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doi:10.2204/iodp.proc.348.102.2015 Methods1H. Tobin, T. Hirose, D. Saffer, S. Toczko, L. Maeda, Y. Kubo, B. Boston, A. Broderick, K. Brown, A. Crespo-Blanc, E. Even, S. Fuchida, R. Fukuchi, S. Hammerschmidt, P. Henry, M. Josh, M.J. Jurado, H. Kitajima, M. Kitamura, A. Maia, M. Otsubo, J. Sample, A. Schleicher, H. Sone, C. Song, R. Valdez, Y. Yamamoto, K. Yang, Y. Sanada, Y. Kido, and Y. Hamada2Introduction and operationsThis section documents the methods used for shipboard measurements and analyses during Integrated Ocean Drilling Program (IODP) Expedition 348. During Expedition 348, we conducted riser drilling from 860.3 to 3058.5 meters below seafloor (mbsf) at Site C0002 (see Table T4 in the “Site C0002” chapter [Tobin et al., 2015]) as a continuation of riser drilling in Hole C0002F begun during Expedition 326 in 2010 (Expedition 326 Scientists, 2011) and deepened during Expedition 338 in late 2012 and early 2013 (Strasser et al., 2014b). Operations began with connection of the riser to the Hole C0002F wellhead and sidetrack drilling of the cement shoe at 860.3 mbsf to establish a new hole, parallel to previous Hole C0002F drilling but laterally offset by ~16 m. This new sidetrack was designated as Hole C0002N to distinguish it from the overlapping interval in Hole C0002F. Previous IODP work at Site C0002 included logging and coring for Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stages 1 and 2 during IODP Expeditions 314 (logging while drilling [LWD]), 315 (riserless coring), 326 (riser tophole installation), 332 (LWD and long-term monitoring observatory installation), and 338 (riser drilling and riserless coring) (Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009b; Expedition 326 Scientists, 2011; Expedition 332 Scientists, 2011; Strasser et al., 2014b). During riser operations, we collected drilling mud gas, cuttings, downhole logs by LWD instruments, downhole pressure data, flow data, core samples, and drilling parameters (including mud flow rate, weight on bit, and torque, among others). Gas from drilling mud was analyzed in near–real time in a mud-gas monitoring laboratory and was sampled for further postexpedition research. Continuous LWD data were collected and displayed in real time (except for a loss of measurement-while-drilling [MWD] telemetry during drilling to total depth in Hole C0002P) for quality control and for initial assessment of borehole environment and formation properties. Recorded-mode LWD data provided higher spatial sampling of downhole parameters and conditions. Cuttings were sampled for standard shipboard analyses and for shore-based research. Coring of a portion of Hole C0002P provided core for standard shipboard and shore-based research. Additionally, Hole C0002M was drilled to 512.5 mbsf as a test of the developmental small-diameter rotary core barrel (SD-RCB) system, and four cores were taken from 475 to 512.5 mbsf. These SD-RCB cores are currently designated as the “R” core type in the data management system, but they differ from normal RCB cores in that they are larger in diameter: 73 mm instead of 66 mm. Site C0002 drilling operationsOperations at Site C0002 during Expedition 348 were entirely riser drilling. With the riser attached to the wellhead, drilling mud was circulated to clean the hole of cuttings, prevent wellbore failure, and maintain borehole pressure to balance stresses and pore pressure in the formation. IODP riser drilling on the D/V Chikyu differs from riserless drilling in ways that impact science, most notably in that cuttings can be collected continuously whenever the drill bit is advancing, and core physical properties and chemistry may be affected by the invasion of components of drilling mud (e.g., Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). Continuous monitoring of mud weight, annular pressure, mud loss, and other circulation data during riser drilling can provide useful constraints on formation pore fluid pressure and state of stress (e.g., Zoback, 2007). Problems related to mud weight or hole collapse may impact successful drilling or casing of the borehole itself, as well as the ability to conduct downhole measurements or to achieve postdrilling scientific objectives, including observatory installations and active source seismic experiments. Because riser drilling remains relatively new in IODP, we follow recent proceedings from Expeditions 319 and 338 to describe key observations related to downhole (borehole) pressure, mud weight, and hole conditions during drilling of Holes C0002N and C0002P. Reference depthsDepths of each measurement or sample are here reported referenced to the drilling vessel rig floor (rotary table) in meters below rotary table (m BRT) and meters below seafloor (mbsf) (Table T1). These depths are determined by drill pipe and wireline length and are correlated to each other by use of distinct reference points. Drilling engineers refer to pipe length when reporting depth and report it as drilling depth below rig floor (DRF) in meters. Core depths are based on drilling depth below rig floor to the top of the cored interval and curated length of the recovered core. During Expedition 348, core depths were converted to core depth below seafloor, method A (CSF-A), which allows overlap relative to cored interval and section boundaries in cases of >100% core recovery due to expansion after coring (see IODP depth terminology at www.iodp.org/program-policies). Cuttings and mud depths are reported as mud depth below rig floor (MRF) or mud depth below seafloor (MSF), based on DRF and the calculated lag depth of the cuttings (see below for further details). In referring to LWD results, depth was measured as LWD depth below rig floor (LRF) and sometimes reported as LWD depth below seafloor (LSF) (see “Logging” for further details). Depths reported in DRF and MRF are converted to depths below seafloor (drilling depth below seafloor [DSF] or CSF-A and mud depth below seafloor [MSF], respectively) by subtracting water depth and the height of the rig floor rotary table from the sea surface (28.5 m), with corrections relative to drilling depth where appropriate. These depths below seafloor (DSF, CSF-A, MSF, and LSF) are therefore all referenced to an equivalent datum. Seismic depths are reported in either time (seconds) or depth (meters). For time sections, a two-way traveltime in seconds is used. For depth sections, seismic depth below seafloor (SSF) or seismic depth below sea level (SSL) are used. Because Holes C0002N and C0002P are sidetracked holes (see “Site C0002 drilling operations” for further details), there is a ~1–2 m difference between the true vertical depth and the measured depth along the hole that is used for all onboard measurements. Therefore, a measured depth (MD-m BRT and MD-mbsf) as well as a true vertical depth (TVD-m BRT and TVD-mbsf) are defined for any position along the boreholes. Because the difference is small, we used measured depth rather than true vertical depth for all measurements reported in this volume, unless otherwise explicitly noted (i.e., in this volume “mbsf” refers to “MD-mbsf” everywhere). Correlations between measured and true vertical depths at key depths, such as casing shoe and unit boundaries, are summarized in Table T2. Although all of these depths are defined explicitly (Table T1), depths are reported throughout the Site C0002 chapter simply in mbsf or m BRT in most cases, unless a specific distinction is drawn among logging, coring, and mud depths for a given value. Cuttings and mud depthsDuring riser drilling, drilling mud circulates down the drilling pipe, out at the drill bit, then up the borehole annulus into the riser pipe, and back up to the drillship. As the drill bit cuts through the formation, cuttings are suspended in the drilling mud and carried with the drill mud, formation fluid, and formation gas back to the ship. A cuttings sample is assumed to be a mixture of rock fragments, sediment, and drilling fluid from the sampled interval. The time between when the formation is cut by the drill bit and when these cuttings arrive at the ship is known as the “lag time,” which is a function of drilling mud pumping rate and annular mud volume, and is used to calculate the “lag depth.” At a constant pump rate, lag time and lag depth increase as the hole is deepened and the volume of circulating mud increases. All of the depths recorded for cuttings and mud gas in Holes C0002N and C0002P have been corrected for this calculated lag. Because cuttings disperse and mix as they are carried to the surface, any given cuttings sample is believed to be representative of a depth-averaged volume; precision of their depth of origin is assumed to be ~5 m under normal conditions, and it is always possible that cavings and material from higher positions in the hole can be present at misleading lag depth. Sampling and classification of material transported by drilling mudA total of 293 cutting samples between 870.5 and 2330 mbsf and 231 cutting samples between 2107.5 and 3058.5 mbsf were collected during drilling in Holes C0002N and C0002P, respectively (see Hole C0002N and C0002P cuttings smear slides in SMEARSLD in “Supplementary material”). Cuttings were collected at every 5 m depth interval from the shale shakers. Drilling mud and mud gas were also regularly sampled during drilling (see “Geochemistry”). Mud gas, fluid, and cuttings samples were classified by drill site and hole using a sequential material number followed by an abbreviation describing the type of material. The material type identifiers are
Additional information for individual samples (e.g., cuttings size fraction) is provided in the comments section of the J-CORES database and reported in the report text as, for example, “348-C0002N-123-SMW, 1–4 mm” (for the 1–4 mm size fraction aliquot of the one hundred twenty-third cuttings sample recovered from Hole C0002N during Expedition 348). Influence of drilling mud composition on cuttingsBecause of the recirculation of drilling mud and continuous production of formation cuttings and fluids, contamination of cuttings samples is common. Expedition 319 Scientists (2010b) discuss the possible effects of contamination on different types of measurements. New observations of contamination and artifacts induced by riser drilling operation and further QA/QC analysis during Expedition 348 were performed and are reported in the individual methods and site chapters. Cuttings handlingEvery 5 m between 870.5 and 2330 mbsf in Hole C0002N and between 2107.5 and 3058.5 mbsf in Hole C0002P, we routinely collected 2500–4500 cm3 of drilling mud containing cuttings from the shale shaker for onboard analysis, long-term archiving, and personal samples for shore-based postexpedition research. The variable initial sampling volume from the shale shaker is due to varying amounts of personal research samples needed at a specific depth. Between 870.5 and 2330 mbsf, the Marine Works Japan technicians processed all samples following the standard shipboard analysis procedure outlined below, excluding lithologic and structural description and micropaleontological investigation, because the science party had not yet boarded. Description of cuttings at every 10 m interval, as well as additional shipboard measurement and analysis at selected depth intervals, was performed after the science party embarked. Archive samples and an archive split of all processed cuttings samples were sent to the Kochi Core Center (KCC) (Japan) for permanent archiving. The standard cutting laboratory flow is summarized in Figure F1. Unwashed cuttings samples were taken for the following objectives:
The remaining cuttings were washed gently with seawater in a 250 µm sieve at the core cutting area. Samples were then further washed and sieved with seawater using 0.25, 1, and 4 mm mesh. During sieving, a hand magnet was used to remove iron contaminants originating from drilling tools and casing. Cuttings were separated by size fraction as 0.25–1, 1–4, and >4 mm. Splits of the 1–4 and >4 mm fractions were used for bulk moisture and density (MAD) measurements. For Hole C0002N, 220 cm3 of the 1–4 and >4 mm fractions was vacuum dried. Aliquots (15 cm3) from each size fraction were sent as bulk samples for grinding for X-ray diffraction (XRD), X-ray fluorescence (XRF), and organic geochemistry analysis (total organic carbon [TOC], total carbon [TC], and total nitrogen [TN]). The remaining cuttings were described and analyzed for structures and lithology, including microscopy-based observations of thin sections from selected cuttings. On the other hand, for Hole C0002P, washed >4 mm fractions before vacuum drying were sent for structural description. The samples were hand-washed to pick up intact cuttings (see “Structural geology”), were described, and the intact cuttings which exhibited no deformation were used for MAD. Occasionally after dividing and description, samples of interest that were divided by major and minor lithology were selected for additional XRD, XRF, TOC, TC, and TN analysis. For description and analysis of cuttings during Expedition 348, cuttings samples were divided into three types: (1) drilling-induced cohesive aggregate (DICA), (2) pillowed cuttings, and (3) intact (or formation) cuttings (Fig. F2). DICAs were first defined during Expedition 338 as an aggregate that contains less-sorted angular mineral grains and fragments of small formation cuttings in a drilling-mud matrix (Strasser et al., 2014a). These were characteristically easily disaggregated when exposed to water. Their properties are considered not representative of the in situ formation. Pillowed cuttings, the most abundant type of cuttings, were characterized by an accordion-like surface, likely formed by drill bit cutting action, and therefore also significantly altered from their in situ condition. Intact cuttings are considered to represent the formation and were collected by handpicking during the washing and sieving processes. Types of cuttings used for onboard description and standard measurements are summarized in Table T3. Drilling mud handlingDrilling mud samples were collected at two locations, the mud tanks and the mud return ditch. Sampling was carried out regularly every 2–3 days. Drilling mud samples were used for measuring background and contamination effects for NGR, interstitial water (IW), and carbon analysis (see “Physical properties” and “Organic geochemistry”). Additional mud-gas samples were collected once every 12 h (100 mL each) for archiving as reference material. Mud-gas handlingMud gas was extracted from drilling mud immediately after the mud returned from the borehole. A degasser with an agitator was installed on the bypass mudflow line, and the gas extracted in the degasser chamber was pumped to the mud-gas monitoring laboratory through a polyvinyl chloride tube. After problems were identified in producing adequate levels of extracted gas, the degasser unit was moved to the mud return ditch, just “upstream” of the shale shakers. Analysis in the unit is described in “Geochemistry.” Core handlingStandard IODP coring tools, including plastic core liners (6.6 cm in inner diameter), were used in Hole C0002P. In addition to the standard tools, SD-RCB coring with both plastic and aluminum core liners was tested at 475–512.5 mbsf in Hole C0002M. Cores were typically cut into ~1.4 m sections at the core cutting area and logged and labeled by the onboard curator. Figure F3 shows the basic core processing flow chart. A small volume (~5–10 cm3) of sample was taken for micropaleontology from the core catcher section. For time-sensitive whole-round sampling for interstitial water analysis, microbiological analysis, and anelastic strain recovery (ASR), selected core sections were first run through the X-ray computed tomography (XRCT) scanner to identify suitable interval for sampling. Core watchdogs then ensured that the samples could be used and did not contain any critical structures. Interstitial water sample lengths varied depending on core recovery and estimated volumetric fluid in the formation. Microbiological and ASR samples were ~10 cm long. All other core sections were taken to the core processing deck for standard XRCT scanning and core logging with the whole-round multisensor core logger (MSCL-W). After XRCT scanning and MSCL-W logging, community and approved personal research whole-round samples as long as ~20 cm were taken where intact, relatively homogeneous sections could be identified. The number of community whole rounds was limited by core recovery and core quality. All whole rounds were stored at 4°C. Adjacent to whole-round samples (including the time-sensitive, community, and personal whole rounds), a cluster sample was taken at least once per section. The cluster sample is used for routine MAD, XRD, XRF, carbon, and nitrogen analyses shipboard. Some cluster samples were used for shore-based research on clay-fraction XRD and grain size analysis. The core sections remaining after whole-round core sampling were split into working and archive halves. The former was used for structural description and sampling and the latter for lithological description. Digital images of archive-half sections were taken with the photo image logger (MSCL-I) before visual core description (VCD) by sedimentologists and color reflectance measurement by the color spectroscopy logger (MSCL-C). Thermal conductivity measurements were performed on samples from the working half of the cores using the half-space method. Discrete cubes for P-wave velocity and impedance analysis were sampled from the working half. Additional samples were taken for MAD, XRD, XRF, and carbon analyses. After the expedition, all cores were transported in refrigerated reefers for archiving at the KCC. Authorship of site chaptersThe separate sections of the site and “Methods” chapters were written by the following shipboard scientists (authors are listed in alphabetical order):
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