Fault Rocks. EARS5136 slide 1

Transcription

1 Fault Rocks EARS5136 slide 1

2 Fault rocks Fault rock types Examples of deformation features in cores Microstructures Porosity and permeability Lithological and lithification control EARS5136 slide 2

3 Deformation Processes Environmental controls: - Stress - Temperature - Fluid Pressure - Fluid Chemistry Lithological controls: - Mineralogy - Grain Size - Porosity - Permeability - Material Properties - Strength Deformation response Change properties EARS5136 slide 3

4 Deformation processes Elastic Grain boundary sliding Dissolution precipitation processes Fracture processes Plastic flow EARS5136 slide 4

5 Deformation Mechanism Cycles Stress Strain rate Fracture Time Deformation mechanism changes: Fault Rock properties EARS5136 slide 5

6 Methodologies of fault rock characterisation Microstructural / Microchemical Evaluation SEM, TEM, CL Deformation Processes Diagenesis PETROPHYSICAL PROPERTY DETERMINATION Porosity Permeability Pore Size Distributions Entry Pressures EARS5136 slide 6

7 Interpretation of backscattered electron micrographs EARS5136 slide 7

8 Fault rock classification (Sibson 1978) Incohesive Predominantly non-foliated Fault breccia Fault gouge Pseudotachylyte Foliated Fault gouge Crush breccia (fragments >0.5cm) Fine crush breccia (fragments cm) Proportion of matrix Cohesive Crush microbreccia (fragments <0.1cm) Cataclasite series Mylonite series Fracture processes dominant Crystal plastic processes dominant Protocataclasite Protomylonite 10-50% Cataclasite Mylonite 50-90% Ultracataclasite Ultramylonite % Blastomylonite EARS5136 slide 8

9 Fault rock classification variables - Clay content - Fragmentation level - Lithification state -Fracturing - Grain sliding - Pressure solution - Cementation EARS5136 slide 9

10 Fault Rock Classification Cemented Non - cemented (<20%) Clay Content <15% > 40% 15-40% Clay Smear Phyllosilicate Framework Fault Rock Disaggregation Zone (low Clay) Cataclasites (low clay) EARS5136 slide 10

11 Detailed Fault Rock Classification EARS5136 slide 11

12 Review of fault rock types Disaggregation zones Cataclasites (Clay poor quartz rich) Clean sands Phyllosilicate framework - Impure sands Clay Smears Cemented faults/fractures EARS5136 slide 12

13 Disaggregation seams Very common in clean sands (>90% of total) Formed at low effective stresses No grain fracture Thickness c.10 times grain size of host Indistinguishable from host-rock at <5% clay Some post-deformation pressure solution at 5-15% Some post-deformation quartz cementation due to cleaning of grains Grade into proto-cataclasites EARS5136 slide 13

14 Disaggregation zones Permeability Reduction Factor Permeability nearly independent of clay content Clay Content (%) EARS5136 slide 14

15 Cements, Clay Contents and Diffusive mass transfer (DMT) Enhanced dissolution by clays at grain contacts Clay coating to grains prevents nucleation EARS5136 slide 15

16 Tectonic diffusive mass transfer A fault rock with sub-vertical grain-to-grain contacts resulting from compaction related pressure solution. EARS5136 slide 16

17 Cataclasites Characterized by grain fracture & grain size reduction Thickness not related to host grain size Composed of grain fragments (up to 50%) Permeability reduced with respect to host (1-6 orders of magnitude) More quartz cemented than host Isolated features or dense clusters Anastomosing seams EARS5136 slide 17

18 Cataclasis Stress concentration during deformation: a) undeformed sediment b) stress concentrated on the centre grain c) centre grain fractured Sites for cementation (after Antonellini et al., 1994b) EARS5136 slide 18

19 Cataclasites: quartz rich -clay poor EARS5136 slide 19

20 Cataclasites EARS5136 slide 20

21 Partially lithified cataclasites: Undeformed EARS5136 slide 21

22 Partially lithified cataclasites: Deformed EARS5136 slide 22

23 Partially lithified cataclasites: CL Image EARS5136 slide 23

24 Well lithified cataclasite: Deformed EARS5136 slide 24

25 Well lithified cataclasite: CL Image EARS5136 slide 25

26 Cataclasite Backscattered CL EARS5136 slide 26

27 Backscattered CL Backscattered CL EARS5136 slide 27

28 EARS5136 slide 28

29 Late Fault Activity, Reactivation Impacts on prospectivity New Growth Can allow leaks New growth prevents stress closure EARS5136 slide 29

30 Quartz cementation and cataclasis Coated grains; No nucleation sites New fracture surfaces act as precipitation sites Cement concentration in fault not from large fluid invasion EARS5136 slide 30

31 Cataclastic faults Permeabilities reduced and threshold pressures increased due to grain-fracturing, enhanced post-deformation quartz cementation, deformation induced mixing of fine grained phyllosilicates with framework grains and enhanced postdeformation grain-contact quartz dissolution. Cataclasites experience enhanced quartz cementation due to the creation of reactive quartz surface area. Quartz cement is generated by local processes (graincontact quartz dissolution) and not as a result of fault-related fluid flow. Cementation and grain-contact quartz dissolution is particularly enhanced when faulting occurs at ~90 o C. EARS5136 slide 31

32 Temperature vs. quartz cementation EARS5136 slide 32

33 Pore Size and Threshold Pressure in Cataclasites (Low Clay) reservoir fault EARS5136 slide 33

34 Cataclasites (low clay) permeabilities reservoir fault Permeability reduction factor: 100 to 10,000 EARS5136 slide 34

35 Permeability vs. Threshold pressure EARS5136 slide 35

36 Plot of permeability vs depth of burial Sand on Sand Cataclastic Faults. Clay content not key control. EARS5136 slide 36

37 Calibration of the sealing capacity of cataclasites 1000 Average Threshold Pressure (psi) Maximum burial depth (km) 5 EARS5136 slide 37

38 Calibration of the sealing capacity of cataclasites Maximum burial depth (m) Difference in hydrocarbon column height (ft) EARS5136 slide 38

39 Phyllosilicate Framework Fault Rocks Formed in impure sandstones Clay Contents 15 40% EARS5136 slide 39

40 Phyllosilicate framework fault rocks Less grain fracturing Appear darker than host More homogeneous clay distribution and less macroporosity than host More grain contact quartz dissolution than hosts in reservoirs buried to >3km More grain contact quartz dissolution in 15-25% clay content sands. More mechanical compaction at >25% EARS5136 slide 40

41 Phyllosilicate-framework fault rocks Developed in impure sandstone (clay contents ~ 14 to to 40%) Poroperm properties reduced due to a combination deformation induced mixing of fine grained clays with framework grains and post-faulting grain-contact quartz dissolution Experience enhanced post-faulting grain-contact quartz dissolution due to the deformation induced emplacement of clays between quartz grains Sealing capacity increases with time and temperature when buried > 90 o C EARS5136 slide 41

42 Anastomosing phyllosilicate-rich faults EARS5136 slide 42

43 Phyllosilicate framework fault rocks EARS5136 slide 43

44 Micro Damage Zone EARS5136 slide 44

45 Phyllosilicate Framework Fault Rock EARS5136 slide 45

46 Phyllosilicate Framework Fault Rock EARS5136 slide 46

47 Pore Size and Threshold Pressure in PFFR s EARS5136 slide 47

48 Phyllosilicate-framework fault rocks Maximum burial depth (m) Fault permeability (md) EARS5136 slide 48

49 Clay smears Domains of aligned phyllosilicates Very variable microstructure Continuity highly variable i.e. distance from source can be 3-15 times layer thickness Extremely low permeabilities EARS5136 slide 49

50 Clay smear fault rocks EARS5136 slide 50

51 Polished Clay Smears EARS5136 slide 51

52 Clay smear & heterolithic fault rocks EARS5136 slide 52

53 Clay Smearing EARS5136 slide 53

54 Clay smear EARS5136 slide 54

55 Clay Smearing: EARS5136 slide 55

56 Cementation Had a higher permeability than the host prior to cementation i.e. channelled fluids Low permeabilities Carbonate, sulphate and halite cements found Cementation concentrated in dilational areas of reactivated faults e.g. jogs Not the main seal control in hydrocarbon reservoirs EARS5136 slide 56

57 Cemented cataclasites EARS5136 slide 57

58 Salt cemented fault rocks EARS5136 slide 58

59 Cementation EARS5136 slide 59

60 Reactivation and cementation EARS5136 slide 60

61 Cemented Breccia Implosion and Breccia Zone EARS5136 slide 61

62 Polyphase cemented breccia EARS5136 slide 62

63 Fault property controls Which are critical? Fault Throw Clay contents Geohistory Cementation EARS5136 slide 63