Characterizing Mechanisms Of Clay Gouge Formation
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CHARACTERIZING MECHANISMS OF CLAY GOUGE FORMATION AND IMPLICATIONS FOR PERMEABILITY, MOAB FAULT, UTAH NWACHUKWU ANYAMELE 1 of 30
description
Clay composition and content profoundly impacts the strength and sealing capacity of a fault zone, reducing frictional resistance to sliding and permeability by as much as 7 orders of magnitude. Previous approaches, including the Shale Gouge Ratio (SGR) and Shale Smear Potential (SSP), have been used to understand and predict the clay content of fault zones. These models are largely limited to mechanical incorporation of detrital clays. This hypothesis stems from field observations of clay gouge and the smearing and associated attenuation of clay-rich shale beds offset by the fault. Recently, diagenesis has been recognized as an additional critical mechanism of clay enrichment in fault zones. My study investigates the relative contributions of both mechanisms of clay enrichment focusing on the implications for fault permeability and strength through structural and elemental mapping of the Moab Fault in Utah. Detailed mapping at Six sites along the Moab Fault in southeast Utah, revealed distinct structural deformation zones as defined by structures and distribution of normally faulted sandstone and shale including: (1) layers of clay-rich gouge separated by slip surfaces that include isolated sandstone breccia; (2) an inner smeared shale adjacent to the gouge showing increasing bed parallel shearing and resulting boudinage closer to the fault, and an outer smear with little shearing but rotation of beds; (3) faulted sandstone hosting deformation bands, slip surfaces, and intersections, joints and veins in locations near relays. Fluid assisted alteration was revealed by a combination of high spatial resolution scan-lines on outcrops element composition and measured sections of measured with a portable X-Ray Fluorescence device. Results to date include: (1) elemental concentrations relative to immobile species (such as Ti) and by structural zone show that Ca, Sr, Rb are preferentially enriched and/or depleted in the fault core, (2) the fault core hosts the greatest alteration; (3) a progressively more extensive and greater density of bed parallel slip surfaces from protolith to gouge where slip surfaces are associated with mixing and disaggregation; (4) stable concentration of elements associated with illite such as K, occurs preferentially in the gouge; (5) localized enrichment and/or depletion reveals solution mass transfer contributed to formation of the fault core and to a lesser extent the damage zones. Elemental mapping clearly demonstrates a compositional evolution of the fault core, and in particular the clay gouge, that cannot be accounted for by mixing of protolithic formations. Thus, observations from elemental mapping show that solution mass transfer influences the formation of clay gouge in the fault zone, in addition to mechanical incorporation of detrital clays from the surrounding protoliths.
Transcript of Characterizing Mechanisms Of Clay Gouge Formation
CHARACTERIZING MECHANISMS OF CLAY GOUGE FORMATION AND IMPLICATIONS FOR PERMEABILITY, MOAB FAULT, UTAH
NWACHUKWU ANYAMELE
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TALK OUTLINE
Introduction to the problem Methods Geologic Setting Results
Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition
Summary of Findings Conclusions
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TALK OUTLINE
Introduction to the problem Methods Geologic Setting Results
Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition
Summary of Findings Conclusions
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CONCEPTUAL MODEL OF A FAULT ZONE
(Chester and Logan, 1986; Caine et al., 1996)
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CLAY-RICH FAULT ROCKS (CROSS FAULT K)
Red
uct
ion
in
Perm
eab
ilit
y
(Davatzes et al.,2005)Effective Confining Pressure [MPa]
Reference:Undeformed Sandstone (Jn or Jmb ~ 10-12 m2)
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CLAY-RICH FAULT ROCKS
(Davatzes et al., 2005)
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SGR AND SSP
SHALE GOUGE RATIO (SGR):
Rocks juxtaposed across the fault slip surface are worn and mixed to provide a gouge Assumes equal mixing of sand and clay Presence of a gouge
SHALE SMEAR POTENTIAL(SSP): Layers of clay offset by the fault are continuous The shale unit undergoes ductile flow and
progressive thinning with increase in throw Both blocks of the fault contribute shale Does not consider slip surfaces (summarized in Eichhubl et al.,
2005)
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SGR AND SSP
SHALE SMEAR POTENTIAL(SSP):
Layers of clay offset by the fault are continuous The shale unit undergoes ductile flow and progressive thinning with increase in throw Both blocks of the fault contribute shale Does not consider slip surfaces
SHALE GOUGE RATIO (SGR): Rocks juxtaposed across the fault slip surface are
worn and mixed to provide a gouge Assumes equal mixing of sand and clay Presence of a gouge
(Lindsay et al., 1993; Faerseth, 2006)
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DIAGENESIS IN FAULT ZONES
Clays form during faulting from geochemical processes
Shown by XRD analysis of mineralogy at the R191 site of the Moab fault by Solum et al. (2005)
Unresolved questions: Does re-mineralization occur within
a closed or open system? Where do these processes occur in
the fault zone? Does this depend on offset or
deformation intensity? Is this diagenesis important
(neoformation and authigenesis)?Clay precipitation
Comminutedgrains
(Solum et al., in review)
(Solum et al., 2005)
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HYPOTHESIS
APPROACH Compare elemental chemistry of undeformed protolith
to structures in the fault zone to determine whether there are chemical changes directly related to faulting.
TEST If there is no change, then the clays were incorporated
by a purely mechanical process. If there are elemental changes associated with
structural zones in the fault or specific structures, then clay content of the fault zone includes both mechanical incorporation and chemical processes (solution mass transfer).
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TALK OUTLINE
Introduction to the problem Methods Geologic Setting Results
Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition
Summary of Findings Conclusions
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DATA ANALYSIS AND PURPOSEDATA PURPOSE
FIELD WORK To map the key structures and structural zones, and determine the macroscopic mechanisms of clay gouge formation in fault rock and evidence for fluid flow history
GEOCHEMICAL DATA SAMPLING
To represent elemental compositions of protolith and each structural zone
STATISTICAL ANALYSIS
To determine whether concentrations of elements were significantly different between the various structural zones and the protolith
ANOVA To determine whether a significant difference exists between the means of the different structural zones
Trend & Box plots
To clarify spatial trends related to structural position
Scatter plots To discriminate mechanical mixing of two protolith compositions from enrichment or depletion in the fault core
CORRELATION Correlate alteration to structural zone and structure types
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IDEALIZED RESPONSE TRANSECT BOX PLOTS CROSS PLOTS
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TALK OUTLINE
Introduction to the problem Methods Geologic Setting Results
Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition
Summary of Findings Conclusions
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MOAB FAULT BACKGROUND
Located in the NW part of the Paradox Basin in Southeast Utah.
N-W striking 45 km system of normal faults.
Maximum offset of 1 km.
Extensive cross sectional exposure of fault zone
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SLIP DISTRIBUTION
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STRATIGRAPHY
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TALK OUTLINE
Introduction to the problem Methods Geologic Setting Results
Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition
Summary of Findings Conclusions
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BARTLETT WASH EAST:SHALE TO SANDSTONE JUXTAPOSITION
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BARTLETT WASH EASTBOX PLOTS
Antimony (Sb) Calcium (Ca)
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BARTLETT WASH EASTSCATTER PLOTS
Antimony (Sb) Calcium (Ca)
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MILL CANYON I:SHALE TO SHALE JUXTAPOSITION
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MILL CANYON I BOX PLOTSAntimony (Sb) Rubidium (Rb)
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MILL CANYON ISCATTER PLOTSAntimony (Sb) Rubidium (Rb)
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TALK OUTLINE
Introduction to the problem Methods Geologic Setting Results
Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition
Summary of Findings Conclusions
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CONCEPTUAL MODEL OF THE MOAB FAULT
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STRUCTURAL CONTROL OF ALTERATION: ALL SITES
Spatial trends in concentration correlate with structural zones
Elemental composition of clay-gouge is consistent with mixing of shale protolith (and lack of sandstone contribution)
Enrichment and depletion in concentration of some elements occur in the fault core and adjacent damage zone (max. in sst. to shale juxtaposition)
Clay-gouge is the most highly altered zone DBZ (similar deformation intensity) Joints & Breccia (associated with
high k) No local source-sink behavior
detected in the fault zone
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IMPLICATIONS
What does this mean for: - permeability?- friction?- deformation and alteration of the fault zone?
Is the chemical alteration important in the development of the fault core?
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CONCLUSIONS
Moab Fault is composed of distinct structural zones: The damage zone (shale: inner and outer smear; sandstone:
DBs and joints) The fault core (clay-rich gouge and the DBZ) The clay-rich gouge and associated slip surfaces
The structural zones exhibit distinct geochemical signatures revealing fluid flow history
The exchange of elemental constituents between the fault core and the protolith indicates an open system minimal in the shale-shale juxtaposition, but enhanced in the sandstone-shale juxtaposition
In addition to mechanical mechanisms of fault rock formation, solution mass transfer participated in the evolution of the fault zone material
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AKNOWLEDGEMENT
Shell International Exploration and Production Inc., for funding this research
Dr. Nicholas Davatzes (Thesis Advisor) Dr. John Solum (Shell International
Exploration and Production Inc.) Dr. David Grandstaff Dr. Dennis Terry Jr.
Thank you for your attention!
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MINERALOGICAL CONTROLS ON FAULT ROCK FRICTION
(Lockner and Beeler, 2002)
(Increasing depth)
Sandstonefault minerals
Clay-rich(“Shale smear”
& clay gouge)fault rocks
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SGR AND SSP
SHALE GOUGE RATIO (SGR): Rocks juxtaposed across the fault slip surface
are worn and mixed to provide a gouge Assumes equal mixing of sand and clay Presence of a gouge
SHALE SMEARPOTENTIAL(SSP):
Layers of clay offset by the fault are continuous
The shale unit undergoes ductile flow and progressive thinning with increase in throw
Both blocks of the fault contribute shale Does not consider slip surfaces
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BARTLETT WASH EAST: TREND PLOTS
Antimony (Sb) Calcium (Ca)
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COURTHOUSE CANYON:TREND PLOTS
Antimony (Sb) Rubidium (Rb)
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PROTOLITH BOX PLOTS
Antimony (Sb) Rubidium (Rb)
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COURTHOUSE CANYON:SHALE TO SHALE JUXTAPOSITION
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COURTHOUSE CANYON : BOX PLOTS
Antimony (Sb)
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COURTHOUSE CANYON : SCATTER PLOTS
Antimony (Sb)
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CLAY-RICH FAULT ROCKS (CROSS FAULT K)
Permeability
Steady-state
(Davatzes et al., 2005)
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