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9 - Interpretation of carbonates.ppt
- 1. Carbonate sedimentology/1 Image log & dipmeter analysis course Sedimentological interpretation of carbonates
- 2. Carbonate sedimentology/2 • Interpretation techniques applied to clastic reservoirs can also be applied to carbonate depositional systems. • Carbonates however, differ from clastics due to the importance of biogenic processes and their susceptibility to diagenetic modifications. Carbonates versus clastics
- 3. Carbonate sedimentology/3 Tucker (1985) Shelf marginal reef Shelf Shelf marginal reef Mud Tidal flats Skeletal pellet mud Lagoon Shelf Skeletal sand Patch reefs Slope Oolite sand bodies Pelagic ooze Basin Re-sedimented carbonates Shoreline Depositional environments
- 5. Carbonate sedimentology/5 Dunham (1962) Carbonate classification
- 6. Carbonate sedimentology/6 Folk (1962) Carbonate classification
- 7. Carbonate sedimentology/8 • All primary sedimentary structures recognised in siliciclastic rocks can occur in carbonates (e.g. cross-bedding in ooid shoals, slumps etc.) • In addition there may be: – Biogenic structures (reefs etc.) – Diagenetic structures unique to carbonates Sedimentary structures
- 10. Carbonate sedimentology/11 • Algal structures – Algal laminites, birds eye/fenestrae, stromatolites etc. • Bioturbation • Reef – Irregular fabrics, could be confused with vuggy horizons, conglomeratic facies etc. Biogenic structures
- 11. Carbonate sedimentology/12 Thalassinoides Rosselia Asterosoma Teichichnus Rhizocorallium Chondrites Terebellina Palaeophycus Ophiomorpha Diplocraterion Conostichus Skolithos Arenicolites Modified from Pemberton et al. (1992) Biogenic structures
- 12. Carbonate sedimentology/13 • Vugs and moulds, in extreme cases karstification • Karstic fissures, breccias and caverns • Hardgrounds • Nodules, nodular bedding • Stylolites and stylobedding • Metastable aragonite and high Mg-calcite highly susceptible to diagenetic modification Diagenetic structures
- 13. Carbonate sedimentology/14 Dolomitisation Karstification Meteoric cementation Overpressuring and hydrocarbon emplacement Stylolitisation Dolomitisation & cementation Fracture porosity Burial cementation Mixing- zone dissolution Submarine lithification and marine cementation Facies controls Diagenetic modification
- 15. Carbonate sedimentology/16 • Need to preserve resistivity or acoustic contrasts: – Porous vs. non-porous foresets in cross-bedded ooid shoals – Grainstone turbidites in deep water pelagic carbonates – Clay lenses/organic rich partings in algal laminites • Variations in physical roughness of the borehole wall etc. related to primary fabric • Differential cementation/replacement may facilitate imaging, but… • Large-scale diagenetic replacement/cementation may result in homogeneous response. Controls on image log response in carbonates
- 16. Carbonate sedimentology/17 Vertical well Carbonate textures and structures - dissolution Sharp irregular base to rudist grainstone Resistive, cemented wackestones Conductive patches represent mouldic porosity after rudists Argillaceous dissolution seams
- 17. Carbonate sedimentology/18 STATIC DYNAMIC Carbonate textures and structures - porosity Vertical well Resistive base to overlying succession Well bedded interval More conductive patches represent vuggy porosity associated with high permeability zone Resistive base to succession with conductive patches - probably represents poorly interconnected vuggy porosity
- 18. Carbonate sedimentology/19 Carbonate textures and structures - laminae Vertical well Well laminated horizon Alternating relatively resistive and conductive horizons corresponding to fining-upward units. Conductive horizons are the coarser, more porous packstone- grainstone facies. Resistive horizons are finer grained packstone to wackestone facies. STATIC DYNAMIC
- 19. Carbonate sedimentology/20 Intensely stylolitised wackestones Carbonate textures and structures – intense stylolitisation Vertical well
- 20. Carbonate sedimentology/21 Conductive coarser, more porous packstone- grainstone facies. Carbonate textures and structures – cross-bedding Vertical well High angle bedding, possible cross- stratification.
- 21. Carbonate sedimentology/22 Carbonate textures and structures - bioturbation Vertical well Burrowed bed contact. Hardground and major unconformity surface. Large sub-vertical burrow shafts. Elevated resistivity response possibly due to cementation associated with unconformity development. Possible burrow gallery.
- 22. Carbonate sedimentology/23 Stylolitisation Carbonate textures and structures - stylolites Vertical well Large amplitude stylolite with resistive halo
- 23. Carbonate sedimentology/24 Large scale slump features Carbonate textures and structures - slumps Vertical well Preferred orientation of resistive patches, possibly bioclasts.
- 24. Carbonate sedimentology/25 Carbonate textures and structures - stylolites Horizontal well Abundant conductive/resistive fractures Stylolitic bed boundary Abundant conductive/resistive fractures
- 25. Carbonate sedimentology/27 Carbonate textures and structures - stylolites Horizontal well Conductive fracture Stylolitic bed boundary Large, scattered angular-subangular resistive patches. Possibly large bioclasts.
- 26. Carbonate sedimentology/28 Abundant dark conductive patches interpreted as vuggy porosity. There is likely to be good connectivity in this example. The bright, resistive patches are probably bioclasts. Carbonate textures and structures - bioclasts Horizontal well
- 27. Carbonate sedimentology/29 Vuggy porosity Acoustic Image Resistivity Image Carbonate textures and structures - vugs Vertical well
- 28. Carbonate sedimentology/30 Transition from bedding (orange) to oversteepened bedding (brown) across a faulted (magenta) interval. Conductive fractures (cyan) are also observed. Carbonate textures and structures
- 29. Carbonate sedimentology/31 Laminated/Debris Flow Hardground Transported Silic. Tight Zone Top Porous Chalk Mottled, bioturbated marl Marl - Laminated Debris Flow Slumped Slumped/Debris Flow Marl - Laminated Marl/Laminated Conductive mottling Minor healed fractures Slump fold mottled fabric Lower Cretaceous Base Tor Hardground Chert Bands Base Ekofisk Debris flow clasts Laminations minor cemented fault heavily mottled fabric of indeterminate transported chalk facies Ekofisk Formation Tor Formation Chalk textures and structures
- 30. Carbonate sedimentology/32 Porosity classification After Nurmi et al. 1990
- 31. Carbonate sedimentology/33 Carbonate textures and structures
- 32. Carbonate sedimentology/34 CORE IS ESSENTIAL TO CALIBRATE BOREHOLE IMAGE FACIES IN CARBONATES • Open hole log response should be used to determine lithology and porosity • Image fabrics should be used carefully: the same fabric may be caused by more than one process Facies interpretation
- 33. Carbonate sedimentology/35 Textural types • Lime mudstones • Wackestones • Packstones • Grainstones • Boundstones - Framestone - Bindstone - Bafflestone • Breccias • Recrystallised Structure/Fabric • Laminated • Bioturbated • Rippled • Cross bedded • Stromatolitic • Fenestral Grain types • Algal • Spiculites • Skeletal - Infauna - Epifauna • Ooids • Lithoclasts - Siliciclastic - Carbonate • Peloids/Pellets • Oncoidal Common facies types
- 34. Carbonate sedimentology/36 GR NPHI RHOB Static Dynamic Core Stacked fining-upward units with packstone- grainstone bases and wackestone tops Carbonate lithofacies Vertical well Resistive fracture Conductive packstones- grainstones Resistive, mottled wackestones
- 35. Carbonate sedimentology/37 Carbonate lithofacies – core calibration Vertical well Boundary between FMI facies FMI7 and FMI4. The burrowed surface is clearly visible (arrowed) as are the packstone-filled burrows below. Burrowed contact (dashed line) between FMIF7 and FMIF4. Burrows (squares) are filled with packstone. Possible hardground. FMIF7 FMIF4 Static image Dynamic image Slabbed core
- 36. Carbonate sedimentology/38 Carbonate lithofacies – core calibration Vertical well Thin, very resistive horizon (FMIF11) corresponding to a subtle lithological change in a mudstone-wackestone succession (FMIF2). The resistivity contrast may be due to differential cementation. Thin mudstone unit with matrix-supported intraclasts (possible debris deposit) within a mudstone-wackestone succession FMIF11 FMIF2 FMIF2 Static image Dynamic image Slabbed core
- 37. Carbonate sedimentology/39 Carbonate lithofacies FMI image facies associations vs Ka 0.01 0.10 1.00 10.00 100.00 1000.00 0 1 2 3 4 5 6 FMI image facies association Ka (mD) FMIA1 FMIA2 FMIA3 FMIA4 FMIA5
- 38. Carbonate sedimentology/40 • Fractures occur in carbonates in the same manner as in clastics. • However tension gashes associated with stylolite bands are a common feature in carbonates. • The interpretation techniques used for clastic sediment fractures are applied to carbonate fractures. Fractures in carbonates
- 39. Carbonate sedimentology/41 Conductive fracture Resistive fracture Fractures in carbonates Vertical well
- 40. Carbonate sedimentology/42 Bedding Fractures Fractures in carbonates Vertical well
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