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Preliminary scientific assessment

Riser drilling at Site C0009 during Expedition 319 allowed a number of new techniques and measurements. These included measurement of gas from drilling mud; analysis of sediment and rock cuttings; and measurement of in situ stress magnitude, formation pressure, and permeability. Important outcomes of Expedition 319 were an assessment of the scope and limitations of these new techniques and improved cuttings sample handling and processing methods. In addition, experience gained from riser drilling with weighted mud during Expedition 319 will be valuable in planning and executing future deep riser drilling. At the riserless Site C0010, the first observatory installation by the Chikyu was conducted. In addition, future observatory instruments and an instrument carrier were tested during run-in to evaluate their design. Knowledge gained from this expedition, in terms of both operations and science, will be valuable for future riser drilling expeditions and for observatory installation.

The goals of this expedition were numerous and involved a wide range of different drilling operations, including coring, drilling mud sampling, wireline logging, LWD/MWD, a VSP experiment involving a second vessel, and observatory installation. The aim was to drill at two primary sites over a period of 114 days including 27 contingency days: one riser drilling site within the forearc basin above the seismogenic part of the active plate boundary, and a riserless drill site in the shallow part of the major megasplay fault system. The scientific objectives were to further understand the processes and properties of the forearc and fault systems while also preparing for and installing observatory systems for long term monitoring of temperature, pore pressure, deformation, and seismicity.

Success relative to planned objectives

Site C0009 within the landward part of the Kumano forearc basin was successfully drilled to the planned total depth. Cuttings were collected for the entire riser drilled interval, with multiple sets in certain intervals. Core recovery was poor in the upper part of the cored section but overall provided material for shipboard analysis and postexpedition geotechnical studies at the depth of planned future observatory installation and for calibration of cuttings measurements and wireline logs. The three planned wireline logging runs were successfully completed to TD including, for the first time in IODP, the successful deployment of the MDT tool for in situ pore pressure and stress magnitude measurement. A late addition to the operational plan, a zero-offset and walkaway VSP experiment, was incorporated into the operational plan (although with a slightly reduced experiment) despite complex scheduling logistics. Ultimately, the walkaway VSP will provide improved resolution of seismic velocity around the borehole and also image the plate boundary and other structures beneath the borehole. The borehole was cased to 1540 m DSF, preserving the hole for future observatory installation.

Site C0010 was drilled through the shallow megasplay thrust fault near its updip terminus and overlying the hypothesized updip limit of the seismogenic plate boundary to 555 m LSF. Data collected were LWD/MWD (gamma ray and resistivity only) down the borehole and across the megasplay fault, including relogging of an interval because of poor data quality during bad weather. We were able to define major unit boundaries and the fault zone on the basis of the limited LWD/MWD data sets, guided by results of previous LWD and coring at nearby Site C0004. The borehole was cased with a screened interval spanning the megasplay fault zone and cemented as planned. Running of the dummy observatory sensor package to test conditions and adapt engineering design for future installations was hindered by current vibrations and loss of part of the package prior to reentry. Therefore, shock and acceleration data are available in the water column but not from reentry into the borehole. A temporary monitoring package (smart plug) was successfully installed to measure formation pore pressure and temperature within the megasplay fault zone and is programmed to record data until retrieval, anticipated in the next 1–2 y.

The integration of core, cuttings, log, and seismic data at and between the two Expedition 319 sites, combined with NanTroSEIZE Stage 1 sites, provides information about sediment properties, lithology, and structure at a wide range of sites and depths across the forearc basin and prism; allows interpretation of structure and development of the Kumano basin and forearc from the late Miocene to present; constrains the hydrological state of the basin and upper prism sediments from in situ measurements; and provides a comprehensive view of in situ stress variability (primarily orientation) across the forearc.

Additional time at the end of the expedition allowed the planned contingency operations to be assessed; LWD/MWD drilling of future coring Site NT1-07 (Site C0011), the primary site for Expedition 322, was chosen as the best science for the time and personnel available. This provided an important advance data set to guide the coring, downhole measurements, and sampling plan for Expedition 322. This is a contribution to the overall NanTroSEIZE program and also constitutes an example of successful operational and scientific flexibility that allowed the best use of contingency time. Site C0011 was drilled to 980 m DSF, collecting gamma ray and resistivity data (including resistivity images). These data are formally reported in Expedition 322 publications.

Problems and challenges

The wide range of different and new drilling operations during Expedition 319 involving different personnel, equipment, and potential problems resulted in a very long and complex planning process. It is therefore unsurprising that operations and logistics were complex during this expedition. At Site C0009, several operational problems were encountered during riser preparations and drilling, resulting in lost time. These included recovery of a lost BHA/DAT, riser system installation problems, and DPS malfunctions. However, with specific operations taking both more and less time than anticipated, Site C0009 drilling ultimately ended on schedule but used all allocated contingency time. The constantly changing drilling operations schedule also made planning and conducting the walkaway VSP experiment with Kairei very complicated and almost impossible. The flexibility of schedules and vessel availability will need to be reviewed carefully when planning future experiments of this kind.

An additional operational challenge we faced was in cementing both the riser and riserless boreholes. Successful cementing is essential to ensure sealing and good coupling between the casing and formation both for operations (i.e., successful deepening of the borehole and continued control of borehole pressure) and scientific objectives (e.g., integrity of observatory installations and obtaining high-quality VSP data). We encountered some problems in cementing both the 13⅜ inch casing at Site C0009 and the 9⅝ inch casing at Site C0010. In the case of the riser hole, cement loss and low cementing efficiency may be related to mud losses during drilling; merging of data from scientific measurements and operations in postexpedition research will be useful in evaluating this possibility. For the riserless site, some uncertainty in the cement job success resulted from problems with running and landing the cement dart. In both cases, the outcomes also highlight the value of running CBLs to define the top of cement and evaluate coupling of casing to the formation.

Riserless drilling of Site C0010 was operationally simple and took less time than originally scheduled, despite the high velocity of the Kuroshio Current (up to 5 kt) during operations and temporary evacuation during a typhoon. Vibrations induced by the current resulted in the destruction of one instrument and the loss of two others within the sensor package during the observatory dummy run. This reduces the amount of data available for future observatory installation planning but indicates the potential impact of the current on the drill string. The significant vibration sustained while running the instruments in the water column and through the Kuroshio Current may prompt a redesign of the observatory systems and deployment strategies.

This expedition was the first to process and analyze drilling mud samples, including gas and cuttings. Therefore, considerable planning was undertaken and patience and versatility on the part of the science and technical team were needed to analyze samples and maximize scientific achievements. In both cases, all groups worked toward providing valuable experimental guidelines for future IODP drilling mud sample handling.

Expedition 319 was also unusual in splitting the Science Party into two groups but working continuously and collectively on shipboard data analysis and report preparation, with four co-chief scientists and specialty coordinators working together across the halves of the expedition to unify the two groups and the scientific results. This aspect of Expedition 319 was extremely challenging for the co-chiefs, expedition project managers, and also for the science party members and required patience, cooperation, and scientific involvement from all parties both before and after shipboard participation. Improved Internet bandwidth for file sharing between shore-based and shipboard science groups will be essential to ensure the success of similar future expeditions. Experiences during this expedition indicate that the entire science party should meet prior to the expedition to improve planning and communication throughout the expedition. This model will be required for future riser drilling operations; therefore, the experience and lessons learned during Expedition 319 will be critical in guiding and planning these efforts.