Moe Kyaw Thu

Growth of borehole breakouts with time after drilling: Implications for state of stress, NanTroSEIZE transect, SW Japan

Resistivity at the bit tools typically provide images of wellbore breakouts only a few minutes after the hole is drilled. In certain cases images are taken tens of minutes to days after drilling of the borehole. The sonic caliper can also image borehole geometry. We present four examples comparing imaging a few minutes after drilling to imaging from about 30 min to 3 days after drilling. In all cases the borehole breakouts widen with time. The tendency to widen with time is most pronounced within a few hundred meters below the seafloor (mbsf), but may occur at depths greater than 600 mbsf. In one example the widening may be due to reduced borehole fluid pressure that would enhance borehole failure. In the three other cases, significant decreases in fluid pressure during temporal evolution of breakouts are unlikely. The latter examples may be explained by time-dependent failure of porous sediments that are in an overconsolidated state due to drilling of the borehole. This time-dependent failure could be a consequence of dilational deformation, decrease of pore fluid pressure, and maintenance of sediment strength until migrating pore fluids weaken shear surfaces and allow spallation into the borehole. Breakout orientations, and thus estimates of stress orientations, remain consistent during widening in all four cases. In vertical boreholes, breakouts wider than those initially estimated by resistivity imaging would result in higher estimates of horizontal stress magnitudes. Because the vertical overburden stress is fixed, higher estimated horizontal stresses would favor strike-slip or thrust faulting over normal faulting.

Re-evaluation of temperature at the updip limit of locked portion of Nankai megasplay inferred from IODP Site C0002 temperature observatory

In 2010, the first long-term borehole monitoring system was deployed at approximately 900 m below the sea floor (mbsf) and was assumed to be situated above the updip limit of the seismogenic zone in the Nankai Trough off Kumano (Site C0002). Four temperature records show that the effect of drilling diminished in less than 2 years. Based on in situ temperatures and thermal conductivities measured on core samples, the temperature measurements and heat flow at 900 mbsf are estimated to be 37.9°C and 56 ± 1 mW/m2, respectively. This heat flow value is in excellent agreement with that from the shallow borehole temperature corrected for rapid sedimentation in the Kumano Basin. We use these values in the present study to extrapolate the temperature below 900 mbsf for a megasplay fault at approximately 5,200 mbsf and a plate boundary fault at approximately 7,000 mbsf. To extrapolate the temperature downward, we use logging-while-drilling (LWD) bit resistivity data as a proxy for porosity and estimate thermal conductivity from this porosity using a geometrical mean model. The one-dimensional (1-D) thermal conduction model used for the extrapolation includes radioactive heat and frictional heat production at the plate boundary fault. The estimated temperature at the megasplay ranges from 132°C to 149°C, depending on the assumed thermal conductivity and radioactive heat production values. These values are significantly higher, by up to 40°C, than some of previous two-dimensional (2-D) numerical model predictions that can account for the high heat flow seaward of the deformation front, including a hydrothermal circulation within the subducted igneous oceanic crust. However, our results are in good agreement with those of the 2-D model, which does not include the advection cooling effect. The results imply that 2-D geometrical effects as well as the influence of the advective cooling may be critical and should be evaluated more quantitatively. Revision of 2-D simulation by introducing our new boundary conditions (37.9°C of in situ temperature at 900 mbsf and approximately 56 mW/m2 heat flow) will be essential. Ultimately, in situ temperature measurements at the megasplay fault are required to understand seismogenesis in the Nankai subduction zone.

Age‐calibrated three‐dimensional views of the crust

Scientific drilling provides detailed, highresolution, direct observations of the Earths crust. Ocean drilling science has a nearly 40-year history, starting with the Deep Sea Drilling Project (1968–1983), running through the Ocean Drilling Program (1985–2003), and continuing as the Integrated Ocean Drilling Program (IODP2003 to present). Ocean drilling science also has growing ties to the International Continental Drilling Project (1996 to present). Over the course of these scientific drilling programs, intensive analysis of a wide array of global geological and geophysical systems has been accomplished, including oceanic crustal architecture; tectonics and fluid flow along convergent and divergent plate margins; oceanic plateau composition, structure, age, and genesis; volcanic and nonvolcanic rifted margin tectonics, structure, and evolution; sedimentary records of ocean circulation, climate change, and evolution; fault zone geometry and dynamics.