5:15 PM - 6:30 PM
[SCG51-P01] For examining steady-state solution for pore pressure distribution in a plate-subduction zone incorporating fluid release by chemical reaction
Keywords:Plate-subduction zone, Dehydration reaction, Smectite, Pore pressure, Steady-state solution
Progressive compaction and dehydration of subducting sediments in a plate-subduction zone have been considered to affect its pore pressure condition. Representative dehydration reactions include smectite-illite-muscovite conversion and amorphous silica-Opal CT-quartz conversion reactions. Some previous studies modelled quantitative effects of compaction and these dehydration reactions on steady-state pore pressure and permeability. However, these studies ignore the volumetric change in solid phase by reaction, and thus the steady-state solutions incorporating the spatial heterogeneity of such volumetric change should be examined.
The ultimate goal of this study is to examine the steady-state solutions for pore pressure distribution incorporating progressive compaction and dehydration reactions based on a reasonable physical model. Because the permeability of fault rocks is generally much higher than that of surrounding intact rocks, we consider the very simple model in which fluid flows only along a fault zone. Dirichlet boundary conditions of pore pressure were postulated at a surface and a down-dip limit of seismogenic crust. We focus on Kumano Basin in Nankai Trough as an example of plate-subduction zones, and various physical properties were set according to the previous studies examined in this area. Amorphous silica content in the core samples is reported to be less than 1 wt%, so we focus only on the transformation reactions of smectite. As we could not find any literatures reporting kinetics of illite-muscovite conversion, only illitization of smectite is considered here.
The qualitative effect of fluid release due to chemical reaction on pore pressure depends on the balance between the rates of fluid release and pore volume created by reaction. On the basis of some reaction equations reported in previous studies, illitization of smectite can have both positive and negative effects on pore pressure. However, if we consider the constraints on the reaction equation such that the mole number of reacted smectite and generated illite must be equivalent, illitization of smectite may have positive effect on pore pressure. The preliminarily calculation shows that smectite dehydration can decrease pore pressure by up to ~10 MPa. In this presentation, we would like to discuss the calculation results obtained before the conference, in addition to these preliminarily results.
The ultimate goal of this study is to examine the steady-state solutions for pore pressure distribution incorporating progressive compaction and dehydration reactions based on a reasonable physical model. Because the permeability of fault rocks is generally much higher than that of surrounding intact rocks, we consider the very simple model in which fluid flows only along a fault zone. Dirichlet boundary conditions of pore pressure were postulated at a surface and a down-dip limit of seismogenic crust. We focus on Kumano Basin in Nankai Trough as an example of plate-subduction zones, and various physical properties were set according to the previous studies examined in this area. Amorphous silica content in the core samples is reported to be less than 1 wt%, so we focus only on the transformation reactions of smectite. As we could not find any literatures reporting kinetics of illite-muscovite conversion, only illitization of smectite is considered here.
The qualitative effect of fluid release due to chemical reaction on pore pressure depends on the balance between the rates of fluid release and pore volume created by reaction. On the basis of some reaction equations reported in previous studies, illitization of smectite can have both positive and negative effects on pore pressure. However, if we consider the constraints on the reaction equation such that the mole number of reacted smectite and generated illite must be equivalent, illitization of smectite may have positive effect on pore pressure. The preliminarily calculation shows that smectite dehydration can decrease pore pressure by up to ~10 MPa. In this presentation, we would like to discuss the calculation results obtained before the conference, in addition to these preliminarily results.