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Methods and materials

Coal-bearing formation samples

We conducted permeability experiments on two lignite coal samples taken at 1919.57 and 1922.95 meters below seafloor (mbsf) and a grain-size analysis porosity measurement on one sand sample taken at 1925.38 mbsf from lithologic Unit III (1826.5–2046.5 mbsf) in a coal-bearing formation dominated by several coal horizons at Site C0020 (Inagaki et al., 2012). Water content, color, and vitrinite reflectance measurements suggest the coal has low maturity (Inagaki et al., 2012; Gross et al., 2015; Tanikawa et al., 2016). The sand sample is unconsolidated and extremely soft (i.e., beach sand) (see the “Site C0020” chapter [Expedition 337 Scientists, 2013]). The sand layers, which were almost impossible to retrieve intact by riserless drilling, were successfully recovered by riser drilling using high-density mud (Inagaki et al., 2012). Half-round core samples that yielded the coal samples were sealed by a gas barrier film (Escal Neo; Mitsubishi Gas Chemical Company, Inc., Japan) after collection and stored at 4°C to maintain their natural water content until sampling.

Permeability measurements

Permeability experiments were conducted on two cylindrical samples of coal retrieved from 1919.57 and 1922.95 mbsf (Table T1). Samples were taken directly from the sealed core and trimmed parallel to the core axis to allow assessment of vertical permeability. The sample from 1919.57 mbsf is 2.01 cm long and 1.6 cm in diameter, and the sample from 1922.95 mbsf is 1.80 cm long and 1.5 cm in diameter. Each specimen was placed in impermeable, flexible heat-shrink tubing and then loaded into a pressure vessel. The pressure vessel is connected to three pumps (Figure F1; Ohtomo et al., 2013). One pump controlled the pressure at the top of the specimen, one pump controlled the pressure at the bottom of the specimen, and one pump controlled the isostatic confining pressure around the specimen. Deionized water was used as the confining fluid and the permeant.

After the backpressure and confining pressure reached at 30 and 40 MPa, respectively, flow-through permeability tests were conducted by imposing a steady pressure gradient across the specimen while maintaining the isostatic effective stress at the center of the specimen. We didn’t conduct the backpressure saturation or consolidation of specimen before the tests.

Experimental conditions were selected to be similar to the in situ condition at the depth where the sample was collected. Confining pressure represents the total overburden above a specified depth. The total overburden was estimated as follows:

W = ρw × dw + ρb × ds, (1)


  • W = total weight,
  • ρw = density of water,
  • dw = water depth,
  • ρb = bulk density of sediment in water to consider buoyancy, and
  • ds = sediment depth.

Considering the water depth at Site C0020 (1180 m), the density of seawater (1.02 g/cm3), the sediment depth where the sample was collected (~1900 mbsf), and the bulk density of the sediments from Site C0020 ranging from 1.7 to 2.3 g/cm3 (see the “Site C0020” chapter [Expedition 337 Scientists, 2013]), the total overburden in situ is estimated to be 44.3–55.7 MPa. Thus, we conducted the experiment at 40 and 50 MPa confining pressure with effective stress of 10 and 20 MPa, respectively, as similar conditions to the in situ pressure for the coal-bearing formation. After the experiments at 40 MPa, we increased the confining pressure to 50 MPa. The backpressure of 30 MPa was selected to be similar to the hydrostatic pressure at the specified depth (~3000 meters below sea level [mbsl]; ~30.6 MPa).

The temperature of the confining fluid was fixed at 50°C, which is similar to the ambient temperature at ~1900 mbsf (Inagaki et al., 2012; Tanikawa et al., 2016). The temperature of the confining fluid in the pressure vessel was regulated by a constant-temperature fluid circulating in which a silicon oil is circulated through the heater (Julabo, Baden-Württemberg, Germany, HE) and around the pressure vessel (Figure F1). The in situ temperature during the experiment is monitored by a thermocouple sensor attached to stoppers of the column.

All experiments were run after a steady state was achieved in which inflow and outflow volumes were equal while the sample length and pressure gradient remained constant. From the steady-state flow rate, sample dimensions, and pressure difference across the specimen, intrinsic permeability was calculated by rearranging Darcy’s law:

k = –Qµ/A × Δl/ΔP, (2)


  • k = permeability,
  • Q = steady-state flow rate,
  • µ = fluid viscosity,
  • A = cross-sectional area of the specimen,
  • Δl = length of the specimen, and
  • ΔP = fixed pressure difference across the specimen.

Fluid viscosity (µ) was referred to a physical table to adjust for temperature, which was measured nearly continuously during the experiments.

Grain-size distribution and porosity of sandy sediment

We analyzed the grain-size distribution and porosity in the sand sample from 1925.38 mbsf because the sample was too soft to conduct a permeability test (Table T1). The sand sample is unconsolidated beach sand (see the “Site C0020” chapter [Expedition 337 Scientists, 2013]), which is not sandstone at all and would be disturbed. It was impossible to trim the sample in a cylindrical form for the permeability test. Furthermore, the large flow rate is beyond the capacity of the cylinder pump equipped with our instrument was anticipated, even with a small pressure gradient across the specimen.

The porosity of the sand sample was estimated by gravimetric measurements. The sample was carefully placed into a cylindrical plastic chamber of known volume (1.7 cm3) by a spatula. Assuming that the sample was saturated by water, its saturated unit weight was estimated by weighing. The sample was then dried at 60°C and weighed again. The pore volume was estimated from the difference in weight between the wet and dry sample and the density of water. We used the density of pure water (1.0 g/cm3) for the estimation because the coal-bearing formation would be saturated by low-chlorinity water or freshwater and the absolute salt content was unknown (see the “Site C0020” chapter [Expedition 337 Scientists, 2013]). Porosity e was then estimated as follows:

e = Vv/V, (3)


  • Vv = pore volume and
  • V = total sample volume.

For the measurement of grain-size distribution, the sample (~30 cm3) was dried at 60°C and the grain-size distribution was then determined by wet sieving at successive mesh sizes of 75, 90, 125, 150, 180, 250, 425, and 700 µm, followed by drying at 60°C for 3 days and weighing. The gradient of weight difference divided by dry time was checked to be small enough.