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.

1. CHARACTERIZING MECHANISMS OF CLAY GOUGE FORMATION AND IMPLICATIONS FOR PERMEABILITY, MOAB FAULT, UTAH NWACHUKWU ANYAMELE 1 of 30

2. Talk Outline Introduction to the problem Methods Geologic Setting Results Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition Summary of Findings Conclusions 2

3. Talk Outline Introduction to the problem Methods Geologic Setting Results Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition Summary of Findings Conclusions 3

4. Conceptual Model of a Fault Zone 4 (Chester and Logan, 1986; Caine et al., 1996)

5. Clay-rich fault rocks (cross fault k) 5 Reference: Undeformed Sandstone (Jn or Jmb ~ 10-12 m2) Reduction in Permeability (Davatzeset al.,2005) Effective Confining Pressure [MPa]

6. Clay-rich fault rocks 6 (Davatzes et al., 2005)

9. The shale unit undergoes ductile flow and progressive thinning with increase in throw

10. Both blocks of the fault contribute shale

11. Does not consider slip surfaces7 (summarized in Eichhublet al., 2005)

15. Presence of a gouge8 (Lindsay et al., 1993; Faerseth, 2006)

16. 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)? 9 (Solum et al., 2005) Clay precipitation Comminuted grains (Solum et al., in review)

17. 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). 10

18. Talk Outline Introduction to the problem Methods Geologic Setting Results Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition Summary of Findings Conclusions 11

19. DATA ANALYSIS AND PURPOSE 12

20. IDEALIZEDRESPONSE 13

21. Talk Outline Introduction to the problem Methods Geologic Setting Results Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition Summary of Findings Conclusions 14

22. 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 15

23. SLIP DISTRIBUTION 16

24. Stratigraphy 17

25. Talk Outline Introduction to the problem Methods Geologic Setting Results Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition Summary of Findings Conclusions 18

26. BARTLETT WASH EAST:Shale to Sandstone Juxtaposition 19

27. BARTLETT WASH EASTBOX PLOTS 20

28. BARTLETT WASH EASTSCATTER PLOTS 21

29. MILLCANYON I:Shale to Shale Juxtaposition 22

30. MILL CANYON I BOX PLOTS 23

31. MILL CANYON ISCATTER PLOTS 24

32. Talk Outline Introduction to the problem Methods Geologic Setting Results Sandstone-Shale Juxtaposition Shale-Shale Juxtaposition Summary of Findings Conclusions 25

33. CONCEPTUAL MODEL OF THE MOAB FAULT 26

34. 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 27

35. 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? 28

36. 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 29

37. 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. 30 Thank you for your attention!

38. Mineralogical controls on fault rock friction 31 Sandstone fault minerals Clay-rich (“Shale smear” & clay gouge) fault rocks (Increasing depth) (Lockner and Beeler, 2002)

40. Assumes equal mixing of sand and clay

42. The shale unit undergoes ductile flow and progressive thinning with increase in throw

43. Both blocks of the fault contribute shale

44. Does not consider slip surfaces32

45. BARTLETTWASH EAST: TREND PLOTS 33

46. COURTHOUSE CANYON:TREND PLOTS 34

47. PROTOLITH BOX PLOTS 35

48. COURTHOUSECANYON:Shale to Shale Juxtaposition 36

49. COURTHOUSE CANYON : BOX PLOTS 37

50. COURTHOUSE CANYON : SCATTER PLOTS 38

51. Clay-rich fault rocks (cross fault k) 39 Permeability Steady-state (Davatzes et al., 2005)

52. 40