Other articles in the category "Fault seals and top seals"

 

Clay Gouge Evolution in Mechanically Stratified Sequences

 

Results of water-saturated model experiments

Joyce Schmatz and Janos Urai

Project funded by ExxonMobil Upstream Research Company, supervised by P.J. Vrolijk

Abstract

This study focuses on clay smear processes during fault gouge evolution in sand-clay sequences at depths up to 2 km. A clay-rich fault gouge can dramatically lower the fault’s permeability, and prediction of this process is there fore relevant in groundwater modeling and hydrocarbon geology. We constructed an ‘underwater’ sandbox to deform layered sand-clay models of 20 x 40 x 20 cm above a 70º dipping rigid basement fault (Fig. 1).

The experiments are run completely water-saturated to allow deformation of wet clay and cohesionless sand. Water content of the clay is used to control its shear strength and state of consolidation. We defined three strength classes: soft clay (up to 55 wt% water), normal clay (around 40 wt% water) and firm clay (less than 30 wt% water). Soft and firm clay represent underconsolidated and overconsolidated endmembers, respectively. The normal clay is in equilibrium with the overburden load. Material properties were carefully characterized by a series of geotechnical measurements.

The interface on top of the model material is either water or aluminum plates precut along the kinematically ideal plane. We systematically studied the eect of clay strength, thickness, number and position of the clay layers and thickness of the cover sand (Fig. 2). Around 300 high-resolution digital images for each experiment were processed into time-lapse movies and analyzed using PIV (Particle Image Velocimetry). PIV calculates the displacement field (Fig. 3) at the scale of individual sand grains, and allows calculation of all components of the incremental strain field.

The main effects of the different parameters are as follows: (1) absence of the top precut plate makes the fault zone much wider, and renders the formation of a continuous clay gouge more difficult. (2) in models containing soft clay and a precut top plate, a continuous clay smear is formed irrespective of the details of geometry, and deformation approaches simple shear with mechanical mixing of sand and clay (Fig. 4). (3) lateral clay injection is a rare process in our experiments, even for soft clay; (4) stiff clay behaves in a brittle fashion, fault motion is associated with rotation of rigid blocks and no continuous clay gouge is formed (Fig. 3); (5) for the same amount of clay in the sequence the presence of many thin layers prefers the formation of a continuous clay gouge.


Picture_925 - Joyce sandbox broken clay PIV

Fig. 3 Image sequence showing evolving fractures in a firm clay layer. PIV overlay of the rotational component around z displays fault evolution.

 




Picture_922 - Joyce sandbox sketch01

Fig. 1 Sketch showing "underwater"-sandbox.

Picture_923 - Joyce sandbox sketch02

Fig. 2 Sketch showing model setup with 30 mm thick, soft clay layer and top plates.

Picture_924 - Joyce sandbox index

Fig. 4 Picture showing layered clay gouge with continuous clay smear in a multilayer setup.



Clay smear publications

 

Theses

Joyce Schmatz
Experimental study on clay gouge evolution in mechanically stratified sequences. Diplomarbeit, RWTH Aachen University. (2006)
 

Book chapters

Joyce Schmatz, Janos Urai
Clay smear in Sandbox experiments. In: Dynamics of complex intracontinental basins: The Central European Basin System. Littke, R., Bayer, U., Gajewski, D. & Nelskamp, S. Eds.). Springer-Verlag, Berlin-Heidelberg, 520 p. (2008)
 

Papers

Joyce Schmatz, Peter Vrolijk, Janos Urai
Clay smear in normal fault zones - the effect of multilayers and clay cementation in water-saturated model experiments Journal of Structural Geology 32 - Fault zones (11), 1834-1849. (2010)
 
Wouter van der Zee, Janos Urai
Processes of normal fault evolution in a siliciclastic sequence: a case study from Miri, Sarawak, Malaysia. Journal of Structural Geology, 27(12),2281-2300 (2005)
 

Abstracts

Steffen Abe, Janos Urai, Joyce Schmatz
DEM Simulation of Clay Smear along Faults. EGU General Assembly 2009, Vienna, Austria, 19 - 24 April 2009, 11. (2009)
 
Joyce Schmatz, Marc Holland, Wouter van der Zee, Janos Urai
Fault gouge evolution in layered sand-clay sequences - first results of water-saturated sandbox experiments. Fault and Top Seals: What do we know and where do we go?, EAGE, Montpellier, France, 8-11 September 2003, O22. (2003)