9 - Interpretation of Carbonates

8/3/2019 9 - Interpretation of Carbonates

1/43

Carbonate sedimentolo

Image log & dipmeterImage log & dipmeter

analysis courseanalysis course

Sedimentologicalinterpretation of

carbonates


2/43


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


3/43


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


4/43


Carbonate grain typesCarbonate grain types


5/43Carbonate sedimentolo

Dunham (1962)

Carbonate classificationCarbonate classification



Folk (1962)

Carbonate classificationCarbonate classification



Qualifiers:

v = vuggy

c = cemented

Image facies schemeImage facies scheme



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



Sedimentary structuresSedimentary structures


10/43Carbonate sedimentolog

Cross-stratification

SedimentarySedimentary

structuresstructures



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



Thalassinoides

Rosselia

Asterosoma

Teichichnus

Rhizocorallium

Chondrites

Terebellina

Palaeophycus

Ophiomorpha

Diplocraterion

Conostichus

Skolithos

Arenicolites

Modified from Pemberton et al. (1992)

Biogenic structuresBiogenic structures


13/43

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


14/43


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


15/43


Dolomitisation

DiageneticDiagenetic

structuresstructures


16/43


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


17/43


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


18/43


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


19/43


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


20/43


Intensely

stylolitisedwackestones

Carbonate textures and structures Carbonate textures and structures

intense stylolitisationintense stylolitisationVertical well


21/43


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.


22/43


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.


23/43


Stylolitisation

Carbonate textures and structures - stylolitesCarbonate textures and structures - stylolites

Vertical well

Large amplitude stylolitewith resistive halo


24/43


Large scale

slump features

Carbonate textures and structures - slumpsCarbonate textures and structures - slumps

Vertical well

Preferred orientation of

resistive patches,

possibly bioclasts.


25/43



Horizontal well

Abundant

conductive/resistive

fractures

Stylolitic bed boundary

Abundant


fractures

C b d l li


26/43



Horizontal well

Abundant


fractures


C b d l liC b t t t d t t t l lit


27/43



Horizontal well

Conductive fracture


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


28/43


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


29/43


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


30/43


Transition from bedding

(orange) to oversteepened

bedding (brown) across a

faulted (magenta) interval.

Conductive fractures (cyan)

are also observed.


Ch lk t t d t tChalk textures and structures


31/43


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


32/43


Porosity classificationPorosity classification

After Nurmi et al. 1990



33/43



Facies interpretationFacies interpretation


34/43


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


35/43


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


36/43


GR

NPHI

RHOBStatic Dynamic Core

Stacked fining-upward

units with packstone-

grainstone bases and

wackestone tops


Vertical well

Resistive fracture

Conductive packstones-

grainstones

Resistive, mottled

wackestones

Carbonate lithofacies core calibrationCarbonate lithofacies core calibration


37/43



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



38/43



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



39/43



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


40/43


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.




41/43


Conductive fracture

Resistive fracture


Vertical well



42/43


Bedding

Fractures


Vertical well


43/43


The ultimate goalThe ultimate goal

9 - Interpretation of Carbonates

Documents

Transcript of 9 - Interpretation of Carbonates