools reached the surface. MWD measurements included ROP, downhole torque, inclination/orientation, weight on bit (WOB), NGR emissions, and annular pressure while drilling. During drilling operations, these measurements were combined with surface rig floor parameters for drilling monitoring (e.g., WOB, torque, etc.) and quality control.

LWD logging tool: geoVISION

During LWD operations, Schlumberger's geoVISION tool (Fig. F19B) was run, with the mud pulse system transmitting a limited set of data to the surface in real time. The geoVISION resistivity tool is based on resistivity-at-the-bit (RAB) technology. It provides resistivity measurements and electrical images of the borehole wall similar to the wireline Formation MicroScanner (FMS) but with complete coverage of the borehole wall and lower vertical/horizontal resolution. In addition, the RAB tool contains a scintillation counter that provides a total gamma ray measurement. The geoVISION tool is connected directly above the drill bit and uses the lower portion of the tool and the bit as a measuring electrode. A 1.5 inch (3.8 cm) electrode located 136 cm from the bottom of the tool provides a focused lateral resistivity measurement (ring resistivity) with a vertical resolution of 2–3 inches (5–7.6 cm) and a depth of investigation of ~7 inches (17.8 cm). In addition, button electrodes provide shallow-, medium-, and deep-focused resistivity measurements, as well as azimuthally orientated images. The azimuthal resistivity measurements are acquired with a ~7° resolution as the geoVISION tool rotates. The button electrodes are ~1 inch (2.5 cm) in diameter and reside on a clamp-on sleeve. The buttons are longitudinally spaced along the geoVISION tool to render staggered depths of investigation of ~1, 3, and 5 inches (2.5, 7.6, and 12.7 cm). This spacing provides multiple depths of investigation for quantifying invasion profiles and fracture identification (drilling induced versus natural). Resolution and depth of investigation for each resistivity measurement are shown in Table T14. For environmental correction of the resistivity measurements, drilling fluid resistivity and temperature were also measured (Schlumberger, 1989).

The tool's orientation system uses Earth's magnetic field as a reference to determine the tool position with respect to the borehole as the drill string rotates, thus allowing both azimuthal resistivity and natural gamma radiation measurements. The gamma ray sensor has a range of operability of 0–250 gAPI, an accuracy of ±7%, and a statistical resolution of ±3 gAPI at 100 gAPI and an ROP of 30 m/h. Its vertical resolution is 1.5 inches.

Once obtained, resistivity and gamma ray data were processed onboard Chikyu, using Schlumberger's software GeoFrame, GeoMechanics International, Inc.'s GMI Imager, and Paradigm, Ltd.'s software Geolog for further analysis and interpretation.

Wireline logging

During Expedition 322, wireline logging was planned for Sites C0011 and C0012. Our initial wireline logging plan consisted of four primary tool strings:

  1. High-Resolution Laterolog Array (HRLA) tool for laterolog resistivity; NGR tool; and dummy tools with dimensions and weight that match the tools in the following runs to ensure the safety of operation with radioactive sources;

  2. Hostile Environment Litho-Density Sonde (HLDS) for density, photoelectric factor (PEF), and caliper; Highly Integrated Gamma Ray Neutron Sonde (HGNS) for neutron porosity and NGR; and Hostile Environment Natural Gamma Ray Sonde (HNGS) for spectroscopy gamma ray;

  3. FMS for resistivity imaging, Sonic Scanner for sonic velocity (P- and S-wave velocities), and NGR tool; and

  4. Versatile Seismic Imager (VSI) for seismic velocity by check shot.

Vertical resolution and depth of investigation for each tool are shown in Table T15. However, slow ROP and degradation of the drill bit at Hole C0011B resulted in the postponement of the logging plan until Site C0012. Unfortunately, because of difficult hole conditions and expiration of time-on-site in advance of a typhoon evacuation at Hole C0012A, we were limited to one attempt at logging with a single tool string, composed of HRLA and FMS. After several unsuccessful attempts to pass a difficult spot just 10 m below pipe depth, all logging operations were canceled, with no usable data.

Wireline logging tools

Wireline logging data are typically recorded and stored digitally and monitored in real time using Schlumberger's Multitask Acquisition Imaging System (MAXIS) 500 (Fig. F20). Onboard logging processing includes (1) depth shifting all logs relative to a common datum (e.g., the seafloor), (2) corrections specific to individual tools, and (3) quality control. FMS image data are usually processed onboard Chikyu using Schlumberger's GeoFrame (version 4.3) software package and imported into GMI Imager software for further analysis. The processing steps include conversion of data format, inclinometry quality check, equalization, resistivity calibration, and normalization.

High-Resolution Laterolog Array tool

The HRLA tool combines various electrodes to provide five independent, actively focused, depth- and resolution-matched measurements to resolve true formation resistivity (Rt) in thinly bedded and deeply invaded formations in addition to a shallowest mode (Mode 0) for mud resistivity. Modes 1 to 5 have increasing depths of investigation. The supplementary mode helps improve the mud invasion profile and correct the raw resistivity data to retrieve the true formation resistivity. It is less affected by shoulder beds than traditional laterolog measurements because of the active focusing and multifrequency operation, together with the symmetric tool design. In addition, the tool employs software focusing to improve the accuracy of true resistivity estimates through advanced 2-D inversion processing.

Highly Integrated Gamma Ray Neutron Sonde

Neutrons interact differently with matter depending on their energy. Fast neutrons (E > 10 keV) are scattered elastically, primarily by H atoms. They can also induce inelastic scattering, in which case the excited atoms release gamma rays. If the neutron energy becomes small enough (thermal neutron, E < 0.1 eV), they can be absorbed by the medium by thermal capture. The most efficient absorbers are Cl, B, and H. Thermal capture also releases gamma rays. The HGNS tool contains an Am-Be radioactive source that bombards the formation with fast neutrons (>10 keV). These neutrons are slowed by scattering and then captured. Epithermal neutron detectors quantify elastic scattering, and thus the H index and a porosity estimate are generated. Gamma ray detectors and thermal neutron detectors document the thermal capture. The measurement by the HGNS sensor is dependent on hydrogen content, and hence water content. We note that most clay-rich rocks contain some intracrystalline water. The measured value is also affected by water-based mud; therefore, the data are corrected with mud properties.

Hostile Environment Natural Gamma Ray Sonde

40K and isotopes of the decay chains of radioactive isotopes of Th and U emit photons at different energies. The HNGS tool uses spectroscopic analysis to determine the concentration of radioactive 40K (in weight percent), Th (in ppm), and U (in ppm). The HNGS tool also measures U-free gamma ray emissions (in gAPI, also called computed gamma ray emissions). The HNGS uses two bismuth germanate scintillators for gamma ray detection. The tool response is affected by the tool standoff (distance between the sensor and the borehole wall) and the weight and concentration of bentonite or KCl in the drilling mud (KCl may be added to the drilling mud to prevent hydrous clays from swelling and obstructing the well). Spectral analysis filters out gamma ray energy below 500 keV to reduce sensitivity to bentonite and KCl in the drilling mud and to improve measurement accuracy. Environmental corrections are usually made during data processing. This tool has a 24 cm depth of investigation.

Hostile Environment Litho-Density Sonde

The HLDS measures formation density using a radioactive 137Cs gamma ray source (662 keV) and far and near gamma ray detectors mounted on a shielded skid that is pressed against the borehole wall. Gamma rays emitted by the source experience both Compton scattering and photoelectric absorption. Compton scattering involves the ricochet of gamma rays off electrons in the formation via elastic collision, transferring energy to the electron in the process. The number of scattered gamma rays that reach the detectors is related to the number of electrons in the formation, which is a function of the bulk density. Porosity can also be derived from this bulk density if the matrix (grain) density is known from MAD measurements on cores. The HLDS also measures photoelectric absorption of the gamma rays (PEF), which occurs when they drop to <150 keV after being repeatedly scattered by electrons in the formation. PEF varies according to the chemical composition of the formation (Gardener and Dumanoir, 1980). Some examples of PEF values are pure pyrite = 16.97 b/e, calcite = 5.08 b/e, illite = 3.03 b/e, quartz = 1.81 b/e, and kaolinite = 1.49 b/e. Good contact between the tool and borehole wall is essential for acquisition of high-quality HLDS logs. Poor contact results in an underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.

Formation MicroScanner

The FMS provides high-resolution resistivity images of the borehole wall (~25% of a 25 cm diameter borehole can be imaged on each pass). The tool has four orthogonal arms with pads, each containing 16 button electrodes that are pressed against the borehole wall during recording. The electrodes are arranged in two diagonally offset rows of eight electrodes. A focused current is emitted from the button electrodes into the formation with a return electrode located near the top of the tool. The intensity of the current passing through the button electrodes is measured. The maximum extension of the caliper arms is 15 inches; therefore, in holes or sections of holes with a larger diameter, pad contact will be inconsistent and the FMS images may appear out of focus and too conductive. Irregular borehole walls will also adversely affect the image quality if they lead to poor pad/wall contact. Processing transforms these measurements, which reflect the microresistivity variations of the formation, into continuous, spatially oriented, high-resolution images that map the geologic structures of the borehole wall. The depth of investigation is ~2.5 cm, and the electrode buttons produce images with a vertical resolution of 5 mm. The quality of the image enables assessment of rock composition and texture, detailed structure (e.g., bedding and fracturing), and fluid content. Local contrasts in FMS images were improved by applying a dynamic normalization to the FMS data. The General Purpose Inclinometry Tool (GPIT) accompanies the FMS and integrates both a three-axis accelerometer and a three-axis magnetometer to determine the orientation and inclination of the tool string during logging. This provides a means of correcting the FMS images for rotation and irregular vertical tool motion, allowing the true dip and direction (azimuth) of structures to be determined. It can also provide the geometry of the borehole path. We planned to use a slim FMS during Expedition 322.

Sonic Scanner

The Sonic Scanner is a new generation acoustic measurement tool and is a successor of the Dipole Sonic Imager (DSI). In addition to axial and azimuthal measurements, the Sonic Scanner makes a radial measurement to probe the formation for near- and far-field slowness. The depth sensitivity is equal to two to three times the borehole diameter. Other attributes of the Sonic Scanner include: (1) a wide-frequency spectrum ranging from 300 Hz to 9 kHz, (2) a longer azimuthal array (five more receiver stations, 2 ft longer than the DSI), (3) a borehole-compensated monopole with long (11–17 ft) and short (1–7 ft) spacing, and (4) cross-dipole acquisition. The Sonic Scanner provides accurate radial and axial measurements of stress-dependent properties near the borehole. These data are converted into: (1) P- and S-wave velocities, (2) anisotropy of propagation, (3) Stoneley wave velocity, and (4) cement bond quality.

The Sonic Scanner receiver configuration produces a long azimuthal array (i.e., 8 azimuthal receivers at each of the 13 stations). With the two near-monopole transmitters straddling this array and a third transmitter beyond, the short- to long-monopole transmitter-to-receiver spacing combination provides a radial monopole profile.

Versatile Seismic Imager

The VSI is an array of seismometers clamped to the borehole wall to receive seismic waves from an air gun. The VSI produces data that can be interpreted as a check shot survey, a low-resolution velocity-depth function, and a vertical seismic profile. The VSI records seismograms using a three-component geophone in the tool and a surface source and hydrophone. The source was a trio of 250 in3 G-guns to be suspended from Crane 1 ~60 m horizontally from the rotary table and fired at 1700–2000 psi at 6 meters below sea level (mbsl) (e.g., Expedition 314 Scientists, 2009). Time correlations of the shots are ensured with high-precision clocks at both the surface hydrophone and the downhole hydrophones. The surface hydrophone was to be suspended 5 m below the air guns (total of 11 m below mean sea level), and the zero times of the waveforms corrected to mean sea level. At each data acquisition level, a number of shots were to be fired by the surface source. The tool records the acceleration seismograms obtained for each shot.

Logging data quality

Logging data quality may be seriously degraded by changes in hole diameter in sections where the borehole diameter greatly decreases or is washed out. Nuclear measurements (density and neutron porosity) are more sensitive to borehole conditions because of their shallower depths of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data to reduce some of these effects, but for very large washouts, data cannot be corrected.

Azimuthal measurements and associated images (RAB and FMS images) are of low quality when the measuring tool is not rotating ("stick") or when its rotation exceeds 250 rotations per minute (rpm) ("slip"). In zones of high stick-slip, tool rotation can vary greatly locally, resulting in images of lower quality. As all measurements by the same tool are not necessarily sampled at the same time, improper heave compensation and irregular movement of the BHA can result in local depth shifts between measurements by several tens of centimeters.

LWD/MWD data quality is mostly assessed by cross-correlating available logs of two types: drilling control parameters (e.g., ROP and stick-slip indicator) and geophysical control logs (e.g., gamma ray). Time-depth relationships are considered, to identify the sequence of drilling events and to further assess their possible impact on data quality. Then operational surface data and downhole data are compared for a detailed assessment of drilling conditions on log quality.

Natural gamma ray logs from the Enhanced Digital Telemetry Cartridge (EDTC), HGNS, and HNGS provide a means of depth correlation between logging runs. Logs from different tool strings may, however, still have minor depth mismatches caused either by cable stretch or ship heave during recording.

Log characterization and interpretation

LWD and wireline measurements provide in situ petrophysical information on sediments, rocks, and interstitial water while the hole is being drilled, or shortly thereafter in the case of wireline logging. These measurements are sensitive to changes in composition (changing curve magnitudes), textures, and structures (log shape, peak amplitude, and frequency, as well as information from log images). Changes in the log response (values and/or frequency of the signal) are commonly associated with lithologic unit boundaries.

The characterization of logging data, including LWD and wireline logging measurements, as well as imaging tool response, allows the borehole to be zoned into distinct logging units. Once representative petrophysical properties for the logging units were defined, they were incorporated into the log-based lithologic units. For Site C0011, the aim was to provide a preliminary assessment of expected lithologies from LWD data prior to coring and to provide more detailed properties from wireline logging after coring. At Site C0012, where logging was to follow coring operations, the goal was to refine unit boundaries (where core recovery or quality was lacking) and to provide a basis for comparison with Site C0011. However, since wireline logging data were not collected during this expedition, we will not address the details of wireline data analysis here.

LWD data analysis

During Expedition 322, initial log-based lithologies at Site C0011 were interpreted using only the LWD logs obtained during Expedition 319. Following log characterization and classification, logs were lithologically and geologically interpreted using a combination of log characteristics and borehole images.

Compositionally influenced logs, such as gamma ray logs from the geoVISION tool, were used to predict lithology from unit scale (hundreds of meters) to bed scale (centimeters to meters). In particular, the identification of probable sand-rich intervals, clay-rich intervals, and alternating beds of sand and clay was a primary element of the interpretation. Borehole images provided useful information on mesoscopic features such as sedimentary structures (e.g., slumps), bed boundaries, angular unconformities, fractures, and faults. The orientation of breakouts, or borehole enlargements, was also mapped, as the direction of enlargement can be related to the direction of maximum horizontal stress within the formation. Additionally, lithologic descriptions from ODP Legs 131, 190, and 196, in the Nankai Trough off Shikoku Island, were an important aid in interpreting lithology (Taira, Hill, Firth, et al., 1991; Moore, Taira, Klaus, et al., 2001; Mikada, Becker, Moore, Klaus, et al., 2002).

Electrical resistivity images

Structural analysis from logging data was performed on the geoVISION images using GMI Imager (GeoMechanics International, Inc.) and GeoFrame (Schlumberger) software. These software packages present resistivity image data of the borehole wall as planar "unwrapped" 360° images with depth. The software also allows visualization of the data in a 3-D borehole view.

Resistivity image data were displayed as both statically and dynamically normalized images. Static normalization displays the image with a color range covering all resistivity values for the entire logged interval; this highlights relative changes in resistivity throughout the borehole and is useful for identifying lithologic or facies changes. Dynamic normalization scales the color range for resistivity values over a constant depth interval (1 m) and is commonly used for detailed identification of fractures and structures that are more subtle changes.

In unwrapped resistivity images, sinusoidal lines are planar surfaces, while curved lines are nonplanar surfaces. Features were classified according to type, to distinguish between fractures, bedding planes, faults, and borehole breakouts. We compared initial image interpretations from the logging data with structural data from cores and seismic reflection data as a check on the validity of our interpretations.

Core-Log-Seismic integration

During Expedition 322, we used LWD data to establish ties between data from cores (continuous and discrete samples) and the seismic data set (Park et al., 2008). Table T16 shows planned measurements and observations for CLSI during Expedition 322.

Log-seismic integration

Using the available density and P-wave velocity data from discrete core samples, we were able to create a reflection coefficient series for Site C0011. This series was then convolved with the extracted source wavelet to generate a synthetic seismogram. Displaying the synthetic seismogram beside the seismic data in the vicinity of the borehole provides information about specific boundaries of interest and provides a quality check on velocity and density logs.

Core-log integration

Integration of cores with log data relies primarily on NGR data from the MSCL, as well as lithologic characteristics identified in photos, X-ray CT scans, smear slides, discrete sample measurements, VCDs, and so on. NGR data were used for correlation between the cores and the logs and to conduct necessary depth-shifting to correctly place sections of core or account for core expansion. In particular, LWD data (NGR, resistivity, and resistivity images) were useful for core-log integration at Site C0011.

Core-log-seismic integration

In order to correlate log-seismic integration results with core-log integration results, we used seismic reflection traces at Site C0011 and a synthetic seismogram as key inputs. Detailed CLSI enables us to infer lateral variations of physical properties along the seismic reflection profile and to interpret the seismic data in terms of the measured formation properties. Before expanding those physical properties into 2-D or 3-D, careful identification of continuous, high-amplitude reflectors on seismic profiles, which are key packages of beds or horizons, is essential.