Logging, downhole measurements, and in situ sampling plan

In order to fill the gap of information between spot coring intervals, the logging program is essential to document geophysical, geochemical, hydrogeological, and geobiological properties in the Shimokita coalbed hydrocarbon system. In addition, in situ sampling of formation fluids as well as in situ measurement of pH, hydrocarbon composition, and pCO2 in the formation fluid using logging tools will provide information that is important for the understanding of the coalbed hydrocarbon system.

During Expedition 337, we will conduct wireline logging operations at two depth intervals (647–1220 and 1220–2200 mbsf). The current logging plan consists of four runs in the first shallow interval and seven runs in the second interval. Tool sketches and configuration figures are shown in Figure F10. The following tools will be used during Expedition 337:

  • High-Resolution Laterolog Array (HRLA) tool for lateroresistivity.

  • Platform Express (PEX) for density, porosity, 1-arm caliper, microresistivity, and photo electric factor and Hostile Environment Natural Gamma Ray Sonde (HNGS) for gamma ray spectroscopy.

  • Fullbore Formation MicroImager (FMI) for electrical borehole image and Dipole Sonic Imager (DSI) for P and S sonic velocity.

  • Versatile Seismic Imager (VSI) for vertical seismic velocity profile by check shot.

  • Combinable magnetic resonance (CMR) tool for Nuclear Magnetic Resonance (NMR) porosity, permeability, and pore-size distribution.

  • Modular Formation Dynamics Tester (MDT) for collecting in situ formation fluid samples using Quicksilver Probe and measurement of resistivity, pressure, temperature, hydrocarbon composition (C1–C6), pH, and pCO2, using InSitu Fluid Analyzer (IFA) at the sampling line.

  • Mechanical Sidewall Coring Tool (MSCT) for retrieval of multiple minicores from the borehole sidewall.

The first two runs will provide basic physical property data, such as natural gamma ray, resistivity, density, photoelectric factor, and porosity, providing useful data for characterizing high-resolution stratigraphy within the formation. The caliper log will allow us to assess the hole conditions and chances of success of subsequent logging runs.

The high-resolution FMI images in the third run will provide fine-scale stratigraphy and the best information about the extent of deformation and brecciation. DSI will provide the first measurements of in situ formation sonic velocity, allowing the generation of synthetic seismograms for detailed seismic log correlations and characterization of the petrophysical properties.

The fourth run will be a check shot using vertical seismic profile to tie the well data to the seismic survey data. Data will be recorded with a VSI containing a three-axis geophone.

The subsequent three runs will only be used in the second interval. High-resolution NMR measurement provides the porosity and the distribution of pore size within the formation. These data are processed to estimate the permeability and pore throat size, which is essential to evaluate the chance of success and operation plan of the subsequent formation fluid sampling.

MDT using Quicksilver Probe, which is an in situ fluid sampling module, will provide formation fluid samples with minimal contamination. IFA measures fluid resistivity, temperature, pressure, pH, pCO2, and hydrocarbon concentrations in the sampling port line of the MDT. Prior to opening the valve for bottle sampling, the real-time monitoring of the quality of sampled fluids will be performed with IFA, allowing the best timing for sampling of pristine formation fluids. These in situ fluid measurement and sampling operations will be carried out at intervals of representative lithology, including intervals across the unconformity and coal-bearing (or organic-rich) permeable layers. The actual sampling strategy, however, will depend on borehole conditions, and measurement target depths will be selected after observations of FMI images, Environmental Measurement Sonde (EMS) calipers and other logs from previous runs.

If time and budget allow, the MSCT will be attempted to retrieve minicore samples (0.92 inch in diameter) from the scientifically important horizons (e.g., unconformity layers, coalbed, coal-associated sands, and breccia) and/or the RCB spot coring gaps. The sidewall minicore samples will be used for the study of sedimentological, geochemical, and microbiological characteristics. Deployment of the MSCT is still uncertain, however, depending on further review of operational constraints together with scientific priorities.

All logging data will processed on board the ship to interpret sedimentary facies and to integrate the data with those from core, mud logging, and seismic profiles to construct geological and hydrogeological models. These models will be fundamental for constraining the flux of substrates from the coalbed into shallower strata and the surrounding area.

Details of logging operations are subject to change for operational reasons.