session 02-pertamina 2
TRANSCRIPT
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)