Fracture porosity in the décollement zone of Nankai accretionary wedge using Logging While Drilling resistivity data

Abstract

Fracture porosity in the décollement zone of Nankai accretionary wedge is estimated by comparison of porosity measured on cores during Ocean Drilling Program Leg 131 and porosity calculated from resistivity logs acquired during Leg 196 using Logging While Drilling. Resistivity is converted to formation factor considering both pore fluid conductivity and surface conductivity of clay particles. Pore fluid conductivity is calculated from temperature and ion concentration in interstitial water, whereas surface conductivity is calculated from cationic exchange capacity data and exchangeable cation concentrations. Finally the formation factor is converted to porosity using the generalized Archie’s law. The décollement appears as a zone of compacted rock where dilatant fractures have developed. The contrast between resistivity–porosity and core porosity is used to estimate fracture porosity in the décollement, assuming that the total conductivity is the result of fracture network and rock fragment conductivities, behaving as resistors in parallel, in the direction of the fracture network. Fracture porosity increases downward in the décollement zone from 1.8% to 8.5%. This suggests pore pressure in the décollement zone is higher than the pore pressure estimated from compaction curves (excess pore pressure ratio of 0.47). A possible explanation is that dilatancy is associated with a high pressure transient. The migration of a pressure wave along the décollement could occur at a velocity of 500 m/yr if the permeability of the dilated zone is higher than 10 −12 m 2. The characteristic time for transient dissipation by diffusion in the footwall and hanging wall of the décollement is estimated to be 100–1000 years. Coexistence of dilatant and compactive shear localization structures is observed within the wedge and in the main fault zones. However, only the décollement is currently dilated by fluids. We propose that fluids are injected into the décollement zone during or after fracturing and that initial shear localization is always compactive and occurs ahead of the fluid injections. This sequence of events could occur during each fluid migration and slip event, constituting an increment of décollement propagation.