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doi:10.2204/iodp.proc.338.102.2014 Methods1M. Strasser, B. Dugan, K. Kanagawa, G.F. Moore, S. Toczko, L. Maeda, Y. Kido, K.T. Moe, Y. Sanada, L. Esteban, O. Fabbri, J. Geersen, S. Hammerschmidt, H. Hayashi, K. Heirman, A. Hüpers, M.J. Jurado Rodriguez, K. Kameo, T. Kanamatsu, H. Kitajima, H. Masuda, K. Milliken, R. Mishra, I. Motoyama, K. Olcott, K. Oohashi, K.T. Pickering, S.G. Ramirez, H. Rashid, D. Sawyer, A. Schleicher, Y. Shan, R. Skarbek, I. Song, T. Takeshita, T. Toki, J. Tudge, S. Webb, D.J. Wilson, H.-Y. Wu, and A. Yamaguchi2IntroductionThis chapter documents the methods used for shipboard measurements and analyses during Integrated Ocean Drilling Program (IODP) Expedition 338. Riser drilling was conducted, including cuttings, mud gas, logging while drilling (LWD), and measurement while drilling (MWD) from 852.33 to 2005.5 meters below seafloor (mbsf) in IODP Hole C0002F, which had been suspended for 2 years since being drilled during IODP Expedition 326 by the D/V Chikyu in 2010 (Expedition 326 Scientists, 2011). Due to damage incurred to the intermediate flex joint of the upper riser assembly during an emergency disconnect sequence after the passing of a cold weather front with associated high winds and rapid changes in wind direction while in the high-current area, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC)/Center for Deep Earth Exploration (CDEX) decided to discontinue riser operations at Site C0002 on 23 November 2012 (see “Operations” in the “Site C0002” chapter [Strasser et al., 2014b]). In light of this decision, we completed riserless coring in IODP Holes C0002H (1100.5–1120 mbsf), C0002J (902–926.7 mbsf), C0002K (200–286.5 mbsf), C0002L (277–505 mbsf), C0021B (0–194.5 mbsf), and C0022B (0–419.5 mbsf). Riserless LWD operations were completed in IODP Holes C0012H (0–710 mbsf), C0018B (0–350 mbsf), C0021A (0–294 mbsf), and C0022A (0–420.5 mbsf) (Table T1 in the “Expedition 338 summary” chapter [Strasser et al., 2014a]). Previous IODP work at Site C0002 included logging and coring during Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stages 1 and 2. LWD operations provided data from 0 to 1401.5 mbsf (Hole C0002A; Expedition 314 Scientists, 2009a) and 0 to 980 mbsf (Hole C0002G; Expedition 332 Scientists, 2011). Coring at Site C0002 previously sampled 0–203.5 mbsf (Holes C0002C and C0002D) and 475–1057 mbsf (Hole C0002B) (Expedition 315 Scientists, 2009b). During riser operations, we expanded the data sets at Site C0002. Gas from drilling mud was analyzed in near real time in a mud-gas monitoring laboratory and was sampled for postcruise research. Continuous LWD/MWD data were collected in real time 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 shore-based research. Riserless coring in Holes C0002H and C0002J–C0002L provided additional core samples (whole round and discrete) for standard shipboard and shore-based research. Riserless operations at Site C0012 provided an extensive LWD data set for characterization of the sediment and basement conditions and properties. These logging data, which extend from 0 to 710 mbsf, complemented previous coring work at Site C0012 (Expedition 322 Scientists, 2010c; Expedition 333 Scientists, 2012b) and provided additional data in intervals where core recovery was sparse, especially within the basement. Hole C0018B was the logging complement to coring in Hole C0018A. The LWD hole provided in situ characterization of mass transport deposits (MTDs) that were cored in Hole C0018A (Expedition 333 Scientists, 2012c) as part of the Nankai Trough Submarine Landslide History ancillary project letter. Hole C0018A sampled a stacked series of MTDs that are related to active tectonic processes. Logging data provide additional characterization of the features in the MTDs and the sediments that bound them, which allows additional constraints on the evolution of MTDs. Riserless coring and LWD operations at Site C0021 (proposed Site NTS-1C) targeted a more proximal site for MTDs observed at Site C0018. Combined with LWD and core data obtained at Site C0018, LWD and coring at Site C0021 provide additional information on the nature, provenance, and kinematics of MTDs, as well as constraints on sliding dynamics and the tsunamigenic potential of MTDs. Riserless coring and LWD operations at Site C0022 (proposed Site NT2-13A) were initiated to provide new constraints on the timing of activity along the splay fault. Site C0022 is located between IODP Sites C0004 and C0008 (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009b, 2009c). The objectives of the site were to obtain samples for precise age dating of sediment deformation at the tip of the splay fault to determine the age of activity. Core data provided samples for dating and deformation analysis. Logging data provided in situ conditions and resistivity images of deformation features. Drilling operationsSite C0002Reaming while drilling (RWD) was employed for the first time for scientific ocean drilling during Expedition 338 in order to allow cutting the 12¼ inch diameter pilot hole and opening the hole to 20 inches at the same time. This procedure was employed to facilitate installation of casing strings; however, these strings were not installed because of the early termination of riser operations (see “Operations” in the “Site C0002” chapter [Strasser et al., 2014b]). There is a concentric hole opener between the bit and the underreamer (Fig. F1). The underreamer used to enlarge the hole to 20 inches was the National Oilwell Varco Anderreamer (Fig. F2). The design of the bottom-hole assembly (BHA) also included a complete LWD tool suite (Fig. F3). During riserless coring, no underreamer was used and the bit had a 12¼ inch diameter and used the standard rotary core barrel coring system for Holes C0002H, C0002I, and C0002J. Hole C0002K was cored with a hydraulic piston coring system (HPCS), extended punch coring system (EPCS), and extended shoe coring system (ESCS), whereas Hole C0002L was cored with the ESCS only (Table T1). Site C0012Hole C0012H was drilled with an LWD tool string similar to that used for logging in Hole C0002F (Fig. F3); however, the underreamer was not used. The 12¼ inch polycrystalline diamond compact (PDC) bit was employed to allow drilling in soft and semi-indurated sediment and into basement. Site C0018Hole C0018B was drilled with an LWD tool string similar to that used for logging in Hole C0002F (Fig. F3); however, neither the underreamer nor the sonicVISION were used. The 12¼ inch PDC bit was employed to allow drilling in soft and semi-indurated sediment. Site C0021Hole C0021A was drilled with the same LWD tool string that was used for logging in Hole C0018B (Fig. F3). Hole C0021B was cored with HPCS and EPCS. Site C0022Hole C0022A was drilled with the same LWD tool string that was used for logging in Hole C0018B (Fig. F3). Hole C0022B was cored with HPCS, EPCS, and ESCS. Reference depthsDepths of each measurement or sample are reported relative to both the drilling vessel rig floor (rotary table) and the seafloor (mbsf) (see Table T2). These depths are determined by drill pipe and are correlated to each other by the 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 338, core depths are converted to core depth below seafloor, Method A (CSF-A), which allows overlap relative to the cored interval and section boundaries in cases of >100% core recovery due to expansion after coring (Table T2) (IODP Depth Scales, www.iodp.org/program-policies/procedures/guidelines/). Cuttings and mud depths are reported as mud depth below rig floor (MRF) based on drillers depth (DRF) and the calculated lag depth of the cuttings (see below for additional details). In referring to LWD results, depths are measured as LWD depth below rig floor (LRF) and reported as LWD depth below seafloor (LSF) (see “Logging while drilling” for further details). The depths reported in depths below rig floor (DRF, MRF, and LRF) are converted to depths below seafloor (drilling depth below seafloor [DSF] or CSF-A, mud depth below seafloor [MSF], and LSF, respectively) by subtracting water depth and the height of the rig floor from the sea surface, with corrections relative to drillers depth where appropriate. These depths below seafloor (DSF, CSF-A, MSF, and LSF) are therefore all equivalent. Seismic depths are reported in either time (s) or depth (m). For time sections, a two-way traveltime (s) below sea level scale is used. For depth sections, seismic depth below seafloor (SSF) or seismic depth below sea level (SSL) are used. In this report, meters below sea level (mbsl) or mbsf are used in place of the various depth measures, unless otherwise noted. Cuttings and mud depthsDuring riser drilling, drilling mud circulates within the riser pipe between the drillship and the bottom of the hole. As the drill bit cuts through the formation, cuttings are suspended in the drilling mud and carried with the drilling mud, formation fluid, and formation gas back to the ship. A cuttings sample is assumed to be an average mixture of rock fragments and sediments from a 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 Hole C0002F have been corrected for this lag. Depth precision estimates of cuttingsCuttings were retrieved from 5 m depth intervals, and lag depth was calculated and calibrated as discussed above. Sample depths were recorded as the bit depth at the bottom of the 5 m advance, but samples are assumed to be representative of the 5 m interval. In addition to sampling over a 5 m advance interval, RWD produces cuttings from more than one depth at any moment in time. For the BHA employed during Expedition 338 riser operations, the offset between the bit and the cutting region of the concentric string tool was 38.3 m and the offset between the bit and the cutting portion of the underreamer was 43.8 m. Therefore, if the bit was producing cuttings at 100 mbsf, the concentric string tool was producing cuttings at 61.7 mbsf and the underreamer was producing cuttings at 56.2 mbsf. All cuttings were returned to the drillship for analyses and there was an unavoidable mixing of cuttings produced from three different intervals. This mixing created an uncertainty in the origin depth of the cuttings of at least 43.8 m and also created problems for interpreting thickness of layers that were drilled and percent of different lithologies that comprise those layers. To illustrate this complication, we provide three simplified scenarios. In each scenario, we consider drilling with a 12¼ inch bit and a 20 inch underreamer, neglecting the influence of the concentric string tool. Based on these size cutting tools, on a volumetric percent the bit is producing 38% of the cuttings and the underreamer is producing 62% of the cuttings. We then simulate drilling in three environments:
In Scenario 1, the cuttings indicate 100% silty claystone to 200 mbsf. At 200 mbsf, the first occurrence of sandstone appears but the cuttings indicate a formation that is 62% silty claystone and 38% sandstone (Fig. F4A). This is because as the bit produces cuttings in the sandstone at 200 mbsf, the underreamer produces cuttings in the silty claystone at 156.2 mbsf. Drilling progresses with this pattern until the underreamer and bit are in the sandstone (243.8 mbsf), at which point the cuttings indicate 100% sandstone. The net result is an interpreted lithostratigraphy from cuttings that correctly identifies the top of the sand horizon but does not accurately reflect the true sandstone content until the underreamer and bit are in the sandstone unit. Similar to the first scenario, the cuttings produced in Scenario 2 indicate 100% silty claystone to 200 mbsf. Once the bit enters the sandstone unit, the cuttings indicate 62% silty claystone and 38% sandstone until the underreamer enters the sandstone unit (243.8 mbsf) (Fig. F4B). For the next 6.2 m, the underreamer and bit produce cuttings from the sandstone unit, so cuttings analysis shows 100% sandstone. Below the bottom of the sandstone unit (250 mbsf), the bit produces silty claystone cuttings and the underreamer produces sandstone cuttings, which yields a cuttings-interpreted lithology of 62% sandstone and 38% silty claystone from 250 to 293.8 mbsf. Once the underreamer is deeper than the sandstone layer, the cuttings indicate 100% silty claystone. The net effect is a smeared out sand horizon in the cuttings analysis that does not reflect the depth distribution (200–250 mbsf versus 200–293.8 mbsf) or sand content (100% versus 38%–100%) of the true formation. For Scenario 3, we consider an isolated sandstone layer between silty claystone but assume the sandstone layer is only 15 m thick, which is smaller than the distance between the bit and the underreamer. Above 200 mbsf, the cuttings-inferred lithology is 100% silty claystone, but once the bit crosses into the sandstone (200–215 mbsf), the cuttings-inferred lithology is 62% silty claystone and 38% sandstone (Fig. F4C). Once the bit passes through the sandstone, both the bit and the underreamer produce silty claystone. As the underreamer enters the sandstone (bit at 243.8 mbsf), the underreamer produces sand cuttings whereas the bit produces silty claystone cuttings, so the cuttings-interpreted lithology is a 15 m thick layer that is 62% sandstone and 38% silty claystone. In this scenario, the interpreted lithology would be sandy interbeds (38% sand, 62% sand) within a silty claystone–dominated section, which does not accurately reflect the single, thin, 100% sandstone bed. Although these three scenarios are simplified examples, they provide insight into the first-order complexity of interpreting formation lithology with cuttings that are produced during RWD operations. These complications, which are controlled by the diameter of the different tools, bed thickness, and bed composition, result in uncertainty in assessing the true composition of individual beds and the true depth distribution of beds. Thus, sand content and sand thickness interpreted from cuttings data should be used as a guide but not as an absolute measure of the formation. Beyond the geometry of the system, erosion of the borehole wall from mud circulation adds another level of difficulty for interpretations. Such processes may spread out thickness and concentration variations significantly (see “Physical properties” in the “Site C0002” chapter [Strasser et al., 2014b] for more details). By comparison, LWD data are acquired above the bit but below the underreamer, so the data provide petrophysical measurements over well-defined intervals that are not influenced by the underreamer but could be influenced by borehole enlargement because of borehole erosion. Therefore, cuttings data and logging data should be used in conjunction to help interpret lithology, composition, and bed thicknesses, always keeping in mind that the nature of RWD imparts a minimum of 43.8 m uncertainty and perhaps >80 m of uncertainty (see “Physical properties” in the “Site C0002” chapter [Strasser et al., 2014b]) in the origin depth of any cuttings sample. Sampling and classification of material transported by drilling mudAt total of 312 cuttings samples were collected between 865.5 and 2004.5 mbsf during drilling in Hole C0002F (see Table T1 in the “Site C0002” chapter [Strasser et al., 2014b]). Cuttings were taken at every 5 m depth interval from the shale shakers. Drilling mud and mud gases were also regularly sampled during drilling (see “Geochemistry”). Mud gas, fluids, 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 of individual samples (e.g., cuttings size fraction) is provided in the comments section of the J-CORES database and reported in text as, for example, “338-C0002F-123-SMW, 1–4 mm” (for the 1–4 mm size fraction aliquot of the 123rd cuttings sample recovered from Hole C0002F during Expedition 338). Influence of drilling mud composition on cuttingsBecause of the recirculation of drilling mud and continuous production of formation cuttings and fluids, cuttings samples are contaminated. Expedition 319 Scientists (2010c) discussed the possible effects of contamination on different types of measurements. New observations of contamination and artifacts induced by riser and RWD operations and further quality assurance/quality control analysis were performed during Expedition 338 and reported in the individual methods and site chapters. Cuttings handlingEvery 5 m between 865.5 and 2004.5 mbsf, we routinely collected 3000–5000 cm3 of cuttings material from the shale shaker for shipboard analysis, long-term archiving, and personal samples for postcruise research. Varying initial sampling volume from the shale shaker relates to varying amounts of personal samples taken at a specific depth. Between 860 and 1075 mbsf (i.e., the interval that overlaps with the cored interval in Hole C0002B [Expedition 315 Scientists, 2009b]), all samples were processed following the procedure outlined below. Below 1075 mbsf, every other sample was kept as a “temporary archive” without further processing, and thus standard shipboard analyses were performed on a 10 m depth interval. Specific temporary archive samples were reintroduced into the cuttings processing flow at a later stage during the expedition to refine intervals of special interest identified by preliminary shipboard analysis. Unused temporary archive samples and an archive split of all processed cuttings samples were sent to the Kochi Core Center (KCC) in Kochi, Japan, for permanent archiving. The standard cuttings laboratory flow is summarized in Figure F5. 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 then were further washed and sieved with seawater using a 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 mm, 1–4 mm, and >4 mm. A split of the 1–4 mm and >4 mm fractions was used for bulk moisture and density (MAD) measurements. A volume of 220 cm3 of the 1–4 mm and >4 mm fractions was vacuum-dried. Aliquots (10 cm3) from each size fraction were ground as bulk samples for X-ray diffraction (XRD), X-ray fluorescence (XRF), and geochemistry analysis (carbon and nitrogen). The remaining cuttings were described and analyzed for structures and lithology, including microscopy observation on thin sections from selected cuttings. Occasionally, after dividing and description, samples of interest that were divided by major and minor lithology were selected for additional XRD, XRF, and carbon and nitrogen analysis. Drilling mud handlingDrilling mud samples were collected at two locations: mud tanks (LMT samples) and the mud return ditch. Sampling was conducted regularly every 2–3 days. Drilling mud samples were used for measuring background and contamination effects for NGR and total organic carbon (TOC) analysis (see “Physical properties” and “Geochemistry”). 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 mud-flow line, and the gas extracted in the degasser chamber was pumped to the mud-gas monitoring laboratory via a polyvinyl chloride (PVC) tube. Analysis in the unit is described in “Geochemistry.” Core handlingStandard IODP coring tools, including plastic core liners (diameter = 6.6 cm) were used during Expedition 338. Cores were usually cut into ~1.4 m sections in the core cutting area and logged and labeled by the shipboard curator. Figure F6 shows the basic core processing flow chart. A small (~5–10 cm3) sample was taken for micropaleontology from the core catcher section. Time-sensitive samples for interstitial water analysis, microbiological analysis, and anelastic strain recovery (ASR) were identified as whole core sections in the core cutting area. These time-sensitive whole-round samples were then run through the X-ray computed tomography (CT) scanner, and a core watchdog ensured that the samples could be used and using them would not destroy any critical structures. Once approved, these whole-round samples were identified as core sections. 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 X-ray CT scanning and whole-round multisensor core logger (MSCL-W) measurements. After X-ray CT scanning and MSCL-W measurements, community whole-round samples up to ~20 cm in length 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 and were allocated to individual shipboard and shore-based researchers at the end of the expedition. Adjacent to each community whole-round, ASR, and interstitial water sample, a cluster sample was taken. The cluster sample is used for routine MAD, XRD, XRF, carbon, nitrogen, and sulfur analyses on board the ship. Subsamples of the cluster samples were taken for shore-based research on clay-fraction XRD, grain size analysis, and potential thin sections. The core sections remaining after whole-round core sampling were split into working and archive halves. 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 using the half-space method. Discrete cubes for P-wave velocity, impedance analysis, paleomagnetic measurement with superconducting rock magnetometer (SRM), and unconfined compressive strength (UCS) were sampled from the working half. Additional samples were taken for MAD, XRD, XRF, carbon, nitrogen, and sulfur analyses. After the expedition, all cores were transported in refrigerated storage for archiving at KCC. Authorship of site chaptersThe separate sections of the site chapters and methods chapter were written by the following shipboard scientists (authors are listed in alphabetical order):
Publication: 13 January 2014 |
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