DIAGENESISDIAGENESISDEFINITION:

THOSE NATURAL CHANGES WHICH OCCUR IN SEDIMENTS BETWEEN THE TIME OF INITIAL DEPOSITION AND METAMORPHISM

COMMON CARBONATE MINERALOGIESCOMMON CARBONATE MINERALOGIESMINERAL FORMULA CHARACTERISTICS

ARAGONITE CaCO3 TRACE IMPURITIES;ORTHORHOMBIC

MG-CALCITE CaCO3 4-25% Mg IMPIRITIES;HEXAGONAL

CALCITE CaCO3 TACE IMPURITIES; HEXAGONAL

DOLOMITE CaMg(CO3)2 50% or so Mg; HEXAGONAL

DIAGENETIC PHENOMENA AFFECTING CARBONATES

• MINERALOGIC STABILIZATIONARAGONITE, CALCITE

• NEOMORPHISM (REPLACEMENT)CALCITE, CALCITE

• DOLOMITIZATIONCALCITE, DOLOMITE

• CEMENTATIONVOID-FILLING CALCITE,DOLOMITE or EVAPORITES

• SILICIFICATION• PRESSURE SOLUTION /

COMPACTION• DISSOLUTION /

KARSTIFICATION• BRECCIATION / FRACTURING

FRESH WATER VADOSE ENVIRONMENT

• CEMENTS TEND TO BE– MENISCUS– PENDULOUS– EQUANT CALCITE– RHOMBIC CALCITE

• OTHER CHARACTERISTICS– LEACHING OF ARAGONITE– SLIGHT CEMENTATION– COMMON POROSITY

FRESH WATER PHREATIC ENVIRONMENT

• CEMENTS TEND TO BE– ISOPACHOUS BLADED– EQUANT CALCITE– INTERLOCKING CRYSTALS– COARSER TO PORE CENTER

• OTHER CHARACTERISTICS– SOME LEACHING OF ARAGONITE; LEACHING MAY BE ACCOMPANIED BY

CALCITE REPLACEMENT.– LOW POROSITY– RAPID CEMENTATION– SYNTAXIAL OVERGROWTHS ON ECHINODERMS

MARINE PHREATIC ENVIRONMENT

• CEMENTS TEND TO BE– ISOPACHOUS ARAGONITE NEEDLES– MICRITIC Mg-CALCITE– COMMONLY INTERBEDDED WITH INTERNAL SEDIMENT– SOMETIMES BOTRYOIDAL– SOMETIMES BORED

• OTHER CHARACTERISTICS– NO LEACHING– SLOW CEMENTATION EXCEPT WHERE TIDES PUMP WATER

THROUGH SEDIMENT– POLYGONAL BOUNDARIES– MANY MINOR DISCONFORMITIES

DEEP SUBSURFACE ENVIRONMENT

CHARACTERISTICS:

• DISSOLUTION or CEMENTATION POSSIBLE

• SLOW RATES OF DIAGENESIS CAUSED BY:

– NEAR-SURFACE STABILIZATION OF ARAGONITE & Mg-CALCITE TO FORM CALCITE

– NEAR-SURFACE CEMENTATION REDUCES POROSITY & PERMEABILITY WHICH INHIBITS WATER MOVEMENT IN THE DEEP SUBSURFACE

BASIC REQUIREMENTS FOR DOLOMITE FORMATION

•• SOURCE OF MgSOURCE OF MgSEAWATERMf-RICH CLAYS FOR CEMENTSKELETAL Mg CALCITE

•• FLUID FLOW SYSTEMFLUID FLOW SYSTEM•• SUITABLE Mg/Ca RATIOSUITABLE Mg/Ca RATIO

Effects of early Effects of early diagenesisdiagenesis on reservoir quality,on reservoir quality,Burial Burial diagenesisdiagenesis will modify reservoir Quality (RQ)will modify reservoir Quality (RQ)

further information required in purplefurther information required in purple

INITIALLY POROUSAND PERMEABLE UNIT(Sedimentology)

MARINE DIAGENESIS following depositionCementation, little effect on poroperm.no dissolution

EXPOSED(Sedimentology)

Close tounconformity

METEORIC PHREATIClimited dissolution,cementation by low Mgcalcite around grains

METEORIC VADOSEExtensive dissolutionlimited but patchy cementation(especially at pore throats)

Poroperm decrease, frameworkresistant to mechanical compactionduring burial

Porosity increase, permeabilitylowered, framework resistant tomechanical compaction

COMPACTIONLIMITED?

RQ poor RQ good RQ moderate RQ moderate RQ moderate - good

no yes

no yes

no yes

yes no

Effects of Burial diagenesis on reservoir quality

COMPACTION(Effective Stress)

CEMENTATION(Petrography)

DISSOLUTION(Petrography)

BURIAL DOLOMITISATION(Petrography)

FRACTURING(Well Data)

UPLIFT AND EXPOSURE(Seismic)

Quantificationof effects?

Source of cementExtent of cement

Cementation

Yes(compartmentalise)

No source of cement?Hydrocarbon filling?(Geochemistry)

local

(Gheochemistry)

regional

Reprecipitation as cementElsewhere in basin

No(inc. permeability)

Import of Ca MgCO3

Yes

No

mechanical

chemical

further information required in purple

RQREDUCED

RQREDUCED

RQIMPROVED

RQREDUCED

RQIMPROVED

RQ MAYIMPROVED

RQMAINTAINED

RQREDUCED

RQREDUCED

RQIMPROVED

RQMAINTAINED

Process 1 2A 2B 3A 3B 4A 4B 5A 5BPor. -ve -ve 0 +ve -ve -ve 0 0 0Perm. -ve -ve 0 +ve -ve -ve +ve -ve +ve

1

yes no2A

2B

3A 3B

4A

4B

5A

5B

COMPARISON OF MAJOR SUBSURFACE DIAGENETIC CONTROLS

IMPORTANTUNIMPORTANTTEMPERATURE

VERY IMPORTANTUNIMPORTANTPRESSURE

LONG PERIODSHORT PERIODTIME OF RESIDENCE

LOWVERY HIGHRATE OF WATER INFLUX

SLIGHT VARIATIONWIDE VARIATIONEQUILIBRIUM CONDITIONS

MINIMAL IMPORTANCE

VERY IMPORTANTMINERAL STABILIZATION

VERY IMPORTANTMINIMAL IMPORTANCE

STRUCTURAL CONTROL

BURIAL DIAGNESISNEAR-SURFACE

PROBLEMS withCARBONATE RESERVOIR

Heterogeneous porosity and permeability complex depositional environments diagenetic overprints

What are the most important controls onWhat are the most important controls onreservoir quality in carbonate sequences?reservoir quality in carbonate sequences?

The main controls on reservoir quality (porosity and permeability)in carbonate sequences are :

• depositional fabric (primary lithofacies, texture)• mineral dissolution (creation of secondary porosity)• mineral precipitation (cementation and replacement)• karstification (an important from of mineral

dissolution/precipitation)• compaction• fracturing

Effects of meteoric Effects of meteoric diagenesisdiagenesison reservoir qualityon reservoir quality

Effects on RQif yes

Meteoric Diagenesis

• Was there an aragonite precursor? (I.e. of Pre-Cambrian, Carboniferous, Permian, Triassic or Tertiary age)

• Was it a humid climate at time of exposure? (Palaeogeographic position)

• Was there a high rate of water drainage? (elevation, climate, size of hinterland)

• Was reprecipitation of dissolved CaCO3 as cement limited (due to high drainage)?

+ve

+ve

+ve

+ve

Effects of Effects of karstificationkarstificationon reservoir qualityon reservoir quality

Effects on RQif yes

Sequence Boundary Karst

• Is there a joint/fracture system which may have had high water throuhput? (may be so if in faulted/folded terrain, if uplifted or recognizable on seismic)

• Was there an aragonite precursor? (I.e. of Pre-Cambrian, Carboniferous, Permian, Triassic or Tertiary age)

• Is the reservoir close to the unconformity/above the water table?

• Is the pore system matrix dominated? (vuggy porosity may have poor permeability, caverns may be detrimentalto drilling)

• Can overlying clays/shales be ruled out? (May infiltrate porous zone beneath)

+ve

+ve

+ve

+ve

+ve

RESERVOIR QUALITY

< 10 md< 5 %POOR

10 – 50 md5 – 15 %FAIR

> 50 md> 15 % GOOD

> 100 md> 20 %EXCELLENT

PERMPOROSITY

Recurrent carbonate reservoir types (after Wilson, 1983)

• MIDDLE-SHELF GRAINSTONE BARS WITH PRIMARY (or MODIFIED PRIMARY) POROSITY

• MIDDLE- and OUTER-SHELF REEFS2a. PRIMARY POROSITY IN BOUNDSTONES and

ASSOCIATED GRAINSTONES2b. INTERCRYSTALLINE POROSITY and FRACTURE

POROSITY if dolomitized• GRAINSTONES AND BRECCIAS of SLOPE DEPOSITS• INNER-SHELF DOLOMITES (INTERCRYSTALLINE POROSITY

and VUGGY POROSITY) WITH ANHYDRITE SEALS – TIDAL FLAT DEPOSITS

• DISSOLUTION, PALEOKARST DEVELOPMENT and DOLOMITIZATION BELOW REGIONAL UNCONFORMITIES

• FRACTURED CARBONATE RESERVOIRS• CHALKS with INTERCRYSTALLINE POROSITY and FRACTURE

POROSITY

PERMEABILITAS OF ROCKS AND SEDIMENTS

• Tightly cemented criniodal limestone ……10 md• Uncemented carbonate mud ……0.01 – 10 md• Sucrosic dolomite ……0.1 – 150 md• Cemented quartz or sandstone

or carbonate grainstone ……10 – 300 md• Poorly cemented quartz sandstone

or carbonate grainstone ……300 – 500 md• Unconsolidated quartz sandstone

or carbonate grainstone ……>1000 md• Fractured sandstone or carbonate ……>1000 md

WP POROSITYintraparticle porosity

• Refers to pores formed where soft body parts lived in body cavities (e.g., gatropods) or pores where internal partitioning in otherwise solid material (e.g., rudist wall structures)

• may add considerably to the total porosity of grainstones and packstones

• are commonly enlarged by dissolution to form moldicor vuggy porosity

• an example follows of 10 perm plugs measured from three coral heads (Holocene), yielding average porosities of 47, 63 and 53 %

BC POROSITY intercrystalline porosity

• Forms between crystals of dolomite or limestone

• provides one of the most “evenly distributed”types of porosity in carbonates (except for vugs)

• occurs as mesoporosity and macroporosity in dolomites

• occurs as microporosity in limestone (within the lime mud matrix

BP POROSITYinterparticle porosity

• Intergranular pores from spherical triangles between packed spheres and irregular between platy grain shapes

• rare in lithified rock--not commonly preserved due to cementation

• commonly is modified by thin isopachous rim cements that form in the marine phreatic

• common in jurassic carbonate of the Middle East and accounts for the success of these giant reservoirs

KV POROSITYkeystone vug porosity

• Forms by the natural bridging of sand grains to form a “keystone arch” with pore space below it

• forms in the swash zone of beaches• relatively rare porosity type • recognized in the Pleistocene of the

Bahamas

FENESTRAL POROSITY

• A porosity type commonly associated with algal stromatolite lithofacies

• voids formed within algal laminations by algae forming bridges and growing over other algal layers or by air/gas pockets within algal layers

• “fenestra” means window in latin and refers to the window-like openings within algal layers

• fenestral porosity may not be effective porosity

GF POROSITYgrowth framework porosity

• associated with boundstone fabrics and reefs• created by branches or tubes winding and

bridging together to form pore space betweentheir framework elements (not within them)

• one of the most difficult pore types to indentify• may be relatively unimportant, since detritus

commonly fills such spaces at the time of deposition

MO and VUG POROSITYmoldic and vuggy porosity

• molds retain original particle shape • vugs are irregular in shape• aragonite grains are subject to dissolution and

the formation of molds and vugs• molds can leach further to form vugs• the term MV porosity is coined for moldic-vuggy

porosity combinations that are often difficult to separate

• moldic porosity (especially oomoldic) may represent isolated pores with non-effective porosity

FR POROSITYfracture porosity

• Brittle versus ductile behavior: dolomites fracture more readily than limestones

• Orientation of most natural fractures is verticle• Maximum amount of porosity due to fracturing (e.g.,

in the Monterrey Shale of California) is about 6 %• It is commonly beneficial to induce fracturing in the

area surrounding the borehole to increase daily production rates: acid-fracs with propants

• Presence of fractures is critical for reservoir faciesdevelopment in tight boundstones that grade to wackestones and in chalk deposits (coccolithmudstones)