9 - interpretation of carbonates
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Carbonate sedimentolo
Image log & dipmeterImage log & dipmeter
analysis courseanalysis course
Sedimentologicalinterpretation of
carbonates
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Carbonate sedimentolo
Interpretation techniques applied to clasticreservoirs 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 clasticsCarbonates versus clastics
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Carbonate sedimentolo
Tucker (1985)
Shelf marginal reef
Shelf
Shelf marginal reef
Mud
Tidal flats
Skeletal
pellet mud
LagoonShelf
Skeletal
sand
Patch reefs
Slope
Oolite sand bodies
Pelagic ooze
Basin
Re-sedimented
carbonates
Shoreline
Depositional environmentsDepositional environments
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Carbonate sedimentolo
Carbonate grain typesCarbonate grain types
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Dunham (1962)
Carbonate classificationCarbonate classification
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Folk (1962)
Carbonate classificationCarbonate classification
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Qualifiers:
v = vuggy
c = cemented
Image facies schemeImage facies scheme
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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 structuresSedimentary structures
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Sedimentary structuresSedimentary structures
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Cross-stratification
SedimentarySedimentary
structuresstructures
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Algal structures
Algal laminites, birds eye/fenestrae,
stromatolites etc.
Bioturbation Reef
Irregular fabrics, could be confused with vuggy
horizons, conglomeratic facies etc.
Biogenic structuresBiogenic structures
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Thalassinoides
Rosselia
Asterosoma
Teichichnus
Rhizocorallium
Chondrites
Terebellina
Palaeophycus
Ophiomorpha
Diplocraterion
Conostichus
Skolithos
Arenicolites
Modified from Pemberton et al. (1992)
Biogenic structuresBiogenic structures
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Carbonate sedimentolog
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 structuresDiagenetic structures
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Carbonate sedimentolog
Dolomitisation
Karstification
Meteoric
cementation
Overpressuring
and
hydrocarbon
emplacement
StylolitisationDolomitisation &cementation
Fracture porosity
Burial
cementation
Mixing-zone
dissolution
Submarine lithification
and marine
cementation
Faciescontrols
Diagenetic modificationDiagenetic modification
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Carbonate sedimentolog
Dolomitisation
DiageneticDiagenetic
structuresstructures
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Carbonate sedimentolog
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 inhomogeneous response.
Controls on image log response in carbonatesControls on image log response in carbonates
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Carbonate sedimentolog
Vertical well
Carbonate textures and structures - dissolutionCarbonate textures and structures - dissolution
Sharp irregularbase to rudist
grainstone
Resistive,
cemented
wackestones
Conductive patches
represent mouldic
porosity after rudists
Argillaceous
dissolution seams
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Carbonate sedimentolog
STATIC DYNAMIC
Carbonate textures and structures - porosityCarbonate textures and structures - porosity
Vertical well
Resistive base tooverlying 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
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Carbonate sedimentolog
Carbonate textures and structures - laminaeCarbonate 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 wackestonefacies.
STATIC DYNAMIC
Carbonate textures and structuresCarbonate textures and structures
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Carbonate sedimentolog
Intensely
stylolitisedwackestones
Carbonate textures and structures Carbonate textures and structures
intense stylolitisationintense stylolitisationVertical well
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Carbonate sedimentolog
Conductive coarser,
more porous packstone-
grainstone facies.
Carbonate textures and structures cross-beddingCarbonate textures and structures cross-bedding
Vertical well
High angle bedding,
possible cross-
stratification.
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Carbonate sedimentolog
Carbonate textures and structures - bioturbationCarbonate 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 unconformitydevelopment.
Possible burrow
gallery.
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Carbonate sedimentolog
Stylolitisation
Carbonate textures and structures - stylolitesCarbonate textures and structures - stylolites
Vertical well
Large amplitude stylolitewith resistive halo
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Carbonate sedimentolog
Large scale
slump features
Carbonate textures and structures - slumpsCarbonate textures and structures - slumps
Vertical well
Preferred orientation of
resistive patches,
possibly bioclasts.
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Carbonate sedimentolog
Carbonate textures and structures - stylolitesCarbonate textures and structures - stylolites
Horizontal well
Abundant
conductive/resistive
fractures
Stylolitic bed boundary
Abundant
conductive/resistive
fractures
C b d l li
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Carbonate sedimentolog
Carbonate textures and structures - stylolitesCarbonate textures and structures - stylolites
Horizontal well
Abundant
conductive/resistive
fractures
Stylolitic bed boundary
C b d l liC b t t t d t t t l lit
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Carbonate sedimentolog
Carbonate textures and structures - stylolitesCarbonate textures and structures - stylolites
Horizontal well
Conductive fracture
Stylolitic bed boundary
Large, scattered
angular-subangular
resistive patches.
Possibly large
bioclasts.
C b t t t d t t bi l tC b t t t d t t bi l t
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Carbonate sedimentolog
Abundant dark
conductive patchesinterpreted as vuggy
porosity. There is likely
to be good connectivity
in this example. The
bright, resistive patches
are probably bioclasts.
Carbonate textures and structures - bioclastsCarbonate textures and structures - bioclasts
Horizontal well
C b t t t d t tC b t t t d t t
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Carbonate sedimentolog
Vuggy porosity
Acoustic ImageResistivity Image
Carbonate textures and structures - vugsCarbonate textures and structures - vugs
Vertical well
C b t t t d t tC b t t t d t t
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Carbonate sedimentolog
Transition from bedding
(orange) to oversteepened
bedding (brown) across a
faulted (magenta) interval.
Conductive fractures (cyan)
are also observed.
Carbonate textures and structuresCarbonate textures and structures
Ch lk t t d t tChalk textures and structures
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Carbonate sedimentolog
Laminated/Debris Flow HardgroundTransportedSilic. Tight Zone
Top Porous
Chalk
Mottled,
bioturbated marl
Marl - Laminated Debris Flow SlumpedSlumped/Debris FlowMarl - Laminated
Marl/Laminated
Conductive
mottling
Minor healed
fractures
Slump fold
mottled fabric
Lower
Cretaceous
Base Tor
HardgroundChert Bands
Base
Ekofisk Debris flow
clasts
Laminations
minor
cemented fault
heavily mottled
fabric of indeterminate
transported chalk facies
EkofiskFormation
T
orFormation
Chalk textures and structuresChalk textures and structures
Porosity classificationPorosity classification
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Carbonate sedimentolog
Porosity classificationPorosity classification
After Nurmi et al. 1990
Carbonate textures and structuresCarbonate textures and structures
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Carbonate sedimentolog
Carbonate textures and structuresCarbonate textures and structures
Facies interpretationFacies interpretation
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Carbonate sedimentolog
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 interpretationFacies interpretation
Common facies typesCommon facies types
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Carbonate sedimentolog
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 typesCommon facies types
Carbonate lithofaciesCarbonate lithofacies
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Carbonate sedimentolog
GR
NPHI
RHOBStatic Dynamic Core
Stacked fining-upward
units with packstone-
grainstone bases and
wackestone tops
Carbonate lithofaciesCarbonate lithofacies
Vertical well
Resistive fracture
Conductive packstones-
grainstones
Resistive, mottled
wackestones
Carbonate lithofacies core calibrationCarbonate lithofacies core calibration
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Carbonate sedimentolog
Carbonate lithofacies core calibrationCarbonate lithofacies core calibration
Vertical well
Boundary between FMI facies FMI7 and FMI4. The
burrowed surface is clearly visible (arrowed) as are thepackstone-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
Carbonate lithofacies core calibrationCarbonate lithofacies core calibration
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Carbonate sedimentolog
Carbonate lithofacies core calibrationCarbonate 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 amudstone-wackestonesuccession
FMIF11
FMIF2
FMIF2
Static image Dynamic image
Slabbed core
Carbonate lithofaciesCarbonate lithofacies
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Carbonate sedimentolog
Carbonate lithofaciesCarbonate 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 6FMI image facies association
Ka(mD) FMIA1
FMIA2
FMIA3
FMIA4
FMIA5
Fractures in carbonatesFractures in carbonates
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Carbonate sedimentolog
Fractures occur in carbonates in the samemanner as in clastics.
However tension gashes associated with stylolitebands are a common feature in carbonates.
The interpretation techniques used for clasticsediment fractures are applied to carbonatefractures.
Fractures in carbonatesFractures in carbonates
Fractures in carbonatesFractures in carbonates
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Carbonate sedimentolog
Conductive fracture
Resistive fracture
Fractures in carbonatesFractures in carbonates
Vertical well
Fractures in carbonatesFractures in carbonates
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Carbonate sedimentolog
Bedding
Fractures
Fractures in carbonatesFractures in carbonates
Vertical well
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Carbonate sedimentolog
The ultimate goalThe ultimate goal
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